The tu´ngara frog, Physalaemus pustulosus, is a small, common species of frog found throughout much of the lowland tropics of Central America and northern South America. Male advertisement calls are accompanied by conspicuous gular sac inflation. Tu´ngara frogs are nocturnal; males call from shallow, stagnant pools of water in groups of varying size. For example, mean group size was 27 (average nearest-neighbour distance 0.22 m) in 1978 in a 1!2m artificial pool on Barro Colorado Island, Panama. Females attend to interindividual variation in male call characteristics, preferring more complex and lower-frequency calls. On clear nights, the pulsing of a vocal sac is readily detectable to a human observer, suggesting that the vocal sac might serve to facilitate a difficult acoustic task, identifying and localizing a preferred male out of a complex acoustic background.