Listening to frogs can offer clues to their decline [Commentary]

Amphibians such as frogs are excellent bioindicators of their environment, as they react to changes in land, water and air.
Each species of frog has a unique song that can stand out and be identified.
The duration and intensity of frog vocalisations is on a declining trend, indicating the decline of amphibians.
The views in the commentary are that of the author.

Amphibians are vertebrate animals that need both terrestrial and aquatic habitats for their survival, reproduction and metamorphosis. Traditionally, amphibians have been broadly categorised as frogs, toads, salamanders and newts. Frogs, toads and all amphibians whose adults have four legs, lack a tail and can stay out of water, are called anurans. But there are also legless amphibians called caecilians.

However, the binary division of anurans as frogs and toads alone, has led to a lot of confusion as there are many toad-like frogs and frog-like toads and a few that may not qualify as either. The simplistic delineation of anurans as frogs or toads is based on the skin and development of the hind legs in adults. Generally, frogs have smooth skins that are moist and slimy and have well-built hindlegs that enable them to leap high and far. Toads, on the other hand, have dry and coarse skin and weak hindlegs that make them rather slow-moving anurans incapable of long and high leaps.

Strictly speaking, while toads belong to just one family of anurans, Bufonidae, all other anurans are frogs.

Amphibians are diverse

There are close to 9,000 species of amphibians known in the world today. Around 300 are salamanders and newts (tailed amphibians), and 180 are caecilians – legless and burrowing, wormlike amphibians. The rest are anurans. Salamanders and newts are the only amphibians that have diversified in the temperate regions. Most species of anurans and all caecilians are tropical.

Four hundred species of amphibians are known in India, which is rather low considering that it is a mega-diverse country. However, what is interesting is that around 260 species (65%) are found in the Western Ghats making the southwest Indian ecoregion an ‘amphibian hotspot’ with remarkable levels of endemism. Although there are no salamanders or newts in the Western Ghats, it is home to at least 24 species of caecilians. Almost all of the caecilians found in the Western Ghats are endemic. As with the rest of the world, anurans are the key to amphibian diversity here also. Amphibian enthusiasts feel there are many more species of anurans waiting to be discovered in this globally recognised biodiversity hotspot.

An endemic bi-coloured frog photographed in the Western Ghats. It possesses weak hindlegs, much like toads. India is home to 400 species of amphibians, with 65% of these found in the Western Ghats, and many of these are endemic. Image by R.J. Ranjit Daniels.

Frogs are vocal

Frogs (hereafter used as a surrogate for anurans) can vocalise and their vocalisation is only surpassed by birds, among vertebrates. Although frog vocalisation is mainly a masculine trait, there are species in which females also sing. Males use songs to advertise, defend their territories and attract females. Females of some species reciprocate, albeit with a less complex song. Reciprocal singing by females was first reported in treefrogs by an amphibian researcher Debjani Roy in northeast India. Also, both male and female frogs produce loud squeals when in distress.

The magnitude of the difference in the size of adult frogs is rather remarkable. While the smallest can comfortably sit on the thumbnail of an adult human, the largest such as the goliath frog of Africa can weigh more than a kilogram. What is interesting however is that even the smallest frog can sing loudly. This is enabled by an apparatus called vocal sac. Vocal sacs are located in the throat and in many species, they can be inflated to look like fully blown balloons that amplify the sound.

As with birds, each species of frog has a unique song that can stand out and be identified, although not always by the human ear, by its mate or rival. Even when hundreds of frogs sing in a chorus (more appropriately, a cacophony) from a small breeding pool and in total darkness, each individual is successful in its business. It is this character of frogs that has attracted the attention of many researchers interested in animal acoustics and communication and also people who are interested in nature’s soundscapes.

A Nilgiris cricket frog, a typical frog with well-built hindlegs. Frogs serve as bioindicators that react to even small environmental changes, and are songsters, with their vocalisation being surpassed only by birds. Image by E. Kunhikrishnan.

Frogs as bioindicators

When Rachel Carson published her landmark book Silent Spring in the 1960s, she for the first time drew attention to nature’s soundscapes and how unmindful human actions can silence the songsters. The culprit then was the unregulated use of the highly persistent pesticide DDT in the United States of America. Today there are many more of such ‘silencers’ doing their rounds throughout the biosphere.

