Sex-specific patterns of aging on sexual ornaments and gametes

Sex differences in age-dependent mortality and reproductive success are predicted to drive the evolution of sexually dimorphic patterns of reproductive investment over life. However, this prediction has not been fully explored because it is difficult to measure primary and secondary sexual traits over the life spans of males and females. Here we studied a population of fowl, Gallus gallus, to gain longitudinal data on a sexual ornament (the comb), quantity of gametes produced, and gamete quality (sperm velocity and egg mass) of males and females. Our results reveal pronounced differences between the sexes in age-specific patterns of reproductive investment. In males, comb size decreased linearly with age, high sperm quality early in life was associated with reduced sperm quality late in life, and high sperm production was related to early death. In contrast, female comb size and egg mass were maximized at intermediate ages, and fecundity was independent of life span. Finally, the way traits were related in males did not change over life, whereas in females the association between fecundity and comb size changed from positive to negative over the lifetime of a female, indicating that aging may lead to trade-offs in investment between traits in females. These results show that males and females differ in reproductive investment with age, in terms of both the expression of individual traits and their phenotypic covariance.

年龄依赖性死亡率与繁殖成功率的性别差异，被预测会推动生命进程中性二态性繁殖投入模式的演化。然而，由于难以在雌雄个体的完整生命周期内测量第一与第二性征，这一预测尚未得到充分探索。本研究以原鸡（Gallus gallus）种群为研究对象，获取了雌雄个体的性装饰器官（鸡冠）、配子生成量以及配子质量（精子速度与卵重）的纵向追踪数据。研究结果揭示了两性在年龄特异性繁殖投入模式上的显著差异。在雄性个体中，鸡冠尺寸随年龄呈线性减小；生命早期较高的精子质量与生命晚期较低的精子质量存在关联，且高精子生成量与早亡相关。与之相反，雌性的鸡冠尺寸与卵重在中年阶段达到峰值，且繁殖力与寿命并无关联。最后，雄性个体的性状关联模式在整个生命历程中未发生改变；而雌性个体的繁殖力与鸡冠尺寸之间的关联，则随其生命进程由正相关转为负相关，这表明衰老可能会导致雌性个体不同性状间的繁殖投入权衡。上述结果表明，无论从单一性状的表达还是表型协方差来看，雌雄个体随年龄变化的繁殖投入模式均存在差异。