Abstract
Fear of predation can induce profound changes in the behaviour 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, but while some species exploit social defences or produce cryptic acoustic signals, deep-diving beaked whales, well known for mass-strandings induced by navy sonar, seem enigmatically defenceless against their main predator, killer whales. Here we test the hypothesis that the stereotyped group diving and vocal behaviour of beaked whales has benefits for abatement of predation risk and thus could have been driven by fear of predation over evolutionary time. Biologging data from 14 Blainville’s and 12 Cuvier’s beaked whales show that group members have an extreme synchronicity, overlapping vocal foraging time by 98% despite hunting individually, thereby reducing group temporal availability for acoustic detection by killer whales to <25%. Groups also perform a coordinated silent ascent in an unpredictable direction, covering a mean of 1 km horizontal distance from their last vocal position. This tactic sacrifices 35% of foraging time but reduces by an order of magnitude the risk of interception by killer whales. These predator abatement behaviours 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.
Highlights
Deep-diving marine mammals are expected to maximise time spent foraging in deep prey layers to offset the energetic and physiological costs of diving[1]
When other toothed whales dive to similar depths, they do not display such a diving behaviour: both pilot whales that are similar in size to these beaked whales and the larger sperm whales ascend nearly vertically from their deep foraging dives[8,9] and often emit calls during the ascent to mediate reunion with non-diving group members[10,11,12,13]
We propose that deep waters are a refuge where beaked whales are safe from killer whale attacks, and we predict that groups of beaked whales will coordinate their sound production and movements so as to minimise acoustic and visual detection when abandoning the deep refuge to return to the surface
Summary
Deep-diving marine mammals are expected to maximise time spent foraging in deep prey layers to offset the energetic and physiological costs of diving[1]. Smaller species of beaked whales have adopted neither strategy: they produce medium frequency clicks[23] that are detectable at considerable ranges[30] and live in small groups that offer scant social defence[31] This apparent lack of a predator abatement strategy is at odds with their intense reactions to playbacks of killer whale and mid-frequency sonar sounds: even sound exposure levels close to the ambient noise can cause intense behavioural responses in beaked whales[32,33,34,35] suggesting that sonar-related mortalities[36,37] are rooted in an extreme anti-predator response[38]. We propose that fear of predation shapes the minute-by-minute behaviour of these long lived, elephant-sized marine mammals which pay this heavy cost to access a privileged foraging niche and mitigate interception by a stealthy large-brained cosmopolitan predator
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