Do river dolphins hunt prey using many methods?

The Gangetic river dolphin Platanista gangetica in the Ganga river in Bihar.Kadamabari Deshpande  

Asia's river dolphins may be nearly blind, but they compensate by using many methods to catch prey. Though chiefly known as echolocators, researchers now suggest that these dolphins could also be listening for surface-swimming fish and sensing electrical signals emitted by bottom-dwelling prey on the river-bed.

Eyes are of no use for river dolphins (genus Platanista ) in the naturally murky waters of the Indus and Ganga. Instead, they have evolved to use echolocation to navigate and catch fish: the small clicks they produce underwater echo back at them, helping them identify a prey or obstacle on their path. Despite increasing pressures on their habitat – from ships’ underwater noise (which could affect dolphin echolocation) to dams that alter the river’s flow – dolphins still survive in many heavily human-used river stretches.

Wondering if these aquatic mammals have other methods to catch prey and thereby survive, a team of researchers from institutes including Bengaluru’s Asoka Trust for Ecology and Evolution (ATREE) first dug through 105 studies and historical references for information about dolphin anatomy and physiology as well as that of their prey (shrimp and fish). They also complemented this by studying the prey they found in dead dolphins’ stomachs and acoustic information on the echolocation clicks Gangetic river dolphins use in varying depths of a 100-km stretch of the Ganga in Bihar.

The results show that dolphins choose their prey based on size; bottom-dwelling fish dominate their diets. Dolphins grasp their prey and potentially suck them into their mouths using their unusually large tongues. The study, published in Mammal Review, is the first to calculate the distance that these dolphins can detect a fish from: their echolocation enables them to ‘see’ a fish the size of the finger from 20 metres away.

Depths matter

There is also a clear difference in the way dolphins catch fish at various river depths: at the surface they listen for fish movement; prefer echolocation at the middle depths (dolphins produced the most number of clicks here) and sensitive snout-whiskers, especially in calves and juveniles, could help sense weak electrical signals emitted by bottom-dwelling fish and shrimp. This could make Platanista dolphins one of the few mammal groups in the world that use this method, says ATREE's Nachiket Kelkar, lead author of the study.

“The use of these different feeding strategies together could perhaps also explain why we see Platanista dolphins even in some highly disturbed habitats,” adds Kelkar.