1. Lactation length and weaning age provide important information about maternal investment, which can reflect the health and nutritional status of the mother, as well as broader reproductive strategies in mammals. Calcium‐normalized strontium (Sr) and barium (Ba) concentrations in the growth layers of mammalian teeth differ for nursing animals and those consuming non‐milk foods, thus can be used to estimate age‐at‐weaning. To date, this approach has been used only for terrestrial animals, and almost exclusively for primates.
2. The goal of this study was to determine whether Sr and Ba concentrations in the cementum of Pacific walrus Odobenus rosmarus divergens teeth can be used to estimate weaning age. Teeth from 107 walruses were analysed using laser ablation inductively coupled plasma mass spectrometry, and calcium‐normalized ⁸⁸Sr and ¹³⁷Ba concentrations were quantified.
3. For most walruses, both Sr and Ba concentrations exhibited rapid changes in early life. Ba concentrations matched closely with expected patterns in the published literature, rapidly declining from high to low concentrations (typically from ~10 ppm to ~5 ppm). In contrast, Sr exhibited a pattern opposite to that presented in studies of terrestrial mammals, appearing nearly identical to Ba (typically declining from ~400 ppm to ~200 ppm). To explain these findings, we present conceptual models of the factors generating weaning signals in Sr and Ba for terrestrial mammals, as well as a new, hypothetical model for walruses. Both a visual and mathematical approach to weaning age estimation indicated a median weaning age of walruses at the end of the second year of life (in the second dark layer of the tooth cementum), with many walruses estimated to have weaned in their third year of life, and a smaller group weaning in their fourth or fifth year. This is later than expected, given a published estimate of walrus weaning at 18–24 months.
4. These results do not conclusively support the use of tooth Sr and Ba for estimating weaning age in walruses, and further research is warranted to better understand the drivers of the observed patterns of Ba and Sr accumulation in walrus teeth.

中文翻译：

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评估牙齿的锶和钡作为太平洋海象断奶年龄的指标

1. 哺乳期长度和断奶年龄提供了有关母亲投资的重要信息，可以反映母亲的健康和营养状况，以及哺乳动物更广泛的生殖策略。哺乳动物牙齿生长层中钙标准化的锶（Sr）和钡（Ba）浓度对于哺乳动物和食用非奶食品的动物来说是不同的，因此可以用来估计断奶年龄。迄今为止，这种方法仅用于陆生动物，并且几乎专门用于灵长类动物。
2. 本研究的目的是确定太平洋海象Odobenus rosmarus divergens牙齿牙骨质中的 Sr 和 Ba 浓度是否可用于估计断奶年龄。使用激光烧蚀电感耦合等离子体质谱法对 107 只海象的牙齿进行了分析，并对钙归一化的⁸⁸ Sr 和¹³⁷ Ba 浓度进行了定量。
3. 对于大多数海象来说，锶和钡的浓度在生命早期都表现出快速变化。Ba 浓度与已发表文献中的预期模式密切匹配，从高浓度迅速下降到低浓度（通常从约 10 ppm 降至约 5 ppm）。相比之下，Sr 表现出与陆地哺乳动物研究中呈现的模式相反的模式，与 Ba 几乎相同（通常从约 400 ppm 下降到约 200 ppm）。为了解释这些发现，我们提出了陆生哺乳动物 Sr 和 Ba 中产生断奶信号的因素的概念模型，以及海象的新假设模型。断奶年龄估计的视觉和数学方法表明，海象的断奶年龄中位数是在生命第二年末（牙骨质的第二暗层），许多海象估计在第三年断奶。生命，还有一小群在第四年或第五年断奶。鉴于已发布的海象断奶估计为 18-24 个月，这比预期要晚。
4. 这些结果并不能最终支持使用牙齿 Sr 和 Ba 来估计海象的断奶年龄，需要进一步研究以更好地了解海象牙齿中观察到的 Ba 和 Sr 积累模式的驱动因素。