Sleeping off a tough winter to survive

Winter is upon us. Many birds fly south to avoid the cold wet darkness, but migration isn’t an option for mammals. They have to make do with thicker coats and extra layers of body fat, writes Richard Collins

Pygmy shrews do not hibernate in winter

An animal failing to pay its heating bills, goes to the wall. Size matters; large objects retain heat better than small ones, so being big and bulky helps.

Wild creatures feed up in autumn when food is plentiful. Hibernation makes body-fats go much further, but storing enough fuel, in advance, for an entire winter isn’t easy.

You would expect bears and badgers to remain active through the winter; their large bodies retain heat well. Bears take to their beds in late autumn, lowering their body temperatures by about 10 degrees centigrade.

This is dormancy rather than true hibernation; females, while remaining torpid, give birth in the dens. Badgers try to have it both ways. Slumbering in their underground setts, they check on the weather occasionally and venture out if it’s mild.

For small creatures, hibernation seems the obvious strategy. Some butterflies bees, and the dormice recently introduced to Ireland, sleep through the winter but, oddly, house mice and field mice don’t. The pygmy shrew, our smallest mammal, might seem an ideal candidate for hibernation.

Its tiny body, the weight of a €1 coin, loses heat so quickly that a shrew must feed every few hours, or die of starvation. Individuals consume between half and twice their body weight each day. Getting that much food is a challenge but shrews, although capable of temporary torpor, don’t hibernate. Nature works in mysterious ways.

Shrews belong to an ancient order of mammals, the insectivores. Were their ancestors too primitive to evolve hibernation? No.The shrew’s failure to do so isn’t down to backwardness; their relatives, the hedgehogs, drop their temperatures by about 90% in winter.

Research results, just published in the journal Current Biology, reveal the extraordinary lengths shrews go to coping with the trials of winter.

In the 1950s, Polish zoologist August Dehnel discovered that shrews caught in winter are smaller than ones found in summer. Their skulls, he found, were also smaller. Why? To answer that question, Javier Lazaro and colleagues at the Max Planck Institute for Ornithology, trapped shrews. Anaesthetising the animals, they took X-ray images of them and measured each one’s body mass.

The shrews were then tagged and released to the wild.

The team managed to retrap some shrews several times during the subsequent winter. Repeating the measurements, researchers found that skull sizes fluctuated with the seasons, peaking in summer, shrinking in winter and growing again in spring.

“Individuals decreased the size of their braincases in anticipation of winter by an average of 15.3%” and “partially re-grew in spring by 9.3%”, the authors claim. Body mass “decreased by 17.6% and then dramatically increased by 83.4% in spring”. “These are conservative assessments”, they note, “because it is unlikely that any individual was captured at the exact time of size extremes”.

Since it’s an advantage to be large in cold environments, how does being smaller, and shrinking their brains, help shrews to survive? There’s a trade-off. “An average decrease in body-mass of 19% leads to a winter reduction of 18.2% in a shrew’s absolute resting metabolic rate,” the scientists explain.

“This might also reduce the absolute food requirements” and “improve the probability of survival of high-metabolic shrews”. Brains have high running costs. Human ones consume a fifth of the body’s energy. Reducing brain size and the food needed to power it is, it would appear, a more effective strategy for shrews in winter than maintaining their normal size.

Javier Lazaro et al. Profound reversible seasonal changes of individual skull size in a mammal. Current Biology. October 2017.


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