A mechanism for resource allocation among sympatric heteromyid rodent species.

Laboratory feeding experiments were conducted with Dipodomys ordii and Perognathus flavus in an attempt to discover a mechanism which might result in seed size selection. There was no marked difference in the proportions of four seed types collected whether the rodents foraged in the presence or absence of one another. However, analysis of the variability in weight of each of the seed types collected by the two species showed that when alone, the larger kangaroo rat was less effective at harvesting all of a uniformly distributed mixture of seeds. When in the presence of one another both species could harvest enough of the mixed, uniformly distributed seed to coexist indefinitely, but when the food source was presented as four large clumps the kangaroo rat's foraging effectiveness increased tremendously so that the pocked mouse was almost entirely unable to harvest any seed. These data, in light of mobility differences between large and small heteromyids, suggest a mechanism whereby the larger, more mobile kangaroo rats forage for the most readily available (large or clumped) seeds over a relatively large area. The smaller pocket mice, by virtue of their relative efficiency in harvesting seeds can utilize the less detectable seeds which are energetically too demanding for the larger kangaroo rats to harvest. Behavioral dominance of the larger animals may help prevent the smaller from utilizing the most readily available seeds. The patterns of seed size and foraging site selection described in the literature may be easily accounted for by this difference in foraging strategy.