Burley Nancy Tyler, Foster Valerie Suzanne
Department of Ecology and Evolutionary Biology, University of California, Irvine 92697-2525, USA.
Proc Biol Sci. 2004 Feb 7;271(1536):239-44. doi: 10.1098/rspb.2003.2562.
The steroid environment encountered by developing vertebrates has important organizational effects on physiology and behaviour that persist throughout an organism's lifetime. Optimal allocation of maternal steroids to zygotes may be difficult to achieve because of the sexually antagonistic effects of steroids; thus, for example, a hormone environment beneficial to a developing male may be much less beneficial to a developing female. Research into the important topic of how mothers might adaptively adjust steroid titres experienced by particular young has been constrained by the difficulty of measuring the steroid environment experienced by the embryo at critical times in development. A potential approach to this problem has been suggested by research on variation in digit ratios in humans, where the ratio of the length of the second and fourth digits reflects the steroid environment experienced by the foetus; notably, digit 4 lengthens in response to androgens. In light of the conservative nature of homeobox genes regulating early development in tetrapods, we questioned whether a sex difference in digit ratio exists in a passerine bird, the zebra finch, Taeniopygia guttata castanotis, and whether observed variation in the ratio is consistent with the previously reported pattern that androgen allocation to zebra finch egg yolk declines across laying order. We established an aviary population of outbred, wild-type zebra finches, and allowed them to breed freely. Hatchlings were marked to correspond to their egg order, and their digit ratios were measured after birds reached adulthood. We found that digit ratio increased across egg order, which is consistent with a pattern of decreasing androgen allocation. Moreover, digit ratios differed between the sexes. We also investigated whether variation in digit ratio among adult females predicted variation in their performance in mate-choice tests. Digit ratio accounted for almost 50% of the variance in strength of female preference for an attractive male trait: specifically, females with higher (presumably less 'androgenized') ratios had stronger preferences for attractive males. Digit ratio may prove to be an extremely useful tool for addressing a wide range of questions about vertebrate differentiation and behaviour.
发育中的脊椎动物所接触的类固醇环境，对其生理和行为具有重要的组织效应，且这种效应会贯穿生物体的一生。由于类固醇具有性别拮抗效应，所以母体类固醇向受精卵的最佳分配可能难以实现；例如，对发育中的雄性有益的激素环境，对发育中的雌性可能益处要少得多。关于母亲如何适应性地调整特定幼崽所经历的类固醇水平这一重要课题的研究，一直受到在发育关键时期测量胚胎所经历的类固醇环境的困难的限制。对人类手指比例变化的研究提出了一种解决该问题的潜在方法，其中第二和第四指长度的比例反映了胎儿所经历的类固醇环境；值得注意的是，第四指会因雄激素而变长。鉴于调控四足动物早期发育的同源框基因具有保守性，我们质疑在雀形目鸟类斑胸草雀（Taeniopygia guttata castanotis）中是否存在手指比例的性别差异，以及观察到的比例变化是否与先前报道的斑胸草雀卵黄雄激素分配随产卵顺序下降的模式一致。我们建立了一个由远交野生型斑胸草雀组成的鸟舍种群，并让它们自由繁殖。对雏鸟进行标记以对应其卵的顺序，并在鸟类成年后测量它们的手指比例。我们发现手指比例随卵的顺序增加，这与雄激素分配减少的模式一致。此外，两性之间的手指比例存在差异。我们还研究了成年雌性之间手指比例的差异是否能预测她们在配偶选择测试中的表现差异。手指比例几乎占了雌性对有吸引力的雄性特征偏好强度方差的50%：具体来说，比例较高（可能雄激素化程度较低）的雌性对有吸引力的雄性有更强的偏好。手指比例可能被证明是解决关于脊椎动物分化和行为的广泛问题的极其有用的工具。
