Why aren't all animals cold blooded?

Why aren't all animals cold blooded?

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At least, why don't all animals produce no more heat than the tiny amount that's a byproduct of walking or a bonobo climbing up a vine when the outside temperature is above 0°C? They won't freeze anyway.

After all, the amount of food controls the population size at whatever size makes each individual have just enough food, so shouldn't those with a lower energy demand be more likely to get enough food to rear more children? Shouldn't mutations that make a warm blooded animal able to thrive with a lower core temperature and produce less heat be selected for because there's more likely to get enough food to rear more children?

Also, shouldn't cheetahs evolve to produce extremely little heat in the day during rest even on not very hot days so they will be less hot and can chase their prey for longer? Have they already evolved that way?

If a species did evolve to produce less heat, would that also evolve to walk slower because it takes a lower speed to minimize the energy burnt per distance because they're producing less heat? I know there there will be a nonzero speed that minimizes the absolute minimum possible energy burnt per distance possible to evolve for that speed and a lower speed will actually increase the energy burnt per distance because the animal is walking for a longer time but the rate of energy burnt per time varies less rapidly with speed at such low speeds because of the energy burnt to fight against gravity.

Cold/warm blooded are misleading categories

The terms cold blooded and warm blooded is very misleading as it gives one the sensation that animals may warm up their internal temperature for the only reason of warming it up.

The terms homeo-, poikilo-, exo- and endo- terms are much more useful in thinking about control of body temperature. Have a look at this post and this post to understand these terms. Make sure to really read these posts to correctly understand these terms and how they interact.

Reformulating your question

What people, in general mean by cold vs warm blooded animals is whether they actively control their body temperature or not. That is cold blooded would mean exotherm while warm blooded would mean endotherm. As such the question becomes "why would any animal care about being endotherm if the body temperature won't go under freezing point anyway?"


Any metabolic pathway is affected by temperature. A change of temperature of only a few degrees can drastically affect the rate of chemical reactions. Homeotherm individuals (often homeotherm thanks to endothermy) are able to keep a quasi constant level of activity despite external change in temperature. It allow them to forage during hot and cold whether for example. This is a very important advantage but it often comes at the important energy cost. It is a trade-off between the benefit of homeothermy vs the energy cost of endothermy that will define the best interest of an organism.

Why are there fewer venomous animals in colder climates?

This week's question comes to us from Paul Colbourne, in St.John's, Nfld. He asks:

I'm just wondering why there aren't as many venomous creatures in North America, particularly Canada, than there are in places like Australia, South America or Central America?

Bethany Nordstrom graduated with a Masters in biology from Dalhousie University in Halifax where she focused on the predator-prey dynamics of leatherback sea turtles and jellyfish. Nordstrom says most venomous animals are ectotherms or cold-blooded animals such as snakes, spiders and jellyfish. Since they're unable to regulate their own temperature, fewer are found in cooler climates.

Why Are Reptiles Cold Blooded?

Cold blooded does not mean reptiles have cold blood. They are, however, referred to as “cold-blooded” animals because of how they regulate their body temperatures through a process called thermoregulation.

In thermoregulation, reptiles are able to regulate their own body temperatures by moving to different types of environments. If a reptile feels the need to warm up, it heads to an area where it senses warmer temperatures, such as a warm rock or an area with a warmer channel of air. It then stays in that place to readjust its internal body temperature. In turn, if the reptile requires cooling down, it seeks out a cooler location, such as a shady place or a body of water. Another way reptiles maintain body temperature is by huddling together with other reptiles to either warm up or cool down.

Reptiles differ from mammals because mammals must sweat to cool down or shiver their muscles to warm up. Sweating or shivering to regulate body temperature is referred to as endothermy. Mammals and even insects, such as bees, mimic traits of thermoregulation by seeking out warmth or cold. However, unlike reptiles, amphibians, insects, and other cold-blooded animals, mammals are not able to inherently regulate their body temperatures.


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Cold-bloodedness, also called Poikilothermy, Ectothermy, or Heterothermy, the state of having a variable body temperature that is usually only slightly higher than the environmental temperature. This state distinguishes fishes, amphibians, reptiles, and invertebrate animals from warm-blooded, or homoiothermic, animals (birds and mammals). Because of their dependence upon environmental warmth for metabolic functioning, the distribution of terrestrial cold-blooded animals is limited, with only a few exceptions, to areas with a temperature range of 5–10° to 35–40° C (41–50° to 95–104° F). For cold-blooded animals living in the arctic seas, temperatures may range from below 0° C to 10–15° C (below 32° F to 50–59° F). Poikilotherms do maintain a limited control over internal temperature by behavioral means, such as basking in sunlight to warm their bodies.

