“Sneaker males affect fighter male body size and sexual size dimorphism in salmon”

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Laura Weir, Holly Kindsvater, Kyle Young, and John Reynolds

Small ‘sneaky’ males reduce the advantage of large size for aggressive ‘fighter’ males during competition for mates

Sneaky sex drives size differences in salmon

Mating in Atlantic salmon (Salmo salar). The large fish in the foreground is a female in the process of releasing eggs. The colorful large fish with a hooked jaw in the background is a fighter male. A sneaker male can be seen near the back of the female just above a large rock, moving so fast that he appears as just a streak.
(Credit: Anders Lamberg, Scandinavian Nature Surveillance)

It’s a common head scratcher in evolutionary biology: why are males bigger than females in certain populations or species, while the opposite is true in others? Typically, this question has been addressed by determining which sex has a greater need to leverage its size when engaged in the mating game.

Large females often produce more offspring, while large males can be better fighters, but some species have small “sneaker” males who rely more on their stealth than their strength. “Sneakers” will wait until fighter males are about to mate, then dart past the fighter into position to release sperm and fertilize eggs. This creates a David-versus-Goliath scenario, where the large fighter Goliath cannot defeat a more sneakily skilled David by using his typically aggressive approach.

This scenario predicts that fighter size should be reduced in populations where a large number of sneakers exist, since a fighter’s size doesn’t help in a contest against a sneaker.

Researchers at Saint Mary’s University, Rutgers University, Simon Fraser University, and the University of Zurich have demonstrated that this is the case for salmon species. In populations where there are many sneaker males, fighter males are relatively small compared to other populations where sneaker males are uncommon. This result matches the expectation of a reduction in the largest fighters’ ability to win the mating game by relying solely on size and aggression.

This discovery offers new insight into the mechanisms that drive differences in female and male size among populations by introducing the influence of sneaker males into the mix. Read the Article