Data from: Olfaction at depth: cribriform plate size declines with dive depth and duration in aquatic arctoid carnivorans

It is widely accepted that obligate aquatic mammals, specifically toothed whales, rely relatively little on olfaction. There is less agreement about the importance of smell among aquatic mammals with residual ties to land, such as pinnipeds and sea otters. Field observations of marine carnivorans stress their keen use of smell while on land or pack ice. Yet, one dimension of olfactory ecology is often overlooked: while underwater, aquatic carnivorans forage “noseblind,” diving with nares closed, removed from air-borne chemical cues. For this reason, we predicted marine carnivorans would have reduced olfactory anatomy relative to closely-related terrestrial carnivorans. Moreover, because species that dive deeper and longer forage farther removed from surface scent cues, we predicted further reductions in their olfactory anatomy. To test these hypotheses, we looked to the cribriform plate (CP), a perforated bone in the posterior nasal chamber of mammals that serves as the only passageway for olfactory nerves crossing from the periphery to the olfactory bulb and thus covaries in size with relative olfactory innervation. Using CT scans and digital quantification, we compared CP morphology across Arctoidea, a carnivoran clade at the interface of terrestrial and aquatic ecologies. We found that aquatic species from two lineages that independently reinvaded marine environments (Pinnipedia, Mustelidae), have significantly smaller relative CP than terrestrials. Furthermore, within aquatic lineages, diving depth and duration were strongly correlated with CP loss, with the extreme-diving elephant seals showing the greatest reductions. These observations suggest that CP reduction in carnivorans is an adaptive response to shifting selection pressures associated with re-invasions of marine environments, particularly foraging at great depths. Because the CP is fairly well preserved in fossils, using methods presented here to quantify CP morphology in extinct mammals could help define evolutionary patterns of olfactory loss across lineages that have independently committed to life in water.学界普遍认为，专性水生哺乳动物（尤其是齿鲸类）对嗅觉的依赖程度相对较低。而对于仍与陆地存在残存联系的水生哺乳动物（如鳍足类与海獭），其嗅觉的重要性尚未达成共识。针对海洋食肉目动物的野外观测显示，它们在陆地或浮冰上时会敏锐地利用嗅觉。然而，嗅觉生态学中有一个维度常被忽视：水生食肉动物在水下时处于无嗅觉状态，它们会闭合鼻孔潜水，无法获取空气中的化学线索。基于此，我们推测相较于亲缘关系较近的陆生食肉目动物，海洋食肉目动物的嗅觉解剖结构会出现退化。此外，由于下潜更深、时长更久的物种觅食时距离地表气味线索更远，我们预测这类物种的嗅觉解剖结构会出现更显著的退化。为验证上述假说，我们选取了筛板（cribriform plate，CP）作为研究对象：该结构是哺乳动物鼻腔后部的穿孔骨骼，是嗅觉神经从外周通往嗅球的唯一通道，因此其大小与相对嗅觉神经支配程度呈正相关。我们利用CT扫描与数字化量化技术，对犬型总科（Arctoidea）的筛板形态进行了比较——该类群是处于陆生与水生生态过渡地带的食肉目演化支。我们发现，两个独立重新入侵海洋环境的演化支（鳍足亚目Pinnipedia、鼬科Mustelidae）中的水生物种，其相对筛板尺寸显著小于陆生同类。此外，在水生演化支内部，下潜深度与时长与筛板退化程度呈显著正相关，其中极端深潜的象海豹表现出最为显著的筛板尺寸缩减。上述观测结果表明，食肉目动物的筛板退化是对重新入侵海洋环境所带来的选择压力变化的适应性响应，尤其是深海觅食的类群。由于筛板在化石中保存状况较好，利用本文提出的方法量化已灭绝哺乳动物的筛板形态，将有助于阐明那些独立转向水生生活的演化支的嗅觉退化演化模式。