Most animals face extreme endurance tests during their lives, whether it's surviving cold winters or suckling a family of five. But for most creatures there seems to be an internally set limit to the amount of energy that they can turn over, no matter how extreme the conditions. According to Teresa Valencak and her colleagues from the University of Veterinary Medicine, Austria, there are two possible explanations for this limit. Valencak explains that mammals produce heat as a by-product of metabolism, but overheating is dangerous, so some animals' energy turnovers are limited by the amount of heat that they can dissipate. Alternatively, an animal's energy turnover could be set by other factors, such as the amount of food that they can consume or the amount of energy that they are prepared to invest in the next generation. Knowing that small mammals' and dairy cows' energy turnovers are restricted by the amount of heat they can lose, Valencak, Klaus Hackländer and Thomas Ruf decided to find out what sets a lactating European hare's limit (p. 2832).
Knowing that hare leverets only suckle once a day and are left untended the rest of the time, the trio took hare mums with litters of three leverets, separated the young from their mothers and then kept some mums warm (22°C) while their young were kept in the cold (5°C). Other mums were kept at the same low temperature (5°C) as their young, while a third group of families were kept at 22°C. Reuniting the leverets with their mothers once a day to suckle, the team weighed everything that went into and out of the mums, as well as weighing the leverets before and after they suckled to find out how much milk the mothers produced. The team reasoned that if the hare mums were limited by their ability to dissipate heat, then the warm mums could not turn over more energy to produce enough milk to meet the high energy demands of their cold young, while cold mothers would have no problem suckling their cold leverets. However, if something else was limiting the mothers' energy turnover, then the warm mothers might be able to produce enough milk to suckle their cold and ravenous young.
Monitoring the mothers' energy budgets over 4 weeks, the team found that the warm hare mums suckling cold young were able to increase their energy intake just as much as the cold mums during the second week of lactation. The warm mums' energy turnover was not restricted by the amount of heat that they can dissipate. Valencak and her colleagues suspect that hare mums actively restrict their energy turnover by limiting their investment in their current litter to maximise their lifetime reproductive success.