How do Ganges and Indus river dolphins eat their food? New research on feeding strategies in South Asian river dolphins
The evolutionary history of South Asian river dolphins sets them apart not only from all other dolphins, but indeed from many other mammals. This includes the Indus and Ganges river dolphins of the genus Platanista that evolved almost 23-25 million years ago in the naturally sediment-rich waters of the Indus-Ganga-Brahmaputra basins. These dolphins are effectively blind, which is an evolutionary adaptation for life in extremely murky river waters. River dolphins use echolocation to ‘see’: they emit high-frequency click sounds that help them perceive their habitat and catch fish prey. Today these dolphins are highly endangered, not because of their naturally evolved blindness, but because of the intensive use and abuse of rivers in the Indian subcontinent by millions of people. Dams and barrages reducing river flows, accidental bycatch in fishing nets, hunting, pollution, and underwater noise due to vessel traffic are the main threats to our river dolphins. In such a situation it is rather remarkable that they still persist in these rivers. One important aspect of their lives, which remains poorly understood, is the role of their sensory perceptions in foraging and finding food.
A new multi-author study, published in the esteemed international journal Mammal Review, synthesises a thorough review of literature on Platanista dolphins with field-based ecological studies on behaviour, acoustics, and fish prey characteristics. The objective of the study was to identify what sensory abilities and environmental conditions might necessitate the use of different foraging strategies. In the 1970s, teams of researchers from Switzerland, USA, and Japan, had independently carried Indus and Ganges river dolphins to their aquariums for detailed anatomical and behavioural studies. The findings of these somewhat forgotten researches needed to be linked to questions on river dolphin feeding ecology, which the study attempted. The study also incorporated novel acoustic research to estimate the feeding depth of dolphins.
ATREE PhD candidates Nachiket Kelkar and Kadambari Deshpande are co-authors of this study. Nachiket has been studying river dolphins with Subhasis Dey, Sushant Dey, and Prof. Sunil Kumar Choudhary of the Vikramshila Biodiversity Research and Education Centre (VBREC), T.M. Bhagalpur University, (Bhagalpur, Bihar), and Dr. Tadamichi Morisaka, a dolphin behavioural ecologist from Mie University (Japan), who were also co-authors of the study. Kadambari, who studies bats, said that the initial idea for the paper emerged from discussions on functional similarities between dolphins and bat echolocation, which made all authors to look deeper and integrate knowledge across diverse fields in biology, including new work on acoustics and fish prey anatomy-morphology, which helped link stimuli with responses of feeding dolphins. Dr. Morisaka’s acoustics research led to another key contribution, which was the estimation of fish prey target detection distance for Ganges river dolphins.
An exciting outcome of the work, according to Nachiket, was the discovery that Ganges and Indus river dolphins might detect small-sized fish and shrimp prey by using electroreception at the river bottom. Electroreception is the ability to sense electric discharges emanating from fish and shrimps due to their movements. Dolphin calves might be more dependent on this than adults, the latter using mostly echolocation to locate and catch prey. In 2013, an experimental aquarium study identified that Guiana dolphins detected fish prey using electric perception from nerve fibres in the hair follicles on their snouts. Thus, electroreception was always a possibility in other dolphin species as well. The authors asserted that the research findings would be highly relevant for future studies on river dolphin ecology, behaviour, and assessments of their responses to human-induced threats.
Citation: Kelkar, N., Dey, S., Deshpande, K., Choudhary, S.K., Dey, S., & Morisaka, T. (2018) Foraging and feeding ecology of Platanista: an integrative review. Mammal Review, 48(3), 194-208.