Back on our bathroom scales after an extensive feast, we all know the obvious negative effects of body reserves. However, all animals, including us,rely on these reserves to fuel periods of large energy demands and low food availability. As the incidence of these periods fluctuates, so does the fitness value of hidden reserves. Large fat depots have direct costs in terms of nutrient acquisition, tissue maintenance, impaired locomotion and an increased predation risk. Thus, body reserves should be regulated according to the animal's current needs, which change over time. While many studies discuss the benefits of large fat depots, the costs of being fat have largely been ignored, especially in large ungulates. To fill this gap, Per Fauchald and his colleagues from the Polar Environmental Centre in Norway decided to over-feed female semi-domesticated reindeer to see how they accumulate and use body reserves during seasonal and life-cycle changes.
Female reindeer are especially challenged when it comes to building appropriate body reserves; they have to accumulate fat to resist cold and food shortage in winter, while simultaneously maintaining sufficient reserves to invest in calves in spring. The risk of starvation is small in spring, so at this time of year females might be expected to spend more time protecting their offspring from predators, rather than eating to build up body reserves.
The Norwegian team were particularly curious about the effects of female feeding during winter on calving and the first weeks of lactation during spring. To see if fatter females invested more in their calves, they allocated 60 female reindeer to two experimental groups: one group was fed an unlimited supply of commercial reindeer pellets in winter, while the other group was kept on reindeer's natural winter diet.
Surveying the reindeer's body weights in spring, the team observed that animals on unlimited pellets had increased their initial body weight by 12%,while females that had fed on natural pasture lost 6% of their body weight in the same time span. Surprisingly, despite the fact that the females with unlimited food during the winter had clearly fattened up and might be expected to invest more in reproduction, Fauchald and his colleagues did not find any effects of winter feeding on gestation, parturition date, calf body weight or survival of either females or calves. Even more impressively, they found that shortly after parturition there was no difference in body weight between the two groups, indicating that the previously fed animals lost their excess body fat in spring. Fauchald's team concludes that reindeer apparently had no use for excess body fat in spring and that body reserves accumulated during winter were not invested in calves later in the year.
Fauchald's study provides experimental evidence for adaptive body mass regulation in female reindeer. Reindeer body reserves are primarily used to overcome the risk of starvation in winter but not to fuel reproduction. The Norwegian team believe that reindeer mothers face a trade-off between building up reserves to avoid starvation in winter and vigilance to protect their calves in spring. Rather than accumulating new fat depots when new pasture is available in spring, female reindeer watch their weight, presumably to be able to protect their calves from predators. Perhaps the need to protect young from predation risk really is the key to staying slim and trim.
