A state-based model of sperm allocation in a group-breeding salamander

Abstract

We developed a dynamic program of optimal sperm allocation for group-breeding species. Using the small-mouthed salamander, Ambystoma texanum, as a model organism, we considered how spermatophore deposition is affected by sperm reserves, male and female number in breeding aggregations, and time during the breeding season. Parameters for part of the model were based on field data of breeding-pond arrival times for both sexes and on laboratory spermatophore deposition data. Our model included simulations of three different seasonal patterns of female arrival rate: decreasing (as in A. texanum), increasing, and uniform. General predictions are (1) Increased male competitor numbers at breeding aggregations should cause a reduction in spermatophore allocation. (2) Increased female numbers at breeding aggregations should increase spermatophore allocation. (3) The effect of current sperm reserve levels on sperm allocation depends on the seasonal distribution of the mean number of females per male during the breeding season: (3a) If relative female availability decreases over time, males with low sperm reserves should limit allocation early in the season but should deposit maximal sperm loads late in the season; (3b) if female availability increases over time, males with low sperm loads should limit allocation throughout the entire breeding season; and (3c) if female availability is constant, sperm reserves are predicted to have little effect on spermatophore allocation tactics. We discuss model predictions in the context of current sperm allocation theory.