Estimating encounter rates as the first step of sexual selection in the lizard Anolis sagrei

Abstract

How individuals move through their environment dictates which other individuals they encounter, determining their social and reproductive interactions and the extent to which they experience sexual selection. Specifically, females rarely have the option of mating with all males in a population-they can only choose among the males they encounter. Further, quantifying phenotypic differences between the males that females encounter and those that sire females' offspring lends insight into how social and reproductive interactions shape male phenotypes. We used an explicitly spatio-temporal Markov chain model to estimate the number of potential mates of Anolis sagrei lizards from their movement behaviour, and used genetic paternity assignments to quantify sexual selection on males. Females frequently encountered and mated with multiple males, offering ample opportunity for female mate choice. Sexual selection favoured males that were bigger and moved over larger areas, though the effect of body size cannot be disentangled from last-male precedence. Our approach corroborates some patterns of sexual selection previously hypothesized in anoles based on describing them as territorial, whereas other results, including female multiple mating itself, are at odds with territorial polygyny, offering insight into discrepancies in other taxa between behavioural and genetic descriptions of mating systems.

Keywords: anole; encounter rates; mating system; polygyny; territoriality; territory.

Figure 1.

Examples of the locations through time for two individuals, U12 and U26 (a), and the probability of their co-occurrence as estimated by the Markov chain model (b). Lighter colours indicate observations made earlier in the breeding season.

Figure 2.

Estimated SVL differences between male pairs estimated to co-occur (coloured bars), compared with pairwise SVL differences between randomly chosen pairs of males, with random males' sizes estimated on the same days as the observed co-occurrences (white bars). (Online version in colour.)

Figure 3.

Relationship between the number of females encountered by males and the males' spatial extent (a; measured as the mean distance to the centroid) and males' mean estimated SVL across encounters (b).

Figure 4.

Maximum male snout–vent length (SVL) and the hour of last encounter for male–female pairs, coloured by whether or not the male sired any of the female's offspring. (Online version in colour.)