Cold-blooded: What’s it mean?

What is the first thing that comes to mind about reptiles and amphibians? Probably it is the term “cold-blooded.” Most of us have learned that one component of being a reptile or amphibian is being “cold-blooded.” And what are mammals and birds? Why, “warm-blooded” of course. If you’ll indulge me for a few moments (okay, if you’ve read my blogs before, then you know maybe more than a few), let’s do a CliffsNotes version (who remembers those?) of what “cold-blooded” and “warm-blooded” mean and why us science-y types are not fond of those terms.

Most reptiles and amphibians (as well as most fish and invertebrates) are examples of ectothermic animals. First off, the origin of the word. Ecto means “outer” or “outside” and therm means “heat.” Therefore, ectothermic animals are those that rely on the environment to maintain body temperature. Now, let’s compare that to most birds and mammals, which are generally considered to be endothermic. We already know what therm means, shall we guess what endo means? If you guessed “internal” or “inside,” you’d be right! Most mammals and birds maintain fairly stable body temperatures despite the environmental conditions around them.

So what is wrong with “warm-blooded” and “cold-blooded?” Well, not that I recommend trying, but if you cut open a snake, icicles don’t drop out and similarly you know that if you, as a mammal, get a cut, hot lava doesn’t flow out. These terms just don’t really work. The term “cold-blooded” implies that these animals are in a never-ending struggle to stay warm. That really isn’t correct. Many species do like it hot, with some monitor lizards basking at temperatures of 120–150 F. I’d certainly call that some warm blood! When at their ideal body temperature (also called thermal optimum), they have metabolisms that function near or exceeding the level of many birds! On the other side of the spectrum, there are many salamanders, which tend to operate best in temperatures in the 50–60 Fahrenheit range or even lower. Some, like spotted salamanders (Ambystoma maculatum) can sometimes be observed swimming or even breeding under the surfaces of frozen ponds. Ectotherms simply rely on the temperature of their environment (read, environment, not just the air) to reach their thermal optimum, whether it be high in the case of the monitor lizards or low in the case of the salamanders. And of course, they can get too hot and too cold just like we can. But instead of sweating, panting, or shivering, they have to move from place to place.

When not at their thermal optimum, their metabolisms don’t function at full capacity. If too hot, they seek places to cool down. If too cold, they may seek a sunny spot to warm up. In colder months, most temperate species seek a place with a lower than optimum yet fairly stable temperature, allowing their metabolisms to slow down so they require little or no food, and wait for better conditions to come back (like spring). Ectotherms can do this because they aren’t having to constantly stoke that internal furnace to maintain that thermal optimum. Most mammals and birds have to slog through those rough months because they can’t simply power the furnace down until things get better. But what about hibernating bears, you may ask. Well, sadly we don’t have time to cover all the oddballs here, of which there are many on both sides!

And of course, this short(ish) discussion is streamlined for the sake of time and space! But hopefully I’ve at least stoked your mental furnace a bit! I hope to see you visiting all of the wonderful ectotherms and endotherms that reside at Zoo Atlanta!
Robert Hill
Assistant Curator of Herpetology

Thermoregulation in animals: ectothermic animals

As their source of heat comes from the outside, these animals depend on the environment to regulate their body temperature. At this point, we should note that these living beings do produce some internal heat. However, it isn’t enough for them to be able to regulate their temperature.

These are some of the mechanisms these animals use to regulate temperature:

  • Sunbathing. There’s no better external heat source than the sun. Thus, a simple and effective method of increasing body temperature when the animal is cold is to expose itself to it and let its body temperature increase. That’s why it’s so common to see lizards on rocks sunbathing.
  • Bathing. What better way to cool off in hot weather than to take a bath? Animals use this strategy when their body temperature rises and they want to cool off.

