It's a long-standing joke that teenagers seem to be nocturnal, but for many animals the switch to a night active body-clock is more than just a lifestyle choice. Vincent van der Vinne from the University of Groningen, The Netherlands, explains that it is a matter of keeping costs down. Mice that are raised with plenty of food are strictly nocturnal, taking advantage of the night to avoid predators. However, when food becomes scarce and the temperatures drop, the tiny rodents switch to riskier daylight activity. Van der Vinne and his colleagues suspected that the small animals are prepared to gamble with fate to conserve energy when resources are limited by being active when it is warm during the day and snuggling up with nestmates during the cold night. Meanwhile, nocturnal animals should have to invest more energy to stay warm when active at night. However, no one had ever measured the true energetic costs of a nocturnal (versus diurnal) lifestyle, so van der Vinne and his colleagues, Jenke Gorter, Sjaak Riede and Roelof Hut set about measuring the metabolic costs of different mouse lifestyles.
Providing some captive mice with wheels while others had none, van der Vinne and Gorter set the air temperature in the animals’ cages at temperatures ranging from a chilly 10°C to a balmy 30°C and measured their energy expenditure while they were active and resting. The wheel-runners burned more calories than the sedentary mice and the active mice conserved more energy when resting than the mice that were less active. Also, the air temperature had a big effect on the mice, with the warm mice using less energy than the chillier animals. Next, the duo tested the effects on energy consumption of snuggling together by measuring the energy use of mice in nests of 1–3 animals with different amounts of bedding at various temperatures at night. Not surprisingly, the mice that had more nestmates and more bedding stayed warmer; however, the duo found that the air temperature had a significant impact on the animals’ energy consumption, with mice in the coldest conditions benefiting most from cosying up in a nest with several others at night. But what impact would these savings have on the animals’ activity patterns?
Van der Vinne built a computer simulation – incorporating factors such as how much warmth active animals generate, the impact of huddling in nests and the effect of environmental temperature – and found that switching to a daytime lifestyle and using the warmth to be active allowed normally nocturnal mice to save up to 10% of their energy budget. And when van der Vinne factored hunger into the equation, the mice could boost their energy savings to an impressive 20% by dropping their body temperature and going into a mini-hibernation at night. Switching their active periods from night to day and cuddling up with nestmates makes perfect metabolic sense for cold mice when food is scarce.
But what does this mean for mice out in the wild? When are they most likely to trade in a low risk but metabolically costly nocturnal way of life for a more risky but low energy cost daytime existence? Collecting temperature data from weather stations across Europe, van der Vinne found that functioning during the day could save mice as much as 14% of their energy budget in Spain in spring and autumn, but only 3% in Norway. So it makes more sense for Spanish mice to become active during the day when food is scarce than Norwegian mice, which are just as well off sticking to life in the dark.