Coexistence between two psammophilous, nocturnal gerbil species in the Negev is based on environmental heterogeneity and a trade-off between the dominance of Gerbillus pyramidum and the foraging efficiency of Gerbillus allenbyi. We studied the effect of changing population density on individual foraging behavior to understand mechanisms underlying population and community patterns. We used thermal imaging cameras to record gerbil foraging behavior in artificial resource patches placed in two 1 ha plots enclosed with rodent-proof fences. Seed harvest rate by both species was lower at high population densities, but total foraging time was not affected by density. This decline in per capita harvest rate was positively correlated with the increasing number of aggressive interactions among gerbils at high densities. Body mass change during the experiments varied greatly among individuals and was correlated with the individual's ability to monopolize resource patches. In 70% of our observations, males dominated resource patches and gained in body mass, while females lost body mass. Within sexes, small individuals gained relatively more mass than did large ones. This observed variability in relative mass gain and resource monopolization provides a quantitative behavioral perspective to the previously described population and community patterns such as temporal and spatial segregation.