Super-Female Bearded Dragons 'More Male than Males'

In addition to other odd characteristics, the animals are born with male chromosomes but can lay eggs.

When is a female a female? And when is a male a male? These are the questions that scientists continue to ponder after the latest research on an Australian lizard that reverses its sex when exposed to high incubation temperatures.

The study shows central bearded dragons (Pogona vitticeps) that are born with male chromosomes, but can lay eggs, have other strange characteristics.

Not only, as found previously, do they behave like super females, laying more eggs than standard females, but they are also bolder and more active than males.

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This could give them an advantage in the arid and semi-arid areas of Australia they inhabit, said Professor Rick Shine of the University of Sydney, who led the research, published today in the Royal Society journal Proceedings B.

The study follows previous research by some of the same scientists that compared the genetic sex of 131 central bearded dragons, determined by blood samples taken from the tails, and their actual sex, as determined by their size, behavior and gonads.

Male bearded dragons usually have two Z chromosomes and females usually have a Z and a W chromosome.

Eleven of the dragons studied previously were sex-reversed females, in other words, females with ZZ chromosomes. These looked more like males than females.

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Back in the laboratory the researchers found that at lower incubation temperatures these ZZ females produced males, and at high temperatures they produced ZZ females.

Like their mothers, these baby ZZ females also had the potential to lay more eggs than standard females.

Without the production of standard ZW females, these super females could rapidly dominate the population.

The sex determination process in the dragons had changed from one governed by chromosomes to one governed by temperature, the researchers said.

"In the latest study I suggested we should find out what this actually meant in terms of behavior," Professor Shine said.

"To do this my colleague Hong Li, a visiting scholar from China, got to work on standardized observational tests."

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Dr Li studied 115 juvenile and hatchling dragons from eggs incubated at various temperatures.

He found that, when it came to activity and boldness, the 20 super female lizards in the group behaved in an even more male-like way than standard males.

Each lizard was housed in a cage with a container to shelter in and tested to see how active they were, and how quickly they emerged from their shelters (a standard sign of boldness).

"We found that sex-reversed females were much more active than both males and standard females, especially when they were presented with an unfamiliar object," Professor Shine said.

"In the boldness test, the sex-reversed females emerged much quicker from their shelters than the others as well.

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Professor Shine and his team also found the super females had higher body temperatures than standard females - temperatures more similar to that of males.

"A bearded dragon can have male sex chromosomes but be a functioning female. Yet it can be more male-like in the way it looks, in its temperature and in its behavior. They are more male-like than the actual males," Professor Shine said.

Because super female dragons lay more eggs and because standard females can be bred out of the population, then why aren't all central bearded dragons either males or super females?

"It's a phenomenon that happens at quite high temperatures - we don't know the exact temperature in the nest," Professor Shine said.

"Maybe being bold can be good in some situations. They could get more food. Sometimes it might mean they become easier prey. Either way, it's a strange animal indeed."

SEE PHOTOS: World's Top Heavyweight Reptiles

Original article on ABC Science Online

This week we learned that back in the last ice age, the first humans to populate Australia likely had to contend with a giant lizard that might have been almost 20 feet long. That got us thinking: Which are the weightiest reptiles among whom modern humans have to live? So here we present the heaviest 10, based on average weight. We'll start small and count up -- getting "fatter," as it were -- until we reach the top heavyweight reptile on Terra Firma. First up, here's the lightest of the top heavies, the false gharial, a freshwater crocodile with a loooong, slender snout. On average, the native Malaysian weighs about 460 pouns (210 kilograms) and runs about 13 feet (4 meters) long.

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Careful if you find yourself in a dark alley with this fellow, the mugger crocodile. This 495-pounder (225 kilograms) is an accomplished hunter that hides itself and lies in wait to ambush unsuspecting birds, fish, other reptiles or mammals. It can grow to almost 11 feet long om average (3.3 meters). It's the most widespread of India's three crocodile species.

