When BBC Earth went onto Facebook and asked our audience if there are any cold-blooded mammals, we got a strong reaction.
"This is a silly question," wrote Clay Walker. "The definition of being a mammal includes being warm blooded."
Mark Josefsberg was similarly taken aback. "You insult our collective intelligence."
We really did not set out to upset anyone! We just wondered if, the natural world being the varied place that it is, there might be a few outliers – and it turns out that there are.
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.
They are certainly not "cold-blooded" in the same way as fish, amphibians and reptiles
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.
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.
"The more we look, the more species we find that do that," says Justin Boyles, a physiological ecologist at Southern Illinois University in Carbondale.
One of the most extreme heterotherms is the Arctic ground squirrel.
Many mammals are capable of some kind of chilling
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," says Boyles.
Kudos to Graham Humphrey and Janakie Balasuriya, who both specifically mentioned the Arctic ground squirrel.
Clearly, the squirrel is a special case. Still, many mammals are capable of some kind of chilling.
For instance, newborn 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.
Madagascar's pygmy mouse lemur will spend around 10 hours a day in torpor
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, as George Uren, Mary Wyman and Indrani Ghosh noted.
Respect also to those who gave a shout out to the naked mole rat.
It's the perfect example of an evolutionary adaptation
"Naked mole rats are interesting from a thermoregulatory standpoint, because they don't control their body temperature very well," says Boyles.
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," says Boyles. "It's the perfect example of an evolutionary adaptation, not a physiological limitation."
Update 13 June 2016: We missed a trick here. On Twitter, Michael Le Page of New Scientist points out that hyraxes also cannot control their internal body temperature. But unlike naked mole rats, they live out in the open and have to deal with changing temperatures. To survive, they must bask in the sun to warm up, as many reptiles do.
Finally, Jennifer Jones highlighted an extinct species of goat called Myotragus balearicus. It has been claimed that this goat was properly ectothermic: that is, entirely dependent on heat from its environment.
We could not find a follow-up to the 2009 study
The evidence is indirect. "The bone microstructure indicates that Myotragus grew unlike any other mammal but similar to crocodiles at slow and flexible rates," wrote researchers in 2009. This is suggestive of ectothermy, but it is a long way short of proof.
M. balearicus lived until around 3,000 years ago on the Balearic Islands, where the supply of food is erratic. This harsh environment may have selected for an "ectotherm-like state" that allowed the goat to conserve energy, the researchers suggest.
It is an intriguing idea. However, we could not find a follow-up to the 2009 study, so for now it is not clear if M. balearicus was truly ectothermic.
Regardless, 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.
I expect we'll find a whole lot more very cool species
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.
Boyles says that smaller and smarter temperature-recording gizmos are revealing new patterns of temperature fluctuations in mammals all the time.
"Over the next 10 or 15 years," he says, "I expect we'll find a whole lot more very cool species." The pun, presumably, is intended.
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