Sex-biased parental care and sexual size dimorphism in a provisioning arthropod

The diverse selection pressures driving the evolution of sexual size dimorphism (SSD) have long been debated. While the balance between fecundity selection and sexual selection has received much attention, explanations based on sex-specific ecology have proven harder to test. In ectotherms, females are typically larger than males, and this is frequently thought to be because size constrains female fecundity more than it constrains male mating success. However, SSD could additionally reflect maternal care strategies. Under this hypothesis, females are relatively larger where reproduction requires greater maximum maternal effort – for example where mothers transport heavy provisions to nests. To test this hypothesis we focussed on digger wasps (Hymenoptera: Ammophilini), a relatively homogeneous group in which only females provision offspring. In some species, a single large prey item, up to 10 times the mother’s weight, must be carried to each burrow on foot; other species provide many small prey, each flown individually to the nest. We found more pronounced female-biased SSD in species where females carry single, heavy prey. More generally, SSD was negatively correlated with numbers of prey provided per offspring. Females provisioning multiple small items had longer wings and thoraxes, probably because smaller prey are carried in flight. Despite much theorising, few empirical studies have tested how sex-biased parental care can affect SSD. Our study reveals that such costs can be associated with the evolution of dimorphism, and this should be investigated in other clades where parental care costs differ between sexes and species

Gregarious nesting of a digger wasp as a “selfish herd” response to a parasitic fly (Hymenoptera: Sphecidae; Diptera: Sacrophagidae)

Females of a digger wasp Crabro cribrellifer nested in a 234 nest aggregation covering 2x65 m in area. Variability in 3 edaphic factors (soil temperature, hardness, and per cent moisture) had no significant influence on the non-random spatial distribution of nests within the aggregation. A significant positive correlation between wasp nest density and the mean number of parasitic flies, Metopia campestris , present was shown (Fig. 1). Yet, based on 20 excavated nests, the probability of cell parasitism decreased as nest density increased (Fig. 2). This may give females nesting within a dense aggregation a selective advantage, and supports the “selfish herd” model developed by Hamilton (1971).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46872/1/265_2004_Article_BF00299384.pd

Pre-social benefits of extended parental care

The evolution of helping, in which some individuals forfeit their own reproduction and help others to reproduce, is a central problem in evolutionary biology. Recently proposed insurance-based mechanisms rely on a pre-existing life history with a long period of offspring dependency relative to the short life expectancies of adult carers1, 2, 3, 4: a lone mother's offspring are doomed if she dies young, whereas after a helper dies, other group members can finish rearing the offspring5, 6. A critical question, however, is how this life history could evolve in ancestral non-social populations, as offspring survival would then depend on a single, short-lived carer. Here, we resolve this paradox by focusing on the extended parental care inherent in prolonged dependency. We show experimentally that in non-social wasps, extended care can significantly reduce the impact of interspecific parasites. Under extended care, offspring are less vulnerable by the time they are exposed to parasites, and costs of parasitism are reduced because mothers have the option to terminate investment in failing offspring. By experimentally simulating aspects of extended care in a species where it is lacking, we demonstrate that neither benefit requires specialized behaviour. Such benefits could therefore offset the disadvantage of prolonged dependency in non-social species, thereby facilitating the evolution of helping