Many herbivores and omnivores adjust their food selection behavior to regulate the intake of multiple nutrients. Carnivores, however, are generally assumed to optimize the rate of prey capture rather than select prey according to nutrient composition. We showed experimentally that invertebrate predators can forage selectively for protein and lipids to redress specific nutritional imbalances. This selection can take place at different stages of prey handling: The predator may select among foods of different nutritional composition, eat more of a prey if it is rich in nutrients that the predator is deficient in, or extract specific nutrients from a single prey item.

Cooperative breeding societies are characterized by alloparental care and unequal distribution of reproduction (skewed reproduction). Competition for resources among group members may determine the reproductive outcome of each individual. In a spider colony, females feed together on prey and therefore may compete over the extraction of specific nutrients required for reproduction. Here we examined the occurrence of skewed reproduction in colonies of the social spider Stegodyphus dumicola (Eresidae) in nature and asked whether competition over specific limiting nutrients is a mechanism leading to unequal distribution of reproduction. We determined the nutritional composition of breeding and nonbreeding females in natural colonies. By supplementing field colonies with prey rich in either proteins or lipids, we examined the effect of these nutrients on female breeding potential. We then determined which nutrients were extracted from prey by breeding and nonbreeding females in a noncompetitive setting. About 40% of the females in natural colonies are potential breeders. Field colonies supplemented with lipid-rich prey had a higher percentage of breeding females than colonies supplemented with protein-rich prey. Furthermore, when feeding alone, nonbreeding females extracted more lipids relative to protein than breeding females. We suggest that lipids are critical for maturation and ovarian development and that competition for lipids determines the skewed reproduction observed in colonies of S. dumicola.