Signal Synchrony and Alternation Among Neighbor Males in a Japanese Stream Breeding Treefrog, Buergeria japonica

The dual benefits of synchronized mating signals in a Japanese treefrog: attracting mates and manipulating predators

In dense mating aggregations, such as leks and choruses, acoustic signals produced by competing male conspecifics often overlap in time. When signals overlap at a fine temporal scale the ability of females to discriminate between individual signals is reduced. Yet, despite this cost, males of some species deliberately overlap their signals with those of conspecifics, synchronizing signal production in the chorus. Here, we investigate two hypotheses of synchronized mating signals in a Japanese treefrog (Buergeria japonica): (1) increased female attraction to the chorus (the beacon effect hypothesis) and (2) reduced attraction of eavesdropping predators (the eavesdropper avoidance hypothesis). Our results from playback experiments on female frogs and eavesdropping micropredators (midges and mosquitoes) support both hypotheses. Signal transmission and female phonotaxis experiments suggest that away from the chorus, synchronized calls are more attractive to females than unsynchronized calls. At the chorus, however, eavesdroppers are less attracted to calls that closely follow an initial call, while female attraction to individual signals is not affected. Therefore, synchronized signalling likely benefits male B. japonica by both increasing attraction of females to the chorus and reducing eavesdropper attacks. These findings highlight how multiple selective pressures likely promoted the evolution and maintenance of this behaviour. This article is part of the theme issue 'Synchrony and rhythm interaction: from the brain to behavioural ecology'.

Visual displays in a nocturnal rhacophorid frog (Buergeria japonica)

We report a visual display, leg-stretching, in the nocturnal Ryukyu Kajika frog (Buergeria japonica) on Iriomote Island in southern Okinawa, Japan. This visual display is produced by males before, during, or after a vocalization and involves the extension of one or both hindlimbs outwards at substrate level. Leg-stretches in this species are performed during male-male agonistic interactions in choruses, and almost exclusively in concert with aggressive vocalizations. These observations are one of the first reports of visual displays for the Rhacophoridae family and provide insights into why visual displays evolve in frogs, especially in nocturnal species. We discuss our observations in the context of underlying mechanisms and selective pressures driving the evolution of anuran visual displays.