Detailing The Evolution Of Echolocation In River Dolphins
Alan McStravick for redOrbit.com – Your Universe Online
A new study by researchers from researchers at the Woods Hole Oceanographic Institute (WHOI) and Aarhus University in Denmark is focusing on one of the most endangered animal species currently known: the river dolphin.
The Ganges river dolphin, after having diverged from other toothed whale species some 30 million years ago, is thought to be one of the oldest species of aquatic animals to employ the technique of echolocation, or biosonar, for the purposes of navigation and foraging. Researchers believed this fact made the Ganges river dolphin a prime subject for gaining a better understanding of the evolution of echolocation among toothed whales.
The new study, “Clicking in shallow rivers”, was published recently in the journal PLoS ONE. Their findings, according to lead researcher Frants Havmand Jensen, a Danish Council for Independent Research — Natural Sciences (FNU) postdoctoral fellow at WHOI, show these freshwater dolphins produce echolocation signals at significantly lower sound intensities and frequencies as compared to marine dolphins.
“Ganges River dolphins are one of the most ancient evolutionary branches of toothed whales,” says Jensen. “We believe our findings help explain the differences in echolocation between freshwater and marine dolphins. Our findings imply that the sound intensity and frequency of Ganges river dolphin may have been closer to the ℠starting point´ from which marine dolphins gradually evolved their high-frequent, powerful biosonar.”
The differences between the freshwater and marine dolphins evolved as a direct result of the unique environments in which they live and the distribution of prey within both environments, the scientist claims.
Where marine dolphins typically traverse vast expanses of open sea in search for food, the river dolphins must utilize a more nuanced technique in their search for food, which includes small fish and crustaceans, in an environment that includes highly turbid water with a severely diminished visibility, sometimes as low as only a few inches.
“Dolphins that range through the open ocean often feed on patchily distributed prey, such as schools of fish,” Jensen says. “They have had a large advantage from evolving an intense biosonar that would help them detect prey over long distances, but we have little idea of how the complex river habitats of freshwater dolphins shape their biosonar signals.”
As it turns out, river dolphins, much like their marine relatives, navigate their environment through the use of echolocation. The river dolphin continuously emits sound pulses into the environment, listening for the faint echoes reflected off obstacles. Any minute change in the returning pulse receives special attention from the creature as it may signify a possible meal.
For the study, researchers focused on recording the echolocation signals of two species of toothed whales inhabiting the same mangrove forest in the southern part of Bangladesh. The first was the Ganges river dolphin, known to exist exclusively in river environments. Interestingly, the Ganges river dolphin is not even part of the dolphin family but rather the Platanistidae family. The second species was the Irrawaddy. The Irrawaddy is a freshwater toothed whale from the dolphin family. This species can be found in both coastal and river systems.
The researchers learned the echolocation signals of these two species were much less intense than signals produced by marine dolphins of similar size. The team claims this could be due to the fact the freshwater dolphins were foraging at much shorter distances than their marine cousins.
While both species produced this lower intensity biosonar, the Ganges river dolphin employed a particularly low frequency. The researchers state the level was only nearly half as high as expected if this species had been a marine dolphin.
”It is very surprising to see these animals produce such low-frequent biosonar sounds. We are talking about a small toothed whale the size of a porpoise producing sounds that would be more typical for a killer whale or a large pilot whale,” says Professor Peter Teglberg Madsen from Aarhus University, an expert on toothed whale biosonar and co-author of the study.
The team believes echolocation in toothed whales began as a short, broadband and low-frequent click. It was with the move by dolphins and toothed whales into open water, and with it a need to detect schools of fish and other prey items, a longer-distance biosonar system was favored by evolution. It was due to the evolution of the open water dolphins to detect higher sound frequencies that the biosonar beam achieved a higher focus allowing the animals to detect prey at greater distances.
As marine dolphins were transitioning to an open water environment, the Ganges river dolphin opted to remain in the shallow, winding river systems where the need for a high-frequency, long-distance sonar system would probably be less important to survival than other factors such as high maneuverability or the flexible neck that helps them to capture prey at close range.
As mentioned above, the endangered freshwater dolphins, with the Ganges river dolphin numbering only around 1000, inhabit some of the more polluted and overfished river systems on Earth. The team contends the results of their study will help other ecologists by providing them with a new tool in their efforts to conserve these highly threatened freshwater cetaceans. Through the use of acoustic monitoring devices, other researchers will be able to more easily identify the local species, helping them to determine how many of the animals remain and to recognize which areas are most important to their continued health.