Age-related changes in somatic condition and reproduction in the Eurasian beaver: Resource history influences onset of reproductive senescence.

Ruairidh D Campbell,Frank Rosell,David W Macdonald,Chris Newman,Christoph Englert

doi:10.1371/journal.pone.0187484

Ruairidh D Campbell, Frank Rosell + Show 3 more

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https://doi.org/10.1371/journal.pone.0187484

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Abstract

Using 15 years of data from a stable population of wild Eurasian beavers (Castor fiber), we examine how annual and lifetime access to food resources affect individual age-related changes in reproduction and somatic condition. We found an age-related decline in annual maternal reproductive output, after a peak at age 5–6. Rainfall, an established negative proxy of annual resource availability for beavers, was consistently associated with lower reproductive output for females of all ages. In contrast, breeding territory quality, as a measure of local resource history over reproductive lifetimes, caused differences in individual patterns of reproductive senescence; animals from lower quality territories senesced when younger. Litter size was unrelated to maternal age, although adult body weight increased with age. In terms of resource effects, in poorer years but not in better years, older mothers produced larger offspring than did younger mothers, giving support to the constraint theory. Overall, our findings exemplify state-dependent life-history strategies, supporting an effect of resources on reproductive senescence, where cumulative differences in resource access, and not just reproductive strategy, mediate long-term reproductive trade-offs, consistent with the disposable soma and reproductive restraint theories. We propose that flexible life-history schedules could play a role in the dynamics of populations exhibiting reproductive skew, with earlier breeding opportunities leading to an earlier senescence schedule through resource dependent mechanisms.

Highlights

Senescent declines in reproductive success, somatic condition and cohort survival rate [1,2] influence the schedule of lifetime breeding success [3,4,5,6,7] in mammal species
We examine the effects of resource availability on senescence in reproductive and somatic traits in relation to four theories that have been proposed to explain the prevalence of senescence, or commonly observed changes in reproduction, somatic condition and survival with age: the disposable soma theory [11,12,13]; the reproductive restraint theory [13]; the constraints theory [15,16,17]; and the selection theory [16]
We found no evidence for somatic senescence in beavers; instead, once fully grown, body weight (BW, controlling for body length and season) increased continuously with age, the rate of increase abated (Table 2; Fig 1)

Summary

Introduction

Senescent declines in reproductive success, somatic condition and cohort survival rate [1,2] influence the schedule of lifetime breeding success [3,4,5,6,7] in mammal species. In wild-living populations, subject to natural selection, the annual and life-time availability of environmental resources can ‘stress’ organisms [18], either directly through actual insufficient availability, or indirectly through physiological constraints on an organism’s capacity to acquire them sufficiently [19,20,21]. This leads to trade-offs in energy allocation, especially when food supply is limited, with a division between reproduction and investment in somatic maintenance and repair, to ensure continued survival (or ‘longevity assurance mechanisms’, [22]). An influence of individual resource histories on senescence is predicted from both DS and RR theories

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