A male blue-eyed yellow bush frog vocalising. Photo: Seshadri.K.S/Wikimedia Commons, CC BY-SA 3.0
Frogs use vocal acoustics to communicate a myriad of information. Males use advertisement calls to attract females. Through the acoustic energy of the sex trills, the female measures the physical strength of the male and his potential to sire offspring. These trills have several spatial and temporal properties – call length, note number, note rate, note duration, pulse number, pulse rate, pulse duration, and derivatives such as call series, note series, pulse group, and frequency. Each of these conveys a distinct message to the females.
Studies have shown that frogs do not simply hear the calls but also listen to the complex cues in an advertisement call. They respond to spectral degradation or reverberation, they can judge the quality of the call – especially how much it is degraded as a result of habitat structure. For example, a male calling in a forest and in an open agricultural land will sound different to females, both for the habitat structure and the sound reflectance property of different components in its environment.
There is no doubt that habitat structure plays the most important role in the communication of frogs. There is an underlying intricate process that makes communication effective. Every advertisement call is spaced and paced at a particular interval by either antiphony or modification of the call properties to avoid overlap, hence making the male audible to the targeted female. Call alternation can also be synchronised, leading to the formation of duets, trios, or other call groups within the chorus.
During fieldwork in the Western Ghats in July 2022, I observed a call that was responded to either after or simultaneously by another male’s advertising call. However, the introductory notes were modified. The call was that of a bright yellow frog of no more than 1 inch in size with a blue ring around its eyes – so it was aptly named the blue-eyed yellow bush frog. This frog is endemic to the Western Ghats. Curiously, we found the frogs only on trees that are endemic too. One tree hosted only one frog, indicating territoriality in the species.
In amphibians, some species exhibit territoriality where males guard a resource which can be either a female or a nest. In such species, territorial males engage in vocal challenges with a strange or familiar male of the same species. Such types of calls, emitted in response to another male’s advertisement calls, are called aggressive calls. The aggressive call is a “territorial call” that signals interspace between the males. Territorial calls are also emitted to claim a space in the reproductive chorus. The aggressive call can also be emitted in response to males invading one’s territory for resource competition. It can also signify acoustic space or “active space” such that the distance is enough for the signal amplitude to remain above the detection threshold of a potential female recipient. Further, aggressive calls can be “encounter calls” where the competitor male has already entered the resident male’s territory. Encounter calls are also synonymously referred to as “jumping calls“ emitted by resident males when jumping toward a male who has entered the territory and is calling in the resident’s territory.
How does this information help conservation?
Individual male calls can reveal information about fitness and individual identity. However, call types can inform us about habitat quality, which is important for conservation. Previous research shows that in a forest population, frog calls show less degradation than in open population ones. Calls of species that inhabit the forests are higher in dominant frequency (where most of the energy is invested), shorter in duration, and faster in pulse rate than those who live in open habitats, which are less acoustically cluttered.
Contrasting results are seen in cricket frogs, where the forest is a challenging habitat as calls degrade and attenuate faster. The primary decision a frog has to make while communicating with a female is the calling site, which has multiple advantages and determines reproductive success. The blue-eyed bush frog was always observed calling from a high perch on the tree. We do not yet know the significance of the calling site for this species. Therefore, habitat acoustics could be a selecting force for the variation of calls between habitats. Appropriate call sites can mask crucial information like the body size of a male frog. For a subordinate male, this means getting the attention of an interested female while his true individual fitness and status remain concealed. Thus the structure of the habitat also is crucial for uninterrupted communication in amphibians.
Globally forest degradation is a rising concern. In the tropics – Central and South America, sub-Saharan Africa, and South and Southeast Asia – there has been a significant loss of 7 million hectares of natural vegetation between 2000 and 2010.
According to global statistics, Asia has the highest proportion of area under agriculture (52%) and the lowest proportion of forest (19%). Four of the world’s biodiversity hotspots are located in India, of which the Western Ghats is a treasure house with a wide variety of habitats from wet evergreen forests, scrub forests, and savannahs to man-made habitats. It harbours several endemic flora and fauna and is listed as a UNESCO World Heritage Site hosting no less than 325 globally threatened (IUCN Red Data List) species (229 plants, 31 mammals, 15 birds, 43 amphibians, 5 reptiles, and 1 fish). Currently, only a third of the forest persists out of the approximately 180,000 square km area. Anthropogenic pressure is the major factor in this decline. The high diversity, the diverse landscapes within the hotspot, and the level of vulnerability make it a conservation priority especially due to the rate of disturbance and loss of habitat.
Amphibians are a fantastic group of animals that play crucial roles in indicating habitat loss. Habitat acoustics has been viewed as a selective force shaping frog communication. According to the Acoustic Adaptation Hypothesis, frogs respond to adjustments in habitats with phenotypic plasticity through behavioural regulation. An increase in territorial calls can be indicative of greater vicinity of males.
Monitoring frog acoustics over a time period can help conservationists understand the quality of the habitat. In a frog population, higher encounters of territorial or aggressive calls would mean greater invasion in a male’s territory – a clear message of lack of suitable habitat, which encourages proximity between the males. Long-term acoustic monitoring of a habitat can indicate the changes occurring over time due to anthropogenic interventions. Such methods are new and have the potential to cause advances in habitat management strategies.
Deyatima Ghosh, post-doctoral researcher, Nanjing Forestry University.