Assured fitness returns in a social wasp with no worker caste

The theory of assured fitness returns proposes that individuals nesting in groups gain fitness benefits from effort expended in brood-rearing, even if they die before the young that they have raised reach independence. These benefits, however, require that surviving nest-mates take up the task of rearing these young. It has been suggested that assured fitness returns could have favoured group nesting even at the origin of sociality (that is, in species without a dedicated worker caste). We show that experimentally orphaned brood of the apoid wasp Microstigmus nigrophthalmus continue to be provisioned by surviving adults for at least two weeks after the orphaning. This was the case for brood of both sexes. There was no evidence that naturally orphaned offspring received less food than those that still had mothers in the nest. Assured fitness returns can therefore represent a real benefit to nesting in groups, even in species without a dedicated worker caste.

Communal nesting in prairie voles (Microtus ochrogaster): an evaluation of costs and benefits based on patterns of dispersal and settlement

We evaluated costs and benefits of group nesting in prairie voles (Microtus ochrogaster) by examining dispersal from communal nests. Nests at which dispersal occurred did not differ in size from those at which no dispersal was recorded, and most animals did not exhibit declines in body mass prior to dispersal. Of those animals which left communal groups that contained at least one potential mate, half left groups at which level of competition for mates was judged to be low as opposed to medium or high. Our data do not support competition for food or mates as important costs of group nesting. One probable cost to individuals living in family groups is lack of mating opportunity; about one-third of all dispersers were from groups composed solely of family members. Several of our findings suggest that energy conservation is not the primary reason for communal nesting in prairie voles. Reductions in winter group size typically did not precipitate dispersal, and most dispersers that settled into a nest in winter joined groups which were smaller than the groups they had left. Dispersal did not increase during snow cover. Benefits of group nesting in natural populations of prairie voles remain to be identified.