Like many marine mammals, porpoises produce clicks—to locate prey and to communicate—but they do it in a very distinct way: differences in frequency, bandwidth and duration separate porpoise clicks from dolphins’, for instance. A recent study comparing sounds across populations supports the idea that predation pressure by killer whales is the main force behind the porpoise clicking style. This suggests that the predators play a much bigger role in determining marine mammal behavior and evolution than previously thought.
(Credit: Flickr / __o[ES]__)
Porpoises and dolphins speak very different languages. Both use echolocation to find prey, emitting clicks whose sound waves will bounce back to them from a potential next meal, but, while dolphins’ communication repertoire also includes whistles, the porpoise version does not. Theirs is limited to clicks, which are also very different acoustically from dolphins’. What could account for this? The prevailing hypothesis points to killer whales: porpoise clicks may have evolved due to strong evolutionary pressure from these predators. A recent study published in PLOS ONE (available on MyScienceWork), suggests the effect may be much greater than previously thought.
This diversity of echolocation clicks among species sharing the same space could have been explained as a strategy to avoid competing for the same prey. Several studies, cited by the authors, have already shown this in certain species of bats. A team led by Line Kyhn and Peter Teglberg Madsen of Aarhus University tested this hypothesis by recording the sounds of two species of porpoises, in three populations found in British Columbia (Canada) and Denmark. Their analysis showed, however, that, compared to the bats, the porpoise clicks were remarkably similar between species living side by side. These results support the “most favoured explanation” for the evolution of the distinctive click found in porpoises and a certain number of other species: selective pressure imposed by a specific predator.
“All cetaceans have a method of avoiding predation by killer whales; what porpoises do is to stay below the radar,” explains Prof. Madsen. They produce high frequency clicks—as would be expected from an animal with their small body size—but they are restricted to this “weird, narrow band” of frequencies, while dolphins are not. “Porpoises make no click energy under 100kHz – that’s weird because dolphins do that all the time and some dolphins are not much bigger than porpoises.” Not coincidentally, perhaps, 100kHz is also the upper limit on the hearing of another marine mammal: the killer whale.
In order to find food effectively with echolocation, using a high frequency signal, dolphins use a click that is very short in duration, but very broadband, covering a large range of frequencies, Peter Teglberg Madsen explains. Porpoises, due to their similarly small size, also need to operate at high frequency. “We assume the evolutionary starting point [of the porpoise’s click] is just like the dolphin’s. But there is selection pressure to avoid frequencies where killer whales can hear. This pushed them to develop clicks with no energy below 100kHz, and the only way to do that is with a much longer click.”
Generally, dolphins do not use this strategy (apart from two genera that did, in fact, independently evolve narrow-band high-frequency clicks similar to porpoises.) Are dolphins not at risk, then, for predation by killer whales, who, if given the chance and sufficient numbers, will even take on blue whales? “It depends which dolphin you are,” according to Peter Teglberg Madsen. They are very social creatures, living in large groups, where predation becomes a numbers game. “Porpoises have a very different social structure. They are solitary or live in groups of two or three. They have no social defense or power in numbers, so they opt for a very different solution; namely, to be acoustically cryptic.”
Find out more:
Listen to harbor porpoise echolocation clicks, modified to be audible to the human ear, on Discovery of Sound in the Sea
How Porpoises Track Prey with Echolocation, in The Journal of Experimental Biology
More papers on toothed whale echolocation