Inbreeding, but not seed availability, affects dispersal and reproductive success in a seed-inhabiting social beetle

Abstract: Societies based on family groups have flexible social structures, where genetic relatedness and ecological or demographic factors exert pronounced influence on the social organization. Individual decisions that play an important role in determining the degree of cooperation and resulting social structure include whether to disperse from the natal patch, to breed within the group, or to help rear the young. We investigated individual decisions regarding dispersal and possible cooperation in offspring rearing using the date stone beetle, Coccotrypes dactyliperda (Scolytidae). In this species, cooperative breeding is suggested by the observation that genetically similar beetles share galleries in a single seed (stone) over several generations. We used beetles from two populations to create offspring of higher and lower relatedness, and provided them with high and low availability of date seeds as food and alternative breeding sites for dispersing individuals. We found that care of the eggs by adult females is obligate for offspring development. Seed availability did not affect individual behaviors, but dispersal of inbred beetles was lower compared to outbred individuals. Although the number of eggs did not differ between breeding treatments, more offspring were found in colonies of inbred females, indicating either a greater level of cooperation or reduced conflict in the inbred beetles. Taken together, these results emphasize the importance of relatedness among individuals in determining the social strategy and the potential reproductive output of a colony. Significance statement: The evolution of cooperative group living is often ascribed to two main factors: high relatedness, promoting indirect fitness benefits, and environmental constraints on dispersal. We investigated the influence of relatedness between individuals and food and breeding site availability in the habitat on individual decisions relating to dispersal and reproductive success in the date stone beetle, Coccotrypes dactyliperda. The beetles share galleries in the seed (stone), and several generations may occupy a single seed. We found more independent young and fewer dispersers in inbred colonies than in outbred colonies, but expected food availability (i.e., additional dates in the surroundings) had no influence on dispersal or reproductive success. We suggest that colony social organization in this species is strongly influenced by the level of relatedness among colony members.