Causes and consequences of an unusually male-biased adult sex ratio in an unmanaged feral horse population

1. The adult sex ratio (ASR) is important within ecology due to its predicted effects on behaviour, demography, and evolution, but research examining the causes and consequences of ASR bias have lagged behind studies of sex ratios at earlier life stages. Although ungulate ASR is relatively well-studied, exceptions to the usual female-biased ASR challenge our understanding of the underlying drivers of biased ASR, and provide an opportunity to better understand its consequences. 2. Some feral ungulate populations, including multiple horse populations, exhibit unusually male-biased ASR. For example, research suggests that the feral horse (Equus ferus caballus) population on Sable Island, Nova Scotia, Canada, may exhibit a male-biased ASR. Such exceptions to the rule provide a valuable opportunity to reveal the contributions of environmental context and trait differences to ASR bias. 3. We aimed to test for bias in Sable Island horse ASR, identify the demographic drivers of bias, and explore its demographic and social consequences. To do this, we used life-history, movement, and group membership information for hundreds of horses followed through a long-term individual-based study between 2007 and 2018. 4. Sable Island horse ASR is male-biased, and this skew has increased over time, reaching 62% male in 2018. Our life table response experiment suggested that ASR skew was driven predominantly by male-biased adult survival. Further analyses pointed to sex-biased survival being driven by reduced female survival post-reproduction. Male-biased ASR was associated with reduced harem sizes, an increase in the number of social groups on the island, and reduced reproduction in young females. 5. Our results support the idea that male-biased ASR in feral ungulate populations may be caused by a combination of high population density and high reproductive output. We suggest that female-biased mortality may be caused by females continuing to reproduce at high density, and thus being more susceptible to resource shortages. Thus, our results highlight the strong context-dependence of ASR. Furthermore, our work indicates the potential for ASR to substantially alter a population's social organisation. Such changes in social structure could have knock-on consequences for demography by altering the formation/stability of social relationships, or competition for matings.

1. 成年性比（adult sex ratio, ASR）在生态学研究中具有核心意义，因其被预测可对动物行为、种群统计学特征与演化进程产生显著影响。然而，针对成年性比失衡的成因与后果开展的研究，却远远落后于针对生命早期阶段性比的相关探究。
2. 尽管有蹄类动物的成年性比研究相对较为充分，但常规的雌性偏倚成年性比的例外情况，却对我们理解成年性比失衡的潜在驱动因素构成了挑战，同时也为深入探究其影响后果提供了绝佳契机。部分野生有蹄类种群（包括多个马种群）便呈现出异常的雄性偏倚成年性比。例如，有研究表明，加拿大新斯科舍省萨布尔岛上的野生马（Equus ferus caballus）种群，便可能存在雄性偏倚的成年性比。这类常规规律之外的例外情况，为揭示环境背景与性状差异对成年性比失衡的作用机制提供了宝贵的研究契机。
3. 本研究旨在验证萨布尔岛野生马种群的成年性比是否存在失衡现象，明确该失衡的种群统计学驱动因素，并探究其对种群动态与社会结构的影响。为此，我们利用了2007年至2018年间一项基于个体的长期跟踪研究中，数百匹野生马的生活史、移动模式以及社群归属信息。
4. 萨布尔岛野生马种群的成年性比确实呈雄性偏倚状态，且该失衡程度随时间推移不断加剧，至2018年时雄性占比已达62%。我们开展的生命表响应实验（life table response experiment）表明，成年性比失衡主要由雄性偏倚的成年存活率所驱动。进一步分析显示，这种性别偏倚的存活率，源于繁殖后雌性个体存活率的下降。雄性偏倚的成年性比，与婚配群规模缩小、岛上社群数量增加以及幼年雌性繁殖率降低存在显著关联。
5. 本研究结果支持了这一假说：野生有蹄类种群的雄性偏倚成年性比，可能由高种群密度与高繁殖产出共同作用所致。我们推测，雌性偏倚的死亡率，可能源于雌性在高密度种群环境下仍持续繁殖，因此更易受到资源短缺的影响。由此可见，本研究结果凸显了成年性比的强环境依赖性。此外，本研究还表明，成年性比有可能显著改变种群的社会组织结构。这类社会结构的变化，可通过改变社会关系的形成与稳定性，或是配偶竞争模式，对种群动态产生连锁效应。