Bio-acoustic investigations in the green frog, rana esculenta (L.)

Abstract

1. At a pond near Tübingen, West-Germany, the calling period of the green frog, Rana esculenta, lasts for a period ranging from the middle to the end of April, to the beginning to the end of July. The calling period is parted because of two changes in the behavior of calling and the changing number of the calling groups in: pre spawning period, spawning period and post spawning period. 2. Male frogs form calling groups. The males of each group cry in a rhythmic chorus. 3. The calling activity is influenced by 3 external factors, weather, water temperature and temperature of the air. The dependance of calling activity by these factors is not linear. During a period of good weather the calling activity increases daily. During a period of bad weather there is a daily decrease. The highest calling activity is on sunny days with a water temperature of between 17.5 and 22°C plus or minus 1 degree. From this level the activity falls with decreasing or increasing water temperature asymptotic to zero. The lower limit of calling activity ranges from 10 to 12°C, the upper one by 36.5 plus or minus 0.5°C. With the growing increase of air temperature the number of calls per time period falls asymptotic to zero. 4. The male green frog emits 6 calls; one mating call, two territorial calls, two release calls and one distress call. The calls consist of impulses, which, in the case of the mating call, are arranged to characteristic impulse-groups. The female green frog also emits calls, but no mating calls. 5. The parameters of mating and territorial calls are influenced by the water temperature. With increasing water temperature the duration of calls decreases linear, on the other hand the sequence of impulses and the repetition rate of impulse-groups show a positive linear regression. Also the size of the frog influences the calling parameters. With the increase of body length the duration of mating calls and territorial calls decreases and the sequence of impulses increases; and with mating calls also the repetition-rate of the impulse-groups decreases. 6. The larynx of the green frog consists of 3 elements of cartilage, only one of Cartilago crico-trachealis and the two Cartilagines arytaenoideae, and also 4 pairs of muscles. The muscles are innervated by two branches of the Nervus vagus, named Nervus laryngeus brevis and Nervus laryngeus longus. The larynx of male and female are built the same, but the male larynx is larger. The larynx of Rana ridibunda ridibunda Pallas is little different from that of the green frog. Also in the case of this species the male larynx is larger than the female larynx. 7. The differentiation of the larynx structure in the green frog ends after the completion of the metamorphose. The young frogs emit calls which sound similar to that of the territorial calls of adult frogs.