Oecologia (Berlin) (1984) 61:420-425 Oecologia 9 Springer-Verlag 1984 Effects of body size, seed density, and soil characteristics on rates of seed harvest by heteromyid rodents Mary V. Price and Kevin M. Heinz Department of Biology, University of California, Riverside, CA 92521, USA Summary. Although it is well established that coexisting heteromyid rodent species forage in different microhabitats, we do not yet know the basis for divergent microhabitat choice. One possibility is that seed harvest rates differ among microhabitats, and each species forages where it can extract seeds most efficiently. Microhabitats vary in several factors that could affect heteromyid foraging efficiency, in- cluding seed density, soil organic content and particle size distribution. We have explored the effect of each of these variables on harvest rates of several species feeding from petri dishes containing known densities of millet seeds em- bedded in soil of known particle size and density. Results indicate that the number of seeds harvested per second in- creases uniformly with seed density and soil density and decreases with soil particle size. Body size affects these rela- tionships: larger animals have higher harvest rates for a given set of conditions and experience a greater relative change in harvest rate for a given change in conditions. This implies that heteromyids can be expected to exhibit species-specific microhabitat preferences while foraging in nature. Introduction Perhaps the most salient features of heteromyid-dominated North American desert rodent communities are the conspic- uous differences among coexisting species in morphology and foraging microhabitat (Rosenzweig et al. 1975; Price 1978; Price and Brown 1983 and references cited therein). These differences are apparently interrelated: the bipedal kangaroo rats (Dipodomys) and kangaroo mice (Microdipo- dops) forage primarily in open spaces, while the quadrupe- dal pocket mice (Perognathus), which have long vertebral columns, short hindfeet, and robust forelimbs relative to the bipedal forms, forage generally under or near the cano- pies of perennial plants. Similar observations have been made with many other animal communities: a comprehen- sive review of resource partitioning (Schoener 1974) indi- cated that habitat or microhabitat differences are the most conspicuous mode of ecological separation among similar coexisting species. Optimality models of foraging behavior (MacArthur and Pianka 1966; Pyke et al. 1977; Krebs 1978) suggest that such interspecific differences could come about if indi- Offprint requests to: M.V. Price viduals of each species preferentially use microhabitats in which they achieve highest fitness, and if the relative fitness value of a microhabitat varies among species. Although the exact ways in which microhabitats affect fitness are likely to change from system to system, they can all be subsumed under two basic factors - risk (of predation, dis- ease, starvation, etc.) and gain (of calories, nutrients, or other resources). Value is presumably some positive func- tion of net rate of gain, discounted by risk. Both gain and risk have indeed been shown to be capable of affecting patch or microhabitat use by foraging animals (Pyke et al. 1977; Krebs 1978; Werner and Hall 1979; Sih 1980; Mittel- bach 1981 ; Real 1981 ; Caraco 1983), but their relative im- portance in generating inter- or intraspecific differences in microhabitat affinity has, to our knowledge, been examined rarely (Pulliam and Mills 1977; Sih 1980; Real 1981; Werner and Mittelbach 1981 ; Stamps 1983). The microhabitats distinguished by heteromyid rodents differ in several characteristics that potentially could affect predation risk, energy loss, and rates with which limiting seed resources can be harvested. All of these factors have been proposed at one time or another to be instrumental in producing interspecific differences in microhabitat affini- ty, but there is as yet no strong, direct evidence that animals indeed experience microhabitat-specific risks or rates of gain, or that the relative value of a particular microhabitat is a function of morphology (Price and Brown 1983). As part of a larger research program exploring the basis for heteromyid microhabitat affinities, we have conducted a series of laboratory experiments to determine whether there is variation among rodents of different morphological type in the rate with which they harvest seeds of a single size and species, when seed spatial distribution and density and size of soil particles in which seeds are embedded are varied. Here we report results of these studies. Methods Heteromyids are remarkably uniform in their foraging ecol- ogy. All are nocturnal, living in burrows during the day and emerging at night to forage for the seeds that comprise most of their diet. Foraging consists of using olfaction to search for loose seeds on or just below the soil surface or, in certain seasons, for fruiting heads still attached to plants. Once located, seeds are extracted with the forepaws, are placed immediately into the external fur-lined cheek pouches that are characteristic of the family, and are trans-