Effects of population density on static allometry between horn length and body mass in mountain ungulates

Little is known about the effects of environmental variation on allometric relationships of condition-dependent traits, especially in wild populations. We estimated sex-specific static allometry between horn length and body mass in four populations of mountain ungulates that experienced periods of contrasting density over the course of the study. These species displayed contrasting sexual dimorphism in horn size; high dimorphism in Capra ibex and Ovis canadensis and low dimorphism in Rupicapra rupicapra and Oreamnos americanus. The effects of density on static allometric slopes were weak and inconsistent while allometric intercepts were generally lower at high density, especially in males from species with high sexual dimorphism in horn length. These results confirm that static allometric slopes are more canalized than allometric intercepts against environmental variation induced by changes in population density, particularly when traits appear more costly to produce and maintain.

Methods
The methods to collect the data are described in detail in the supporting information of the paper.

目前学界对环境变异如何影响条件依赖性状（condition-dependent traits）的异速生长（allometry）关系的认知仍较为有限，在野生种群中更是如此。本研究针对4个种群的山地有蹄类，估算了其角长与体质量间的性别特异性静态异速生长（static allometry）关系；这些种群在研究周期内经历了种群密度差异显著的不同阶段。这些物种在角大小上的性二态性（sexual dimorphism）存在显著差异：羱羊（Capra ibex）和加拿大盘羊（Ovis canadensis）的性二态性较强，而岩羚羊（Rupicapra rupicapra）和美洲雪羊（Oreamnos americanus）的性二态性较弱。种群密度对静态异速生长斜率的影响微弱且缺乏一致性；而在高密度条件下，异速生长截距普遍更低，这一现象在角长性二态性较强的物种的雄性个体中尤为显著。本研究结果证实，相较于异速生长截距，静态异速生长斜率对种群密度变化所引发的环境变异具有更强的发育缓冲（canalization）能力，这在性状生产与维持成本更高的情况下尤为明显。

研究方法
本研究的数据收集方法已在论文的补充材料中进行了详细阐述。