Plants and animals that react to even small changes in the environment are called ‘bioindicators’. The concept of bioindicators is attributed to David Hawksworth an English mycologist. According to him, a bioindicator should be a common organism, easily identifiable and must show visible changes in its form or behaviour whenever there is the slightest change in the environment. Many organisms including frogs qualify as bioindicators by this simple definition.

Amphibians are the only terrestrial vertebrates that can absorb moisture and exchange gases through their skin. Biologists such as William E. Duellman have highlighted the sensitivity of amphibian skins. They have also illustrated how amphibians that are known for their bimodal life histories, can be sensitive to the slightest changes in both water and land. Thus, according to many biologists, amphibians are by far the best bioindicators among vertebrates quickly reacting to changes in land, water and air.

Dying songs signal a decline in frogs

In the 1990s alarm bells were first rung about the decline in amphibians throughout the world. The IUCN set up the Declining Amphibian Population Task Force (DAPTF) to both study and monitor the decline. Various causal agents including habitat loss, UV-B radiation, fungal and viral infections, pesticides, introduced species and over-harvest were identified as the reasons behind the unprecedented declines. Worldwide assessments of amphibian species and their populations thereafter have suggested that around 40% of the species may be faced with the threat of extinction. Does this indicate a larger global ecological crisis?

The eggs of a Bombay night frog (Nyctibatrachus humayuni). Amphibian eggs lack shells, making them vulnerable to pesticides and other pollutants in water. Image by Nikhil More via Wikimedia Commons (CC BY-SA 4.0). — The eggs of a Bombay night frog (*Nyctibatrachus humayuni*). Amphibian eggs lack shells, making them vulnerable to pesticides and other pollutants in water. Image by Nikhil More via Wikimedia Commons (CC BY-SA 4.0).

The first indication that amphibians are declining is the overall reduction in the duration and intensity of frog songs. Urban areas, as that in parts of Chennai, where hundreds of frogs sang in unison after a spell of heavy rain, have fallen silent during recent times. The number of species that made calls has also come down. In Velachery (south Chennai) for instance, not long ago, rains ushered in a loud chorus of at least eight species of frogs including the common toad, common treefrog, paddy-field frog, Jerdon’s bullfrog, variegated ramanella, ornate microhyla, painted frog and Roland’s burrowing frog. Today, only three can be heard – variegated ramanella, painted frog and paddy-field frog. Occasionally, the common toad is also heard.

Even in agricultural landscapes, declining trends in the frog soundscapes is becoming evident. Rampant use of inorganic pesticides may be the primary reason. Industrial pollution can also be detrimental. Unlike in the Western Ghats where many species of frogs, breed above water in bushes and trees, all species of frogs that are found in cultivation and urban areas fully depend on water to breed; the common treefrog being the only exception. Even in the treefrog, that lays eggs inside a foam-nest placed well above the surface of water, the tadpoles that emerge drop into water to complete their metamorphosis.

Amphibian eggs lack shells. They are therefore not only vulnerable to desiccation, but also to pesticides and other inorganic pollutants in water. Direct contact with pesticides and pollutants destroys the eggs. In the rare event of the eggs surviving pollution, tadpoles and adults live with abnormalities such as deformed appendages, blindness, etc. Eggs are also sensitive to changes in the ambient temperature. It is for this reason that amphibians are considered excellent bioindicators. It’s therefore important to pause and hear the frogs; it could be a clarion call.

The author is an ecologist with the Care Earth Trust, a Chennai-based biodiversity and conservation organisation.

Citation:

Duellman, W E and Trueb, L (1986) Biology of Amphibians. Mac Graw Hill Book Company, San Francisco.
Daniels, R J R (2005) Amphibians of Peninsular India. Universities Press, Hyderabad.
Daniels, R J R (2003) Impact of tea cultivation on anurans in the Western Ghats. Current Science 85(10): 1415-1422.

Banner image: A Jerdon’s bullfrog (Hoplobatrachus crassus) spotted in Coochbehar, West Bengal. When it rains, this species is one of many that sings. Image by Kingshuk Mondal via Wikimedia Commons (CC BY-SA 4.0).