Like all animals, snakes like to interact

To test the theories, Amarello set camera traps outside a den near Prescott where Arizona black rattlesnakes nest for the winter. It had largely been thought that snakes only den together during the winter because it’s so cold they can’t get enough heat from anything other than each other’s bodies. Communal denning of the cold-blooded reptiles is most common in the north. When they come out in the spring, they usually hang out long enough to mate, then go their separate ways.

“That's an easy evolutionary explanation for why they would hang out like that,” Amarello said. “But with Arizona black rattlesnakes, number one, this is Arizona, so even where we were working at a mile in elevation approximately just outside of Prescott, it's not that cold.”

Furthermore, Arizona black rattlesnakes mate during the monsoon, not in early spring.

“So no combat, no courtship. Just kind of hanging out,” Amarello said. “We were like, what’s going on there?”

An increasing amount of studies support Amarello’s conclusions. Most recently, a study that assessed the personalities and sociability of eastern garter snakes found that they seek out social contacts and are picky about whom they “hang out” with.

Scientists still have no idea what’s motivating the friendships, though they do know it’s not related to reproduction or mating: The study said snakes did not prefer the opposite sex as friends.

“All animals — even snakes — need to interact with others,” Morgan Skinner, a doctoral candidate in behavioral ecology at Wilfrid Laurier University in Canada, told National Geographic.

Coldblooded Does Not Mean Stupid

Humans have no exclusive claim on intelligence. Across the animal kingdom, all sorts of creatures have performed impressive intellectual feats. A bonobo named Kanzi uses an array of symbols to communicate with humans. Chaser the border collie knows the English words for more than 1,000 objects. Crows make sophisticated tools, elephants recognize themselves in the mirror, and dolphins have a rudimentary number sense.

And reptiles? Well, at least they have their looks.

In the plethora of research over the past few decades on the cognitive capabilities of various species, lizards, turtles and snakes have been left in the back of the class. Few scientists bothered to peer into the reptile mind, and those who did were largely unimpressed.

“Reptiles don’t really have great press,” said Gordon M. Burghardt, a comparative psychologist at the University of Tennessee at Knoxville. “Certainly in the past, people didn’t really think too much of their intelligence. They were thought of as instinct machines.” But now that is beginning to change, thanks to a growing interest in “coldblooded cognition” and recent studies revealing that reptile brains are not as primitive as we imagined. The research could not only redeem reptiles but also shed new light on cognitive evolution.

Because reptiles, birds and mammals diverged so long ago, with a common ancestor that lived 280 million years ago, the emerging data suggest that certain sophisticated mental skills may be more ancient than had been assumed — or so adaptive that they evolved multiple times.

For evidence of reptilian intelligence, one need look no further than the maze, a time-honored laboratory test. Anna Wilkinson, a comparative psychologist at the University of Lincoln in England, tested a female red-footed tortoise named Moses in the radial arm maze, which has eight spokes radiating out from a central platform. Moses’ task was to “solve” the maze as efficiently as possible: to snatch a piece of strawberry from the end of each arm without returning to one she had already visited.


“That requires quite a memory load because you have to remember where you’ve been,” Dr. Wilkinson said.

Moses managed admirably, performing significantly better than if she had been choosing arms at random. Further investigation revealed that she was not using smell to find the treats. Instead, she seemed to be using external landmarks to navigate, just as mammals do.

Things became even more interesting when Dr. Wilkinson hung a black curtain around the maze, depriving Moses of the rich environmental cues that had surrounded her. The tortoise adopted a new navigational strategy, exploring the maze systematically by entering whatever arm was directly adjacent to the one she had just left. This approach is “an enormously great” way of solving the task, Dr. Wilkinson said, and a strategy rarely seen in mammals.

Navigational skills are important, but the research also hints at something even more impressive: behavioral flexibility, or the ability to alter one’s behavior as external circumstances change. This flexibility, which allows animals to take advantage of new environments or food sources, has been well documented in birds and primates, and scientists are now beginning to believe that it exists in reptiles, too.

Anole, a tropical lizard, have a very specific method of acquiring food, striking at moving prey from above. But Manuel S. Leal, a biologist at Duke University, created a situation in which this strategy simply would not work, hiding a tasty insect larva inside a small hole and covering the hole with a tightfitting blue cap.

Two of the six lizards he tested tried to extract the treat by attacking the blue disk from above, to no avail. But the other four puzzled out new approaches. Two lizards came at the disk sideways, using their mouths to bite and lift it, while the others used their snouts as levers to pry it off the baited well.

