Columbia University, Department of Ecology, Evolution and Environmental Biology, 10th Floor Schermerhorn Extension, 1200 Amsterdam Avenue, New York, New York 10027, USA.
Nature. 2009 Dec 10;462(7274):786-9. doi: 10.1038/nature08614.
Male animals are typically more elaborately ornamented than females. Classic sexual selection theory notes that because sperm are cheaper to produce than eggs, and because males generally compete more intensely for reproductive opportunities and invest less in parental care than females, males can obtain greater fitness benefits from mating multiply. Therefore, sexual selection typically results in male-biased sex differences in secondary sexual characters. This generality has recently been questioned, because in cooperatively breeding vertebrates, the strength of selection on traits used in intrasexual competition for access to mates (sexual selection) or other resources linked to reproduction (social selection) is similar in males and females. Because selection is acting with comparable intensity in both sexes in cooperatively breeding species, the degree of sexual dimorphism in traits used in intrasexual competition should be reduced in cooperative breeders. Here we use the socially diverse African starlings (Sturnidae) to demonstrate that the degree of sexual dimorphism in plumage and body size is reduced in cooperatively breeding species as a result of increased selection on females for traits that increase access to reproductive opportunities, other resources, or higher social status. In cooperative breeders such as these, where there is unequal sharing of reproduction (reproductive skew) among females, and where female dominance rank influences access to mates and other resources, intrasexual competition among females may be intense and ultimately select for female trait elaboration. Selection is thereby acting with different intensities on males and females in cooperatively versus non-cooperatively breeding species, and female-female interactions in group-living vertebrates will have important consequences for the evolution of female morphological, physiological and behavioural traits.
雄性动物通常比雌性动物更具华丽的装饰。经典的性选择理论指出，由于精子的生产成本比卵子低，而且雄性通常更激烈地争夺繁殖机会，对亲代照顾的投资也比雌性少，因此雄性通过多次交配可以获得更大的适合度收益。因此，性选择通常会导致次要性征在雄性中出现偏性差异。这种普遍性最近受到了质疑，因为在合作繁殖的脊椎动物中，用于竞争获得配偶（性选择）或与繁殖相关的其他资源（社会选择）的特征的选择强度在雄性和雌性中相似。由于选择在合作繁殖物种中的两性都以相似的强度进行，因此用于种内竞争的特征的性二态性程度应该在合作繁殖者中降低。在这里，我们使用具有社会性差异的非洲椋鸟（椋鸟科）来证明，由于对雌性特征的选择增加，这些特征可以增加繁殖机会、其他资源或更高的社会地位，因此在合作繁殖的物种中，羽毛和体型的性二态性程度降低。在这些合作繁殖者中，雌性之间存在繁殖的不平等分配（繁殖偏斜），并且雌性的支配等级影响着获得配偶和其他资源的机会，因此雌性之间的种内竞争可能很激烈，并最终选择雌性特征的复杂化。因此，在合作繁殖和非合作繁殖物种中，选择对雄性和雌性的作用强度不同，并且群居脊椎动物中的雌性-雌性相互作用将对雌性形态、生理和行为特征的进化产生重要影响。
