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The hypothesis that superfluous killing, partial consumption, and abandonment of prey is a consequence of adaptation to food-limited environments was tested in two feeding trials on a desert spider, Agelenopsis aperta. First, we made comparisons among populations inhabiting sites of high prey (HP) or low prey (LP) availability that differed in their degree of genetic isolation. Typically, A. aperta entirely consumed one or two of the prey items it captured in a feeding bout. Additional prey were partially consumed or abandoned without eating. Spiders from the genetically isolated HP population, however, captured fewer prey and showed a higher incidence of full feeding on prey than did individuals from the other populations. Only one spider from this Population captured a prey item that it failed to feed on, whereas spiders from LP populations failed to feed on high numbers of captured prey. The greatest variability in feeding behavior was exhibited in the HP population that experienced gene flow. The second test was based on the finding that aggressiveness is largely a sex-linked trait in A. aperta: the aggressiveness of the female parent only is inherited by male offspring, whereas both parents contribute to this trait in female offspring. All female F-1 hybrids between LP and HP parental types exhibited high levels of superfluous killing, as did male F-1 hybrids derived from LP females. F-1 hybrid males derived from HP females exhibited extremely low levels of superfluous killing. Superfluous killing thus has its basis in the genetic control of levels of aggression.