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---
created_at: '2016-04-16T21:29:44.000Z'
title: If birds descended from dinosaurs, why are they warm-blooded? (2010)
url: http://www.abc.net.au/science/articles/2010/11/23/3073903.htm
author: networked
points: 53
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---
[Source](http://www.abc.net.au/science/articles/2010/11/23/3073903.htm "Permalink to If birds descended from dinosaurs, why are they warm-blooded? › Ask an Expert (ABC Science)")
# If birds descended from dinosaurs, why are they warm-blooded? › Ask an Expert (ABC Science)
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# If birds descended from dinosaurs, why are they warm-blooded?
If birds descended from dinosaurs, by what means did they become warm-blooded, assuming the dinosaurs were reptilian and cold-blooded?
By Stuart Gary
![A false color image of the Thermopolis Archaeopteryx][43]
Archaeopteryx, the first bird-like ancestor discovered, had teeth and a long tail as well as forelimbs shaped like wings and feathers that contained phosphorous and sulfur. _(Source: K G Huntley/SSRL)_
### Related Stories
* [Birds are dinosaurs after all][44], Science Online, 25 May 2010
* [Dinosaur-bird connection a thing of the past][45], Science Online, 18 May 2010
* [Fossil sheds new light on 'dino-bird'][46], Science Online, 12 May 2010
* [Hunchbacked dino strengthens bird link][47], Science Online, 09 Sep 2010
> If birds descended from dinosaurs, by what means did they become warm-blooded, assuming the dinosaurs were reptilian and cold-blooded? Were there any in between stages in evolution between cold blooded and warm blooded animals?_—Clare_
Modern-day birds descended from a group of dinosaurs called theropods.
If you look at the hind leg of a _Tyrannosaurus rex_, one of the most well-known theropods, it looks just like a big chicken leg: there are three big toes pointing forward and a fourth pointing backward, all tipped with curved, sharp claws that are compressed from side to side.
Theropod dinosaurs and birds share over 250 features and characteristics identified in their skeletons that no other animals have.
_Archaeopteryx_, the first bird-like ancestor discovered, had teeth and a long tail, just like its much bigger cousin _T. rex_. But it also clearly had feathers and forelimbs that looked like wings.
It was also very likely to have been warm-blooded, says Dr Paul Willis, a palaeontologist with [ABC's Catalyst program][48].
"What we can see in the fossil record is a number of bird-like theropods with hair-like structures and simple downy feathers. Being mostly small creatures [they must have generated their own body heat] and evolved hairs and feathers to help retain the body heat they generated," says Willis.
[^ to top][49]
### Warm-blooded reptiles
Animals have developed two ways to maintain their body heat. One method is to generate body heat using internal thermal regulation, these animals such as birds are endothermic. The other method is to take the heat from the environment and store it in their bodies, these animals, such as reptiles, are ectothermic.
But even though we think of them as 'cold-blooded', ectothermic animals can still have warm blood. And the bigger they are, the more likely they are to be able to maintain a warm body temperature.
"Studies with crocodiles [a distant cousin of dinosaurs] show that bigger cold blooded animals don't lose as much heat at night as their smaller siblings, simply because of their greater body mass."
This is something called gigantothermy where the bigger you get the easier it is to hang on to heat.
So by simple definition, all large dinosaurs must have had warm blood, says Willis.
"In fact, because the capacity to store heat increases with size, the problem for really big dinosaurs must have been getting rid of heat and trying to keep cool," he adds.
![Crocodiles, dinosaurs and birds had a common ancestor called an archosaur.][50]
[^ to top][49]
### Switch to internal heating
Just when ectothermic animals became endothermic, however, is difficult to know.
"That probably varies across the different groups of dinosaurs and is almost impossible to determine from fossils.
Given that crocodiles, which share a common ancestor to dinosaurs and birds, are ectotherms, then endothermy must have evolved at least once in that evolutionary path.
"Exactly where on this path endothermy evolved is a matter of considerable debate, and different interpretations result in all dinosaurs being endothermic or just the most advanced and bird-like theropods being endothermic.
"There are lines of evidence in other groups of dinosaurs to suggest they were also endothermic, but at present, there appears to be no solid evidence that can be taken from the fossil record that will tell us which creatures were endothermic, and which were not."
**_Dr Paul Willis was interviewed by Stuart Gary._**
**Tags: [**animals][51], [dinosaurs][52], [fossils][53], [evolution][54], [palaeontology][55], [reptiles][56], [birds][57]
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Published 23 November 2010
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