Lipar J L, Ketterson E D
Department of Biology and Center for the Integrative Study of Animal Behavior, Indiana University, Bloomington 47405, USA.
Proc Biol Sci. 2000 Oct 7;267(1456):2005-10. doi: 10.1098/rspb.2000.1242.
Hatching asynchrony in avian species often leads to the formation of a size hierarchy that places last-hatched nestlings at a significant disadvantage. The hatching muscle (musculus complexus) is responsible for breaking the shell during hatching and for dorsal flexion of the neck during begging. An increase in its strength in last-hatched nestlings could mitigate the effects of hatching asynchrony by reducing the time required for hatching or enhancing the effectiveness of begging for parentally delivered food or both. We have previously found that yolk testosterone concentration increases with laying order in the red-winged blackbird Agelaius phoeniceus. In this study, we investigated the hypothesis that yolk testosterone has anabolic effects on the development of the complexus, thereby influencing competition among asynchronously hatched nestlings. We found that both yolk testosterone concentration and relative complexus mass (complexus mass/nestling body mass) increased with laying order and that these two variables were positively correlated in both newly hatched nestlings and in two-day-old broods. Moreover, direct injections of testosterone into egg yolks resulted in an increase in relative complexus mass, while injections of flutamide, a testosterone antagonist, resulted in a decrease in relative complexus mass. Neither yolk testosterone concentration nor relative complexus mass differed between male and female nestlings.
鸟类物种中的孵化异步现象常常导致形成一种体型等级制度，使得最后孵化出的雏鸟处于显著劣势。孵化肌（复合肌）负责在孵化过程中破壳以及在乞食时使颈部背屈。最后孵化出的雏鸟的孵化肌力量增强，可通过减少孵化所需时间或提高乞食获取亲代提供食物的效率，或两者兼而有之，来减轻孵化异步的影响。我们之前发现，在红翅黑鹂（Agelaius phoeniceus）中，卵黄睾酮浓度随产卵顺序增加。在本研究中，我们调查了这样一个假说：卵黄睾酮对复合肌的发育具有合成代谢作用，从而影响异步孵化雏鸟之间的竞争。我们发现，卵黄睾酮浓度和相对复合肌质量（复合肌质量/雏鸟体重）均随产卵顺序增加，并且这两个变量在新孵化的雏鸟和两日龄雏鸟中均呈正相关。此外，直接向卵黄中注射睾酮导致相对复合肌质量增加，而注射睾酮拮抗剂氟他胺则导致相对复合肌质量减少。雄性和雌性雏鸟的卵黄睾酮浓度和相对复合肌质量均无差异。
