Story and watercolors by Wenfei Tong       

watercolor painting of Piping Plover mother and chicks, by Wenfei Tong

Many parents volunteer in their children’s schools and classrooms, putting in extra time and energy to help support teachers and ensure that their kids receive a good education—people with children have a strong incentive to invest in this particular example of what economists call a public goods problem. But it’s not only in economics that we find public goods problems; evolutionary biology has plenty of them, too. And a big one is generating heat to survive cold weather. In almost any human culture, “warmth” in a person typically refers to their generous and affectionate personality rather than a capacity for generating physical heat. This is probably no accident. Looking at heat as a public goods problem, evolutionary biologists think that one of the reasons for group living could be the mutual benefit of sharing heat through huddling when it’s cold outside, and it takes at least two to huddle. Bees do it, bats do it, even rattlesnakes might do it.


Take birds, for instance. Small migrating birds like the European Blackcap Warbler get a thirty percent energy saving from huddling overnight when compared to solitary individuals. Similarly, Tree, Barn, and Cliff swallows and even Grey Partridges all tend to form groups and huddle together when it gets cold.

five barn owlets in a barn

Barn Owl chicks that live in colder climates are better adapted to generating their own heat; this ability is genetically linked to growing paler feathers. Photo by Chad King,

Barn Owls are a particularly interesting example of the importance of huddling because there appears to be genetic variation in their strategies for conserving heat that is linked to feather color in this geographically widespread species. Scientists haven’t worked out all the genetic links, but blacker owlets tend to be less efficient at generating body heat in isolation and have to huddle more to stay warm. In contrast, paler owlets are better at regulating their body temperature and don’t need to huddle as much to stay warm. Indeed, Barn Owl populations that live further north have paler owlets than those that live closer to the equator. You might think that being darker would keep individuals warmer because darker colors absorb more heat from sunlight, but owls don’t spend much time in the sun.

watercolor of penguin chick by Wenfei TongPenguins are an especially good example of group living and huddling being associated with colder temperatures. Most adults of any penguin species have a well-defined personal space, which is apparent if you ever see the squabbling and squawking in a colony on a nature documentary. In contrast, all young penguins are deposited in crèches akin to human nurseries, where they huddle together for warmth while their parents are out fishing. Emperor Penguins are the only penguin species to retain this huddling behavior in adulthood, and it’s no coincidence that this species experiences the most extreme cold of all the penguin species. Males cluster together to keep each other warm during the harsh Antarctic winter. Each male is effectively fasting and living off fat reserves until his mate returns after four months to take over, and he has to heat not just himself, but the precious egg sitting on his feet. Scientists sticking temperature probes into individual male Emperor Penguins have found that males in a huddle can afford to lower their personal heating bill by keeping their own furnace about two degrees Celsius lower and getting passively warmed by the rest of the group, compared to solitary males who have to pay a higher energetic cost to keep their core warm.

Emperor penguin {Aptenodytes forsteri} males huddle for warmth in minus 40 degrees centigrade while incubating eggs in winter, Antarctica Winter extreme cold survival birds seabirds nesting-behaviour

Emperor Penguins huddle like the chicks of other penguin species to keep out the bitter Antarctic cold. Photo ©Nature Picture Library / Alamy Stock Photo.


Aside from birds, there are examples of mammal species, such as marmots, that support the social heating hypothesis. This rodent family includes woodchucks, which tend to be solitary and live in the warmest climates, and Montana’s native yellow-bellied and hoary marmots, which tend to live in social groups, especially in winter.

The best-studied marmot is the alpine marmot of Europe, which overwinters in groups of up to 20 individuals, often including a breeding pair, juvenile offspring from the same year, and subordinate adults that are mostly offspring from previous years. When there are no juveniles in a group, larger groups have more surviving marmots at the end of winter, suggesting that huddling benefits adults. However, when juveniles make up part of the group, some interesting things happen. The juveniles are always in the center of the group, suggesting that the adults are paying a disproportionately large share of the heating bill to keep their young relatives warm, but is this heating bill equally shared among adults?

It turns out that the main overwintering costs for marmots aren’t the fasting or the hibernating, but having to re-warm every time they emerge from hibernation—rather like having to heat up a cabin after keeping the furnace off for months. Short of putting temperature probes into marmots before they go underground to hibernate, the easiest way for scientists to see who paid the largest amount of energy to the communal heating bill is to weigh marmots before and after hibernation. Those who have lost the most weight probably burned up the most fat to heat the group.

So who does pay more of the heating bill? It turns out that subordinate adults that are unrelated to the family group lose the least weight while the marmot parents lose the most weight when there are juveniles in the group. This makes sense because evolutionarily, it pays to sacrifice some of your own fat stores to keep your children warm and alive. What is more surprising is that some subordinate adult males in the group lose about as much weight as the breeding pair. One possible explanation for this is that some of those subordinate males might actually have snuck a mating or two with the breeding female and so could be more closely related to the juveniles than expected.

four yellow-bellied marmots on a talus slope

Yellow-bellied marmots are more likely to survive the winter if they don’t have to help heat up the younger marmots in the huddle. Photo by Wenfei Tong.

Complex social groups increase the benefits of huddling for warmth, and although there is a general pattern of colder conditions leading to more complex groups, there are some twists. Unlike the alpine marmots, in which larger groups are more likely to survive the winter, in hoary marmots, neither group size nor composition makes much of a difference. Rather, the main factor influencing group survival is the harshness of the climate, suggesting that no amount of group huddling can save a hoary marmot group when conditions are bad enough. One possible explanation for this is that the snowpack in the Alps is always very deep, insulating alpine marmot groups and the heat they generate, whereas hoary marmots in the Canadian Rockies often experience much thinner snow packs, combined with colder external temperatures, suggesting fewer benefits from huddling.