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Here'a a familiar creature, the American alligator, all 530 pounds (240 kilograms) and 11 feet (3.3 meters) of him.

Alligator mississippiensis, as it's known in the language of taxonomy, is a common inhabitant of coastal wetlands from North Carolina, down to Florida, and across the Gulf of Mexico to Texas.

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Remember the false gharial in our first slide? Well, here's a gharial with no false in front of its name. Weighing in at an average 550 pounds (250 kilograms) and stretching nearly 15 feet (4.5 meters) long, it's sometimes known as the fish-eating crocodile. Its name derives from the unique shaping of the end of its snout, which looks like a kind of earthenware pot whose name in Hindi is ghara. The huge hunter is another of India's three crocodile species (like the mugger crocodile we just met).

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No slouch in the big reptile department is the American crocodile. We're taking a leap up in size now, as this creature typically runs about 740 pounds, even though it runs a bit shorter in length (13 feet) than, say, the gharial. Though confined largely to southern Florida and Puerto Rico, it still has managed to bounce back as a species. There are a couple of thousand of them now, up from just a couple of hundred in the 1970s. It can eat prey as big as cattle but in general its diet is largely fish-based.

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Weighing in at an average 770 pounds or so, the black caiman is One. Enormous. Crocodile. The carnivorous beast lives in the Amazon basin and other freshwater habitats in South America. It can be uncomfortable to think about, but the larger of these crocs can and will eat cute mammals like sloths and monkey. Deer aren't safe either, nor are cattle, horses or dogs.

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Did you wonder if we'd ever come to a top heavyweight reptile that wasn't an alligator or crocodile? Here we are! Meet the leatherback sea turtle, the biggest turtle of them all. Unlike other sea turtles, this 800-pound (364 kilogram) creature doesn't have a bony shell. Instead its upside is skin and and flesh. The turtle's range varies widely in the Atlantic and Pacific Oceans. They eat jellyfish, almost exclusively, and are known for migrating many thousands of miles between nesting locations. Lengthwise, they run about the size of an NBA small forward, at 6.6 feet (2 meters).

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It's back to crocodiles now, with the orinoco. This monster runs nearly 850 pounds on average, nearly 400 kilograms. The species is listed as critically endangered and exists, in small numbers, only in freshwater habitats of Colombia and Venezuela, especially the Orinoco River. The apex predator will take a shot at anything -- birds, mammals, reptiles. But it mainly eats fish. On land or in water, it stealthily stalks its prey. There are reports it has eaten other, smaller crocodiles like caimans and even consumed members of its own species.

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"See you later, alligator! In a ... Nile crocodile???" The Nile crocodile is the second-weightiest reptile in our gallery, tipping the scales at an average of 900 pounds (410 kilograms) and stretching the tape measure to just under 15 feet (4.5 meters). It can be found in aquatic habitats all over Sub-Saharan Africa. The apex predator stalks marshes, lakes, and rivers, eating the usual crocodile diet of mammals, other reptiles, fish, and birds. Their vise-like jaws clamp down so hard that prey just have no chance. They have a structured social order that hands the biggest, oldest males the best places to bask in the sun and first dibs on food.

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Finally, we've arrived at the world's biggest extant reptile, the saltwater crocodile (Crocodylus porosus). It weighs on average just under 1,000 pounds and stretches 15 feet long (450 kilograms, 4.5 meters). It can live in saltwater but prefers places like lagoons, deltas, and swamps. They have an incredibly wide range, the widest of any of today's crocodiles -- from northern Australia, thoughout southeast Asia and the east coast of India. And it's our third of India's crocodile species, along with the mugger and the gharial. True to its size, there's almost nothing it won't consider prey. The ambush master will take on mammals, fish, reptile, birds, fish, crustaceans -- you name it. They're thought to have the most powerful bite of any living animal. Steer clear of this one, if you value life and limb.

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