Sexual competition, coercive mating and mate assessment in the one-sided livebearer, Jenynsia multidentata: Are they predictive of sexual dimorphism?

We investigated the mechanisms of sexual selection operating on body size i n the one-sided livebearer (Jenynsia multidentata), a small fish characteri zed by male dwarfism. Mating in the one-sided livebearer is coercive: males approach females from behind and try to thrust their copulatory organ at t he female genital pore. Females counter males' mating attempts by either sw imming away or attacking them. We tested the hypothesis that the components of sexual selection favouring small size in males (sexual coercion) were m ore effective than those favouring a large size (male competition and mate choice). When alone, small males had a significantly higher success in thei r mating attempts than large males. The proportion of successful attempts w as also positively correlated with female size. When two males competed for the same female, the large male had a significant mating advantage over th e small one. With a 1 : 1 sex ratio, the large-male mating advantage vanish ed because each male tended to follow a different female. Large males, howe ver, preferentially defended large females, thus compelling small males to engage with smaller, less fecund females. Males did not discriminate betwee n gravid and non-gravid females, but preferred mating with larger females. This preference disappeared when males were much smaller than the female, p robably in relation to the risk for the male of being eaten or injured by t he female. In a choice chamber, male-deprived females that had their sperm storage depleted remained close to males and showed a preference for large individuals, a behaviour not observed in non-deprived females. Nonetheless, when placed with males in the same aquarium, all females showed avoidance and aggression. Struggling may represent a way by which the female assesses the skill and endurance of males.