A frequent victim of naval sonar muffles her own voice to stay hidden, says S.Ananthanarayanan.

Animals that use sound to communicate end up giving themselves away to predators. The way to stay viable is then to spend time at places where predators cannot venture.

Natacha Aguilar de Soto, fleur Visser, peter L. tyack, Jesús Alcazar, Graeme Ruxton, patricia Arranz, peter T. Madsen and Mark Johnson, from University of La Laguna, Tenerife, Canary Islands, Spain, University of Amsterdam and Utrecht University, The Netherlands, University of St Andrews, Scotland and Aarhus University, Denmark, describe in the journal, Scientific Reports, how the beaked whale has adapted to avoid becoming the prey of the killer whale.

The beaked whale, at 4 to 13 metres long and 1 to 15 tonnes in weight, is a middling whale, and is marked by the nature of its teeth – teeth erupt only in the males and of these there is generally only one pair, rather like tusks, than teeth. What distinguishes beaked whales from other deep diving species of whales, however, is their manner of diving in groups, of vocalising only at depths and the way they return to the surface.

The way they rise to the surface after a dive is at a shallow angle, which takes time. This manner of rising limits the time they can spend at the depths, where they forage. Rising slowly had been considered to be and adaptation to avoid decompression sickness, an effect known to human divers when they rise too fast after a dive. Reasons for this to happen, however, have not been found and other, toothed whales, that dive to similar depths, are seen to make nearly vertical ascents. These other whales also vocalise when they reach shallow water, to re-establish contact with their herd. Whereas the beaked whale typically stays silent when not diving deep. As the beaked whale uses echolocation, or listening for echoes, for locating prey, they are able to actively forage for only 20% of the time, the paper says.

An alternate explanation for the way the beaked whale rises to the surface, and limits hunting to greater depths has been proposed as a means of predator evasion, the paper says. Beaked whales can be attacked by sharks and it is known that they are harassed by dolphins. But their main predator is the killer whale. Killer whales are toothed whales, the largest of the marine dolphin family, and known as apex predators, as no animal preys on them. They are found in all parts of the world and prey on fish, other marine animals like seals and other species of dolphin, even whales.

The killer whale, like many predators of the sea, has sharp hearing to locate its prey. The beaked whale or other animals that use sonar to locate food are thus easily detected by the killer whale from afar. The killer whale, however, is silent and does not provide its victims the same courtesy. As an adaptation, the paper says, some toothed whale species have evolved to echolocate at frequencies that are higher than the limits of the killer whale’s hearing. Other species, dolphins, sperm and pilot whales, the paper says, find social safety in cohesive groups.

The beaked whales, however, have adopted neither strategy. They produce clicks at mid frequencies and can be heard by predators from a distance. And they live in small groups that offer no protection. That the beaked whale has developed no defensive strategy is at odds, the paper says, with the intense reaction they show when exposed to recordings of killer whale sounds or mid-frequency sonar sounds. Data shows that beaked whales are prominent among whales that are beached or stranded as a result of the sonar emissions of naval exercises. This suggests that the mortalities are the result of anti-predator response, the paper says. And a further conclusion that the behaviour of vocalising only after diving deep, and then ascending at a shallow angle, are also for predator evasion.

The authors of the paper note that the beaked whale’s behaviour has a heavy price tag as it limits the time available for echolocation of food. The authors then carry out trials to quantify how far the features of diving, vocalising and manner of ascent contribute to predator avoidance, to make the price worthwhile.

The killer whale, the paper says, is a powerful predator, but with limited diving capacity. Data suggests that they spend most of their time in the top 20m of water. As intense effort is needed to capture whales and dolphins, and the killer whale’s needs to come to the surface every 10 minutes, the killer whale really cannot seek prey at substantial depths. Deep water is hence a safe refuge for the killer whale’s prey, specifically the beaked whale. The authors therefore propose that beaked whales restrict echolocation to deep water, and keep silent, to give the least evidence of their presence, when they need to return to the surface.

This they achieve by coordinating their movements, by group diving, and by not echolocation, which would give them away, till they are too deep for the killer whale to follow. The echolocation that they do use, however, would show the predator where its prey is foraging and it would lie in wait, directly overhead. Hence the slow ascent of the beaked whales, at a shallow angle, and in silence, so that the predator cannot make out where to get at them!

The coordinated diving behaviour was studied by tagging whales with sensors that would transmit depth and orientation data. Pairs of whales in the same social group were tagged to see how soon after the first whale began a dive did the second whale follow. The result, the paper says is that the overlap of the time spent in a deep dive by the pair was 99%. As the whales were selected at random, it can be taken this shows coordination of diving behaviour. And then, the analysis of the vocalisation behaviour, or the time the whales used echolocation, showed an overlap of 98%.

In respect of the slow ascent, again, the positions of the whales from just before the vocalising stops were plotted, using the pitch, roll, direction and depth data, over 101 instances of ascent. The whales covered a horizontal distance of one kilometre during the ascent, which created “a large circular locus of potential surfacing positions that must be searched by killer whales and which they must search visually rather than using echolocation to avoid alerting their prey,” the paper says.

“The unique diving and vocal behaviour of beaked whales could only evolve if the severe costs it imposes are outweighed by survival benefits,” the paper says.

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