Several correlation studies and narratives have shown that beaked whale mass strandings seem to be highly correlated with the prevalence of sonar noise. However, the underlying cause has never fully been understood.
Now, a recent study by Aguilera de Soto et al. has found a possible explanation by analysing their diving behaviour. Beaked whales are very good divers, descending up to 1000 metres deep with a duration of two hours. Foraging takes place in several hundred meters depth using echolocation. Analysis of biologging data from several tagged animals revealed that the animals dived cohesively as groups, always staying close enough together to be in hearing range. Further, their clicking only started after reaching a certain depth. Dives, ascent and descent close to the surface were spent in silence. They also showed a peculiar diving pattern in which they did not ascend vertically like most deep-diving species but at a low pitch angle. This way they appeared at the surface a kilometre off from where they started ascending cohesively from their foraging.
As such, this behaviour was already mostly known in literature though still lacking explanation. By keeping their group cohesion even in deep waters, beaked whales are restricting their individual foraging area to an “acoustic leash” where they can still communicate (278 ± 57 m), leading to a foraging disadvantage. Even more so does their ascending behaviour. By choosing ascent at an angle instead of vertically, they lose 35% of their foraging time. Ascending as a group also means that their time spent foraging is restricted to the group member with the lowest diving ability – usually the young ones.
Looking at this tremendous disadvantage, Aguilera de Soto and colleagues assumed that there must have been a powerful driver for the evolution of such behaviour. And what higher selection pressure is there than the escape from predators – in this case namely Killer whales. Though Killer whales have the ability for deep dives, they tend to stay close to the surface since their diving duration is limited to several minutes. They are thus restricted to allocate the beaked whales by their clicks and wait for them silently until they come to the surface for air. Counteractively, the beaked whales can avoid giving their exact position away by restricting their clicking to deep water and then ascending in an angle. Thus the killer whales do not know where exactly the group will reach the surface and are restricted to visual observation as not to give their own presence away by using echolocation.
That beaked whales have evolved such a costly predator avoidance behaviour indicates how sensitive they are to any Killer whale presence. And that is where the problem of sonar activity comes to play. They are selected to react intensely to any kind of sign that a predator might be present – including all kinds of mid-range sonar noise. The authors assume that the stress response in beaked whales upon detecting predators in the vicinity might force the beaked whales ”beyond their physiological limits and in some cases lead to sonar induced mortalities”. Consequent beachings might then either be based on the animals’ death at sea followed by the carcasses washing ashore, or flight into unsuitable territory such as shallow water resulting in the same dreadful end.
The authors thus conclude that “as such, a successful predator abatement strategy shaped by natural selection has become maladaptive in the face of novel human activities.” The best way to prevent further sonar induced mortalities would be to avoid beaked whale habitats on a large scale when using mid-frequency sonar.
Source: Aguilar de Soto, N., Visser, F., Tyack, P.L. et al. Fear of Killer Whales Drives Extreme Synchrony in Deep Diving Beaked Whales. Sci Rep 10, 13 (2020). https://doi.org/10.1038/s41598-019-55911-3
Comments