Social polyandry shapes sperm morphology

Sexual selection is a major driver of trait variation, and the intensity
of male competition for mating opportunities has been linked with sperm
size across diverse taxa. Mating competition among females may also shape
the evolution of sperm traits, but the interplay between female-female
competition and male-male competition on sperm morphology is not well
understood. We evaluated variation in sperm morphology in two species with
socially polyandrous mating systems, in which females compete to mate with
multiple males. Northern jacanas (Jacana spinosa) and wattled jacanas (J.
jacana) vary in their degree of polyandry and sexual dimorphism,
suggesting species differences in the intensity of sexual selection. We
compared mean and variance in sperm head, midpiece, and tail length
between species and breeding stages, because these measures have been
associated with the intensity of sperm competition. We found that the
species with greater polyandry, northern jacana, has sperm with longer
midpieces and tails, as well as marginally lower intra-ejaculate variation
in tail length. Intra-ejaculate variation was also significantly lower in
copulating males than in incubating males, suggesting flexibility in sperm
production as males cycle between breeding stages. Our results indicate
that stronger female-female competition for mating opportunities may also
shape more intense male-male competition by selecting for longer and less
variable sperm traits. These findings extend frameworks developed in
socially monogamous species to reveal that sperm competition may be an
important evolutionary force layered atop female-female competition for
mates.