Phenotypic correlates of mating success in the sand wasp Bembecinus quinquespinosus (Hymenoptera: Sphecidae)

Females of the sand wasp Bembecinus quinquespinosus nest in dense aggregations, the offspring emerging in great numbers the following year. Males display alternative mating tactics. Large males search and dig for females about to emerge from the ground. Mating success here is correlated with body size, primarily because larger males are better able to carry the female in flight away from the emergence area and avoid further harassment. The largest males, representing about one-quarter of the population, obtained over 90% of the matings initiated in the emergence area, whereas males below average in size were never successful there. Because some females leave the area without mating, a pool of virgins is available to small males, who patrol just outside the emergence area and intercept passing females. Males below average in size (and all females) are black with pale white bands on the abdomen. In males above mean size, the proportion of yellow pigmentation on the dorsal and lateral surfaces increases with body size. Males undertaking alternative tactics experience differing thermal environments. Yellow colouration was correlated with higher cuticular reflectance. Comparative evidence and differences in the activity patterns of males of different colour suggest that yellow pigmentation acts as a thermoregulatory mechanism allowing larger males to increase their tolerance of the high temperatures and solar radiation loads in the emergence area. We hypothesize that the mating tactics represent condition-dependent behavioural tactics, with flexibility maintained by sexual selection, and that the colour patterns represent condition-dependent morphological tactics, with the developmental response favoured by selection for thermoregulatory ability.