Data from: Maternal investment increases with altitude in a frog on the Tibetan plateau

Reproducing females can allocate energy between the production of eggs or offspring of different size or number, both of which can strongly influence fitness. The physical capacity to store developing offspring imposes constraints on maximum clutch volume, but individual females and populations can trade off whether more or fewer eggs or offspring are produced, and their relative sizes. Harsh environments are likely to select for larger egg or offspring size, and many vertebrate populations compensate for this reproductive investment through an increase in female body size. We report a different trade-off in a frog endemic to the Tibetan Plateau, Rana kukunoris. Females living at higher altitudes (n = 11 populations, 2000–3500 m) produce larger eggs, but without a concomitant increase in female body size or clutch size. The reduced diel and seasonal activity at high altitudes may impose constraints on the maximum body size of adult frogs, by limiting the opportunity for energy accumulation. Simultaneously, producing larger eggs likely helps to increase the rate of embryonic development, causing tadpoles to hatch earlier. The gelatinous matrix surrounding eggs, more of which is produced by large females, may help buffer developing embryos from temperature fluctuations or offer protection from ultraviolet radiation. High-altitude frogs on the Tibetan Plateau employ a reproductive strategy that favours large egg size independent of body size, which is unusual in amphibians. The harsh and unpredictable environmental conditions at high altitudes can thus impose strong and opposing selection pressures on adult and embryonic life stages, both of which can simultaneously influence fitness.

可繁殖的雌性个体能够在不同大小或数量的卵或子代的产出之间分配能量，二者均会对适合度产生显著影响。储存发育中子代的生理能力会对最大卵团体积施加限制，但雌性个体与种群可在产出更多或更少的卵/子代，以及它们的相对大小之间进行权衡。恶劣环境通常会倾向于选择更大的卵或子代体型，而许多脊椎动物种群会通过增大雌性体型来弥补这一繁殖投入。我们在青藏高原特有蛙类——高原林蛙（Rana kukunoris）中报道了一种不同的权衡策略。栖息于高海拔区域（n=11个种群，海拔2000~3500米）的雌性个体产出的卵更大，但雌性体型或产卵量并未随之增加。高海拔环境下昼夜与季节活动时长的缩减，会通过限制能量积累的机会，对成年蛙的最大体型施加限制。与此同时，产出更大的卵可能有助于提升胚胎发育速率，使蝌蚪更早孵化。大型雌性个体可分泌更多的卵周胶状基质，该结构能够帮助发育中的胚胎抵御温度波动，或免受紫外线辐射的伤害。青藏高原的高海拔蛙类采用了一种与体型无关的、偏向大卵尺寸的繁殖策略，这在两栖动物中颇为罕见。因此，高海拔地区严酷且不可预测的环境条件，会对成年个体与胚胎这两个生活史阶段同时施加强烈且方向相反的选择压力，而二者均会同时影响适合度。