Mechanical constraint on size-assortative paring success in a temperate frog: an experimental approach

In anurans with axillary amplexus, males may be unable to handle females much different in body size from them due to physical limitation. Such mechanical constraint during the grasping processes is thought to be one of the proximate mechanisms leading to pairs to form size-assortively. Using a pairing experiment, the purpose of this study was to test this prediction for a temperate frog (Rana chensinensis) wherein some size-assortative matings occur in natural populations. We found a reduced probability of pairing success as the difference between sexes. When one female was much larger than one male that attempted to grasp her, she tended to dislodge aggressively him, suggesting a role of mechanical constraint in facilitating female choice against small-sized mates. By contrast, when the male was much larger than the female, he often failed to grasp her effectively or remain her in amplexus for longer, indicating the restriction of mechanical constraint to male pairing attempts and to female preference for large-sized mates.