Selection on Sperm Count, but Not on Sperm Morphology or Velocity, in a Wild Population of Anolis Lizards

Abstract

Sperm competition is a widespread phenomenon that shapes male reproductive success. Ejaculates present many potential targets for postcopulatory selection (e.g., sperm morphology, count, and velocity), which are often highly correlated and potentially subject to complex multivariate selection. Although multivariate selection on ejaculate traits has been observed in laboratory experiments, it is unclear whether selection is similarly complex in wild populations, where individuals mate frequently over longer periods of time. We measured univariate and multivariate selection on sperm morphology, sperm count, and sperm velocity in a wild population of brown anole lizards (Anolis sagrei). We conducted a mark-recapture study with genetic parentage assignment to estimate individual reproductive success. We found significant negative directional selection and negative quadratic selection on sperm count, but we did not detect directional or quadratic selection on any other sperm traits, nor did we detect correlational selection on any trait combinations. Our results may reflect pressure on males to produce many small ejaculates and mate frequently over a six-month reproductive season. This study is the first to measure multivariate selection on sperm traits in a wild population and provides an interesting contrast to experimental studies of external fertilizers, which have found complex multivariate selection on sperm phenotypes.

Sperm Numbers as a Paternity Guard in a Wild Bird

“…Specifically, when the probability of double mating by females is high (risk model), males should invest in large numbers of sperm per ejaculate to gain a numerical advantage in sperm competition, whereas sperm numbers per ejaculate are predicted to decrease when females mate with more than two males (intensity model) [ 98 , 105 , 106 ]. At the same time, when male mating rate is high, large numbers of sperm allow a male to produce a greater number of smaller ejaculates to engage in frequent copulations and avoid sperm depletion [ 104 , 107 ]. Available evidence suggests that fairy-wrens face a high risk of sperm competition and likely allocate large numbers of sperm to a few ejaculates.…”

Selection on sperm size in response to promiscuity and variation in female sperm storage organs

“…Empirical data thus provides a minimum estimate of number of individual partners (Cramer et al ., 2020a). For many species, an average number of copulation partners less than 5 appears realistic (Gage, 1994; Brommer et al ., 2007, 2010; Simmons et al ., 2007; Simmons & Beveridge, 2010; Turnell & Shaw, 2015; Cramer et al ., 2020a; Kahrl et al ., 2021a), though in eusocial insects the average can be over 50 (Tarpy et al ., 2004). The values we chose to investigate were also informed by the expectation that selection strength should asymptote with > about 10 copulation partners (Gomulkiewicz, 1991; Muniz & Machado, 2018).…”

“…However, to our knowledge, no theoretical work on mate choice models the conditions most relevant for sperm-female interactions. Specifically, most mate choice models assume that females copulate with a single male in the population, while empirical data show that females often copulate with multiple males, who then share paternity of their offspring (e.g., Gage, 1994; Simmons et al ., 2007; Simmons & Beveridge, 2010; Turnell & Shaw, 2015; Brouwer & Griffith, 2019; Kahrl et al ., 2021a). In addition, we assume that females copulate with fewer males than they assess during mate choice, implying that females sample the sperm of relatively few males.…”

Selection on sperm size in response to promiscuity and variation in female sperm storage organs

“…Empirical data thus provides a minimum estimate of number of individual partners (Cramer, Kaiser, et al, 2020). For many species, an average number of copulation partners less than 5 appears realistic (Brommer et al, 2007(Brommer et al, , 2010Cramer, Kaiser, et al, 2020;Gage, 1994;Kahrl, Kustra, et al, 2021;Simmons & Beveridge, 2010;Turnell & Shaw, 2015).…”