SEASONAL WATER, ENERGY AND FOOD USE BY FREE-LIVING, ARID-HABITAT MAMMALS

In arid ecosystems, the dry season is probably an especially challengi ng time for animals to find adequate food and water resources. Both st ressful and benevolent times of the year can be identified by measurin g rates of water and energy use (with doubly labelled water), diet and behaviour throughout a year in free-ranging animals. Such field studi es have been completed on four species of arid-land mammals. Jackrabbi ts (Lepus californicus) are strict herbivores, and most Mojave Desert individuals have no access to drinking water, and depend on their food for their water. When vegetation dries up during the summer drought, most jackrabbits apparently die of dehydration and malnutrition, but p opulations are replenished by bountiful reproduction after winter rain s. However, springbok antelope (Antidorcas marsupialis), also strict h erbivores, can maintain water and energy balance throughout the year i n the Kalahari in southern Africa by drinking water if available, but, if not, they obtain enough water from dietary plants, probably by fee ding before dawn, when food items have taken up water from the humid a ir. Antelope ground squirrels (Ammospermophilus leucurus) are omnivoro us in the Mojave Desert, and are active diurnally. They have lower wat er and energy requirements than non-desert mammals of the same body ma ss, like many desert mammals, but their ability to find succulent plan t or animal foods in all seasons appears to be their primary survival tool. On the other hand, Merriam's kangaroo rats (Dipodomys merriami) are essentially nocturnal, and were thought to be strict granivores th at never drank. A recent study, done throughout a year in the Mojave D esert, confirms their lack of drinking, but reveals that they eat much green vegetation in late winter and spring, probably in connection wi th reproduction. They obtain additional water throughout the year by c aching dry seeds in humid burrows, where seeds take up water hygroscop ically before being eaten. Although springbok, antelope ground squirre ls and Merriam's kangaroo rats were in negative energy and water balan ce at times during the year, only jackrabbits were severely stressed b y the dry season. Each species has a unique suite of desert survival m echanisms. Future research should include carnivores as well as arid-h abitat marsupials.