Nature often rewards aggression in sexual selection, the process by which competition for mates drives reproductive success and trait evolution. Male deer clash antlers in fierce battles for mating dominance. Female praying mantises cannibalize weaker male mates after reproducing. Although these rigid sex competition dynamics have proven to be successful, for some species, a more flexible approach to mate selection could improve reproductive success. A new study suggests that in the pajama cardinalfish (Sphaeramia nematoptera), both males and females may compete for and choose mates, adapting their roles based on their environment to increase their rate of reproduction.
Pajama cardinalfish are known to be socially monogamous, in which a male and female will mate and raise offspring together for long periods of time. However, existing research suggests the species exhibits unique mating behaviors that defy traditional, socially monogamous characteristics. These fish remain in one region throughout their lives and are paternally mouthbrooding, meaning that males carry and protect developing eggs in their mouths. However, current research on sexual selection in pajama cardinalfish overlooks how mating dynamics are shaped by local, rather than population-wide, interactions. A team of scientists from the University of Newcastle, led by evolutionary scientist Theresa Rueger, investigated how patterns of sexual selection vary geographically and temporally. The researchers tracked eighteen groups of pajama cardinalfish in Papua New Guinea over a two-year period. To assess reproductive success, they implemented a program designed to trace parentage through the organisms’ genetic makeup, to determine both the number of mates and offspring produced within each group.
Their results supported the hypothesis that sexual selection in pajama cardinalfish is influenced by both group size and sex ratio, defined as the proportion of male to female fish. “We also show that socially monogamous species might not be so monogamous given the chance,” said Rueger. For each group of fish, the team computed the Bateman gradient, which is a statistical measure that relates reproductive success to the number of mating opportunities. In small to medium-sized groups where the number of males was equal to or greater than the females, females demonstrated a steeper Bateman gradient than males. This suggests that in male-dominated environments, females have more opportunities for additional matings with males other than their partner. In contrast, larger and female-dominated groups demonstrated similar Bateman gradients for both sexes, which the researchers believe is due to the increased mating opportunity for male cardinalfish. These findings highlight that, within smaller social groups, local dynamics can significantly shift sexual selection dynamics.
Building on these findings, the researchers intend to explore additional traits and mechanisms that may influence reproductive success in the pajama cardinalfish. They plan to explore how phenotypic plasticity—juveniles’ ability to alter physical traits in response to environmental changes—adapts in response to social context. Most critically, research is needed to connect these dynamics to accelerating climate change caused by human activity. “When habitats degrade, the social environment changes, which can quickly lead to declines in reproduction and thus threat to populations,” Rueger said. The ongoing effort to understand cardinalfish social and mating systems will illuminate how key species in coral reef ecosystems may fare amidst environmental change.