Lifetime reproductive success and longevity of workers in a social insect

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The evolution of social behaviour and the evolution of ageing both form major areas of study in evolutionary ecology. The evolution of social behaviour concerns how animal societies such as those of the eusocial insects (ants, bees, wasps and termites) arise and are maintained. The evolution of ageing concerns the factors that affect the longevity of organisms and the pattern of reproduction over their lifetimes. Recently, researchers have realised that these areas are connected, and that orthodox patterns of ageing can be heavily affected by sociality. A key example involves the relationship between lifetime reproductive success (the number of offspring an individual has over its lifetime) and longevity. The standard evolutionary theory of ageing predicts that sources of extrinsic mortality (factors leading to death in the external environment, such as predators, accidents, and so on) select for earlier reproduction, which in turn is paid for by earlier death. This predicts a negative relationship between lifetime reproductive success and longevity, i.e. that individuals producing more offspring have shorter lives. But, in eusocial societies, which contain both breeding and helping individuals, researchers have predicted that the breeders should exhibit a positive relationship between lifetime reproductive success and longevity, i.e. that individuals producing more offspring have longer lives. This is because breeders in eusocial societies are protected against extrinsic mortality by remaining in the nest. The principal investigator's research recently confirmed this prediction by demonstrating for the first time a positive relationship between lifetime reproductive success and longevity in queens of a eusocial insect, the bumble bee Bombus terrestris.In B. terrestris, as in many other eusocial insects, the workers, which are all daughters of the queen, can produce some offspring of their own. Because of the sex determination system in bees, workers' offspring are always males. Reproductive workers will, like queens, be protected from extrinsic mortality if they tend to remain in the nest, but not if they routinely leave the nest to forage. This allows the hypothesized effect of extrinsic mortality to be isolated, because it leads to the prediction that reproductive workers will exhibit a positive relationship between lifetime reproductive success and longevity if they tend to remain in the nest and a negative relationship if they tend to leave the nest to forage. The aim of this project is to test this hypothesis using B. terrestris as the study organism.The project is feasible because B. terrestris is an annual eusocial insect, i.e. one completing its colony cycle in a single season and hence one in which queens and workers live in the colony for at most a few months. B. terrestris colonies are also commercially available and are easy to keep and observe in the laboratory. The lifetime reproductive success and longevity of all mother queens and a sample of their reproductive worker daughters will be determined in a set of colonies whose workers have been individually marked. Birth and death dates, time spent in the nest and lifetime reproductive success will be measured across all individuals, with lifetime reproductive success being measured both as egg-production (measured from filming of nests) and from parentage analyses of males using genetic markers. This research is novel and fundamental because it will test new concepts at the interface of the important areas of social evolution and the evolution of ageing. For this reason, the work will substantially advance our basic understanding of both sociality and ageing in general.