Rosenfield Robert N, Stout William E, Giovanni Matthew D, Levine Noah H, Cava Jenna A, Hardin Madeline G, Haynes Taylor G
Department of Biology University of Wisconsin Stevens Point Wisconsin 54481.
W2364 Heather Street Oconomowoc Wisconsin 53066.
Ecol Evol. 2015 Aug 28;5(18):4037-48. doi: 10.1002/ece3.1674. eCollection 2015 Sep.
Offspring sex ratios at the termination of parental care should theoretically be skewed toward the less expensive sex, which in most avian species would be females, the smaller gender. Among birds, however, raptors offer an unusual dynamic because they exhibit reversed size dimorphism with females being larger than males. And thus theory would predict a preponderance of male offspring. Results for raptors and birds in general have been varied although population-level estimates of sex ratios in avian offspring are generally at unity. Adaptive adjustment of sex ratios in avian offspring is difficult to predict perhaps in part due to a lack of life-history details and short-term investigations that cannot account for precision or repeatability of sex ratios across time. We conducted a novel comparative study of sex ratios in nestling Cooper's hawks (Accipiter cooperii) in two study populations across breeding generations during 11 years in Wisconsin, 2001-2011. One breeding population recently colonized metropolitan Milwaukee and exhibited rapidly increasing population growth, while the ex-Milwaukee breeding population was stable. Following life-history trade-off theory and our prediction regarding this socially monogamous species in which reversed sexual size dimorphism is extreme, first-time breeding one-year-old, second-year females in both study populations produced a preponderance of the smaller and cheaper sex, males, whereas ASY (after-second-year), ≥2-year-old females in Milwaukee produced a nestling sex ratio near unity and predictably therefore a greater proportion of females compared to ASY females in ex-Milwaukee who produced a preponderance of males. Adjustment of sex ratios in both study populations occurred at conception. Life histories and selective pressures related to breeding population trajectory in two age cohorts of nesting female Cooper's hawk likely vary, and it is possible that these differences influenced the sex ratios we documented for two age cohorts of female Cooper's hawks in Wisconsin.
从理论上讲，在亲代抚育结束时，后代的性别比例应该偏向于成本较低的性别，在大多数鸟类中，成本较低的性别是雌性，即体型较小的性别。然而，在鸟类中，猛禽呈现出一种不同寻常的情况，因为它们表现出相反的体型二态性，雌性比雄性大。因此，理论预测雄性后代会占优势。猛禽和一般鸟类的结果各不相同，尽管鸟类后代性别比例的种群水平估计通常为1:1。鸟类后代性别比例的适应性调整很难预测，部分原因可能是缺乏生活史细节以及短期调查无法解释性别比例随时间的精确性或可重复性。我们在威斯康星州进行了一项新颖的比较研究，研究对象是2001年至2011年期间两个繁殖世代的两个研究种群中巢内的库珀鹰（Accipiter cooperii）的性别比例。一个繁殖种群最近在密尔沃基市区定居，种群数量迅速增长，而原密尔沃基繁殖种群则保持稳定。根据生活史权衡理论以及我们对这种社会一夫一妻制物种的预测，其中反向性体型二态性极为明显，两个研究种群中首次繁殖的一岁、两岁雌性产生的后代中，体型较小且成本较低的雄性占优势，而密尔沃基的成年后（ASY，即两岁以后）、≥2岁的雌性产生的雏鸟性别比例接近1:1，因此可以预测，与原密尔沃基产生大量雄性后代的ASY雌性相比，密尔沃基的这些雌性产生的雌性比例更高。两个研究种群的性别比例调整都发生在受孕时。筑巢雌性库珀鹰两个年龄组的生活史和与繁殖种群轨迹相关的选择压力可能不同，这些差异有可能影响了我们记录的威斯康星州两个年龄组雌性库珀鹰的性别比例。
