Being mammal equals being warm-blooded, or does it?
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| photo credit wikipedia |
Mammals are animals that are (mostly) covered in hair and that nurse their young with milk. They include duck-billed platypuses, mice, elephants and human beings.
It is true that all mammals can produce heat from within, a talent known as endothermy. This means that most mammalian species do indeed have warm blood. They maintain a high and fairly constant body temperature, which allows them to function efficiently across a range of conditions.
This is why entry-level textbooks often refer to mammals as being "warm-blooded". This distinguishes them from "cold-blooded", ectothermic creatures whose body temperature is wholly dependent on their surroundings.
Breaking the rules is, well...natural
The thing is, in biology rules are made to be, at the very least, severely bent.
There are plenty of mammals that take a much more relaxed approach to body temperature. For these "heterothermic" animals, the term "warm-blooded" does not really capture what they are doing.
They are certainly not "cold-blooded" in the same way as fish, amphibians and reptiles are "cold-blooded". But they are capable of some impressive feats of cooling.
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| photo by Matt Heschen |
One of the most extreme heterotherms is the Arctic ground squirrel.
In a classic paper that made the front cover of Science in 1989, physiologist Brian Barnes of the University of Alaska Fairbanks studied the squirrels during hibernation. He found that they drop their core body temperature below zero, in one instance to -2.9C, without freezing solid.
It's hard to get much more cold-blooded than that.
Clearly, the squirrel is a special case. Still, many mammals are capable of some kind of chilling.
For instance, new-born mammals' body temperature is entirely dependent on the temperature of the environment. The ability to produce internal heat only kicks later in development.
Similarly, when mammals sleep their body temperature usually falls by a degree or two.
Energy-saving mode
Smaller mammals – including many rodents, insectivores, bats, marsupials and even some primates – have evolved a way to push this temperature reduction much further. They enter an energy-saving state known as daily torpor.
For instance, the common blossom-bat can lower its body temperature from around 36C at night to just 20C in the day. Similarly, the Brazilian gracile opossum seems able to chill at 16C for hours on end.
In a more extreme case, Madagascar's pygmy mouse lemur will spend around 10 hours a day in torpor, its body temperature falling below 7C.
Some mammals can enter a more prolonged torpor. We call this "hibernation" if they do it in winter, and "aestivation" if it is a summertime thing. Then, their blood runs even colder.
Naked mole rats are interesting from a thermoregulatory standpoint, because they don't control their body temperature very well.
This does not mean they are failed mammals. Rather, it is simply that they spend all their lives in underground tunnels where the temperature is fairly predictable, usually somewhere between 29 and 32C.
They don't have to spend the energy on thermoregulation. It's the perfect example of an evolutionary adaptation, not a physiological limitation.
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These examples all speak to a simple fact: maintaining a high and constant body temperature is a costly exercise. In fact, it is surprising that we do not see more mammals spending more of their time with colder blood.
Species often lose abilities when they are no longer useful. For example, animals that live in dark caves tend to lose the use of their eyes. As the naked mole rat demonstrates, mammals that live in places with steady temperatures may not need to heat themselves internally.
