Is it possible to Breed an Animal to have more Females than Males?

Is it possible to Breed an Animal to have more Females than Males?

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Minute Earth talked about environmental factors that cause animals and humans to have more male or female children:

Presuming that is accurate, I was wondering if you could artificially select for people or animals who have more children of a certain gender?

As far as I know, there are no animals that do produce notably more males than females, or vice versa, except maybe some insects.

Yes. What you are looking for is a meiotic drive, it's a genetic factor that favors its own transmission. Sex linkage is very common in these due to sex chromosomes not undergoing recombination. Once such driver, R2D2 (I shit you not) is a Y linked gene that drastically increases the number of male offspring in a mouse that carries it, to the point it can cause a population collapse from lack of females. Several such drives are known, none quite as drastic as R2D2.

Many animals can give birth without mating

We have written before about the strange but spectacular phenomenon of virgin births, or "parthenogenesis" as it's known.

Some animals are fully asexual and do not need a male to give birth: for instance, some species of whiptail lizards. But there are also animals that can mate with a male, but do not always do so, and they are the ones we are considering.

Here we report four new cases published in the scientific literature in 2015. They all point to the idea that, even in sexually-reproducing species, many animals have long been able to go it alone.

Stick insects

Female Australian giant prickly stick insects will mate with males when it suits them, but they have found ways to repel them so they can have young without any male interference.

In a study published in the journal Animal Behaviour in March 2015, scientists examined why the females sometimes do without a male.

It was not that males are rare or absent, which is thought to be a key driver for parthenogenesis in other species. Instead, the team proposed that sex can be very costly for females, so they might prefer to take their chances alone if they can.

They win sexual conflicts more frequently than females&hellip despite female resistance

Female giant prickly stick insects will even fight off lustful males. First, they emit an anti-aphrodisiac chemical to stave off temptation. If a male is still keen, the female will curl her abdomen and kick her legs to repel him.

"Since females that have started reproducing parthenogenetically are no longer attractive to males, such females appear to have the opportunity to continue to reproduce exclusively via parthenogenesis," the team says.

All the offspring from parthenogenesis are female. So if the female stick insects carry on reproducing alone, the males could be wiped out.

But for now the males still have a fighting chance. They "win sexual conflicts more frequently than females&hellip despite female resistance," the team says.

This may help explain why parthenogenesis remains rare, even in species that are capable of it. In such species, "males typically force females to mate".

Parthenogenesis has been documented in several species of captive snakes, but it was long thought to be something females only did when there were no males around.

That changed in 2012, when Warren Booth of the University of Tulsa in Oklahoma, US discovered that two litters of wild pit vipers had been born via parthenogenesis.

These snakes are half clones of their mother, so they are highly inbred

It was the first time parthenogenesis had been documented in wild-caught snakes, which presumably had access to males. One of the baby snakes has since gone on to have healthy offspring.

This year another team noticed an instance of a pit viper virgin birth, but this time the young did not survive. A captive female gave birth to one stillborn snake and four undeveloped ova. Two years later, the same snake had another virgin birth.

We don&rsquot know for sure why her offspring died, but the incident is telling. It highlights that this form or reproduction can be far from ideal, says lead author Mark Jordan of Indiana University &ndash Purdue University Fort Wayne in Indiana, US.

"These snakes are half clones of their mother, so they are highly inbred," says Jordan. "When parthenogenesis happens, there's a lot of mortality or lack of development."

Nevertheless, Jordan says it is clear that reproducing this way has long been "fundamental to their biology". "It's something they may use periodically in situations where there are no males around to mate with, when populations are low or if they are moving into new habitats."

The animal in question was the endangered smalltooth sawfish, which had never previously been documented reproducing parthenogenetically. Virgin births have been seen in sharks, which are related to sawfish, but only in captive sharks.

In the wild, it is much harder to know whether parthenogenesis has taken place. The evidence came from genetic testing.

The discovery came about by chance. The sawfish population is dropping, so ecologists were studying their genes to understand how this is affecting them. "We were looking at how much genetic variation remains," says co-author Kevin Feldheim of the Field Museum of Natural History in Chicago, Illinois, US.

A last-ditch effort for females to pass on their genes

The young sawfish were healthy and thriving, despite being inbred.

We do not know why the female smalltooth sawfish chose to undergo a virgin birth. But it could be a survival strategy when population levels are low. "If they can't find a mate, it's possible this mechanism kicks in as a last-ditch effort for these females to pass on their genes," says Feldheim.

The team has now taken 130 further samples from wild smalltooth sawfish. They are now analysing them to see how often they use parthenogenesis.

Strictly speaking lizards should not be on this list. We know that, in general, the lizards that have virgin births are all female and asexual. They have no choice but to reproduce alone.

But it turns out the story is not that simple. A study published in the Journal of Herpetology in August 2015 reported that one lizard species, thought to be all female, has males after all.

For this lizard parthenogenesis may be a successful strategy

Eight male Muller's tegus were discovered among 192 adults found in 34 different places in South America. It was the first time males of this species have ever been found, even though it is abundant in several areas.

This suggests that some Muller's tegus reproduce sexually. However, the asexual ones are thought to be strict about their no-males policy.

"We expect that parthenogenetic females do not cross with the males, but normal females do," says lead author Sergio Marques de Souza of The University of São Paulo in Brazil. "In this sense, sexual and asexual lizards are distinct evolutionary units, since we believe that there is no genetic exchange between them."

The existence of these males may provide new clues into how the species became parthenogenetic in the first place.

Muller's tegus have been doing it &ndash or rather, not doing it &ndash for four million years

It is generally believed that parthenogenesis arises in lizards through hybridization: when two related species mate, resulting in a new species. All the offspring of these hybrids are then female.

Now that males have been found, it suggests this may not be the case. Instead parthenogenesis could have arisen spontaneously due to environmental pressures, says de Souza.

His analysis also suggests Muller's tegus have been doing it &ndash or rather, not doing it &ndash for four million years. "It contradicts previous studies, which proposed that parthenogenetic organisms have low genetic variation and, consequently, low evolutionary success," says de Souza.

For this lizard at least, parthenogenesis may be a successful strategy.

Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.


Increasing global temperatures threaten marine turtle populations [1–4]. Most authors’ concerns grow from consideration of temperature-dependent sex determination (TSD), the mechanism by which incubation temperature of the nest directly impacts the sex of the embryo [5,6]. In marine turtles, warmer incubation temperatures tend to produce females, whereas cooler temperatures tend to produce males [7, 8]. Authors are concerned that higher temperatures will cause such a female bias in sex ratios that populations will face extinction [9,10]. Currently however, the magnitude of the sex ratio skew in adults is unknown due to our limited understanding of the proportion of adult males (and males approaching sexual maturity) [11]. Marine turtle individuals are often widely distributed geographically, outside of the nesting season. Dispersed members of populations make detecting sex ratio issues across populations challenging. In addition, adult males are very difficult to access because they rarely come to land. While a determination of adult sex ratio is beyond reach, a functional alternative, breeding sex ratios (BSR: the proportion of males and females that successfully mate at any time) [12] can be used to identify the minimum number of males and females contributing to populations. By estimating BSR at small, growing nesting aggregations a more thorough proportion of the nesting beach can be assessed than at large nesting beaches, and inferences can be made about the impact of climate change on the population as a whole [13, 14].

