Data from: Testis asymmetry in birds: the influences of sexual and natural selection

Gonad size and shape asymmetries are particularly common in birds. Although some obvious size and shape differences between the left and right testes in birds were first documented more than a century ago, little is known about what influences the variation across species in either the degree or the direction of these asymmetries. Here we show that a left bias in size is the most likely ancestral state in most orders and families, and that there is a weak but significant negative relation between the degree of size and shape asymmetries. In extant species, testis size and shape symmetries increase with the degree of sperm competition (relative testes mass), but those relations are significant only in those species with left bias in each of these traits. When space is particularly constrained on the left side of the body cavity due to a large gizzard (e.g. in granivores), the left testis is more elongated and the degree of size symmetry is larger. Despite significant patterns, relative testes mass and gizzard complexity together explain < 10% of the variation in testis asymmetry in species with left biases. Thus our analyses suggest that some other factor is largely responsible for the evolution of gonad asymmetry in birds: 1) that a left bias in testis size might be a correlated response to selection for a left bias in the development of a single (left) ovary in females of most bird species, and/or 2) that physiological efficiency due to the dramatic and rapid increase in testis size of most species during the breeding season might favour enlargement of one testis. Our conclusions highlight the need to rethink and improve our understanding of the physiological processes underlying the investment in gonads by male birds.

性腺（gonad）大小与形态不对称现象在鸟类中尤为普遍。尽管一个多世纪前就已有文献记载了鸟类左右睾丸（testis）间存在显著的大小与形态差异，但目前学界对调控各物种种群性腺不对称的程度与方向变异的关键因素仍知之甚少。本研究证实，多数目与科的鸟类中，睾丸大小呈现左偏侧化极可能是其祖先状态，且大小不对称程度与形态不对称程度间存在微弱但显著的负相关关系。在现生鸟类中，睾丸大小与形态对称性随精子竞争（sperm competition）程度的升高而增强，该程度常以相对睾丸质量（relative testes mass）衡量，但该相关性仅在该两类性状均存在左偏侧化的物种中显著。当体腔左侧因硕大的砂囊（gizzard）而空间受限（如食谷类（granivores）物种）时，左侧睾丸会更趋细长，且大小对称性程度更高。尽管存在上述显著关联模式，但在存在左偏侧化的物种中，相对睾丸质量与砂囊复杂度二者共同仅能解释睾丸不对称变异的不足10%。据此，本研究分析提示，鸟类性腺不对称的演化主要由其他未被纳入考量的因素主导：1）多数鸟类雌性仅发育左侧单一卵巢（ovary），雄性睾丸大小的左偏侧化可能是对雌性左侧卵巢发育偏左侧化选择的相关响应；或2）多数鸟类在繁殖季期间睾丸体积会出现剧烈且快速的增长，由此带来的生理效率优势可能会倾向于促使单侧睾丸增大。本研究结论凸显了重新审视并深化学界对雄性鸟类性腺投入相关生理机制的理解的必要性。