Calling for company
COME May and it is din-creating time for frogs. The month heralds the breeding season which lasts till August. At times, noise levels call be so high that it makes one suspect if the frogs are taking part in a competition to test their ability to croak loud and long. It is indeed a contest of sorts because the males try to lure the female by emitting these loud mating calls (Current Science, Vol 69, No 3).
Tile sounds produced by the males of it certain antiran species is peculiar to them as each of these groups have a chavacteristictfieciucm-y band. Females can also identify the ti pical calls of their IYOUld-be's in the acoustical bedlam. There are many records of whit the males communicate to the females, but few attempts have been made to decipher fiernale responses to male calls.
DeInara Roy and her students of the institute of Self Organising Systems and Biophysics at the North Eastern Hill University in Shillong, Nfeghalava, have studied anmin acoustic commuunication to investigate the same. Their study has for the fit st time demonstrated the role of the female ieciprocal call in the reproduction biology of amphabians. Roy and her team have shown that the female's jesponses serve is a catalyst to enhance reproductive activity in the bleeding colony. The calls were recorded during the active breeding period. While only behavioural observations were made hiring the first year, in the next year, both behaviour and acoustic sItudies were conducted. The litter involves recording the calls, analysing them arid then digitising arid storing them. This requires the preparation of oscillograms, sonograms and mcan spectra which are printed after they are passed through bandpass filters.
Of all the arnman species found in the north cast, females of Rana erythtaca, Rana linniocharis and Rana cyanopliflyctis are known' to emit reciprocal calls. Inspite of being feeble and ofshort duration, they could still be heard over the babel because of tile rapid succession with which they were released, The sound was similar to the chirping of birds.
The arboreal nature of Rana ery thraea makes its Study easy in cornpari son to Rana lillmocharis and Rana cyanophlyctis, which reside in swamps and jump into the watea oil the slightest distur bance. It wa@ noticed that the females of Rana crythraca earterge and approach the calling males after the latter have 0 advertised then presence foi 30 minutes. The incessant male call stops all of a sudden, for a short time, when the female is around 10 cm away. Fhe female then takes over and emits feeble calls after short intervals, listing two-five minutes during the next hour. On hearing the female's response, the rnale - which had lost its voice at her d[)peqraiice - resumes croaking with a much higher intensity. Simultaneously, all neighbouring males join in unison arid croak with even higher intensin,. I hey also become more active and stait jumping ir0Lmd from one place to mother. J-hese )LIMPS are confined to the area around arid across the female which had responded to the mating call.
Females of the other two species responded to male calls while submerged in water and this made it difficult for the scientists to collect sufficient data on the reciprocal calls and behaviour of the animals. All that they report is that the male calls were louder and activity increased considerably, once the female frog responded and approached the beckoning males.
What eludes the researchers is why the female reciprocal calls are so feeble. Possible explanations may be found in the bigger size of the females, the absence of vocal sacs among them, the difference in the structural morphology of the vocal cords and larynx, and the tension in their trunk muscles. Roy concludes by indicating that a detailed study of the morphology and kinematics of the vocal cords of the males and females - of the same species - is necessary to clear these doubts.
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