Understanding the causes and consequences of biases in the sex ratio at birth and of adults in species of mammals that have unusual life histories may help us know whether sex ratios are adaptive responses to ecological and evolutionary forces and may be important in conserving endangered species. For example, have sex ratio biases at birth and in adults in species of mammals evolved as an adaptive response to environmental unpredictability? We investigated the sex ratio of populations of hairy-nosed wombats, Lasiorhinus latifrons, and the endangered Lasiorhinus krefftii. The social structure of Lasiorhinus includes male philopatry and female dispersal, which are unusual traits among mammals. Reproduction in these wombats is often curtailed by unpredictable droughts, and so wombats may be a suitable target group for understanding the causes and consequences of biases in mammalian sex ratios. The sex ratio in populations of adult L. latifrons reported in the scientific literature varied widely, but the mean percentage of females in the 10 studies examined was 54.9 with a 95% confidence interval of 50.3 to 59.5. In the population of L. latifrons that we monitored for nearly twenty years beginning in 1994, the percentage of adult females varied between 66 and 37% in a manner that suggested homeostasis. The growth of the population of critically endangered L. krefftii from 52 males and 29 females in 2000 to 79 males and 91 females in 2016 suggested that more females than males were born. The sex ratio of pouch young of L. latifrons was biased towards females at times. The mean percentage of female pouch young in 19 breeding seasons was 56.4 (95% confidence interval of 50.4–62.4). Variation in the sex ratio of pouch young was examined regarding theories of sex allocation. When the percentage of adult females in the population was lower (37–50%) and had been declining, L. latifrons mothers had more female than male pouch young, but the sex ratio of pouch young was about equal when the percentage of adult females was higher (53–66%). This pattern is partly consistent with Fisher’s principle, which is also known as the homeostatic hypothesis (HH). When the percentage of adult females was lower, the mothers of female pouch young were in better condition than the mothers of male pouch young, which is consistent with a reversed Trivers–Willard (TW) effect in part. The biases in the sex ratio of adult and pouch young wombats may be adaptive responses to their life history and to demographic and environmental cues. Wombats are unusual among mammals in that males are philopatric and females disperse. Hairy-nosed wombats (two species within the genus Lasiorhinus) exist tenuously in semi-arid environments because of infrequent reproduction, habitat degradation, and drought. L. krefftii is critically endangered. This study suggests that following severe droughts, a female bias in the sex ratio of pouch young and of adults may result in populations increasing more rapidly than if the sex ratio was equal. Agencies charged with managing and conserving wombats could make use of this knowledge by undertaking more comprehensive demographic monitoring of populations to evaluate their risk of extinction; a persistent male bias in adults may be a signal of an increased risk of extinction. The unpredictability of rainfall and drought across their distribution has resulted in variation in abundance and in genetic, ecological, and social parameters associated with wombat colonies, making these species valuable for exploring the causes and consequences of biases in mammalian sex ratios.

中文翻译：

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小袋袋袋袋熊和成年毛鼻袋熊（Lasiorhinus latifrons 和 Lasiorhinus krefftii）性别比例的变化

了解具有不寻常生活史的哺乳动物物种出生时和成年时性别比例偏差的原因和后果可能有助于我们了解性别比例是否是对生态和进化力量的适应性反应，并且可能对保护濒危物种很重要。例如，哺乳动物在出生时和成年时的性别比例偏差是否进化为对环境不可预测性的适应性反应？我们调查了毛鼻袋熊、Lasiorhinus latifrons 和濒临灭绝的 Lasiorhinus krefftii 种群的性别比例。Lasiorhinus 的社会结构包括雄性亲和和雌性分散，这是哺乳动物中不寻常的特征。这些袋熊的繁殖常常受到不可预测的干旱的影响，因此，袋熊可能是了解哺乳动物性别比例偏差的原因和后果的合适目标群体。科学文献中报告的成年 L. latifrons 种群的性别比例差异很大，但在所审查的 10 项研究中，雌性的平均百分比为 54.9，95% 置信区间为 50.3 至 59.5。在我们从 1994 年开始监测了近 20 年的 L. latifrons 种群中，成年雌性的百分比在 66% 到 37% 之间变化，表明体内平衡。极度濒危 L. krefftii 的种群数量从 2000 年的 52 只雄性和 29 只雌性增加到 2016 年的 79 只雄性和 91 只雌性，表明出生的雌性多于雄性。L. latifrons 幼袋的性别比例有时偏向雌性。19 个繁殖季节中雌性育幼袋的平均百分比为 56.4（95% 置信区间为 50.4-62.4）。根据性别分配理论检查了育儿袋幼仔性别比例的变化。当成年雌性在种群中的比例较低（37-50%）并且一直在下降时，L. latifrons 母亲的雌性多于雄性育幼袋，但当成年雌性的比例低于雄性时，育幼袋的性别比例大致相等更高（53-66%）。这种模式部分符合费雪原理，也称为稳态假设 (HH)。当成年雌性的百分比较低时，雌性育幼仔的母亲比雄性育幼仔的母亲状况更好，这在一定程度上与反向 Trivers-Willard (TW) 效应一致。成年袋熊和袋熊幼袋熊性别比例的偏差可能是对其生活史以及人口和环境线索的适应性反应。袋熊在哺乳动物中很不寻常，因为雄性是亲民的，而雌性则分散。由于繁殖不频繁、栖息地退化和干旱，毛鼻袋熊（Lasiorhinus 属内的两个物种）在半干旱环境中微弱地存在。L. krefftii 极度濒危。这项研究表明，在严重干旱之后，与性别比例相等的情况相比，育儿袋年轻人和成年人的性别比例中的女性偏见可能会导致人口增长更快。负责管理和保护袋熊的机构可以通过对袋熊进行更全面的人口监测来评估它们的灭绝风险，从而利用这些知识；成年人中持续存在的男性偏见可能是灭绝风险增加的信号。降雨和干旱分布的不可预测性导致了与袋熊种群相关的丰度和遗传、生态和社会参数的变化，使这些物种对于探索哺乳动物性别比例偏差的原因和后果很有价值。