Because of their elusive nature and preference for the ocean depths, beaked whales remain one of the least understood species of cetaceans to science. The enigmatic marine mammals are capable of diving to depths of 3,000 meters (10,000 feet) – a behavior that has long mystified researchers. Now, a new theory suggests that their deep-diving tendencies may be to avoid one of the ocean’s most ferocious predators: the killer whale.
"Beaked whales are extreme divers that beat records of dive depth and duration. For diving mammals, their body is their oxygen tank. This is why it is so remarkable that medium-sized beaked whales can outcompete much larger sperm whales in diving endurance," study author Natacha Aguilar de Soto of the Universidad de La Laguna told IFLScience. But diving to such depths takes a toll on the whales' metabolism, strength, and endurance, making it difficult to ward off potential attacks from predators like killer whales.
A team of international researchers analyzed data obtained by trackers attached to dozens of sized Blainville’s (Mesoplodon densirostris) and Cuvier’s (Ziphius cavirostris) beaked whales, recording information on how deep individuals dove, the steepness of their dives, and the sounds that the animals made throughout their journey. This information was compared with existing literature to determine what sets these two species apart from other deep-diving toothed whales.
Analyses revealed that the whales remained largely silent when in shallower waters where potential predators may lurk, but began echolocating for prey in closely coordinated groups once reaching depths of around 450 meters (1,475 feet). At this point, the animals would split off to hunt individually and reconvene again at about 750 meters (2,460 feet) before silently ascending to the surface at a shallow angle.
"Other deep-diving whales, such as pilot and sperm whales, live in large social groups and are powerful muscular animals," explained Aguilar. "Beaked whales, in contrast, have developed cunning behavioral tactics which allow them to hunt at depth using their biological echosounder: echolocation, and still reduce the fatal encounters with groups of killer whales stalking them acoustically from the surface."
Killer whales are excellent listeners and silent hunters that give little advance notice of their presence, power, and speed, leaving no room for last-minute escapes. The researchers believe that this unique diving behavior may have been an evolutionary adaptation to avoid the black-and-white hunters dating back millions of years.
“Fear of predation can induce profound changes in the behavior and physiology of prey species even if predator encounters are infrequent. For echolocating toothed whales, the use of sound to forage exposes them to detection by eavesdropping predators,” write the authors in Nature, adding that deep-diving beaked whales seem “enigmatically defenseless against their main predator, killer whales.”
The findings may also explain how some whales react to naval sonar, which has been linked to mass stranding events. Both M. densirostris and Z. cavirostris are among the most commonly found in strandings related to naval sonar.
“These predator abatement behaviors have likely served beaked whales over millions of years, but may become maladaptive by playing a role in mass strandings induced by man-made predator-like sonar sounds,” conclude the authors.
Aguilar tells IFLScience that her team hopes to next understand the evolutionary context of beaked whales' sensitivity to killer whale calls, which could explain why beaked whales respond so strongly to naval sonar. In theory, it could be that the whales are misinterpreting sonar as a predator.