Date of this Version
Bonduriansky & Brassil in NATURE (Nov. 28, 2002) 420.
Ageing (senescence) has never been demonstrated convincingly in any insect in the wild, where mean life-spans are probably much shorter than in the laboratory1, and most evidence for senescence in other wild animals (such as mammals) is limited to their reduced survival with age2. Here we show that ageing is detectable in wild populations of a very short-lived insect, the antler fly (Protopiophila litigata), and causes debilitating and costly effects that force a decline not only in survival probability, but also in the reproductive rate of males. Our findings argue against the possibility of a trade-off between fitness components, whereby survival may decline without senescence if investment in reproduction increases with age3, and indicate that ageing rates are subject to intense selection in the wild.
Although theory predicts the evolution of rapid senescence in organisms that experience high extrinsic (age-independent) mortality rates4, it has been suggested that very few individuals in these groups (such as insects or small mammals) survive long enough in the wild to exhibit detectable5,6 senescence.
We tested for senescence in a wild population of the antler fly, a small dipteran that breeds exclusively on discarded antlers of moose and deer. The tendency of adult flies to spend their lives on a single antler, as well as the long duration of their mating (2.3 h; reference 7), facilitate the acquisition of field data on mating success and survival. We surveyed mating aggregations on nine moose antlers every 2 h over 72 days, and recorded the presence and mating status (single or coupled) of each of 609 individually marked males8.