Mammals that hibernate for part of the year tend to have smaller brains than those that can feed all year round.
The finding may help explain why big-brained primates like us mostly evolved near the equator. It may be that our species arose in Africa, rather than neighbouring Europe, because Africa lies on the equator and much of it has only weak seasons.
of the University of Zurich, Switzerland and her colleagues compiled data on 1104 mammalian species. They looked at the size of each species’ brain relative to its body, and whether or not it hibernated.
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Those mammals that hibernated had significantly smaller brains relative to their bodies, even when the team controlled for other factors like the animals’ diets. “Hibernation really is a constraint for brain size,” says Heldstab
Expensive brains
The team argues that this is because it takes a lot of energy to grow and run a large brain, an idea called the “expensive brain hypothesis”. For an animal to grow a big brain, it must put less energy into something else – and if food is tight there are limits to what it can do.
“If you have a seasonality in food availability, if you don’t have a constant high energy supply, you’re just not able to have a large brain,” says Heldstab.
“If ever you don’t feed your brain, you die immediately,” says of the University of Queensland in Brisbane, Australia, who was not involved in the study. “[Even if] for three seasons you might have a lot of food, one season is going to block you from having this big brain.”
Weisbecker says the study confirms what has long been suspected. In 2015 she showed that , after noticing that marsupials on New Guinea had larger brains than their close relatives on Australia. “That really struck me,” she says, but it makes sense because New Guinea is exactly on the equator and has no seasons, whereas Australia does.
Primates, the group that includes monkeys, apes and humans, mostly live in places near the equator – and the fossil record suggests that the first primates evolved in equatorial Asia.
The lack of seasons near the equator, which means no need to hibernate, is probably one of the reasons why so many primates have evolved large brains, says Heldstab.
Even today, only a handful of primates hibernate. “There’s just three orders of lemurs which hibernate, and these three have the smallest brains in primates,” says Heldstab. She suspects that the first primates did not hibernate, and that the behaviour has arisen in only a few groups since then.
However, once a species has evolved a big brain, the additional intelligence it gains may then allow it to colonise more seasonal places. In line with this, there is evidence that .
Journal of Evolutionary Biology
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