Female concave-eared torrent frogs prefer smaller males

Mate choice is a natural selection mechanism whereby the traits of select members of a population are passed on to their offspring. We hypothesized that, for stream-dwelling anurans, females might prefer males with smaller body size and higher call frequency as their vocal calls are more conspicuous in the species' habitat featuring intense but predominantly low-frequency stream noise. To test this hypothesis, we first measured the body size of amplectant male concave-eared torrent frogs (Odorrana tormota) in the field, a species with demonstrated ability of ultrasonic communication. Our field observation showed that, on average, amplectant males had smaller body size compared to that of non-amplectant males (P < 0.05), an unusual finding among anurans. We next performed two-choice amplexus experiments by pairing a female with two males in an aquarium indoors. Results of laboratory experiments similarly revealed female's preference for males with smaller body sizes. Additionally, we recorded and analyzed the advertisement calls of 33 males and confirmed that their call frequency was inversely related to the body size as expected. Together, our studies showed that, for O. tormota, smaller males with higher call frequencies had higher mating success over bigger rivals, that is, females prefer smaller males.