Data from: Elevation and latitude interact to drive life history variation in precocial birds: a comparative analysis using Galliformes

1. Elevational gradients provide a powerful laboratory for understanding the environmental and ecological drivers of geographic variation in avian life history strategies. Environmental variation across elevational gradients is hypothesized to select for a tradeoff of reduced fecundity (lower clutch size and/or fewer broods) for higher offspring quality (larger eggs and/or increased parental care) in higher elevation species and populations. In birds, a focus on altricial species from north-temperate latitudes has prevented an evaluation of the generality of this tradeoff, and how it is affected by latitude and intrinsic factors (development mode). 2. We performed a comparative analysis controlling for body size and phylogenetic relationships on a global dataset of 135 galliform species to test (i) whether higher elevation precocial species have lower fecundity (smaller clutch and/or fewer broods) and invest more in offspring quality (greater egg mass) and (ii) whether latitude influences the traits involved and/or the trade-off, and (iii) to identify ecological and environmental drivers of life history variation along elevational gradients. 3. Life history traits showed significant interaction effects across elevation and latitude: temperate higher elevation species had smaller clutches and clutch mass, larger eggs, and shorter incubation periods, whereas more tropical species had larger clutches, eggs, and clutch mass, and longer incubation periods as elevation increased. Number of broods and body mass did not vary with elevation or latitude. Latitudinal gradient in clutch size was observed only for low elevation species. 4. Significantly, an overlooked latitude-by-elevation interaction confounds our traditional view of clutch size variation across a tropical-to-temperate gradient. Across all latitudes, higher elevation species invested in offspring quality via larger eggs but support for reduced fecundity resulting from smaller clutches was found only along temperate elevational gradients; contrary to expectations, tropical high elevation species showed increased fecundity. Variation in nest predation risk could explain differences between temperate and tropical elevational gradients, but we lack a consistent mechanism to explain why predation risk should vary in this manner. Alternatively, a resource-availability hypothesis based on physical attributes that globally differ between elevation and latitude (seasonality in day-length and temperature) seems more plausible.

1. 海拔梯度为探究鸟类生活史策略地理变异的环境与生态驱动因子提供了绝佳的研究平台。学界假说认为，海拔梯度间的环境差异会对高海拔鸟类类群及其种群产生选择压力，使其在繁殖力（较低的窝卵数和/或更少的繁殖窝数）与后代质量（更大的卵重和/或更高的双亲抚育投入）之间形成权衡。以往针对北温带纬度地区晚成鸟的研究局限，阻碍了对该权衡普遍性及其受纬度与内在因素（发育模式）影响机制的评估。
2. 本研究基于包含135个鸡形目（Galliformes）物种的全球数据集，在控制体型与系统发育关系的前提下开展比较分析，旨在验证三个科学问题：（1）高海拔早成鸟是否具有更低的繁殖力（更小的窝卵数和/或更少的繁殖窝数）且对后代质量的投入更高（更大的卵重）；（2）纬度是否会影响相关生活史性状乃至该权衡关系；（3）厘清海拔梯度上鸟类生活史变异的生态与环境驱动因子。
3. 研究结果显示，鸟类生活史性状存在海拔与纬度的显著交互效应：温带高海拔类群的窝卵数与窝卵总重更低、卵更大且孵化期更短；而随着海拔升高，热带类群的窝卵数、卵重与窝卵总重均更高，且孵化期更长。繁殖窝数与体型并未随海拔或纬度发生显著变化。仅在低海拔类群中观察到窝卵数的纬度梯度变异。
4. 值得注意的是，此前被忽视的纬度-海拔交互效应，颠覆了我们对热带至温带梯度上窝卵数变异的传统认知。在所有纬度区间内，高海拔类群均通过产出更大的卵来提升后代质量投入，但仅在温带海拔梯度上观察到"窝卵数更小对应繁殖力降低"的支持证据；与预期相悖的是，热带高海拔类群的繁殖力反而更高。巢捕食风险的差异或许可以解释温带与热带海拔梯度间的性状差异，但目前尚缺乏一致的机制来阐释捕食风险为何会呈现此种变化规律。相较而言，基于海拔与纬度间全球尺度物理差异（光周期与温度的季节性变化）的资源可用性假说，似乎更为合理。