Predators induce conditions for size-dependent alternative reproductive tactics in a water strider male

Alternative reproductive tactics (ARTs) are commonly associated with differences in morphological, physiological and behavioural traits. The morphological differences can be associated with differences between ARTs in effectiveness of sexual display but the relationship has rarely been documented. We tested it using the Asian water strider Gerris gracilicornis (Heteroptera: Gerridae), in which males have two ARTs: a signalling courtship tactic and a nonsignalling tactic. Many G. gracilicornis males employ an intimidating signalling courtship tactic capitalizing on predators' behaviours and female responses to predators. The males produce courtship ripple signals on the water's surface by vibrating their middle legs after mounting the female. In this study we found that smaller males more often adopted a nonsignalling than a signalling courtship tactic. We hypothesized that smaller males with shorter middle legs and weaker muscles may not be able to produce ripples that are sufficiently strong to attract predators from a distance, and therefore sexual selection favours nonsignalling tactics in smaller but not larger males. We created a 2 × 2 experimental design to test the contribution of male body size to the intimidation effect under different levels of predation risk. We showed that only large males' ripple signals had an intimidation effect on females. We also found that females suffered a higher predation risk when large, but not small, males mounted them. The signal intensity of large males was also stronger than that of small males. Hence, we suggest that size-dependent effectiveness of the intimidation signalling maintains the size-dependent reproductive tactics in G. gracilicornis.