Sowerby’s beaked whale biosonar and movement strategy indicate deep-sea foraging niche differentiation in mesoplodont whales

Closely related species are expected to diverge in foraging strategy,
reflecting the evolutionary drive to optimize foraging performance. The
most speciose cetacean genus, Mesoplodon, comprises beaked whales with
little diversity in external morphology or diet, and overlapping
distributions. Moreover, the few studied species of beaked whales
(Ziphiidae) show very similar foraging styles with slow, energy-conserving
movement during long, deep foraging dives. This raises the question of
what factors drive their speciation. Using data from animal-attached tags
and aerial imagery, we tested the hypothesis that two similar-sized
mesoplodonts, Sowerby’s(Mesoplodon bidens) and Blainville’s (Mesoplodon
densirostris) beaked whales, exploit a similar low-energy niche. We show
that, compared with the low-energy strategist Blainville’s beaked whale,
AQ6 Sowerby’s beaked whale lives in the fast lane. While targeting a
similar mesopelagic/bathypelagic foraging zone, they consistently swim and
hunt faster, perform shorter deep dives, and echolocate at a faster rate
and with higher frequency clicks. Further, extensive nearsurface travel
between deep dives challenges the interpretation of beaked whale shallow
inter-foraging dives as a management strategy for decompression sickness.
The distinctively higher frequency echolocation clicks do not hold
apparent foraging benefits. Instead, we argue that a high-speed foraging
style influences dive duration and echolocation behaviour, enabling access
to a distinct prey population. Our results demonstrate that beaked whales
exploit a broader diversity of deep-sea foraging and energetic niches than
hitherto suspected. The marked deviation of Sowerby’s beaked whales from
the typical ziphiid foraging strategy has potential implications for their
response to anthropogenic sounds, which appears to be strongly
behaviourally driven in other ziphiids.