Lifetime reproductive success in a swarming midge: trade-offs and stabilizing selection for male body size

Body size in male animals has profound effects on fitness; large males are commonly favored by sexual selection. However, if body size is a stable trait, then advantages of large size must be balanced by disadvantages. In males of the midge Chironomus plumosus we identified the trade-offs that result in stabilizing selection on body size. The smallest individuals have the greatest mating advantage in swarms, perhaps due to their greater agility in capturing females. This measure of mating success in the individual's own swarm was corrected to a population-level measure to take account of the positive correlation between body size and swarm size, the relationship between swarm size and mating success, and the distribution of swarm sizes on an evening. The small male advantage is maintained in the population-level measure. In contrast, large males have greater fecundity and, when unfed, live longer and can remain longer in the swarm each evening (due to their greater stamina). Consequently, lifetime reproductive success is greatest for males of intermediate size, and the most successful body-size class dosely matches the most frequent class in the wild. This represents, to our knowledge, the first evidence of stabilizing selection on body size in males of an insect species, as measured by trade-offs in lifetime reproductive success.