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Keywords:

  • deep water ecosystem;
  • foraging ecology;
  • marine mammal;
  • pilot whale;
  • swimming speed

Summary

  • 1
    Empirical testing of optimal foraging models for breath-hold divers has been difficult. Here we report data from sound and movement recording DTags placed on 23 short-finned pilot whales off Tenerife to study the foraging strategies used to catch deep-water prey.
  • 2
    Day and night foraging dives had a maximum depth and duration of 1018 m and 21 min. Vocal behaviour during dives was consistent with biosonar-based foraging, with long series of echolocation clicks interspersed with buzzes. Similar buzzes have been associated with prey capture attempts in other echolocating species.
  • 3
    Foraging dives seemed to adapt to circadian rhythms. Deep dives during the day were deeper, but contained fewer buzzes (median 1), than night-time deep dives (median 5 buzzes).
  • 4
    In most deep (540–1019 m) daytime dives with buzzes, a downward directed sprint reaching up to 9 m s−1 occurred just prior to a buzz and coincided with the deepest point in the dive, suggestive of a chase after escaping prey.
  • 5
    A large percentage (10–36%) of the drag-related locomotion cost of these dives (15 min long) is spent in sprinting (19–79 s). This energetic foraging tactic focused on a single or few prey items has not been observed previously in deep-diving mammals but resembles the high-risk/high-gain strategy of some terrestrial hunters such as cheetahs.
  • 6
    Deep sprints contrast with the expectation that deep-diving mammals will swim at moderate speeds optimized to reduce oxygen consumption and maximize foraging time at depth. Pilot whales may have developed this tactic to target a deep-water niche formed by large/calorific/fast moving prey such as giant squid.