Sperm morphology and count vary with fine-scale changes in local density in a wild lizard population

Matthew C Kustra,Aaron M Reedy,Daniel A Warner,Robert M Cox,Ariel F Kahrl

doi:10.1007/s00442-019-04511-z

Matthew C Kustra, Aaron M Reedy + Show 3 more

Open Access

https://doi.org/10.1007/s00442-019-04511-z

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Abstract

Given that sperm production can be costly, theory predicts that males should optimally adjust the quantity and/or quality of their sperm in response to their social environment to maximize their paternity success. Although experiments demonstrate that males can alter their ejaculates in response to manipulations of the social environment and studies show that ejaculate traits covary with social environment across populations, it is unknown whether individual variation in sperm traits corresponds to natural variation found within wild populations. Using an island population of brown anole lizards (Anolis sagrei), we tested the prediction that sperm traits (sperm count, sperm morphology, sperm velocity) respond to natural variation in the risk of sperm competition, as inferred from the local density and operational sex ratio (OSR) of conspecifics. We found that males living in high-density areas of the island produced relatively larger sperm midpieces, smaller sperm heads, and lower sperm counts. Sperm traits were unrelated to OSR after accounting for the covariance between OSR and density. Our findings broaden the implications of sperm competition theory to intrapopulation social environment variation by showing that sperm count and sperm morphology vary with fine-scale differences in density within a single wild population.

Highlights

Population density and sex ratio affect the probability of encountering both competitors and mates (McLain 1992; Kokko and Rankin 2006; McCullough et al 2018)
From our univariate least-squares regressions of each sperm trait on operational sex ratio (OSR), we found that sperm count and head length increased as zone OSR became more male biased (Table 1; Fig. 2e, f)
Sperm velocity was not correlated with any sperm trait (Table 3). We found that both sperm morphology and sperm count varied with local density in a wild population of Anolis lizards, suggesting that males can respond to fine-scale variation in the abundance of potential mates and/or competitors by altering the phenotypes of their ejaculates

Summary

Introduction

Population density and sex ratio affect the probability of encountering both competitors and mates (McLain 1992; Kokko and Rankin 2006; McCullough et al 2018). The frequency of these encounters can influence the intensity of male–male combat, the likelihood of mate acquisition, and Communicated by Jean-François Le Galliard. Many studies have focused on the effects of density and sex ratio on precopulatory sexual selection (i.e., selection arising from variance in mating success), these aspects of the social environment can influence postcopulatory sexual selection. Recent evidence suggests that the relative importance of postcopulatory processes may increase with population density (McCullough et al 2018)

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