In the scientific book, sounds of frogs and toads is often said to be ‘croaking’. Whatever their musical qualities, vocalizations are conspicuous features of behaviour of most frogs and toads. Acoustic signalling has a great importance in the breeding ecology of most frogs. Acoustic data are used in taxonomic studies of frogs. Thus, researchers today are very interested in studying sounds produced by anurans to know about their behaviour, study their evolution, monitoring population, comparison of species and find ways for their conservation.
In anurans, generally males are involved in bioacoustic signalling. Only a very few female and at very few times are found producing signals. Anuran vocal repertoires can be into various calls namely advertisement calls( mate calls), courtship calls( responsive calls), aggressive calls, release calls, defensive calls. These are useful in male-male and male- female communications. These sounds are also used by them to protect them from predators too. They can estimate distance between opponent and get warned. Further, the produced sound defines the species, it’s size, it’s habitat , it’s dominance, etc in some way or other.
Advertisement calls are often referred to as mating calls and are principal signals given by males during breading season. Experiments have shown that females approach conspecific calls. Advertisement calls are often found to stimulate hormone production in females. It also expertises male’s position to other males.
The vocal behavior of male frogs often changes when females are nearby or are approaching. Courtship call are a sort of continuous trill( unlike two-note advertisement calls). These are simply increased call rates. Female courtship calls also exists. These are given in response to the calls of male, which tends to be at low intensity and therefore, are hard to hear. It enhances the ability of males and females to find each other.
Aggressive calls are produced as a sign of defence for their calling sites against conspecifics. These are often called territorial calls. Then there are release calls given when male anurans are clasped by other males. Further exists distress calls, which are generally loud screams to describe vocalisations given by frogs being attacked by predators. Alarm calls and defensive calls are to warn neighbours of danger or startle the predator, to allow victim to escape.
The pitch(determined by frequency) of sound is an indicator of anuran’s body size. The frogs with higher pitch are found to be small in size while the frogs having low-pitched calls are large and heavy. Call structure is an indicator of habitat of frogs( whether near waterfall or near sream or in forest or in open habitat). For ex: frogs living in dense forest are large in size and produce low pitched calls and vice versa. Thus, there also exists preference in calling site for different species. If male and female, both are at the same level, then high frequency components will suffer greater attenuation than low frequency components. But, if either caller or receiver is elevated, then differential loss of higher frequencies is reduced.
Comparison of frogs living in Amazonian forests with those in ﬂoating meadow habitats revealed that forest species have signiﬁcantly lower-pitched calls. The forest species generally are larger than those in open areas, and call frequency decreases with increasing body size. Calls of many species probably are not adapted for long range propagation, but for ease of localization at short distances. Phylogenetic differences probably are important as well. Forest frogs have very different call structure than open-habitat frogs. They tend to have more narrowly tuned calls with lower pulse repetition rates than open habitat frogs. Call intensity is related to factors other than habitat structure. For ex: frogs that call in open habitats tend to call in much denser choruses than those in forested habitats, and competition among males should favor the evolution of very loud calls.
Thus, understanding sounds of anurans is a very interesting part of study and researchers are working on it, striving to discover more, finding ways to conserve some endangered species( like N. sahyadrensis).