I once heard the fact (I believe from Professor Richard Dawkins) that echolocation has evolved independently on four different occasions: bats, dolphins and two species of cave dwelling birds. How true this is technically, I’m not certain on, considering there are shrews in Madagascar with a much lower amplitude, frequency modulated version of echolocation. There is also evidence that echolocation evolved independently in two separate lineages of bats (and I really am lost about how whales fall into this). Although these acoustic techniques can be used by animals to locate their prey quite easily, it also makes it a lot easier for us to find them, especially in the case of dolphins.
The ocean is a very big place, you might think your local city is quite big, but that’s peanuts compared to the ocean. Considering this, trying to study a marine mammal that doesn’t even have the courtesy to remain close to the coast becomes quite a challenge. Visual imaging is only effective for monitoring the movement of animals when they come close to the sea’s surface which happens quite rarely for some. Passive acoustic monitoring gives us the ability to view the activities, distributions and populations of acoustically active mammals with good time resolution. The areas of ocean away from both the seabed and the shore (known as pelagic sea) find dolphins as their top predators and of course delphinids have many acoustic abilities. Dolphins can use sonar to hunt, navigate and communicate and by picking up and recording these sounds details about local dolphin movement can be found.
As equipment is improved and more and more data is gathered there needs to be a way of effectively filtering the noise that is the produce of the local fish rather than the dolphins under study. This is achieved by improved and more sophisticated algorithms which can automatically detect the dolphin biosonar and the accuracy of this program was compared to that of human detection of dolphins’ clicks. Using this method two years’ worth of data was analysed leading to the conclusion that previously assumed lunar based alterations in dolphin behaviour only occurred close to shore, when tidal forces were active. At least in the Mediterranean sea, where this study occurred, the lunar cycle had no effect on the dolphins’ behaviours.
Paper links: Long-Term Monitoring of Dolphin Biosonar Activity in Deep Pelagic Waters of the Mediterranean Sea
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