Information about the reproductive status of free-ranging pinnipeds provides useful insight into their population dynamics, which is essential to their management and conservation. To determine the reproductive status of individual animals, blood sampling is often required despite being impractical to collect in open water. Hair as an endocrine marker has been used to less invasively assess the reproductive status of terrestrial animals. However, it is unknown whether pinniped reproductive status can be assessed from hair samples. Here, we examine testosterone levels in hair obtained from 57 male northern fur seals and used it to compare their age class and spermatogenesis during the non-breeding season off Hokkaido. We isolated testosterone from the samples using gas chromatography and measured testosterone levels using time-resolved fluoroimmunoassay. Testosterone levels in hair increased towards the breeding season. In May, testosterone levels were the highest in seals aged between 4 and 7 years, followed by those over the age of 8 years and under the age of 4 years. Spermatids, the final phase of spermatogenesis, were present in the seals sampled between April and June, even though testosterone levels were low in April. The seals with spermatids in May showed the highest testosterone levels. Our results demonstrate that seals with higher testosterone levels in May are likely to be mature males (≥4 years). Since hair can be collected using biopsy darts in the field, it will be possible to less invasively determine testosterone levels of male seals in the future.

中文翻译：

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自由放养的雄性北方海狗 (Callorhinus ursinus) 毛发中的睾酮水平与采样月份、年龄等级和精子发生的关系

有关自由放养鳍足类动物繁殖状况的信息为了解它们的种群动态提供了有用的见解，这对它们的管理和保护至关重要。为了确定个体动物的生殖状态，尽管在开放水域采集血液不切实际，但通常需要进行血液采样。毛发作为内分泌标志物已被用于以较小的侵入性方式评估陆生动物的生殖状况。然而，尚不清楚是否可以从毛发样本中评估鳍足类动物的生殖状态。在这里，我们检查了从 57 只雄性北方海狗身上获得的毛发中的睾酮水平，并用它来比较它们在北海道非繁殖季节的年龄等级和精子发生。我们使用气相色谱法从样品中分离出睾酮，并使用时间分辨荧光免疫测定法测量睾酮水平。头发中的睾酮水平随着繁殖季节而增加。5 月，4 至 7 岁海豹的睾酮水平最高，其次是 8 岁以上和 4 岁以下海豹。尽管 4 月份的睾酮水平较低，但在 4 月至 6 月采样的海豹中仍存在精子，即精子发生的最后阶段。5 月份带有精子细胞的海豹显示出最高的睾酮水平。我们的研究结果表明，5 月睾酮水平较高的海豹很可能是成熟的雄性（≥4 岁）。由于可以在现场使用活检飞镖收集头发，