The loggerhead sea turtle (Caretta caretta) is listed globally as vulnerable by the International Union for the Conservation of Nature (IUCN) [15]. However, along the continental US and adjacent waters in the Northwest Atlantic Ocean, it is listed as threatened [16]. The Northwest Atlantic contains one of only two marine turtle nesting aggregations of greater than 10,000 individuals nesting annually [15, 17]. Florida nesting loggerheads make up approximately 90% of that aggregation [18–20]. Florida Fish and Wildlife Conservation Commission (FWC) estimated that 184,064 loggerhead nests were laid in the 2016 nesting season and the overall nesting trend is an increasing one across the state [21].

Due to their accessibility, nesting females, nest success, and hatchlings are frequently examined and used for demographic studies and population models [22–24]. Data on nesting females and hatchlings are supplemented with in-water capture/recapture and satellite tag studies, which provide additional information on the number of turtles [25,26]. The Turtle Expert Working Group estimated that the female loggerheads return to nest every 2.5 years on average [19] however, using mark-recapture data over a 20-year data set, Phillips et al. estimated it at an average of 3.2 years for turtles nesting in Southwestern Florida [27]. From tagging and resighting data, it has been estimated that loggerheads lay on average from 3–4.1 nests per season [22, 28, 29] while satellite tagging suggests that within the Gulf of Mexico, the average is closer to 5.4 nests per season [30]. Nest frequency is an important metric because it can be used to calculate how many females nest each year. Unfortunately, information regarding adult male behavior and number is lacking. Many in-water capture studies do not identify the sex of the turtles [31]. Studies that do identify the sex of captured individuals tend to examine juvenile sex ratios [32–35] or are focused on migration or distribution [36,37]. Consequently, male sea turtles’ reproductive behavior is poorly understood and sex ratio cannot be estimated directly.

A variety of methods have been used to infer aspects of male reproductive behavior. In all seven-extant species of marine turtles, it has been shown that sperm from more than one male can fertilize a single clutch (multiple paternity) [12, 38–43]. Furthermore, in at least one species, a single male may mate with more than one female [44]. Little is known about mate choice, and while direct observations of multiple matings occur [45, 46], assigning which male(s) successfully father young from observed copulations may not be accurate. Hormonal studies suggest that loggerhead males could mate annually [47] and satellite tracking of adult males suggests that about 40% remain close to nesting beaches during a breeding season and may therefore mate more than once [11, 37]. Together, these findings suggest that males contribute to multiple nests during a nesting season and might breed more frequently than females. The number of males fathering each clutch can be determined genetically and used to estimate the minimum BSR [12, 48]. Whether the BSR or reproductive behavior vary among populations is unknown.

The primary goal of this study was to estimate the breeding sex ratio for the loggerhead turtle nesting on a small nesting beach on the southwestern coast of Florida. To this end, paternal genotypes were identified through exclusion analysis and were used to estimate the number of males contributing to this population.


Mules generally have a somewhat shorter lifespan than their donkey counterparts. Mules may live between thirty and forty years while a donkey's lifespan can range from thirty to as much as fifty years. The lifespans for both animals is largely dependent upon their individual living and working conditions. Animals who toil under heavy loads for extended periods of time may only live to an age of between twelve and fifteen years.

A spotted mule. Image credit: MuleGirl/

Mules are hybrid animals which are born after a female horse (or mare) mates with a male donkey (called a jack). Physically they more closely resemble horses more than do their donkey counterparts. This is particularly true when it comes to examining the mule's ears which are smaller in size than a donkey's and more in line with the physical structure of horse ears. This is also true in terms of the mule's tail which looks a lot like that of a horse's tail. Conversely, the tail of a donkey is coarse and looks much more similar to that of a cow's. Mules are characterized by their short manes and heads as well as thin legs. Mules are also taller than donkeys. The two animals also display subtle physical differences in regards to the appearance of their teeth, neck shape, and coat.

Mules are used in a variety of industries and environments throughout the world including mining, farming, and transporting heavy loads as draft animals.

Male mules are called jack or john while females are referred to as molly.

Mules are said to have harder hooves than that of horses. This is another reason why they are able to handle carrying heavy loads over long distances.

The mule's coat is usually brown, grey, or black but a small percentage may be white, buckskin, or palomino.

There are approximately twelve million mules in the world with most living in countries such as China. Mexico, and Brazil.

Pathogenic animals: Animals that can reproduce asexually

There are over 80 known species of fish, reptiles and amphibians that reproduce parthenogenetically. These species rely on facultative parthenogenesis only under dire circumstances, including when females are isolated from males.

Here are a few examples of parthenogenetic species from different taxa.

Komodo dragons

Let&rsquos be honest&hellip who would have ever thought the world&rsquos largest lizard, the Komodo dragon (Varanus komodoensis), could reproduce via parthenogenesis? But yes, it&rsquos true! In 2006, a female Komodo dragon housed in Chester Zoo, U.K. laid a clutch of 25 eggs, even though she had never mated or been in the presence of a male dragon.

Similarly, in the London Zoo, another captive-bred female produced four eggs two-and-a-half years after her last interactions with a male dragon. This individual laid another clutch after mating with a male dragon, which revealed that the species used facultative parthenogenesis to reproduce.

Komodo dragon populations are dwindling across the world and rely on captive breeding programs for their survival. (Photo Credit : Sergey Uryadnikov/Shutterstock)

Komodo dragons are only found in some parts of the world, and they are under severe threat from poaching. As a result, their populations are often skewed&mdashwith fewer males and more females (or vice versa). Here, it is likely that female dragons were forced to adopt facultative parthenogenesis due to the lack of male dragons in captivity.

Stick insects

In 2013, a group of Australian scientists investigated the life history of spiny leaf stick insects (Extatosoma tiaratum). They found that the females resisted mating with males by using one of three approaches. They would either kick their legs and curl their abdomens, or they would alter their pheromones&mdashthe odor molecules organisms release to communicate&mdashto appear inconspicuous to males, or they would secrete anti-aphrodisiac chemicals that repelled males.

The study concluded that female stick insects, under some circumstances, benefit by not mating. This, researchers suspect could further lead to the evolution of facultative parthenogenesis in the species.

