Man has bucked the trend when it comes to over wintering. As the nights draw in and the temperatures drop, we retreat to our homes, and turn the heating up. But other mammals don’t have that choice. For them it’s adapt or die. For small animals that have a large surface-to-volume ratio and cool rapidly, the problem is even more urgent. Stuart Egginton assumed that animals which pass the winter in hibernation use different approaches to preparing for the cold than animals that keep active to stay warm. He tricked hamsters and rats into preparing for winter and looked at the way they modified their muscles with the unexpected result that rats put on muscle, while the hamster’s muscles wasted (p. 829).
Small rodents have two choices; either to cool down and hibernate like hamsters, or to burn more calories to keep warm like rats. Active animals generate much of their heat in their skeletal muscle, while dormant hamsters have to maintain their muscles during their inactivity, ready for when they reawaken.
If you want to know how an animal really prepares for the winter, you’ve got to slow them down gradually. Just putting an animal into a fridge is nothing like the real onset of winter, it’s ‘cold’ shock. Egginton and his graduate student Durmus Deveci set about designing ‘rodent biospheres’ where they could control the temperature and day length to convince the animals to prepare for winter. Over a month, they cooled the animals from long warm late summer days down to a day that lasted just 1 hour at 5°C.
Once the animals had spent another month in their artificial winter conditions, they compared the winter animals with their summer cousins. Deveci had a surprise when he looked at the rats; their muscles were built up, while everyone else who’d ever looked at cold rats had watched the animal’s muscles wither.
The rats need bigger muscles to generate the heat to keep them warm. Deveci saw that the muscle fibres had expanded. Bigger muscles also need more fuel and generate more waste, so the rats also have to plumb-in more blood vessels to supply the bigger muscles. But the increased blood flow doesn’t increase the muscle’s capillary density, because the muscle had expanded at the same rate as the increase in the number of capillaries.
Hamsters, on the other hand, have an entirely different set of problems. As they prepare for their long winter nap, they cool down and their blood thickens, which makes it harder to pump through tiny capillaries. At the same time, the animal’s muscle fibres shrivel because it is immobile. The hamsters doesn’t put more blood vessels into their muscles to overcome the problem of pumping thicker blood, they concentrate the same number of capillaries into a smaller muscle to increase the muscle’s capillary density and maintain the essential fuel supply to the resting muscle.
Egginton’s rodent biosphere has turned out to be the key to this truly comparative study of the animal’s approaches to over wintering. Both animal’s successful survival hinges on a good blood supply, but they use entirely different body building strategies to get them through to spring.