Data from: Despite catch-up, prolonged growth has detrimental fitness consequences in a long-lived vertebrate

Individuals experiencing poor growth early in life may later make up their size deficit. Compensatory growth or growth prolongation may lead to such catch-up, involving different life-history tradeoffs under natural conditions. Frequent recaptures and detailed monitoring of animals surviving to asymptotic size are required to compare growth tactics and their fitness consequences. No study to date has obtained such detailed information for wild animals. We used repeated mass measurements (mean 11.6/animal) spanning the lifetime of 104 bighorn ewes (Ovis canadensis) to quantify growth tactics and identify the determinants and life-history costs of these tactics. Growth prolongation, not compensatory growth, led to partial catch-up: mass difference at age 7 was reduced to 4%, for two groups that differed by nearly 20% as yearlings. Ewes that had been light as yearlings prolonged their growth regardless of density or age of primiparity. Growth prolongation did not affect fecundity or longevity. Ewes that experienced poor early growth prolonged growth at the expense of reproductive fitness, weaning a smaller proportion of their lambs. By tracking multi-year growth patterns and comparing events at different life-history stages, we quantified a trade-off between growth and reproduction that would be overlooked if only the adult phenotype was considered. Compensatory growth in long-lived animals appears unlikely when early growth restrictions are mostly density-dependent.

生命早期生长迟缓的个体，后续可弥补其体型缺口。补偿生长（compensatory growth）或生长延长（growth prolongation）可实现此类体型追赶，在自然条件下这涉及不同的生活史权衡（life-history tradeoffs）。为比较生长策略（growth tactics）及其适合度后果（fitness consequences），需要对存活至渐近体型（asymptotic size）的动物进行频繁重捕与详细监测。迄今为止尚无针对野生动物的此类详细研究。我们对104只大角羊母羊（Ovis canadensis）开展贯穿其一生的重复体重测量（平均每只11.6次），以此量化生长策略并明确其决定因素与生活史成本。研究发现，实现部分体型追赶的是生长延长而非补偿生长：在一岁龄时体型差异近20%的两个群组，到7岁时体重差异已缩小至4%。一岁龄体重偏轻的母羊，无论种群密度或初产年龄（age of primiparity）如何，均会延长其生长时长。生长延长并未对繁殖力（fecundity）或寿命（longevity）产生影响。早期生长迟缓的母羊通过牺牲繁殖适合度（reproductive fitness）来延长生长，其断奶的羔羊比例更低。通过追踪多年度生长模式并对比不同生活史阶段的事件，我们量化了生长与繁殖之间的权衡——若仅关注成年表型（adult phenotype），这一权衡将被忽略。当早期生长受限主要由密度依赖性因素导致时，长寿动物的补偿生长似乎不太可能发生。