Even the smallest of creatures can rely on parthenogenesis for reproduction. (Photo Credit : Aedka Studio/Shutterstock)

Copperhead snake

Several snake species can reproduce via parthenogenesis. One Copperhead (Agkistrodon contortrix) in Indiana, USA, gave birth to a stillborn offspring and four infertile eggs. They had captured this individual from the wild and kept her in an enclosure, one she had never shared with another snake. In fact, she had not mated in nine years.

Would you have ever imagined a snake being able to reproduce without mating? (Photo Credit : Creeping Things/Shutterstock)

Desert grassland whiptail lizards

Desert grassland whiptail lizards (Aspidoscelis uniparens), as their name suggests, are found in desert and grassland ecosystems in the United States of America.

Harsh environments, such as deserts, often force species to alter their reproductive ways. (Photo Credit : Nina B/Shutterstock)

This species is unique, as they are an all-female species. Therefore, as you might have already guessed, they can only reproduce using parthenogenesis. Desert whiptails reproduce offspring via meiosis, and because all of their chromosomes come from the mother, all the offsprings are clones and female.

This species is also known to exhibit male-like behavior in order to initiate pseudo-copulation with other females, which stimulates reproduction.

One of the major advantages of parthenogenesis in this species is that it can reproduce much faster than those species that reproduce sexually, allowing for a rapid increase in population when conditions are ideal.

Zebra shark

A group of Australian researchers investigating captive zebra sharks (Stegostoma fasciatum) found that the species could switch from sexual reproduction to parthenogenetic reproduction in captive environments.

In 1999, the researchers introduced a female zebra shark captured from the wild to a captive male zebra shark. During this time, the two mated, after which they separated the pair. This reuniting and separating went on for a few more years until 2012, when the male was permanently separated from the female. Once the mating stopped, so did her production of eggs.

The next year, in 2013, the researchers introduced the female&rsquos daughter into her tank. Interestingly, during this time, the mother began laying eggs again! However, an even bigger surprise came from her daughter, who had reached maturity herself and began laying her own eggs, even though she had never mated with a male!

Most incidents of parthenogenesis, including in zebra sharks, have been observed in captive environments, such as aquariums or zoos. (Photo Credit : Tatiana Belova/Shutterstock)

The researchers believe that the embryos in the mother&rsquos egg developed because the shark could store the male&rsquos sperm for a long time, or because it was parthenogenetic. On the other hand, parthenogenesis seemed most likely for the daughter, as she had never mated.

There are a few reasons animals choose (or are forced) to reproduce without mating. To begin with, parthenogenesis eliminates the cost of sexual reproduction entirely by avoiding any investments by males or in courtship. This saves animals a lot of time and energy.

Second, it helps species like komodo dragons thrive in uninhabited islands, as a single female can create a population on her own. Finally, such processes are likely the last resort for reproduction for many reptiles, insects, and amphibians living in harsh environments, such as deserts.

However, parthenogenetic species are often termed as &lsquodead-ends&rsquo, since they produce clones, which do not have any new trait combinations. Because all the offspring are clones and are incapable of adapting to changing environments, they succumb faster to disease, which can ultimately threaten or reduce their populations in drastic ways.

The mixed-up world of hybrid animals

If a zoo keeps a male lion and a female tiger in the same enclosure, a liger can result. It has a mix of its parents’ traits.

Алексей Шилин/Wikimedia Commons

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September 13, 2018 at 5:45 am

Deep in the Amazon rainforest live two green birds. The snow-capped manakin, has a splash of white on its head. The opal-crowned manakin looks very similar. But this species’ crown can appear white, blue or red depending on the light. It’s “like a rainbow,” says Alfredo Barrera-Guzmán. He is a biologist at the Autonomous University of Yucatán in Mérida, Mexico.

Thousands of years ago, these two species of birds started mating with each other. The offspring initially had crowns that were dull whitish-grey, Barrera-Guzmán suspects. But in later generations, some birds grew yellow feathers. This bright color made males more attractive to females. Those females may have preferred mating with yellow-capped males rather than snow-capped or opal-crowned males.

Eventually, those birds became separate enough from the two original species to be their own, distinct species: the golden-crowned manakin. It’s the first-known case of a hybrid bird species in the Amazon, he says.

Usually, different species don’t mate. But when they do, their offspring will be what are called hybrids.

The molecules of DNA in each of an animal’s cells hold instructions. These guide what an animal looks like, how it behaves and the sounds it makes. When animals mate, their young get a mixture of the parents’ DNA. And they can end up with a mixture of the parents’ traits.

If the parents are from the same species, their DNA is very similar. But DNA from different species or species groups will have more variations. Hybrid offspring get more variety in the DNA they inherit.

So what happens when the DNA of two animal groups mix in a hybrid? There are many possible outcomes. Sometimes the hybrid is weaker than the parents, or doesn’t even survive. Sometimes it’s stronger. Sometimes it behaves more like one parent species than the other. And sometimes its behavior falls somewhere in between that of each parent.

Scientists are trying to understand how this process — called hybridization (HY-brih-dih-ZAY-shun) — plays out. Hybrid birds may take new migration routes, they found. Some hybrid fish appear more vulnerable to predators. And rodents’ mating habits may affect what their hybrid offspring can eat.

Wise to hybridize?

Hybridization happens for many reasons. For instance, the territory of two similar types of animals may overlap. This happens with polar and grizzly bears. Members of the two groups of animals have mated, producing hybrid bears.

When the climate changes, a species’ habitat can shift to a new area. These animals may encounter other, similar species. The two groups may mate by accident. For instance, researchers have found hybrids of southern flying squirrels and northern flying squirrels. As the climate warmed, the southern species moved north and mated with the other species.

When animals can’t find enough mates from their own species, they may select a mate from another species. “You have to make the best out of the situation,” says Kira Delmore. She is a biologist at the Max Planck Institute for Evolutionary Biology in Plön, Germany.

Scientists have seen this happen with two antelope species in southern Africa. Poachers had thinned out the populations of giant sable antelope and roan antelope. Later, the two species bred with each other.

People can unwittingly create opportunities for hybridization, too. They might put two closely related species in the same enclosure at a zoo. Or as cities expand, urban species may increasingly encounter rural ones. People may even set loose animals from other countries, accidentally or on purpose, into a new habitat. These exotic species now may encounter and mate with the native animals.

Many hybrid animals are sterile. That means they may be able to mate, but they won’t create offspring. For example, mules are the hybrid offspring of horses and donkeys. Most of these are sterile: Two mules can’t make more mules. Only a horse mating with a donkey can make another mule.

Biodiversity is a measure of the number of species. In the past, many scientists assumed that hybridization wasn’t good for biodiversity. If many hybrids were produced, the two parent species could merge into one. That would reduce the variety of species. That’s why “hybridization was often viewed as a bad thing,” Delmore explains.

