Intrinsic worker mortality depends on behavioral caste and the queens’ presence in a social insect

Abstract

According to the classic life history theory, selection for longevity depends on age-dependant extrinsic mortality and fecundity. In social insects, the common life history trade-off between fecundity and longevity appears to be reversed, as the most fecund individual, the queen, often exceeds workers in lifespan several fold. But does fecundity directly affect intrinsic mortality also in social insect workers? And what is the effect of task on worker mortality? Here, we studied how social environment and behavioral caste affect intrinsic mortality of ant workers. We compared worker survival between queenless and queenright Temnothorax longispinosus nests and demonstrate that workers survive longer under the queens' absence. Temnothorax ant workers fight over reproduction when the queen is absent and dominant workers lay eggs. Worker fertility might therefore increase lifespan, possibly due to a positive physiological link between fecundity and longevity, or better care for fertile workers. In social insects, division of labor among workers is age-dependant with young workers caring for the brood and old ones going out to forage. We therefore expected nurses to survive longer than foragers, which is what we found. Surprisingly, inactive inside workers showed a lower survival than nurses but comparable to that of foragers. The reduced longevity of inactive workers could be due to them being older than the nurses, or due to a positive effect of activity on lifespan. Overall, our study points to behavioral caste-dependent intrinsic mortality rates and a positive association between fertility and longevity not only in queens but also in ant workers.

Gene expression is more strongly associated with behavioural specialization than with age or fertility in ant workers

“…Brood carers that labour inside the safe nest are selected to upregulate repair processes (Schmid‐Hempel & Schmid‐Hempel, ), whereas this selection diminishes in foragers exposed to high extrinsic mortality (Amdam & Page, ; Ricklefs, ). In line with this, a previous study on T. longispinosus workers revealed a lower intrinsic mortality of young brood carers compared to old foragers (Kohlmeier et al, ).…”

Gene expression is stronger associated with behaviour than with age and fertility in ant workers

“…Old workers in contrast overexpress viral transcripts, indicative of a higher viral load in aged workers, which was previously documented in honey bees, in which a sugar-rich diet further increased the viral load of workers [105]. High viral load might contribute to the increased intrinsic mortality of old compared to young workers as well as foragers compared to brood carers [77] Interestingly, we did not detect any typical aging pathways or genes, such as ROS pathways.…”

“…Six brood carers (observed tending the brood) and six foragers (found outside the nest) were then individually labelled with metal wire and their investment into either brood caring and foraging was observed and recorded ten times a day for three days. In Temnothorax ants, a single observation is sufficient to classify workers into brood carers and foragers [77] (Kohlmeier et al subm.). Behavioural observations revealed a clear age-polyethism with young workers focussing on brood care and older workers caring for the adult nestmates and taking over foraging (for details on the methods and results of the behavioural observations see Kohlmeier et al subm.).…”

Abdominal microbial communities in ants depend on colony membership rather than caste and are linked to colony productivity

“…We assumed that individuals close to the brood were brood carers and those outside the nest were foragers. Assignment to worker caste based on a single observation is highly reliable in Temnothorax ants, including our focal species T. nylanderi (Beros, Enders, Menzel, & Foitzik, ; Kohlmeier, Feldmeyer, & Foitzik, ; Kohlmeier et al, ). The sampled workers were stored in clean 1.7 ml Eppendorf tubes at −20°C until DNA extraction.…”