Abstract
In most species the reproductive system ages at the same rate as somatic tissue and individuals continue reproducing until death. However, females of three species – humans, killer whales and short-finned pilot whales – have been shown to display a markedly increased rate of reproductive senescence relative to somatic ageing. In these species, a significant proportion of females live beyond their reproductive lifespan: they have a post-reproductive lifespan. Research into this puzzling life-history strategy is hindered by the difficulties of quantifying the rate of reproductive senescence in wild populations. Here we present a method for measuring the relative rate of reproductive senescence in toothed whales using published physiological data. Of the sixteen species for which data are available (which does not include killer whales), we find that three have a significant post-reproductive lifespan: short-finned pilot whales, beluga whales and narwhals. Phylogenetic reconstruction suggests that female post-reproductive lifespans have evolved several times independently in toothed whales. Our study is the first evidence of a significant post-reproductive lifespan in beluga whales and narwhals which, when taken together with the evidence for post-reproductive lifespan in killer whales, doubles the number of non-human mammals known to exhibit post-reproductive lifespans in the wild.
Highlights
Why a female should cease reproducing before their expected end of life is a long-standing question in evolutionary biology[1,2,3]
In three species a linear relationship between corpora count and age suggests that there is no decrease in reproductive effort with age and no reproductive senescence (Table 1)
We found no correlation between the number of corpora and age, suggesting that ovarian corpora are not a good measure of reproductive senescence in the species
Summary
Why a female should cease reproducing before their expected end of life is a long-standing question in evolutionary biology[1,2,3]. Recent research has suggested that the demographic consequences of certain social structures are important in life-history evolution[8], and the evolution of post-reproductive lifespans in particular[4,9]. The diversity of social structures and reproductive strategies in the toothed whales makes them an important target group to understand the evolution of post-reproductive lifespans. We use these published physiological data to infer the rate and timing of reproductive senescence in female cetaceans. Using these data we analyse the rates of reproductive senescence as a population level trait and reveal the prevelance of post-reproductive life history in the toothed whales
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