But hybridization sometimes can boost biodiversity. A hybrid might be able to eat a certain food that its parent species cannot. Or maybe it can thrive in a different habitat. Eventually, it could become its own species, like the golden-crowned manakin. And that would increase — not decrease — the variety of life on Earth. Hybridization, Delmore concludes, is “actually a creative force.”

Going their own way

Hybrids can be different from their parents in many ways. Appearance is just one. Delmore wanted to know how hybrids might behave differently than their parents. She looked to a songbird called the Swainson’s thrush.

Over time, this species has split into subspecies. These are groups of animals from the same species that live in different areas. However, when they do encounter each other, they can still breed and produce fertile young.

One subspecies is the russet-backed thrush, which lives on the west coast of the United States and Canada. As its name implies, it has reddish feathers. The olive-backed thrush has greenish-brown feathers and lives farther inland. But these subspecies overlap along the Coast Mountains in western North America. There, they can mate and produce hybrids.

One difference between the two subspecies is their migration behavior. Both groups of birds breed in North America, then fly south in winter. But russet-backed thrushes migrate down the west coast to land in Mexico and Central America. Olive-backed thrushes fly over the central and eastern United States to settle in South America. Their routes are “super different,” Delmore says.

The birds’ DNA contains instructions for where to fly. Which directions do hybrids get? To investigate, Delmore trapped hybrid birds in western Canada. She placed tiny backpacks on them. A light sensor in each backpack helped record where the birds went. The birds flew south to their wintering grounds, carrying the backpacks on their journey.

The next summer, Delmore re-captured some of those birds back in Canada. From the sensors’ light data, she figured out what time the sun had risen and set at each point along the bird’s journey. The length of the day and timing of midday differs depending on location. That helped Delmore deduce the birds’ migration paths.

Some hybrids roughly followed one of their parents’ routes. But others didn’t take either path. They flew somewhere down the middle. These treks, though, took the birds over rougher terrain, such as deserts and mountains. That could be a problem because those environments might offer less food to survive the long journey.

Another group of hybrids took the olive-backed thrush’s route south. Then they returned via the russet-backed thrush’s path. But that strategy might also cause problems. Normally, birds learn cues on their way south to help them navigate back home. They might notice landmarks such as mountains. But if they return by a different path, those landmarks will be absent. One result: The birds migration might take longer to complete.

These new data might explain why the subspecies have remained separate, Delmore says. Following a different path may mean that hybrid birds tend to be weaker when they reach the mating grounds — or have a lower chance of surviving their yearly journeys. If hybrids survived as well as their parents, DNA from the two subspecies would mix more often. Eventually these subspecies would fuse into one group. “Differences in migration could be helping these guys maintain differences,” Delmore concludes.

Perils of predators

Sometimes, hybrids are shaped differently than their parents. And that can affect how well they avoid predators.

Anders Nilsson recently stumbled onto this finding. He is a biologist at Lund University in Sweden. In 2005, his team was studying two fish species named common bream and roach (not to be confused with the insect). Both fish live in a lake in Denmark and migrate into streams during winter.

Explainer: Tagging through history

To study their behavior, Nilsson and his colleagues implanted tiny electronic tags in the fish. These tags allowed the scientists to track the fish’s movements. The team used a device that broadcast a radio signal. Tags that received the signal sent back one of their own that the team could detect.

At first, Nilsson’s team was interested only in roach and bream. But the researchers noticed other fish that looked like something in between. The main difference was their body shape. Viewed from the side, the bream appears diamond-shaped with a taller middle than its ends. The roach is more streamlined. It’s closer to a slim oval. The third fish’s shape was somewhere between those two.

“To the untrained eye, they just look like fish,” Nilsson admits. “But to a fish person, they are hugely different.”

Roach and bream must have mated to produce those in-between fish, the scientists thought. That would make those fish hybrids. And so the team began tagging those fish, too.

Fish-eating birds called great cormorants live in the same area as the fish. Other scientists were studying the cormorants’ predation of trout and salmon. Nilsson’s team wondered if the birds were eating roach, bream and hybrids as well.

Cormorants gobble fish whole. Afterward, they spit out unwanted parts — including electronic tags. A few years after the researchers had tagged the fish, they visited the cormorants’ nesting and roosting sites. The birds’ homes were pretty gross. “They throw up and defecate all over the place,” Nilsson says. “It’s not pretty.”

But the researchers’ search was worth it. They found a lot of fish tags in the birds’ mess. And the hybrids appeared to fare the worst. For their efforts, the team found 9 percent of the bream tags and 14 percent of the roach tags. But 41 percent of the hybrids’ tags also turned up in the nests.

Nilsson isn’t sure why hybrids are more likely to be eaten. But perhaps their shape makes them easier targets. Its diamond-like shape makes bream hard to swallow. The roach’s streamlined body helps it quickly swim away from danger. Since the hybrid is in between, it may not have either advantage.

Or maybe hybrids just aren’t very smart. “They could be sort of stupid and not react to the predator threat,” Nilsson says.

Picky mating

Just because scientists find hybrids doesn’t mean the two species will always breed with each other. Some animals are choosy about which mates they’ll accept from another species.

Marjorie Matocq studied this question in rodents called woodrats. Matocq is a biologist at the University of Nevada, Reno. She started studying California’s woodrats in the 1990s. Matocq found these creatures interesting because they were very common, but scientists knew so little about them.

In a recent study, her team focused on two species: the desert woodrat and Bryant’s woodrat. Both live in the western United States. But desert woodrats are smaller and inhabit dry areas. The bigger Bryant’s woodrats live in shrubby and forested areas.

At a site in California, the two species overlapped. The animals here were mating and producing hybrids, but Matocq didn’t know how common this was. “Is it just a chance accident, or is this happening all the time?” she wondered.

To find out, the researchers brought woodrats to their lab. They set up tubes shaped like a T. In each experiment, the scientists placed a female desert woodrat or Bryant’s woodrat at the bottom of the T. Then they put a male desert woodrat and a male Bryant’s woodrat in opposite ends of the top of the T. The males were restrained with harnesses. The female could then visit either male and decide whether to mate.

Female desert woodrats almost always mated with their own species, the scientists found. These females may have avoided Bryant’s woodrats because those males were bigger and more aggressive. Indeed, the males often bit and scratched the females.

But the female Bryant’s woodrats didn’t mind mating with male desert woodrats. Those males were smaller and more docile. “There wasn’t as much danger,” Matocq observes.

Scientists Say: Microbiome

The researchers suspect that many wild hybrids have a desert woodrat father and a Bryant’s woodrat mother. That could be important because mammals, such as woodrats, inherit bacteria from their mothers. These bacteria stay in the animal’s gut and are called their microbiome (My-kroh-BY-ohm).

An animal’s microbiome may affect its ability to digest food. Desert and Bryant’s woodrats likely eat different plants. Some of the plants are toxic. Each species may have evolved ways to safely digest what they chose to eat. And their microbiomes may have evolved to play a role in that as well.

