Effects of Food Preference on Scatter-Hoarding by Kangaroo Rats (Dipodomys Merriami)

Two laboratory studies were conducted to determine whether Merriam's kangaroo rats invest greater effort in the caching of a more preferred food. As predicted, more of the preferred food was cached and yet the individual caches were smaller. The second experiment showed wider dispersion of the preferred food, and these caches were placed further away from the source. These findings imply that investment in protecting food from pilferage is adjusted in relation to the animal's evaluation of that food.

Seasonal Water, Energy and Food Use by Free-Living, Arid-Habitat Mammals

In arid ecosystems, the dry season is probably an especially challenging time for animals to find adequate food and water resources. Both stressful and benevolent times of the year can be identified by measuring rates of water and energy use (with doubly labelled water), diet and behaviour throughout a year in free-ranging animals. Such field studies have been completed on four species of arid-land mammals. Jackrabbits (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 populations are replenished by bountiful reproduction after winter rains. However, springbok antelope (Antidorcas marsupialis), also strict herbivores, can maintain water and energy balance throughout the year in the Kalahari in southern Africa by drinking water if available, but, if not, they obtain enough water from dietary plants, probably by feeding before dawn, when food items have taken up water from the humid air. Antelope ground squirrels (Ammospermophilus leucurus) are omnivorous in the Mojave Desert, and are active diurnally. They have lower water and energy requirements than non-desert mammals of the same body mass, like many desert mammals, but their ability to find succulent plant 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 that never drank. A recent study, done throughout a year in the Mojave Desert, confirms their lack of drinking, but reveals that they eat much green vegetation in late winter and spring, probably in connection with reproduction. They obtain additional water throughout the year by caching dry seeds in humid burrows, where seeds take up water hygroscopically before being eaten. Although springbok, antelope ground squirrels and Merriam's kangaroo rats were in negative energy and water balance at times during the year, only jackrabbits were severely stressed by the dry season. Each species has a unique suite of desert survival mechanisms. Future research should include carnivores as well as arid-habitat marsupials.

Sexually differentiated effects of radio transmitters on predation risk and behaviour in kangaroo rats Dipodomys merriami

In a 12-year study involving 191 radio-tracked Merriam's kangaroo rats and 337 subcutaneous radio implantations, females were killed by predators at a rate of 0.0054 per radio-bearing night and males at a rate of 0.0116. Both the mortality rate and the sex difference therein declined over the course of several nights after radio implantation. Females reduced their excursions from the day burrow for the first few nights after radio implantation, whereas males exhibited little if any such inhibition of movement. This sexually differentiated behavioural response to the transmitters is a likely source of the sexually differentiated mortality patterns.