Social context and courtship flexibility in male sailfin mollies, Poecilia latipinna (Pisces: Poecillidae)

Abstract

Male sailfin mollies exhibit patterns of sexual behaviour directly related to body size: large males exhibit higher rates of courtship displays and lower rates of forced insemination attempts. Male sailfin mollies from three size classes were examined with a repeated-measures design to investigate the extent to which an individual will alter the rates at which different sexual behaviours are expressed in response to short-term changes in male size distributions in small groups. Small and large males did not respond to changes in male size distributions, but intermediate-sized males did. When alone intermediate-sized males exhibited rates of behaviour that were intermediate between rates exhibited by small and large males. When other males were present, regardless of their body sizes, the same intermediate-sized males shifted their behaviour patterns toward those of small males. When intermediate-sized males were placed with males of different body sizes, they exhibited behaviour patterns that were indistinguishable from those of small males. Significant differences among individuals were exhibited within all three size classes, but intermediate-sized males exhibited the highest level of differences among individuals. These results suggest that behavioural variation in natural populations will be more discontinuous in character than has been supposed previously, with variation resembling more a set of alternative tactics conditional on size than a gradation of variable behaviours correlated with size.