Abstract
The age trajectory of reproductive performance of many iteroparous species features an early - life increase in performance followed by a late - life senescent decline. The largest contribution of lifetime reproductive success is therefore gained at the age at which reproductive performance peaks. Using long term data on North American red squirrels we show that the environmental conditions individuals encountered could cause variation among individuals in the “height” and timing of this peak, contributing to life history variation and fitness in this population that experiences irregular resource pulses. As expected, high peak effort was positively associated with lifetime reproductive output up to a high level of annual effort. Furthermore, individuals that matched their peak reproductive effort to an anticipated resource pulse gained substantial fitness benefits through recruiting more offspring over their lifetime. Individual variation in peak reproductive effort thus has strong potential to shape life history evolution by facilitating adaptation to fluctuating environments.
Highlights
Reproductive performance typically changes non-linearly throughout life due to age-specific trade-offs between resource allocation into self-maintenance and reproduction[1, 2]
We examined the fitness consequences of variable maximum reproductive effort in an iteroparous species, the North American red squirrel (Tamiasciurus hudsonicus)
We show that a high maximum reproductive effort, when effort was timed to coincide with anticipated high fitness payoffs, generally predicts higher lifetime reproductive success up to a biologically feasible level of effort
Summary
Reproductive performance typically changes non-linearly throughout life due to age-specific trade-offs between resource allocation into self-maintenance and reproduction[1, 2]. The realized age trajectories of reproductive performance can, vary widely among individuals[15,16,17] as well as species[9] To maximize their lifetime reproductive success, a major fitness component, individuals should attempt to maximize their payoffs for reproductive effort by adjusting their resource allocation according to the potential fitness payoffs. The optimal age profile of reproductive effort may differ among individuals[11], but individual differences in the timing of maximum reproductive effort and investment in the peak phase, its fitness consequences, or the implications for life history evolution have not been previously studied When fitness is likely to be improved more by an increased effort under beneficial intrinsic or extrinsic conditions than by a constant effort among reproductive events, adjustable effort should be selected for. The actual expression of effort is likely constrained by individual quality or condition, and contrasting selective pressures that confer fitness benefits under suboptimal conditions may constrain the maximum possible effort
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