Creatures that navigate by echolocation have two problems: differentiating their calls and echoes from those of related species that live in the same location and reducing interference from echoes generated by cluttered environments. Bats are past masters at dealing with these issues, but what about dolphins? Did closely related species adjust their calls so that they could be distinguished or are the clicks adapted to reduce interference from acoustic clutter? Line Kyhn and her colleagues from Aarhus University, Denmark, and the Woods Hole Oceanographic Institute wondered how two species of toothed whales, Peale's and Commerson's dolphins, have adapted their calls in the waters that they both inhabit around the Falkland Islands (p. ).
Designing an array of six hydrophones, the team recorded over 2000 clicks from Peale's and Commerson's dolphins as the animals came to ride the bow waves of vessels. But in order to investigate the characteristics of the clicks, the team had to directly compare clicks that had been recorded when the dolphins were head-on, leaving the team with only 94 Commerson's dolphin clicks and 87 Peale's dolphin clicks.
Analysing the frequency distribution of the animals' clicks, Kyhn and her colleagues found that they were close to 130 kHz. The Commerson's dolphins' clicks were slightly higher pitched (133 kHz) than those of the larger Peale's dolphins (129 kHz), but both were well above the hearing range of killer whales that prey on dolphins. The team also found that both species use narrow sonar beams, reducing the problem of unwanted reflections from clutter, although Commerson's dolphins are quieter than Peale's dolphins and their softer clicks are better suited to foraging in the kelp forests where they hunt.
Considering whether the dolphins have modified their clicks because of their overlapping habitats, the team calculated the frequency distribution that they would expect larger Peale's dolphins to produce and found that the difference between the dolphins' clicks was smaller than expected, based on size alone. The team suspects that the dolphins have deliberately shifted their frequency ranges and that the shifts are sufficient for the animals to distinguish between Commerson's and Peale's dolphins' clicks. However, as the species do not hunt together and their beams of sound are highly focused, the team concluded that it has not been necessary for either to evolve more specialised acoustic mechanisms to overcome jamming by the other's clicks.
