FORAGING IN HETEROMYID RODENTS - THE ENERGY-COST OF SCRATCH-DIGGING

North American heteromyid rodents are alike in their primary seed harvesting behavior (scratch-digging), which is a potentially important component of their foraging energetics. The present study was designed to quantify the energy cost and efficiency ([Benefit - Cost]/Cost) of scratch-digging throughout the body size range represented among the Heteromyidae. The resting energy expenditure of bipeds was generally greater than that of quadrupeds on an equal body size basis, whereas their energy expenditure during scratch-digging (DMR) was equivalent. Seed concentration and substrate bulk-density did not markedly affect DMR, but seed concentration did alter the rate of seed harvest. Species differences in seed harvest rate were correlated with mode of locomotion, supporting the hypothesis that bipedality is correlated with specialization of the forelimbs for seed handling. In heteromyids, resting metabolic rate (RMR) was elevated by a factor of 1.5 over basal metabolic rate (BMR); DMR was elevated by a factor of 3.4 over BMR and by a factor of 2.3 over RMR. DMR, seed harvest rate, and the energy cost of scratch-digging on a per seed basis scaled with body mass to the 0.65, 0.31, and 0.33 powers, respectively. Although large bipedal heteromyids harvested seeds at more than twice the rates of small quadrupedal heteromyids, the unequal scaling of DMR and harvest rate makes them less efficient at scratch-digging. In comparison with other heteromyids, the kangaroo mouse (Microdipodops megacephalus) was exceptionally efficient at scratch-digging. Its higher efficiency may allow it to forage on sparsely distributed seeds in open areas where the energy return would be inadequate for larger bipeds. The pocket mouse (Chaetodipus baileyi) was exceptionally inefficient at scratch-digging. Its low efficiency in sand may be correlated with specialization for foraging in pebbly soils. For heteromyids of a given size, factors affecting pouching rate are most likely to determine foraging efficiency because DMR is essentially a fixed cost.