Ontogenetic profiles of stable isotopes (15N and 13C) in dentine growth layer groups (GLGs) of narwhal embedded teeth

Narwhal embedded canines were collected by Inuit during subsistence hunts near Pond Inlet and provided to Fisheries and Oceans Canada through a voluntary collaborative sampling program with the Pond Inlet Hunters and Trappers Association. These harvested narwhals belonged to the Baffin Bay narwhal population, which is monitored by DFO for stock assessment. Stable isotope analysis (SIA) of sequential dentine growth layer groups from narwhal embedded canine teeth can be used to estimate the lifetime diet of individuals and infer major ontogenetic shifts such as the completion of nursing. We used SIA of dentine to investigate ontogenetic dietary patterns, with a focus on nursing duration. We also determined whether nursing duration differed between sexes and between 2 periods during which narwhals may have undergone dietary shifts due to warming. Embedded teeth from both sexes were collected near Pond Inlet, Nunavut, Canada, in 1982 and 1983 (n = 17) and 2015 and 2017 (n = 14). Nursing duration ranged from ~2 to ~6 yr, with 60% of narwhals being nursed beyond the previously published estimate of <2 yr. The proportion of individuals nursed <2 yr versus >2 yr did not differ between sexes or periods. This study not only revealed that narwhals vary extensively in their nursing duration, but also indicated that extended nursing (>2 yr) with gradual introduction of solid food over this period was common. These findings provide insights into narwhal life history strategies, as extended nursing may be another feature of a long-lived, slow-reproducing mammal adapted to unique polar conditions that are threatened by global warming.

本研究中的独角鲸嵌入犬齿样本，由因纽特人在庞德因莱特（Pond Inlet）附近的生计狩猎中采集，并通过与庞德因莱特猎人与捕手协会合作的自愿采样项目，捐赠给加拿大渔业与海洋部（Fisheries and Oceans Canada, 简称DFO）。这些被捕猎的独角鲸属于巴芬湾（Baffin Bay）独角鲸种群，该种群由DFO开展监测以进行资源种群评估。对独角鲸嵌入犬齿的连续牙本质生长层组开展稳定同位素分析（Stable Isotope Analysis, SIA），可用于估算个体的终生食性，并推断其主要的个体发育转变，例如哺乳阶段的结束。本研究通过牙本质稳定同位素分析，探究独角鲸的个体发育食性模式，重点关注哺乳期时长。同时我们还分析了哺乳期时长是否存在性别差异，以及在两个因气候变暖可能发生食性转变的时期中，该时长是否存在差异。研究采集了加拿大努纳武特（Nunavut）地区庞德因莱特附近的雌雄独角鲸嵌入齿样本：1982年与1983年采集17例（n = 17），2015年与2017年采集14例（n = 14）。独角鲸的哺乳期时长约为2至6年，其中60%的个体哺乳期超过了此前已发表的短于2年的估算值。哺乳期短于2年与长于2年的个体占比，在不同性别间以及不同时期均无显著差异。本研究不仅揭示了独角鲸的哺乳期时长存在显著个体差异，同时表明，在哺乳期内逐步引入固体食物的延长哺乳期（>2年）模式较为普遍。这些发现为独角鲸的生活史策略研究提供了新视角：鉴于全球变暖正威胁着独特的极地环境，延长哺乳期或许是适应极地环境的长寿、低繁殖率哺乳动物的又一特征。