Biology Index
By Richard Bruce BA, MA, and PhC in Economics

Crocodiles Succeeded By Switching from Cold to Warm Blood and Back
Here's Why

Crocodilians, crocodiles, alligators and their relatives are cold-blooded, but they are descended from warm-blooded animals. Cold blooded reptiles evolved into warm-blooded archosaurs which eventually evolved into cold-blooded crocodilians. By switching back and forth crocodilians gained a unique physiology. This unique physiology protects them from communicable diseases.

We Are the Environment

We are the environment for the pathogens that cause our communicable diseases. Those diseases have evolved to thrive in our physiology and have difficulty dealing with a physiology that is different. A unique physiology different from the other animals in the environment provides immunity from many communicable diseases.

Different Not Better

Note that this is not quite like evolving greater speed, or more acute vision. The advantage is achieved simply by being different. Normal adaptation is like an archer hitting or at least getting close to a bullseye. This is like scoring points for missing the target, the farther away the better. Of course, the physiology has to work. So a balance must be struck between immunity to disease and a physiology that allows the animal to catch food, avoid predation, and generally survive and reproduce.

Many organisms exist not because they have special adaptions, but because they are different enough from the other animals in their environment that they are automatically immune to many of the diseases that those other animals carry. These animals tend to be large and predators. Crocodilians are both large and predators.

Large vs Small

The Virginia white-tailed deer illustrates the principle well. The white-tail carries several pathogens that are not deadly to the white-tail but are deadly to larger deer. The best known of these pathogens is a brain worm. The result is that the larger deer, for example, elk (similar to red deer in Europe) and moose (called elk in Europe) are mostly extinct in the range of the white-tail deer.

So a small relative can drive larger relatives into extinction within its range. This is because close relatives have very similar physiologies and share the same diseases. For example, humans and our closest relatives, the chimps, share every single communicable disease that either species is subject to.

Smaller organisms are frequently more numerous than their larger relatives and have shorter life cycles. So the smaller organisms tend to evolve immunities to diseases more rapidly. This allows them to carry diseases that are deadly to their relatives without being harmed themselves. So it is important for large organisms to be distantly related to other organisms in their environment.

There are many examples. Sequoias and giant coastal redwood trees are living fossils. They have no relatives in their environment with a common ancestor more recent than the Mesozoic. The largest birds ostriches, emus, and other ground-dwelling ratites are part of the earliest surviving branch off the bird family tree. Similarly, the largest spiders, the tarantulas are the second earliest surviving branch off the spider family tree. These are but a couple of examples of a trend.


Predators also need to be immune from diseases, particularly the diseases of their prey. A key adaptation of herbivores is immunity to the poisons of the plants they eat, similarly, a key adaptation of predators is immunity to the diseases of the prey they eat. Being distantly related to the prey, or have a different physiology can provide an automatic immunity to many diseases.

Lizards and Similar Animals

Lizards, tuatara, and crocodilians are similar. All are reptiles, with long tails, four legs, and no shell. The tuatara is the largest lizard-like animal in New Zealand. Even if the New Zealand lizards are generally superior to the tuatara it can survive on a combination of its larger size and immunity to lizard specific diseases.

Similarly, even the smallest currently existent crocodilians are larger than almost all currently existent lizards, and the largest crocodilians are larger than even the largest lizards. If a large lizard were to compete with the crocodilians it would have to deal with the diseases carried by the smaller lizards. If the crocodilians or tuataras were to try to compete with lizards at smaller sizes they would fail because the lizards are actually superior.

Ok, yes I know that crocodilians are usually semi-aquatic and they have special adaptions for that environment, and lizards are better adapted to terrestrial environments. This is part of the story, the part that biologists would normally focus on. But in science, we try to be original. "To boldly go where no one has gone before." as they said in the opening monologue of Star Trek: The Next Generation. So this is another part of the story.

The Large and the Predatory are Inferior

Note that biologists generally see larger animals and predators as more advanced. For example, they will say that dinosaurs dominated mammals in the terrestrial environment of the Jurassic and Cretaceous. As you can see from the above, I have a different view. Large organisms and predators are inferior. Small and herbivore are superior. Essentially, when two clads compete for the same niches the superior clad takes those niches that support a large number of individual organisms, the loser takes those niches that support relatively few organisms.

So paleontologists tell us that therapsids and mammals were forced to burrow, climb trees, hide, and be nocturnal by the successful archosaurs and dinosaurs. I disagree. Therapsids were moving into those niches before the archosaurs, let alone the dinosaurs even evolved. All those small burrowing, tree climbing, hiding, nocturnal therapsids and mammals were carrying diseases that killed the larger therapsids and mammals and that is why the archosaurs and dinosaurs were able to claim the large and predatory niches. (Note, mammals are therapsids and dinosaurs are archosaurs.)

Crocodilians no doubt have special adaptions to the aquatic lifestyle, but given that they are large and predatory, a large portion of their success is probably due to their unique physiology and the immunity from communicable diseases that it affords them.

Aquadic Terrestrial Switch Back

Crocodilians are the only case I know of where animals so completely switched back from warm-blooded to cold-blooded, but I might note that many animals from the land have invaded the sea, and all of those are descended from fish that invaded the land. Once again many of those, the whales, seals, and the oceanic reptiles of the Mesozoic were large. Both today and in the Mesozoic, the largest animals in the ocean were descended from terrestrial animals. Once again disease is probably a major factor in this. If you are at the top of a food chain of bony fish it is best not to be a bony fish.

So switching back and forth to gain a unique physiology that resists communicable disease may be a more general strategy that applies beyond crocodilians. I will have to be on the look out for examples.

This web page is one of a number on this general subject, you can find many more in the biology index, or in the right column.

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