If true, hybrids may have inherited bacteria that help them digest the plants that Bryant’s woodrats typically consume. That means these animals might be better-suited to dine on what a Bryant’s woodrat eats. Matocq’s team is now feeding different plants to the parent species and their hybrids. The researchers will monitor whether the animals get sick. Some hybrids might fare better or worse depending on their mix of DNA and gut bacteria.

What’s exciting about hybrids is that you can think of each one “as a little bit of an experiment,” Matocq says. “Some of them work, and some of them don’t.”

Power Words

aggressive (n. aggressiveness) Quick to fight or argue, or forceful in making efforts to succeed or win.

autonomous Acting independently. Autonomous vehicles, for instance, pilot themselves based on instructions that have been programmed into their computer guidance system.

bacteria (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).

behavior The way something, often a person or other organism, acts towards others, or conducts itself.

biodiversity (short for biological diversity) The number and variety of species found within a localized geographic region.

biology The study of living things. The scientists who study them are known as biologists.

breed (noun) Animals within the same species that are so genetically similar that they produce reliable and characteristic traits. German shepherds and dachshunds, for instance, are examples of dog breeds. (verb) To produce offspring through reproduction.

climate The weather conditions that typically exist in one area, in general, or over a long period.

climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

colleague Someone who works with another a co-worker or team member.

defecate To discharge solid waste from the body.

diet The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health. (verb) To adopt a specific food-intake plan for the purpose of controlling body weight.

digest (noun: digestion) To break down food into simple compounds that the body can absorb and use for growth. Some sewage-treatment plants harness microbes to digest — or degrade — wastes so that the breakdown products can be recycled for use elsewhere in the environment.

DNA (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

docile An adjective meaning calm, cooperative, submissive or deferential.

environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).

evolutionary An adjective that refers to changes that occur within a species over time as it adapts to its environment. Such evolutionary changes usually reflect genetic variation and natural selection, which leave a new type of organism better suited for its environment than its ancestors. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

exotic An adjective to describe something that is highly unusual, strange or foreign (such as exotic plants).

fertile Old enough and able to reproduce.

generation A group of individuals (in any species) born at about the same time or that are regarded as a single group. Your parents belong to one generation of your family, for example, and your grandparents to another. Similarly, you and everyone within a few years of your age across the planet are referred to as belonging to a particular generation of humans.

habitat The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

hybrid An organism produced by interbreeding of two animals or plants of different species or of genetically distinct populations within a species. Such offspring often possess genes passed on by each parent, yielding a combination of traits not known in previous generations. The term is also used in reference to any object that is a mix of two or more things.

gut An informal term for the gastrointestinal tract, especially the intestines.

insect A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

mammal A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.

microbiome The scientific term for the entirety of the microorganisms — bacteria, viruses, fungi and more — that take up permanent residence within the body of a human or other animal.

migration (v. migrate) Movement from one region or habitat to another, especially regularly (and according to the seasons) or to cope with some driving force (such as climate or war). An individual that makes this move is known as a migrant.

molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

monitor To test, sample or watch something, especially on a regular or ongoing basis.

native Associated with a particular location native plants and animals have been found in a particular location since recorded history began. These species also tend to have developed within a region, occurring there naturally (not because they were planted or moved there by people). Most are particularly well adapted to their environment.

navigate To find one’s way through a landscape using visual cues, sensory information (like scents), magnetic information (like an internal compass) or other techniques.

population (in biology) A group of individuals from the same species that lives in the same area.

predation A term used in biology and ecology to describe a biological interaction where one organism (the predator) hunts and kills another (the prey) for food.

predator (adjective: predatory) A creature that preys on other animals for most or all of its food.

radio To send and receive radio waves, or the device that receives these transmissions.

rainforest Dense forest rich in biodiversity found in tropical areas with consistent heavy rainfall.

rodent A mammal of the order Rodentia, a group that includes mice, rats, squirrels, guinea pigs, hamsters and porcupines.

salmon A popular game fish that tends to live most of its life in the ocean, then enters coastal rivers (and freshwater) to breed and lay eggs.

sensor A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly.

species A group of similar organisms capable of producing offspring that can survive and reproduce.

sterile (in biology) An organism that is physically unable to reproduce.

strategy A thoughtful and clever plan for achieving some difficult or challenging goal.

subspecies A subdivision of a species, usually based on geographic separations. Over time, this separation may have allowed some of the genes in a population of a species to vary, creating differences in those organisms’ appearance or adaptation to the local environment.

tag (in conservation science) To attach some rugged band or package of instruments onto an animal. Sometimes the tag is used to give each individual a unique identification number. Once attached to the leg, ear or other part of the body of a critter, it can effectively become the animal’s “name.” In some instances, a tag can collect information from the environment around the animal as well. This helps scientists understand both the environment and the animal’s role within it.

terrain The land in a particular area and whatever covers it. The term might refer to anything from a smooth, flat and dry landscape to a mountainous region covered with boulders, bogs and forest cover.

toxic Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

trait A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.

urban Of or related to cities, especially densely populated ones or regions where lots of traffic and industrial activity occurs. The development or buildup of urban areas is a phenomenon known as urbanization.

Why Is Captive Breeding So Hard?

The Panda House at the National Zoo will be off-limits to the public for the next three days, as zookeepers attempt to coax the giant pandas Tian Tian and Mei Xiang into sexual activity. Keepers hope “to create expectation between the two“—by separating the animals until hormone tests reveal that Mei Xiang has reached peak fertility. Why is it so hard to get animals in the mood?

We don’t really know what turns them on. Breeding some critters is easy, and zookeepers work to make sure they don’t reproduce too often. The endangered giant pandas happen to be quite finicky: Even a female in heat rarely elicits a response from a captive male panda. The reason for this remains unclear, but studies have shown that giant pandas breed most successfully when they’ve had direct physical contact with keepers, as well as access to climbable trees and private areas away from public scrutiny.

For other animals, missing social cues can cause problems. For many years, zookeepers had trouble breeding the white rhinoceros. Though they were often exhibited in male-female pairs, the animals rarely reproduced. In the wild, the white rhino lives in small herds it turns out that a male needs to interact with a number of females in order to be properly aroused. Much of the difficulty breeding white rhinos disappeared as zoos began to keep them in larger groups.

A number of other factors can contribute to problems with captive breeding. Keepers might clean up waste too quickly and remove an important odor that signals fertility. The social tensions particular to zoo life can distract males from reproducing—a male guenon in a dysfunctional family group, for example, can become so preoccupied with aggressive behavior that he ignores the females. Aggression might even be directed out of the animal’s enclosure and toward animals of a different species in a nearby cage.

