Abstract
Increased individual resources (condition) can be correlated with either increased or decreased longevity. While variation in resource acquisition and allocation can account for some of this variation, the general conditions that select for either pattern remain unclear. Previous models suggest that nonlinearity of payoffs from investment in reproduction (e.g., male secondary sexual traits) can select for high‐condition individuals that sacrifice longevity to increase reproductive opportunity. However, it remains unclear what mating systems or patterns of sexual competition might select for such life‐history strategies. We used a model of condition‐dependent investment to explore how expected payoffs from increased expression of secondary sexual traits affect optimal investment in lifespan. We find that nonlinearity of these payoffs results in a negative relationship between condition and lifespan under two general conditions: first, when there are accelerating marginal benefits from increasing investment; second, when individuals that invest minimally in secondary sexual trait expression can still achieve matings. In the second scenario, the negative relationship occurs due to selection on low‐condition individuals to extend lifespan at the cost of secondary sexual trait expression. Our findings clarify the potential role of sexual selection in shaping patterns of condition‐dependent ageing, and highlight the importance of considering the strategies of both low‐ and high‐condition individuals when investigating patterns of condition‐dependent ageing.
Highlights
In some species, when access to resources is increased, for example through higher quality diet, expected lifespan increases
While Kokko showed that increasing nonlinearity of payoffs from investment in secondary sexual traits can select for high-condition individuals to sacrifice investment in longevity, we ask whether this finding is generalizable to different function shapes representing other patterns of sexual competition, such as mating systems where lowcondition males engage in alternative reproductive tactics that do not require secondary sexual trait expression
THE MODEL As we are primarily interested in how secondary sexual trait expression affects mating success and fitness, our model focuses on investment decisions by males
Summary
In some species, when access to resources is increased, for example through higher quality diet, expected lifespan increases. The specific mating system parameters and patterns of sexual competition that select for different patterns of condition-dependent ageing are still unclear We extend these models to explore how variation in the payoff function for investment in secondary sexual traits can affect patterns of optimal investment in lifespan (via resource allocation to somatic maintenance) in individuals of varying condition. While Kokko showed that increasing nonlinearity of payoffs from investment in secondary sexual traits can select for high-condition individuals to sacrifice investment in longevity, we ask whether this finding is generalizable to different function shapes representing other patterns of sexual competition, such as mating systems where lowcondition males engage in alternative reproductive tactics that do not require secondary sexual trait expression. Low-condition males of some species (e.g., some horned beetles: Emlen 1994) can suppress secondary sexual trait expression almost completely; by contrast, in other species (such as some cervids: Suttie and Hamilton 1983), even males in low condition still express the secondary sexual trait to some
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