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）大小与形状的不对称现象在鸟类中尤为普遍。早在一个多世纪前，学界就首次记录到鸟类左右睾丸（testis，复数testes）存在显著的大小与形态差异，但目前对于影响不同鸟类类群此类不对称的程度与方向变异的因素，仍缺乏深入认知。本研究证实，睾丸大小的左侧偏倚是多数鸟类目、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。在现存鸟类类群中，睾丸大小与形状的对称性随精子竞争（sperm competition）程度（相对睾丸质量（relative testes mass））的升高而增强，但该相关性仅在那些自身两项性状均存在左侧偏倚的物种中显著。当鸟类体腔左侧因大型砂囊（gizzard，如食谷动物granivores）而空间极度受限之时，左侧睾丸会更呈细长形态，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存在左侧偏倚的鸟类类群中，不足10%的睾丸不对称变异。因此，本研究分析表明，另有其他因素在鸟类性腺不对称的演化中发挥主导作用：其一，多数雌性鸟类仅左侧卵巢发育，雄性睾丸大小的左侧偏倚可能是对该左侧发育偏倚选择的相关响应；其二，多数鸟类繁殖季睾丸会出现剧烈且快速的体积增长，由此带来的生理效率优势可能会偏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。