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.

In promiscuous species, fitness estimates obtained from genetic parentage may often reflect both pre- and post-copulatory components of sexual selection. Directly observing copulations can help isolate the role of pre-copulatory selection, but such behavioral data are difficult to obtain in the wild and may also overlook post-copulatory factors that alter the relationship between mating success and reproductive success. To overcome these limitations, we combined genetic parentage analysis with behavioral estimates of size-specific mating in a wild population of brown anole lizards (Anolis sagrei). Males of this species are twice as large as females and multiple mating among females is common, suggesting the scope for both pre- and post-copulatory processes to shape sexual selection on male body size. Our genetic estimates of reproductive success revealed strong positive directional selection for male size, which was also strongly associated with the number of mates inferred from parentage. In contrast, a male’s size was not associated with the fecundity of his mates or his competitive fertilization success. By simultaneously tracking copulations in the wild via the transfer of colored powder to females by males from different size quartiles, we independently confirmed that large males were more likely to mate than small males. We conclude that body size is primarily under pre-copulatory sexual selection in brown anoles, and that post-copulatory processes do not substantially alter the strength of this selection. Our study also illustrates the utility of combining both behavioral and genetic methods to estimate mating success to disentangle pre- and post-copulatory processes in promiscuous species. By combining behavioral and genetic measures of fitness in a wild lizard population, we show that sexual selection favors large body size because it enhances the mating success of males, not because larger males mate with more fecund females or have higher fertilization success. Although females frequently have offspring with multiple males, we find no evidence that post-mating competition alters the pattern of selection for larger body size that is driven by competition for mates.

Parasites interact with nearly all free‐living organisms and can impose substantial fitness costs by reducing host survival, mating success, and fecundity. Parasites may also indirectly affect host fitness by reducing growth and performance. However, experimentally characterizing these costs of parasitism is challenging in the wild because common antiparasite drug formulations require repeated dosing that is difficult to implement in free‐living populations, and because the extended‐release formulations that are commercially available for livestock and pets are not suitable for smaller animals. To address these challenges, we developed a method for the long‐term removal of nematode parasites from brown anole lizards (Anolis sagrei) using an extended‐release formulation of the antiparasite drug ivermectin. This treatment eliminated two common nematode parasites in captive adult males and dramatically reduced the prevalence and intensity of infection by these parasites in wild adult males and females. Experimental parasite removal significantly increased the sprint speed of captive adult males, the mating success of wild adult males, and the growth of wild juveniles of both sexes. Although parasite removal did not have any effect on survival in wild anoles, parasites may influence fitness directly through reduced mating success and indirectly through reduced growth and performance. Our method of long‐term parasite manipulation via an extended‐release formulation of ivermectin should be readily adaptable to many other small vertebrates, facilitating experimental tests of the extent to which parasites affect host phenotypes, fitness, and eco‐evolutionary dynamics in the wild.

Dispersal is a costly, though potentially rewarding, behavior with important fitness consequences for juveniles. When the costs or benefits of juvenile dispersal differ between sexes, sex-biased dispersal should be favored, though such sex-specific consequences are rarely measured for multiple components of lifetime fitness in the wild. Here, we use detailed mark–recapture data from 4 annual cohorts of juveniles with associated estimates of juvenile survival and adult reproductive success from genetic parentage to measure natural selection on 2 dispersal phenotypes (binary dispersal propensity and continuous dispersal distance) in an island population of lizards (Anolis sagrei). Juvenile dispersal was consistently male-biased, with males exhibiting a higher propensity to disperse and dispersing twice as far as females. Males that dispersed had higher survival to adulthood and total fitness than males that remained philopatric, whereas fitness components did not differ between dispersing and philopatric females. Although this result indicates that the fitness benefits of dispersal are sex-specific, we found no difference in fitness components between dispersing and philopatric males after accounting for body size, which was positively correlated with dispersal. Likewise, we did not consistently detect direct selection on dispersal distance in either sex when including body size in multivariate selection analyses, nor did we find consistent correlational selection on combinations of body size and dispersal distance in either sex. We conclude that selection on juvenile dispersal propensity is sex-specific, but likely indirect and mediated through its positive association with body size in males.

In promiscuous species, fitness estimates obtained from genetic parentage may often reflect both pre- and post-copulatory components of sexual selection. Directly observing copulations can help isolate the role of pre-copulatory selection, but such behavioral data are difficult to obtain in the wild and may also overlook post-copulatory factors that alter the relationship between mating success and reproductive success. To overcome these limitations, we combined genetic parentage analysis with behavioral estimates of size-specific mating in a wild population of brown anole lizards ( Anolis sagrei ). Males of this species are twice as large as females and multiple mating among females is common, suggesting the scope for both pre- and post-copulatory processes to shape sexual selection on male body size. Our genetic estimates of reproductive success revealed strong positive directional selection for male size, which was also strongly associated with the number of mates inferred from parentage. In contrast, a male’s size was not associated with the fecundity of his mates or his competitive fertilization success. By simultaneously tracking copulations in the wild via the transfer of colored powder to females by males from different size quartiles, we independently confirmed that large males were more likely than small males to mate. We conclude that body size is primarily under pre-copulatory sexual selection in brown anoles, and that post-copulatory processes do not substantially alter this pre-copulatory selection. Our study also illustrates the utility of combining both behavioral and genetic methods to estimate mating success to disentangle pre- and post-copulatory processes in promiscuous species.