Behavioral estimates of mating success corroborate genetic evidence for pre-copulatory selection

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.

Methods
We studied an island population of brown anole lizards (Anolis sagrei) in the Guano Tolomato Matanzas Natural Estuarine Research Reserve in northern Florida (29°37′53′′ N, 81°12′ 46′′ W) using procedures approved by the University of Virginia Animal Care and Use Committee (protocol 3896). To assay the reproductive success of males, we sampled all adults and juveniles of the population at four different times during the breeding season (March, May, July, and October) in 2019. This population has been the focus of a long-term mark-recapture study since 2015, such that most adults in the 2019 breeding season were first captured, marked, and genotyped as juveniles in 2017 or 2018. We measured the snout-vent length (SVL, nearest 1 mm) and body mass (nearest 0.01g) of all individuals prior to releasing them at their exact site of capture the following day. We extracted DNA from adults captured in the population and used SNPPIT 2.0 (Anderson 2012) to assign genetic parentage. At two points in the middle of the breeding season (May and July), we also tracked copulations in the wild by dusting the venters of adult males with fluorescent powder as we released them and tracked the transfer to females to evaluate if and the extent to which body size mediates higher reproductive success through mating success.