Data from: Environmental and evolutionary effects on horn growth of male bighorn sheep

The development of male secondary sexual characters such as antlers or horns has substantial biological and socio-economic importance because in many species these traits affect male fitness positively through sexual selection and negatively through trophy hunting. Both environmental conditions and selective hunting can affect horn growth but their relative importance remains unexplored. We first examined how a large-scale climate index, the Pacific Decadal Oscillation (PDO), local weather and population density influenced both absolute and relative annual horn growth from birth to 3 years of male bighorn sheep (Ovis canadensis) over 42 years. We then examined the relative influence of environmental conditions and evolution mainly driven by trophy hunting on male horn length at 3 years of age. Horn growth was positively influenced by low population density and warm spring temperature, suggesting that ongoing climate change should lead to larger horns. Seasonal values of PDO were highly correlated. Horn growth increased with PDO in spring or summer at low density, but was weak at high density regardless of PDO. The interaction between population density and PDO in spring or summer accounted for a similar proportion of the observed annual variation in horn growth (32% or 37%) as did the additive effects of spring temperature and density (34%). When environmental conditions deteriorated, males allocated relatively more resources to summer mass gain than to horn growth, suggesting a conservative strategy favoring maintenance of condition over allocation to secondary sexual characters. Population density explained 27% of the variation in horn length, while evolutionary effects explained 9% of the variance. Thus, our study underlines the importance of both evolution and phenotypic plasticity on the development of a secondary sexual trait.雄性第二性征（secondary sexual characters）如鹿角或牛角的发育，具有显著的生物学与社会经济价值：在诸多物种中，这类性状通过性选择对雄性适合度（male fitness）产生正向影响，却又因战利品狩猎（trophy hunting）对其产生负向影响。环境条件与选择性狩猎均可影响牛角生长，但二者的相对重要性尚未得到系统探究。本研究首先针对42年研究周期内、出生至3岁龄的雄性大角羊（Ovis canadensis），探究了大型气候指数——太平洋年代际振荡（Pacific Decadal Oscillation, PDO）、局地天气与种群密度，如何影响其从出生到3岁期间的绝对与相对年度牛角生长量。随后，本研究进一步分析了环境条件与以战利品狩猎为主导的演化过程，对3岁龄雄性大角牛角长的相对影响。研究结果显示，牛角生长量随种群密度降低与春季气温升高呈正向增长，这表明持续的气候变化将促使牛角尺寸增大。各季节的PDO指数高度相关：在低密度种群中，春季或夏季的PDO指数升高会促进牛角生长；但在高密度种群中，无论PDO指数如何变化，牛角生长均无明显响应。春季或夏季的种群密度与PDO的交互作用，可解释32%或37%的观测到的牛角生长年度变异量，这一比例与春季气温和种群密度的加性效应（34%）相近。当环境条件恶化时，雄性大角羊会将更多资源分配至夏季体重增长，而非牛角生长，这表明其采取了保守策略：优先维持自身身体状况，而非将资源投入第二性征发育。种群密度可解释27%的角长变异量，而演化效应仅解释了9%的方差。综上，本研究强调了演化与表型可塑性（phenotypic plasticity）在第二性征发育中的共同重要性。