In front of a mirror: visual displays may not be aggressive signals in nocturnal tree frogs

Some evidence indicates that in anuran amphibians, visual signalling can be important during social interactions such as territorial disputes among males, especially in diurnal species. The correct identification of a signal is not a trivial matter. A visual signal provides a visual cue during a social interaction, and to be effective it must elicit an immediate response in the receiver. We tested the hypothesis that visual displays in an agonistic context constitute aggressive signals, in three nocturnal species of Hylidae. We predicted that the production of visual displays would increase in the presence of a conspecific intruder male. Males of Hypsiboas raniceps, Dendropsophus nanus and Lysapsus limellum were submitted to two treatments: (1) Self Image, a reflection in a mirror, simulating the presence of an intruder; and (2) Control, a black rectangle covering the mirror. We observed three visual displays: vocal-sac display (inflate the vocal sac and maintain it inflated for some time), limb lifting (rapid up-and-down movements of one or more limbs), and toe/finger trembling (rapid up-and-down movements of one or more toes and/or fingers). This last display was observed only in H. raniceps males. Contrary to our hypothesis, the emission rates of all visual displays of the focal animals did not differ between treatments; and the behavioural response did not differ among species. Therefore, we suggest that these behaviours could not be used directly for communication in agonistic contexts, and may represent displacement activities (involuntary responses). Alternatively, an aggressive bimodal stimulus may be necessary to trigger a behavioural response by using visual signals during territory defence in these three species.