When individual animals seem unable to reproduce, keepers can call in physiologists to diagnose possible biological problems. The reproductive tract of an animal (especially among hoofstock) might begin to break down if she hasn’t bred regularly once reaching sexual maturity. Or cysts in the reproductive tract might make pregnancy impossible. The physiologist will also test semen samples for volume and concentration. (Cheetah semen is notorious for its poor quality.) Sometimes, zookeepers try to identify potential problems at the outset: When the zoo needs to bring in female elephants from the wild, for example, they will tranquilize more animals than they need and then let scientists perform ultrasound exams to determine which are most likely to reproduce.

But even an animal that seems perfectly healthy might not reproduce. Some individuals are just better at breeding than others, and zookeepers still haven’t figured out why. At the Chengdu Research Base of Giant Panda Breeding, a panda named Mei Mei (and nicknamed “the Heroine Mother”) gave birth to 10 cubs in her 21-year life.

Explainer thanks Barbara Durrant and Larry Killmar of the San Diego Zoo.

Reproduction in Poultry

The cock has no penis but a small opening near the vent through which sperms are emitted. Cock has testes within the body.

The hen has elongated oviduct for formation of an egg. Fertilization occurs internally.

During mating the cloaca of the hen and the vent of the cock fit into each other and then semen is poured into the cloaca ,then sucked to the oviducts.

TheReproductiveSystem of a Hen

The reproductive system has the following parts
i). Ovary
ii). Funnel(infundibulum).
iii). Magnum
iv). Ishtumus
V). Uterus/Shell gland
vii) Vaginal
viii). Cloaca

Hen has two ovaries but one functional. Ova is formed in ovaries.
About 3500-4000 ova present inside ovary held by follicle. Mature ovum released via rapture of follicle. It moves into oviduct received by the funnel.

Fertilization occurs here. Chalazae also added to yolk.
It also collects the ovum and stores the sperm. Time here is 15 minutes and it is 11.6cm long.

Thick albumen is added and stays for 3hrs. its 33cm long.

Its 10.6cm long, Shell membranes added and determines shape of egg.
Water, mineral salts and vitamins added and takes 15 minutes.

Uterus(shell gland)
Calcium deposits added around the egg. Pigments added.
Addition of albumin finished and stays here for 18-22hours.

Short, 6.9cm long and for temporal storage of egg before laying

Egg moves out of cloaca through the vent and the cloaca extents out to prevent the egg from breaking.
Egg formation not depended on fertilization. Egg formation takes 24-26hours.
The components of egg are obtained from body reserves of the hens body.

Animal Diversity Web

Boa constrictor is an exclusively New World species which has the largest distribution of all neotropical boas. Boa constrictors range from northern Mexico south through Central and South America. In South America the range splits along the Andes mountains. To the east of the Andes, B. constrictor is found as far south as northern Argentina. On the west side of the mountains, the range extends into Peru. Boa constrictors are also found on numerous islands off the Pacific coast and in the Caribbean. Islands included in the boa constrictor range are: the Lesser Antilles, Trinidad, Tobago, Dominica, and St. Lucia. Some islands off the coast of Belize and Honduras are also inhabited by this species. (Chiaraviglio, et al., 2003 Mattison, 2007 O'Shea, 2007 Stafford, 1986)


Boa constrictors occupy a variety of habitats. Primary habitat is rainforest clearings or edges. However, they are also found in woodlands, grasslands, dry tropical forest, thorn scrub, and semi-desert. Boa constrictors are also common near human settlements and often found in agricultural areas. Boa constrictors are commonly seen in or along streams and rivers in appropriate habitats. Boa constrictors are semi-arboreal, although juveniles tend to be more arboreal than adults. They also move well on the ground and can be found occupying the burrows of medium-sized mammals. (Mattison, 2007 Montgomery and Rand, 1978 O'Shea, 2007 Stafford, 1986)

  • Habitat Regions
  • tropical
  • terrestrial
  • Terrestrial Biomes
  • savanna or grassland
  • chaparral
  • forest
  • rainforest
  • scrub forest
  • Aquatic Biomes
  • rivers and streams
  • Other Habitat Features
  • agricultural
  • riparian
  • Range elevation 0 to 1,000 m 0.00 to ft

Physical Description

Boa constrictor has long been famous as one of the largest species of snake. In reality, boa constrictors are fairly modest-sized boids and are dwarfed by the other competitors for this title. The maximum length reported in B. constrictor was slightly over 4 meters. Individuals are generally between 2 and 3 meters in length, although island forms are commonly below 2 meters. Within populations, females are usually larger than males. However, the tails of males may be proportionally longer than those of females because of the space taken up by the hemipenes. Boa constrictor coloration and pattern are distinctive. Dorsally the background color is cream or brown that is marked with dark "saddle-shaped" bands. These saddles become more colorful and prominent towards the tail, often becoming reddish brown with either black or cream edging. Along the sides, there are rhomboid, dark marks. They may have smaller dark spots over the entire body. The head of a boa constrictor has 3 distinctive stripes. First is a line that runs dorsally from the snout to the back of the head. Second, there is a dark triangle between the snout and the eye. Third, this dark triangle is continued behind the eye, where it slants downward towards the jaw. However, there are many variations on appearance. At least 9 subspecies are currently recognized by some authorities, although many of these are poorly defined and future research will undoubtedly modify this taxonomy. Currently acknowledged subspecies include: B. c. constrictor , B. c. orophias , B. c. imperator , B. c. occidentalis , B. c. ortonii , B. c. sabogae , B. c. amarali , B. c. nebulosa (Dominican boa, recently elevated to full species), and B. c. longicauda . Most of these subspecies are distinguished largely by their range rather than appearance, but regional (subspecific) variation in form, size, and coloration does occur. (Chiaraviglio, et al., 2003 Mattison, 2007 O'Shea, 2007 Stafford, 1986)

As in most members of the family Boidae, boa constrictors possesses pelvic spurs. These are hind leg remnants found on either side of the cloacal opening. They are used by males in courtship and are larger in males than in females. Males possess hemipenes, a double-penis, of which only one side is commonly used in mating. Although heat-sensing pits are common in Boidae, they are absent in B. constrictor . Thus, this species is presumed to have no specialized thermosensory abilities. The teeth of boa constrictors are aglyphous, meaning they do not possess any elongated fangs. Instead, they have rows of long, recurved teeth of about the same size. Teeth are continuously replaced particular teeth being replaced at any one time alternate, so that a snake never loses the ability to bite in any part of its mouth. Boas are non-venomous. Boa constrictors have two functional lungs, a condition found in boas and pythons. Most snakes have a reduced left lung and an extended right lung, to better match their elongated body shape. (Mattison, 2007 O'Shea, 2007 Pough, et al., 2004)

  • Other Physical Features
  • heterothermic
  • polymorphic
  • Sexual Dimorphism
  • female larger
  • Range length 1 to 4 m 3.28 to 13.12 ft
  • Average length 2-3 m ft


