“The evolution of reproductive phenology in broadcast spawners: frequency-dependent sexually antagonistic selection and the maintenance of polymorphism”

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Colin Olito, Dustin J. Marshall, and Tim Connallon

Sexual conflict in broadcast spawners: females want more kids, males want more kids than the other guys

Females want more offspring, while males just want more than the other guy: sexual conflict over the timing of reproduction

Galeolaria geminoa is a broadcast spawning marine tube worm common on the east coast of Australia, and a particularly tractable experimental species for studies of reproduction in broadcast spawners. This female has recently spawned in a laboratory setting – her eggs can be seen in background, and attached to her setae.
(Credit: Laura McLeod / Marshall lab)

Choosing the best time to reproduce is a major challenge faced by all organisms. For broadcast spawning species that release eggs and sperm into water, the consequences of getting reproductive timing wrong can be profound for both sexes. If females spawn when there are too few mates, there is not enough sperm to fertilize all their eggs. If females spawn when there are too many mates, egg mortality due to polyspermy (where eggs are fertilized by multiple sperm) can be severe. For males, as more individuals spawn at the same time, there are more eggs but there are also more competitors. On top of all of this, there are often better times to spawn than others because environmental conditions may sometimes be hostile to offspring. Broadcast spawners must therefore balance many conflicting factors when it comes to timing their reproduction, and it’s unclear what factors matter the most.

In their article appearing in The American Naturalist, the authors use mathematical models to show that males and females often prefer to spawn at different times, depending on how many individuals are participating in spawning events. Females favor spawning at the specific times that maximize egg fertilization and offspring survival. In contrast, males often favor spawning at multiple times as a means of competing more effectively against other males for fertilizations, even if this results in lower overall fertilization success of the population. These predictions help explain several well-documented, yet counterintuitive, patterns in aquatic species, including unexpectedly long spawning seasons, different spawning behaviors by males and females, and spawning during poor environmental conditions. The models also provide a fascinating example of how conflict between the sexes over a shared trait (reproductive timing) can emerge from fundamental processes of egg fertilization, the timing of reproduction, and the density of spawning individuals. Read the Article