Lessons from Tenrecs and Treeshrews: Heterothermy is More Common Than We Thought

by Etti Cooper

Dr. Danielle Levesque (upcoming 2023 speaker for SICB) chuckles as she describes collecting Greater hedgehog tenrecs (Setifer setosus) in the forests of Madagascar. “We just kind of chased after them in the forest,” she recounts. “Their spines are their defense mechanism. One saw me coming and curled up into a ball so I just picked it up.”

A greater hedgehog tenrec (Setifer setosus) in Madagascar. Photo by Danielle Levesque.

If you’re as unfamiliar with tenrecs as I was just a few weeks ago, picture a hedgehog that’s not really a hedgehog, with beadier eyes and a longer, wetter snout. In reality, only a few of the 31 extant species could be confused for hedgehogs. The lowland streaked tenrec (Hemicentetes semispinosus), for example, is incredibly bizarre, with yellow and black spines that stick up in every direction and what could only be described as a permanently shell-shocked expression. However, it turns out that tenrecs are even more unusual than their appearance would suggest.

In reality, tenrecs are not particularly closely related to hedgehogs; they are members of the clade Afrotheria, which includes such charismatic African fauna as elephants and aardvarks. Tenrecs provide an excellent system with which to study evolution because the vast majority live on the island of Madagascar. It is hypothesized that a common ancestor of all extant tenrec species arrived on the island from mainland Africa millions of years ago. Since then, a massive adaptive radiation has allowed tenrec species to diversify and occupy a wide variety of niches on the island. But to Levesque, yet another unique characteristic of the tenrec is most intriguing: their highly variable body temperature.

One familiar characteristic of the mammals as a phylogenetic group is our ability to maintain a high and constant body temperature, called homeothermic endothermy. In humans, this temperature is approximately 98.6 degrees Fahrenheit or 37 degrees Celsius. In the grand scheme of life on Earth, homeothermic endothermy is actually very rare and is shared only with birds. Therefore, all other organisms (with a few notable exceptions) must rely on heat from the environment to keep their body temperature within an appropriate range. For decades, homeothermic endothermy was considered to be a hallmark of the mammals to the same extent as having hair, giving birth to live young, or lactating.

That’s not to say that heterothermy—or a body temperature that is fluctuating rather than stable—is not present in mammals. The traditional example is a ground squirrel, which become torpid (a more precise term for “hibernation” in a colloquial sense) for months at a time in the winter. In fact, Levesque first became hooked on thermal physiology when introduced to her undergraduate mentor Murray Humphries’ work on North American hibernators. In this case, torpor is used as a tool to save energy as opposed to expending the energy required to stay warm in the coldest months. The squirrels’ circannual rhythms trigger torpor in the winter months and the winter months only; Levesque comments that “no matter what you do to a ground squirrel in July, it won’t use torpor at all…there are very few things you can do to stop it from using torpor in the winter.”

However, recent work has challenged our understanding of what it means to be a mammal by suggesting that mammalian heterothermy is far more common than it seems. In fact, Levesque remarks, the only clades that lack evidence of heterothermy entirely are the lagomorphs, or rabbits and hares, and the ungulates, the large hoofed mammals. Even more surprising? The finding that many mammals living in tropical habitats also utilize torpor. This may seem odd, given that the tropics are neither as cold nor as variable as the ground squirrels’ frigid Canadian home. In this case, torpor is not used to save the energy required to stay warm. So why is this behavior beneficial? Levesque wanted to learn more.

This line of inquiry took her to far-flung, exotic field sites: Australia, to study echidnas; Madagascar, to study tenrecs; and most recently, the Bornean jungle to research the large treeshrew (Tupaia tana) and the pygmy treeshrew (Tupaia minor), among other small mammals. Treeshrews are tough to describe to an audience with a North American frame of reference. Neither marsupials nor rodents, they belong to the order Scandentia, members of which are native only to south and southeast Asia. Picture a squirrel with a thinner tail, shiny gray and brown fur, and a face with wide, round eyes and strangely human-like ears. The treeshrew’s long, downturned snout creates an almost comically worried expression.

As different as these species may appear from one another, these creatures have one common feature: none are nearly as endothermic nor as heterothermic as they should be according to our traditional understanding of mammalian physiology. Echidnas and tenrecs have lower ambient body temperatures than most other mammals, which is probably an energy-saving strategy. Alternatively, large treeshrews have very high ambient body temperatures, which, although counterintuitive, appears to be a way to stay cool and retain water in the hot Bornean jungle.

Also important is the fact that some species of tenrecs and other small mammals are so-called daily heterotherms; rather than becoming torpid for an entire winter like the ground squirrel, these species instead become torpid whenever the temperature dips a little too low, which usually occurs on a nightly basis. Because so many groups of mammals are able to utilize torpor, including species that are evolutionarily similar to the ancestral mammal (like the tenrecs), the researchers who work on the evolution of heterothermic endothermy, like Levesque, have come to a major conclusion: we are the anomaly. More precisely, maintaining a high and constant body temperature like we do is objectively a huge waste of energy. Despite the fact that we often view homeothermy as an inherent feature of being a mammal, the common ancestor of the mammals was probably a heterotherm. By studying these tropical mammals, we can learn more about our common ancestor.

Levesque still has lots of questions. Usually, thermal physiology as a field tends to focus more on everyone but mammals and birds. As a result, there is a lot left to learn. She is wondering which species must spend weeks or months preparing to enter torpor and which ones can become torpid at the drop of a hat. She wants to continue cataloging the thermal physiology of the unique and understudied mammalian fauna in Borneo. Most of all, she wants to continue to untangle the evolutionary processes that made mammals mammals.

You can find Dr. Danielle Levesque on her lab website and on Twitter. Check out a recent publication here.

Previous ICB paper Levesque co authored:

Understanding Drivers of Variation and Predicting Variability Across Levels of Biological Organization 

https://doi.org/10.1093/icb/icab160

Connect with sci comm blogger
Etti Cooper who is interested in plasticity in thermal tolerance and metabolism in insects, reptiles, and fishes. Connect with her on Twitter at @EttiCooper. 

Author: suzannecrmiller

Author of Queen, Wage, The Selections on Amazon, Fly on site and soon to be Souvenir through @Inkdedingray publishing

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