In yellow-bellied marmots, which experience less extreme winters than alpine or hoary marmots, and are correspondingly less social, there actually seems to be a cost to being too friendly. Biologists have found that marmots with very strong, affectionate social ties with other individuals are the ones most likely to die during the winter, whereas the most antisocial individuals are most likely to survive. This surprising and somewhat disheartening finding could be due to the very high energetic costs of re-warming, and the fact that yellow-bellied marmot groups are less cohesive than alpine marmot groups. So if the group isn’t good at re-warming all at once, some individuals—the ones with strong social ties—pay a disproportionately high price to warm the group and are consequently more likely to lose so much weight they die. Juvenile marmots, in particular, seemed to disrupt the group’s re-warming schedule by trying to warm up too early. In contrast, the antisocial marmots that hibernated alone instead of in a huddle don’t have to be sensitive to someone in the group trying to warm up too soon and are more likely to survive just paying their personal heating bill at the optimal time.

So how does all this expensive re-warming take place? Marmots and other mammals, like humans, have an extra way to warm up that hasn’t evolved in other warm-blooded animals like birds. It’s called brown adipose tissue, or brown fat, a highly specialized tissue in which every cell is like a little heat production machine. These specialized cells produce heat by short-circuiting the reaction all cells use to make the energy. Instead of fueling all the little chemical reactions that keep us alive, brown fat cells dump this energy out as heat. By mobilizing their brown fat stores, hibernating marmots begin to re-warm their muscles, which can then continue the re-warming process by shivering.


Not surprisingly, brown fat is especially common in human infants as well as in hibernating mammals. What is more intriguing to many in the health industry is that adult humans have brown fat too, and that individuals vary considerably in how much brown fat they have. We know that thinner people tend to have more brown fat—what you might typically call a “high metabolism”—presumably because people with a lot of brown fat are converting more calories into heat. However there isn’t yet a consensus on what causes this variation in the proportion of brown fat in adult humans, or even if it’s the main cause of not putting on weight easily.

A curious, but possibly related aspect of human biology is our hairlessness. One of the other strikingly hairless adult mammals (also highly social) is the naked mole rat of Africa, and they spend a lot of time huddling and looking like a very large litter of pink young mice or rats. When you think about it, a thick winter coat makes lots of sense if one is trying to keep precious body heat in, but if one is trying to share the heat in a huddle, an insulating barrier or hair or down would simply get in the way of efficiently transferring heat.

Hairless, pink young like songbird chicks or young mice and rats are typically helpless and not terribly good at generating their own heat. They are hairless for the very good reason that any heat produced by one is more efficiently shared by all. During the breeding season, many adult songbirds develop a naked area of exposed skin with lots of blood vessels near the surface called a brood patch, to transfer heat more efficiently to their eggs and chicks. In contrast, the typically endearing downy young like young penguins, ostriches, chicks, or ungulates are born much more independent in many ways, including being able to stay warm without social assistance.

Watercolor of bat-eared fox family by Wenfei Tong


And now we’re back to the public goods problem. With huddling, everyone in the group benefits equally, but some individuals may contribute more (thereby paying a greater cost) than others. It pays the most individually to be a cool rat pup in a warm huddle. But then what determines who pays?

Recall that in alpine marmot families, the main heat donors tend to be the parents or close relatives of the juveniles that need the most help staying warm. This is best explained by the fact that marmots able to selectively help relatives with heat donations beget more offspring than those that indiscriminately sacrifice their own heat resources to aid the survival of unrelated individuals. Perhaps there’s a sort of genetic switch with a rule, where if you are hibernating with close relatives who smell similar, you produce more heat, whereas if you aren’t related, your heat-producing genes stay off, leaving you more energy and fat stores for surviving the spring, while passively absorbing heat from the group.

If the key condition for genes to be switched on or off is how likely the gene is to be present in other members of a huddling group, then in a group of huddlers, heat-producing genes should have evolved to donate more heat to related individuals in the group, but to be switched off and selfish if they are surrounded by non-relatives. When parents are perfectly faithful to each other, then on average, their children all share half their genes. But not all parents are perfectly faithful couples. In many rodent species, females mate with more than one male, so a litter will share the same mother, but not everyone will have the same father. As one might predict, genes for turning on heat production in mouse pup brown fat cells tend to be switched on when inherited from mothers, while the copy of the same gene inherited from fathers is switched off.

In the economics of evolution, individual genes can switch on or off to be more or less selfish in a public goods game involving the sharing of heat. Human societies encounter all sorts of public goods problems, from education to transportation to clean water. Some can or are supposed to be solved by taxes; others, like the environment and all the ecosystems services nature provides, from clean water to pollinating our crops, are trickier. If one of the evolutionary benefits we derive from living in highly complex social groups is the sharing of heat, including with unrelated adults and children, humans just might be able to “switch on” our capacity to cooperate in other public goods problems, such as our shared dependence on a healthy natural environment.

Dr. Wenfei Tong is a research associate at the departments of Organismic and Evolutionary Biology at Harvard and the Division of Biological Sciences at the University of Montana. She works for the Avian Science Center in the College of Forestry, studying the effects of fire and bison grazing on songbirds at the National Bison Range.


This article was originally published in the Winter 2017-2018 issue of Montana Naturalist magazine, and may not be reproduced in part or in whole without the written consent of the Montana Natural History Center. ©2017 The Montana Natural History Center.

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