Martin Thomas E, Bassar R D, Bassar S K, Fontaine J J, Lloyd P, Mathewson H A, Niklison A M, Chalfoun A
U.S. Geological Survey, Montana Cooperative Wildlife Research Unit, University of Montana, Missoula, Montana 59812, USA.
Evolution. 2006 Feb;60(2):390-8.
Broad geographic patterns in egg and clutch mass are poorly described, and potential causes of variation remain largely unexamined. We describe interspecific variation in avian egg and clutch mass within and among diverse geographic regions and explore hypotheses related to allometry, clutch size, nest predation, adult mortality, and parental care as correlates and possible explanations of variation. We studied 74 species of Passeriformes at four latitudes on three continents: the north temperate United States, tropical Venezuela, subtropical Argentina, and south temperate South Africa. Egg and clutch mass increased with adult body mass in all locations, but differed among locations for the same body mass, demonstrating that egg and clutch mass have evolved to some extent independent of body mass among regions. A major portion of egg mass variation was explained by an inverse relationship with clutch size within and among regions, as predicted by life-history theory. However, clutch size did not explain all geographic differences in egg mass; eggs were smallest in South Africa despite small clutch sizes. These small eggs might be explained by high nest predation rates in South Africa; life-history theory predicts reduced reproductive effort under high risk of offspring mortality. This prediction was supported for clutch mass, which was inversely related to nest predation but not for egg mass. Nevertheless, clutch mass variation was not fully explained by nest predation, possibly reflecting interacting effects of adult mortality. Tests of the possible effects of nest predation on egg mass were compromised by limited power and by counterposing direct and indirect effects. Finally, components of parental investment, defined as effort per offspring, might be expected to positively coevolve. Indeed, egg mass, but not clutch mass, was greater in species that shared incubation by males and females compared with species in which only females incubate eggs. However, egg and clutch mass were not related to effort of parental care as measured by incubation attentiveness. Ecological and life-history correlates of egg and clutch mass variation found here follow from theory, but possible evolutionary causes deserve further study.
卵重和窝卵数的广泛地理模式描述甚少，其变异的潜在原因在很大程度上仍未得到研究。我们描述了不同地理区域内和区域间鸟类卵重和窝卵数的种间变异，并探讨了与异速生长、窝卵数、巢捕食、成体死亡率和亲代抚育相关的假说，将其作为变异的相关因素和可能的解释。我们在三大洲的四个纬度研究了74种雀形目鸟类：北温带的美国、热带的委内瑞拉、亚热带的阿根廷和南温带的南非。在所有地点，卵重和窝卵数都随成体体重增加，但相同体重的鸟类在不同地点存在差异，这表明卵重和窝卵数在一定程度上已在各区域独立于体重而进化。正如生活史理论所预测的，卵重变异的很大一部分可由区域内和区域间与窝卵数的反比关系来解释。然而，窝卵数并不能解释卵重的所有地理差异；尽管南非的窝卵数较小，但其卵却是最小的。南非这些小卵可能是由于巢捕食率高所致；生活史理论预测，在后代死亡风险高的情况下，繁殖投入会减少。这一预测在窝卵数上得到了支持，窝卵数与巢捕食呈反比，但在卵重上却未得到支持。尽管如此，窝卵数的变异并未完全由巢捕食来解释，这可能反映了成体死亡率的相互作用影响。由于检验力有限以及直接和间接效应相互抵消，巢捕食对卵重可能影响的测试受到了影响。最后，亲代投资的组成部分，定义为每个后代的投入，可能会预期正向协同进化。事实上，与仅由雌性孵卵的物种相比，雌雄共同孵卵的物种卵重更大，但窝卵数并非如此。然而，卵重和窝卵数与以孵卵专注度衡量的亲代抚育投入并无关联。此处发现的卵重和窝卵数变异的生态和生活史相关因素符合理论，但可能的进化原因值得进一步研究。
