Abstract
Recent examples of actuarial senescence in wild insect populations have challenged the long-held assumption that the brevity of wild insect life spans precludes senescence. We investigate age-related patterns in mating behaviour in adults of a short-lived damselfly, Coenagrion puella and the implications of this mating. Using capture histories for 1033 individuals over two field seasons, we conduct both pooled and stratified analyses of variations in breeding activity. Pooled analyses suggest that there is strong age-related variation in the probability of being present at the mating rendezvous. However, no age-related variation was observed in the probability of mating. Stratified approaches confirmed a general pattern of age-related declines in survival probability, but provided only equivocal evidence of an effect of age on transition between temporary breeding states. Mating males and females showed greater survival than non-mating individuals, possibly as a consequence of higher body condition. Older males that were not currently breeding were less likely to commence breeding on the next day, but showed no patterns in breeding cessation. Overall, transitions between both breeding states declined with age, suggesting that males that breed tend to continue breeding while those that do not breed continue to be unsuccessful. Female mating rates were consistently high across all ages with no age-related decline apparent. While previous research has demonstrated actuarial senescence in this population, as does this study, we find little evidence of either age-related declines in reproductive behaviour or breeding-related declines in survival, which might indicate functional senescence or costs of mating, respectively. Indeed, the greater survival in mating individuals of both sexes suggests that variations in individual quality may mediate both reproductive success and longevity. Contrary to recent studies, we found no compelling evidence for reproductive senescence or a cost of mating in an important and well-studied model odonate. The possible link between condition and ageing suggests that individual quality needs to be taken into account when studying senescence. We recommend the use of multistrata models for the future investigation of these phenomena.
Highlights
Senescence is considered to be a ubiquitous aspect of an organism’s life history, whether it manifests as an age-related increase in mortality ("actuarial senescence") or a decline in performance ("functional senescence")
While it has been proposed that senescence would be rare in wild populations due to high natural rates of mortality and most individuals die before they deteriorate in condition (Rose 1991; Kirkwood & Austad 2000), there is ample evidence of both actuarial and functional senescence in long-lived wild vertebrates, and several recent studies have demonstrated similar effects in short-lived invertebrates (Nussey et al 2013)
Older female cockroaches produce fewer and smaller clutches than younger cockroaches in the laboratory (Moore & Moore 2001) and older male antler flies mate at a lower rate than younger antler flies in the field
Summary
Senescence is considered to be a ubiquitous aspect of an organism’s life history, whether it manifests as an age-related increase in mortality ("actuarial senescence") or a decline in performance ("functional senescence"). 1991), the relationship between age and mortality rates varies markedly between species (Jones et al 2014), and there is evidence that certain clonal taxa may avoid senescence altogether While it has been proposed that senescence would be rare in wild populations due to high natural rates of mortality and most individuals die before they deteriorate in condition (Rose 1991; Kirkwood & Austad 2000), there is ample evidence of both actuarial and functional senescence in long-lived wild vertebrates, and several recent studies have demonstrated similar effects in short-lived invertebrates (Nussey et al 2013). Age-related declines in the functioning of the reproductive system (“reproductive senescence”) have been demonstrated in several taxa and in both sexes. A recent study of males of the damselfly Hetaerina americana compared “mature” and “young” males (defined loosely according to wing flexibility) and showed that males in the mature age class were more likely to copulate
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