Scientists have found that even in animals with high nutrient demands, such as lactating females, energy intake is bound to reach a limit. This, despite the abundant food supply. The new theory effectively contradicts Charles Darwin's original ideas in the subject.
Darwin and his contemporaries postulated that food consumption in birds and mammals was limited by resource levels, which meant that animals would eat as much as they could while food was plentiful and produce as many offspring as this would allow them to.
Scientists at the Research Institute of Wildlife Ecology in Vienna have now suggested that energy intake reaches a limit due to active control of maternal investment in offspring in order to maintain long-term reproductive fitness.
The new research led by Dr Teresa Valenca showed that Brown hares could increase their energy turnover and rate of milk production above normal levels when their energy reserves were low, or when their offspring were kept in cooler temperatures.
That indicated that, ordinarily, the hares were operating at below their maximum capacity.
It also showed that this is not due to any kind of physiological constraint, such as length of digestive tract or maximum capacity of mammary glands.
As the hares were also provided with plentiful food, there could be no limitation of energy turnover due to food availability.
The way that females regulated their energy expenditure according to pup demand and their own fat reserves but did not exceed certain levels was in line with the group's theory that using energy at close to the maximum rate has costs for animals which may compromise their ability to successfully reproduce in the future.
For example, if a hare puts most of its energy into a litter of pups then it will have little left over for growth and body repairs, which may shorten its life or make it less able to produce or care for young in the future.
Thus, by actively limiting the rate of energy turnover, a mother can prevent this and maintain a higher level of reproductive success over her lifetime.
The study will be presented at the Society for Experimental Biology Annual Meeting in Glasgow.