When did mammals become endothermic, or ‘warm-blooded’?
On these cold winter nights, nothing is better than sitting down to a good science book with a warm cat in my lap.
It feels so good because cats and humans, as with all mammals, are so-called “warm-blooded” animals.
I say because the blood of “cold-blooded” animals actually can be warmer. A better term for “warmblooded” is endothermic.
An endothermic animal has a high metabolism and a body temperature controlled physiologically, so that it doesn’t change much as the outside temperature varies.
On the other end of the spectrum are ectothermic animals, such as reptiles, with body temperatures that are not controlled physiologically, but by such behaviors as basking in the sun. That means their temperatures can vary significantly, depending on their surroundings.
Mammals evolved from the same lineage as reptiles, but when did they become endothermic? Some research recently published in the journal Current Biology had a fascinating answer.
Unlike the other cells in your body, red blood cells ( like those of all other mammals) do not contain nuclei. This allows them to be smaller and move faster and farther, carrying vital oxygen to cells in muscles, the brain and other organs, fueling the high metabolism.
But how can we tell the size of blood cells in ancient animals?
The new research looked at the small canals that penetrate bone. Rather than being an inert framework, your skeleton is a living organ that needs nourishment and oxygen. Those needs are supplied by tiny blood vessels that occupy small canals in bone.
The new research looked at the bones of 14 modern species including six mammals, two birds, two amphibians and four reptiles. When observed under a microscope, they found that the canals in the bones of endothermic animals, such as birds and mammals, tend to be much smaller than those of ectotherms, such as amphibians and reptiles.
Bones of prehistoric mammals had small canals, indicating endothermy. The real surprise came when the researchers looked at the fossil bones of the ancestors of mammals and found that small canal size originated much earlier than anticipated.
The first true mammals appeared late in the Triassic Period, about 200 million years ago, but small canal size was present in the bones of mammal ancestors more than 50 million years earlier.
What about dinosaurs? Despite a lot of suggestive evidence, the question of whether dinosaurs were endothermic or ectothermic has yet to be determined to everyone’s satisfaction. The new research showed that their bones, as with those of mammals, have small vascular canals. And like mammals, those small canals appeared early in their lineage.
Interestingly, in both groups, the appearance of small canal size (and thus the origin of endothermy) coincides with the greatest mass extinction in our planet’s history, the one that occurred at the boundary between the Permian and Triassic Periods 252 million years ago.
The authors suggest that endothermy might have been one of the factors that allowed the ancestors of both dinosaurs and mammals to survive. Both groups would eventually dominate terrestrial ecosystems — the dinosaurs for 150 million years and the mammals for 65 million years after that.