Fertilization is internal, with mating facilitated by the pelvic spurs of males. Boa constrictors are ovoviviparous embryos develop within their mothers' bodies. Young are born live and are independent soon after birth. Newborn boa constrictors resemble their parents and do not undergo any metamorphosis. As in other snakes, boa constrictors shed their skins periodically as they age, allowing them to grow and preventing the scales from becoming worn. As a boa grows, and its skin is shed, its coloration may gradually change. Young snakes tend to have brighter colors and more contrast between colors, but most changes are subtle. (Mattison, 2007 O'Shea, 2007 Pough, et al., 2004 Stafford, 1986)


Males are polygynous each male can mate with multiple females. Females may also have more than one mate in a season. Females are usually widely scattered and courting males must invest energy into locating them. Most female boa constrictors do not appear to reproduce annually. Usually about half of the female population is reproductive each year. Furthermore, females likely become reproductive only when they are in good physical condition. While a higher percentage of males seems to reproduce each year, it is likely that the majority of males also do not reproduce annually. (O'Shea, 2007 Stafford, 1986)

Boa constrictors generally breed during the dry season, usually from April to August, though the timing of the dry season varies across their range. Gestation lasts for 5 to 8 months depending on local temperatures. The average litter has 25 young but can be anywhere from 10 to 64 young. (Bertona and Chiaraviglio, 2003 Chiaraviglio, et al., 2003 Mattison, 2007 O'Shea, 2007 Stafford, 1986)

  • Key Reproductive Features
  • iteroparous
  • seasonal breeding
  • sexual
  • fertilization
  • viviparous
  • Breeding interval Females perhaps every other year, or less often, depending on condition.
  • Breeding season Breeding occurs during the dry season (April-August), birth occurs 5-8 months later.
  • Range number of offspring 10 to 64
  • Average number of offspring 24 (in <<B.c. occidentalis>>)
  • Range gestation period 5 to 8 months
  • Average time to independence after only a few minutes
  • Average age at sexual or reproductive maturity (female) 2-3 years
  • Average age at sexual or reproductive maturity (male) 2-3 years

Maternal investment in young is considerable and requires the mother to be in good physical condition. Since young boa constrictors develop within the mother's body, they are able to develop in a thermoregulated, protected environment and they are provided with nutrients. Boa constrictor young are born fully developed and are independent within minutes of birth. Male reproductive investment is largely spent in finding mates. (Andrade and Abe, 1998 O'Shea, 2007 Stafford, 1986)

  • Parental Investment
  • pre-fertilization
    • provisioning
    • protecting
      • female
      • provisioning
        • female
        • female


        Boa constrictors are potentially long-lived, perhaps averaging around 20 years old. Captive boas tend to live longer than wild ones, sometimes by as much as 10 to 15 years. (O'Shea, 2007 Stafford, 1986)

        • Range lifespan
          Status: wild 30 (high) years
        • Range lifespan
          Status: captivity 40 (high) years
        • Average lifespan
          Status: wild 20 years
        • Typical lifespan
          Status: captivity 25 to 35 years


        Boa constrictors are solitary, associating with conspecifics only to mate. However, Dominican populations which will occasionally den together. Boa constrictors are nocturnal or crepuscular, though they bask in the sun to warm themselves in cool weather. They periodically shed their skins (more frequently in juveniles than adults). A lubricating substance is produced under the old skin layer. When this occurs, the snake's eye can be seen to cloud up as this substance comes between its eye and the old eye-covering. The cloudiness affects their vision and boas will often become inactive for several days until the shedding has completed and their vision is restored. During shedding, the skin splits over the snout and eventually peels back from the rest of the body. Boa constrictors are most often observed in trees or on the ground near streams and rivers. (Bartlett and Bartlett, 2003 Chiaraviglio, et al., 2003 Montgomery and Rand, 1978 O'Shea, 2007 Stafford, 1986)

        • Key Behaviors
        • arboreal
        • terricolous
        • nocturnal
        • crepuscular
        • sedentary
        • solitary
        • territorial

        Home Range

        Boa constrictors defend territories that change over time. Territories may be abandoned if resources or conditions decline. (O'Shea, 2007 Stafford, 1986)

        Communication and Perception

        Like most snakes, boa constrictors rely on strong vomeronasal senses. Their tongues flick continuously, bringing odor molecules into contact with the chemosensory (vomeronasal) organ in the top of their mouths. In this manner, they constantly sense chemical cues in their enviornment. Boa constrictors have good vision, even into the ultraviolet spectrum. In addition, they can detect both vibrations in the ground and sound vibrations through the air through their jaw bones. They do not have external ears. Unlike most boids, boa constrictors lack thermosensory pits. (Mattison, 2007 O'Shea, 2007 Sillman, et al., 2001 Stone and Holtzman, 1996)

        • Communication Channels
        • tactile
        • chemical
        • Perception Channels
        • visual
        • tactile
        • acoustic
        • vibrations
        • chemical

        Food Habits

        Boa constrictors are carnivorous generalists. The main bulk of their diet consists of small mammals, including bats, and birds. However, they will eat any animal they can capture and fit in their mouths. Boa constrictors capture prey through ambush hunting, although occasionally they actively hunt. They can rapidly strike at an animal that passes by a branch that they are suspended from, for example. They are non-venomous and prey is dispatched through constriction. Boa constrictors wrap their prey in the coils of their body and squeeze until the prey asphyxiates. This is especially effective against mammals and birds whose warm-blooded metabolism demands oxygen at a rapid rate. Once dead, the prey is swallowed whole. Interestingly, if captive boa constrictors are presented with dead prey, they still constrict the food item before consuming it. It takes boa constrictors 4 to 6 days to fully digest a meal. (Bartlett and Bartlett, 2003 Mattison, 2007 O'Shea, 2007 Stone and Holtzman, 1996)

        • Primary Diet
        • carnivore
          • eats terrestrial vertebrates
          • Animal Foods
          • birds
          • mammals
          • amphibians
          • reptiles
          • eggs


          When threatened, boa constrictors will bite to defend themselves. Though there are few references to predation on boa constrictors in nature, they are certainly killed and consumed by numerous reptilian, avian, and mammalian predators. Young boas are especially vulnerable. (O'Shea, 2007 Pough, et al., 2004)

          Ecosystem Roles

          Boa constrictors are predators on birds and small mammals, including bats. They are important predators of rodents and opossums, especially, which can become pests in some areas and carry human diseases. (Mattison, 2007 O'Shea, 2007 Stone and Holtzman, 1996)

