Dietary shifts in vertebrates: factors affecting seed selection in desert rodents

Abstract

This study uses foraging theory to predict that small vertebrates forage to maximize their net rate of energy gain or minimize risk of starvation. It focuses on an Australian desert rodent, the sandy inland mouse (Pseudomys hermannsburgensis), due its abundance and tractability for hypothesis testing, and because it provides an opportunity to compare its foraging with other desert rodents. My experiments on foraging and seed selection focused on diet shifts under the risk of predation, diet choice in relation to seed characteristics and the background seed base, and the foraging characteristics of animals when seed access was manipulated by burying seeds at different depths and with added moisture.

Pseudomys hermannsburgensis is omnivorous, but seeds dominate the diet as shown by observations of foraging animals and analysis of stomach contents. Giving-up density and cafeteria trials with different seeds showed animals to be risk-averse and less active in open compared with sheltered areas. Under low predation risk animals selected preferred seeds and under high risk they were less choosy and quickly took any seeds they encountered. Mice used olfaction to detect seeds buried at depths up to 5 cm, but preferred seeds on the soil surface; adding moisture did not affect seed detection or consumption. Familiar seeds were preferred compared with rare seeds, although other factors such as seed quality are also important. Experimental exclusion of rodents from plots showed that seed predation reduces seed species diversity and abundance, but these effects became clear only after a major rainfall event that triggered an eruption in rodent numbers and predation impact.

My research is the first detailed study of the diet and foraging behaviour of an Australian desert rodent. It uncovers a range of novel strategies that allow P. hermannsburgensis to persist in its variable desert environment, and contrasts these strategies with those used by rodents in other world deserts.