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.亲缘关系紧密的物种通常会在觅食策略上产生分化，这一现象反映了优化觅食表现的进化驱动力。物种多样性最高的鲸类属——中喙鲸属（Mesoplodon）——所包含的喙鲸类群，在外貌形态与食性上差异极小，且分布范围相互重叠。此外，目前已被研究的少数喙鲸科（Ziphiidae）物种，其觅食风格高度一致：在漫长的深层觅食潜水中，它们以缓慢且节能的方式移动。这一现象引出了一个核心问题：究竟是何种因素推动了该类群的物种分化？ 本研究借助动物附着式标签（animal-attached tags）与航空影像数据，对两种体型相近的中喙鲸——索氏喙鲸（Sowerby's beaked whale，Mesoplodon bidens）与布兰维尔喙鲸（Blainville's beaked whale，Mesoplodon densirostris）——是否共享相似的低能量生态位这一假说展开了验证。研究结果显示，与采取低能量觅食策略的布兰维尔喙鲸相比，AQ6索氏喙鲸的生活节奏明显更快。尽管二者的觅食目标区域均为相似的中层/深层带（mesopelagic/bathypelagic zone），但索氏喙鲸的游动与狩猎速度始终更高，深水潜水时长更短，回声定位速率更快，且发出的回声定位咔哒声（echolocation clicks）频率也更高。 此外，深水潜水之间存在大量近水面巡游的现象，这对‘喙鲸的浅水区觅食间歇潜水是缓解减压病（decompression sickness）的应对策略’这一主流解释提出了挑战。频率显著更高的回声定位咔哒声，并未带来明显的觅食优势。与之相反，我们认为高速觅食模式会影响潜水时长与回声定位行为，从而使该物种得以利用独特的猎物种群。 本研究结果表明，喙鲸对深海觅食与能量生态位的利用范围，比此前学界所推测的更为广泛。索氏喙鲸与典型喙鲸科物种觅食策略的显著差异，可能会影响其对人为声响的响应——而在其他喙鲸科物种中，这类响应似乎强烈受行为因素驱动。