          Economic Importance for Humans: Positive

          Boa constrictors are popular in the pet trade. It is easy to obtain boa constrictors that have been captive bred for generations, increasing their affinity for humans. They are relatively undemanding pets, as long as their large adult size and space needs are accounted for. Proper levels of heat and humidity (boas usually need a dry climate, otherwise their scales will develop rot) need to be observed. Boa constrictors can be fed dead mice and rats and only require food and defecate about once a week. Proper care should be observed in handling them, especially the larger varieties. Boa constrictors, whole or in parts, are also seen in local markets within their range, presumably as food or medicine. They are sometimes harvested for the skin trade. In some areas boas constrictors can play a large role in controlling populations of pest rodents and opossums (Didelphidae). Opossums in the tropics can be carriers for the human disease leishmaniasis, which is transferred by blood-feeding sand flies (Psychodidae) that parasitize the opossums. Boa constrictor predation pressure may help to regulate opossum populations and decrease potential trasmission of leishmaniasis to humans. (Bartlett and Bartlett, 2003 Mattison, 2007 O'Shea, 2007)

          • Positive Impacts
          • pet trade
          • food
          • body parts are source of valuable material
          • controls pest population

          Economic Importance for Humans: Negative

          Little negative impact on humans is known. Boa constrictors rarely, if ever, attack humans except in self-defense. Humans, even children, are far outside the range of prey size taken by boas. Boa constrictor bites are painful bure are unlikely to be dangerous as long as standard medical care is obtained. Boa constrictors are not venomous. Large captive snakes must always be handled with extreme care, especially when being fed, as a hungry snake strikes and constricts in a largely automatic sequence of behaviors. Very large snakes should handled and fed only with more than one person present. (Bartlett and Bartlett, 2003 Mattison, 2007 O'Shea, 2007)

          Conservation Status

          Overcollection for the pet trade and needless direct persecution has had an impact on some B. constrictor populations. Some populations have been hit harder than other, and various wild populations are now endangered, particularly those on offshore islands. On the mainland, boa constrictors have been harvested for their skins, meat and body parts. Furthermore, habitat loss and road mortality has reduced populations. Most boa constrictors are on the CITES Appendix 2 list. The subspecies B. c. occidentalis is on Appendix 1 of CITES. (O'Shea, 2007 Pough, et al., 2004)

          • IUCN Red List Not Evaluated
          • US Federal List No special status
          • CITES Appendix I Appendix II
          • State of Michigan List No special status

          Other Comments

          As mentioned above, the species Boa constrictor is divided into many subspecies. These subspecies are highly variable and over the years the taxonomy has changed. Currently there are at least 9 recognized subspecies: Colombian or common boa constrictors ( B. c. constrictor ), St. Lucia boa constrictors ( B. c. orophias ), Imperial or Central American boa constrictors ( B.c. imperator ), Argentine boa constrictors ( B.c. occidentalis ), Peruvian boa constrictors ( B.c. ortonii ), Taboga Island boa constrictors ( B.c. sabogae ), Bolivian boa constrictors ( B.c. amavali ), Dominican or clouded boa constrictors (sometimes considered a full species, B.c. nebulosa ), and long-tailed boa constrictors ( B.c. longicauda ). Subspecies that are occasionally cited, but are not as widely acknowledged or are often combined with a previously listed subspecies are: Mexican boa constrictors ( B.c. mexicana ), black-bellied boa constrictors ( B.c. melanogaster ), and Tres Marias Islands boa constrictors ( B.c. sigma ). As apparent by the names, most subspecies are recognized by their range. In many cases, a boa constrictor of unknown geographical origin may be impossible to assign to a subspecies. Additionally, pet trade breeders have created many new color morphs that are not seen in wild populations. (Andrade and Abe, 1998 Bartlett and Bartlett, 2003 Mattison, 2007 O'Shea, 2007 Stafford, 1986)


          Tanya Dewey (editor), Animal Diversity Web.

          Laurel Lindemann (author), Michigan State University, James Harding (editor, instructor), Michigan State University.


          living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

          living in the southern part of the New World. In other words, Central and South America.

          uses sound to communicate

          living in landscapes dominated by human agriculture.

          Referring to an animal that lives in trees tree-climbing.

          an animal that mainly eats meat

          Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

          uses smells or other chemicals to communicate

          having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment being difficult to see or otherwise detect.

          union of egg and spermatozoan

          A substance that provides both nutrients and energy to a living thing.

          forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

          having a body temperature that fluctuates with that of the immediate environment having no mechanism or a poorly developed mechanism for regulating internal body temperature.

          offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

          the area in which the animal is naturally found, the region in which it is endemic.

          the business of buying and selling animals for people to keep in their homes as pets.

          the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

          "many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.

          rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

          Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

          scrub forests develop in areas that experience dry seasons.

          breeding is confined to a particular season

          reproduction that includes combining the genetic contribution of two individuals, a male and a female

          uses touch to communicate

          defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

          the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

          A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

          A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

          A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

          movements of a hard surface that are produced by animals as signals to others

          uses sight to communicate

          reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.


          Andrade, D., A. Abe. 1998. Abnormalities in a litter of Boa constrictor amarali. The Snake , 28: 28-32. Accessed December 05, 2008 at

          Bartlett, R., P. Bartlett. 2003. Red-tailed Boas and Relatives: Reptile Keeper's Guide . Hauppauge, NY: Barron's Educational Series, Inc..

          Bertona, M., M. Chiaraviglio. 2003. Reproductive biology, mating aggregations, and sexual dimorphism of the argentine boa contrictor (Boa constrictor occidentalis). Journal of Herpetology , 37(3): 510-516. Accessed December 05, 2008 at

          Chiaraviglio, M., M. Bertona, M. Sironi, S. Lucino. 2003. Intrapopulation variation in life history traits of Boa constrictor occidentalis in Argentina. Amphibia-Reptilia , 24/1: 65-74. Accessed November 07, 2008 at

          Mattison, C. 2007. The New Encylcopedia of Snakes . Princeton, New Jersey: Princeton University Press.

          O'Shea, M. 2007. Boas and Pythons of the World . Princeton, New Jersey: Princeton University Press.

          Pough, F., R. Andrews, J. Cadle, M. Crump, A. Savitzky, K. Wells. 2004. Herpetology, third edition . Upper Saddle River, NJ: Benjamin Cummings.

          11 Praying Mantis: Post-Coitus Snack Anyone?

          It's common knowledge that female praying mantises eat the heads of their mates during intercourse, but as it turns out, this isn't always the case. In some species, head-eating is a required part of the interaction, as it makes the male ejaculate more quickly. But in most cases, the cannibalism is actually a relatively rare behavior that occurs anywhere from 5-31% of the time. In these cases, the female will only eat the male because she's hungry and needs more sustenance in order to go on living. Remember, most animals only mate to keep their species going and a dead female isn't going to help the mantis survive as a species. When cannibalism doesn't occur, the mating ritual is actually a bit romantic, including a long mating dance and soft antennae stroking. Who knew these guys were such softies?

          Watch the video: Κόκορας κάι σκυλάκι (August 2022).