Endocrine physiology of the division of labour in Pogonomyrmex californicus founding queens

Abstract

The proximate controls of a behaviour in extant species can inform us about the evolutionary route towards that behavioural phenotype. In social insects, different behavioural phenotypes often correlate with divergent hormone levels, and, in honeybees ( Apis mellifera ), this insight has lead to the hypothesis that behavioural biases, or division of labour, emerged via co-option of endocrine regulatory systems that paced behavioural change during the reproductive cycle of solitary ancestors. Founding queens of the California harvester ant Pogonomyrmex californicus show discrete behavioural changes during colony founding, with a dichotomy between nest-biased behaviour and field-biased behaviour. Additionally, a division of labour can develop if queens found nests together, with one queen being nest-biased and another being field-biased. To determine whether behavioural diphenism can be associated with reproductive endocrine regulators in an ant, we measured ecdysteroid and juvenile hormone (JH) content in (1) single-founding queens showing normal behavioural progression and (2) cofounding queens showing a division of labour. We found that ecdysteroid levels did not correlate with behaviour. JH titres, on the other hand, were elevated during the foraging life stage of single-founding queens as well as in the cofounding queens with a behavioural bias towards foraging. Our results suggest that JH affects the propensity for foraging task replication in P. californicus , and provide evidence for a common evolutionary route towards social behaviour in ants and bees.