Abstract
The genus Gambusia represents approximately 45 species of polyandrous livebearing fishes with reversed sexual size dimorphism (i.e. males smaller than females) and with copulation predominantly via male coercion. Male body size has been suggested as an important sexually selected trait, but despite abundant research, evidence for sexual selection on male body size in this genus is mixed. Studies have found that large males have an advantage in both male-male competition and female choice, but that small males perform sneaky copulations better and at higher frequency and thus may sire more offspring in this coercive mating system. Here, we synthesized this inconsistent body of evidence using pre-registered methods and hypotheses. We performed a systematic review and meta-analysis of summary and primary (raw) data combining both published (n=19 studies, k=106 effect sizes) and unpublished effect sizes (n=17, k=242) to test whether there is overall selection on male body size across studies in Gambusia. We also tested several specific hypotheses to understand the sources of heterogeneity across effects. Meta-analysis revealed an overall positive correlation between male size and reproductive performance (r=0.23, 95% confidence interval: 0.10-0.35, n=36, k=348, 4,514 males, three Gambusia species). Despite high heterogeneity, the large-male advantage appeared robust across all measures studied (i.e. female choice, mating success, paternity, sperm quantity and quality), and was considerably larger for female choice (r=0.43, 95% confidence interval: 0.28-0.59, n=14, k=43). Meta-regressions found several important factors explaining heterogeneity across effects, including type of sperm characteristic, male-to-female ratio, female reproductive status and environmental conditions. We found evidence of publication bias; however, its influence on our estimates was attenuated by including a substantial amount of unpublished effects, highlighting the importance of open primary data for more accurate meta-analytic estimates. In addition to positive selection on male size, our study suggests that we need to rethink the role and form of sexual selection in Gambusia and, more broadly, to consider the ecological factors that affect reproductive behaviour in livebearing fishes.
Highlights
Body size is one of the most important traits affecting the fitness of organisms (Roff, 2002)
We predict a positive correlation when (a) females can choose between males without physical interaction; (b) experimental density is low, allowing large males to physically dominate small males; (c) habitat complexity is high, allowing females to avoid or reduce sexual harassment, and to be preferentially choosy and (d) sex ratio is male-biased, which leads to increased male–male competition
We found that male size and reproductive performance are positively correlated across studies of Gambusia
Summary
Body size is one of the most important traits affecting the fitness of organisms (Roff, 2002). Male-biased ratios have been suggested both to benefit large males (Bisazza & Marin, 1995) and to play no clear role in the relationship between male body size and reproductive success in eastern mosquitofish (Head et al, 2017). We predict a positive correlation when (a) females can choose between males without physical interaction (e.g. in dichotomous female mate choice test); (b) experimental density is low, allowing large males to physically dominate small males; (c) habitat complexity is high, allowing females to avoid or reduce sexual harassment, and to be preferentially choosy and (d) sex ratio is male-biased, which leads to increased male–male competition. Since the mating system is similar across Gambusia species, (c) we do not predict large differences among species
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