Disentangling the causes of temporal variation in the opportunity for sexual selection

Abstract

In principle, temporal fluctuations in the potential for sexual selection can be estimated as changes in intrasexual variance in reproductive success (i.e. the opportunity for selection). However, we know little about how opportunity measures vary over time, and the extent to which such dynamics are affected by stochasticity. We use published mating data from multiple species to investigate temporal variation in the opportunity for sexual selection. First, we show that the opportunity for precopulatory sexual selection typically declines over successive days in both sexes and shorter sampling periods lead to substantial overestimates. Second, by utilising randomised null models, we also find that these dynamics are largely explained by an accumulation of random matings, but that intrasexual competition may slow temporal declines. Third, using data from a red junglefowl (Gallus gallus) population, we show that declines in precopulatory measures over a breeding period were mirrored by declines in the opportunity for both postcopulatory and total sexual selection. Collectively, we show that variance-based metrics of selection change rapidly, are highly sensitive to sampling durations, and likely lead to substantial misinterpretation if used as indicators of sexual selection. However, simulations can begin to disentangle stochastic variation from biological mechanisms.

Fig. 1. Short-term temporal dynamics in the opportunity for precopulatory sexual selection across species.

Mean (95% c.i) values for the opportunity for precopulatory sexual selection (I_M) across vertebrate and invertebrates species over time (days) during a breeding period. Females (♀) and males (♂) are represented on the left and right panels, respectively. Species from top to bottom represent: a, b Hawaiian swordtail crickets (Laupala cerasina) n groups = 1. c, d Strawberry poison-dart frogs (Dendrobates pumilio) n groups = 1; e, f howler monkeys (Alouatta caraya) n groups = 6. g, h Jackdaws (Corvus monedula) n groups = 1. i, j Red junglefowl^,,, (Gallus gallus) n groups = 20. k, l Squirrel monkeys (Saimiri oerstedi) n groups = 1. m, n Water striders (Aquarius remegis) n groups = 40. Green points show I_M values calculated cumulatively and represent the maximum potential strength of precopulatory sexual selection on a given day taking into account patterns of mating over all preceding time units, and purple points show instantaneous values where opportunity for precopulatory sexual selection is assessed independently for each time unit (day). Bars represent the 95% confidence intervals (95% c.i) for studies with replicate groups. The lower range of c.i were capped at 0 to exclude negative values. Green shaded areas represent expectations for cumulative measures of I_M based on 10,000 random mating simulations (i.e. the 95% range of simulated estimates). Observed cumulative values with c.i. lying within shaded areas were considered not to differ from null expectations. Source data are provided as a Source Data file.

Fig. 2. Short-term temporal dynamics in multiple opportunity for selection episodes in a polygynandrous bird.

Patterns of opportunity estimates across different components of reproductive success in male and female red junglefowl (Gallus gallus) n groups = 20. Points represent observed mean opportunity estimates, bars represent 95% confidence intervals (95% c.i.) and point colour reflects the sampling method (green: cumulative; purple: daily). Green shaded areas represent expectations for cumulative measures of opportunity for selection based on 10,000 random mating simulations (i.e. the 95% range of simulated estimates). Observed cumulative values with c.i. lying within shaded areas were considered not to differ from null expectations. Females (♀): a Standardised variance in reproductive success (opportunity for total selection, I_T). Males (♂): b Opportunity for total selection (I_T), c Standardised variance in partner’s fecundity (opportunity for sexual selection on partner fecundity, I_N), d Standardised variance in paternity share (opportunity for postcopulatory sexual selection, I_P). Source data are provided as a Source Data file.