Then Dr. Leal increased the difficulty by hiding the larvae under a new cap, this one blue and yellow. He used the solid blue disk to cover an adjacent, empty well. In tests of four lizards, two recognized the switch and learned that getting the bait now required flipping the multicolored disk instead of the blue one.

Other studies have documented similar levels of flexibility and problem solving. Dr. Burghardt, for instance, presented monitor lizards with an utterly unfamiliar apparatus, a clear plastic tube with two hinged doors and several live mice inside. The lizards rapidly figured out how to rotate the tube and open the doors to capture the prey. “It really amazed us that they all solved the problem very quickly and then did much better the second time,” Dr. Burghardt said. “That’s a sign of real learning.”

So how did we miss this for so long? Scientists say that many early studies of reptile cognition, conducted in the 1950s and ’60s, had critical design flaws.

By using experiments originally designed for mammals, researchers may have been setting reptiles up for failure. For instance, scientists commonly use “aversive stimuli,” such as loud sounds and bright lights, to shape rodent behavior. But reptiles respond to many of these stimuli by freezing, thereby not performing.

Scientists may also have been asking reptiles to perform impossible tasks. Lizards do not use their legs to manipulate objects, Dr. Leal said, “so you cannot develop an experiment where you’re expecting them to unwrap a box, for example.”

What’s more, because they are coldblooded, reptiles are particularly sensitive to environmental conditions. Rats and mice can run a maze just fine in a 70-degree lab, but many reptilian species need a much warmer environment — with air temperatures in the mid-80s or 90s. “They seem to learn the quickest at body temperatures that are very uncomfortable for us,” Dr. Burghardt said.

Now that scientists have gotten better at designing experiments for reptiles, they are uncovering all kinds of surprising abilities. Some of the most intriguing work involves social learning. The conventional wisdom is that because reptiles are largely solitary, asocial creatures, they are incapable of learning through observation.

New research calls that assumption into question. In another study of red-footed tortoises, Dr. Wilkinson deposited a tortoise on one side of a wire fence and a piece of strawberry on the other, in sight but just out of reach. To get their snouts on the treat, the tortoises needed to take a long detour around the edge of the fence.

Not one tortoise figured this out on its own. (Unable to reach the reward, some of the animals simply decided to nap.) But when they watched a trained tortoise navigate around the fence, all the observers learned to follow suit.

Other studies of reptiles have turned up similar results, challenging the popular theory that social learning evolved as a byproduct of — and a special adaptation for — group living. Instead, Dr. Wilkinson said, social learning may be merely an outgrowth of an animal’s general ability to learn.

The field of reptile cognition is in its infancy, but it already suggests that “intelligence” may be more widely distributed through the animal kingdom than had been imagined. As Dr. Burghardt put it, “People are starting to take some of the tests that were developed for the ‘smart’ animals and adapting them to use with other species, and finding that the ‘smart’ animals may not be so special.”

Evolution Of Warm-blooded And Cold-blooded Animals

Scientists have, over the years, proposed various theories that try to explain why endotherms evolved relatively high and stable body temperatures. They include the need to aid physiological processes, the need for animals to maintain activity over more extended periods, and because it allows some animals to take care of precocial offspring. Such theories have, however, not received strong support in the scientific community. While various theories have some truth to them, some experts argue that the likely cause was something that significantly impacted their chances of survival and reproduction. Otherwise, traits of endothermy would have been too costly as a strategy and would therefore not be favored during natural selection. The assertion is debatable. A more popular theory suggests that pathogens may have facilitated the evolution of warm-blooded animals. Scientists in favor of this hypothesis believe that the ability to maintain a high core temperature would have allowed animals to mount a rapid fever response to invading pathogens compared to cold-blooded animals. Cold-blooded organisms would have to rely on external sources of heat to obtain fever-like temperatures. That means that cold-blooded animals would have to search for the ideal microclimate to initiate a fever they would also struggle to mate and forage and would be exposed to predators. The theory emerged as a result of recent discoveries in fields such as animal physiology and immunology. Scientists, however, agree that rigorous tests, experiments, and data collection has to be carried out to strengthen the hypothesis. For example, the theory also suggests that species that maintain the most stable and warmest temperature should also experience a higher frequency of virulent pathogens or disease outbreaks. That is yet to be confirmed.