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）大小与形状不对称现象在鸟类中尤为普遍。早在一个多世纪前，学界就已首次记录到鸟类左右睾丸（testes）间存在显著的大小与形态差异，但目前对于影响不同物种间此类不对称程度与方向变异的驱动因素，仍缺乏足够了解。本研究表明，睾丸大小偏向左侧，是鸟类多数目与科级类群最可能的祖先状态，且睾丸大小不对称程度与形状不对称程度之间存在微弱但显著的负相关关系。在现生物种中，睾丸大小与形状的对称性随精子竞争（sperm competition）程度（以相对睾丸质量（relative testes mass）衡量）的升高而增强，但此类相关性仅在那些具备左侧偏向性的物种中显著。当体腔左侧因大型砂囊（gizzard）而空间尤为受限时，例如食谷动物（granivores）类群，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，但相对睾丸质量与砂囊复杂度，仅能解释具备左侧偏向性物种中睾丸不对称变异的不足10%。据此，本研究的分析表明，另有其他主要因素驱动了鸟类性腺不对称的演化：其一，睾丸大小的左侧偏向性，可能是多数鸟类雌性仅发育左侧单一卵巢的选择压力所带来的相关响应；其二，多数鸟类繁殖季节中睾丸体积急剧快速增长所带来的生理效率优势，可能会倾向于单侧睾丸的增大。本研究结论凸显了重新审视并深化对雄性鸟类性腺投入相关生理过程的理解的必要性。