Faculty of Environmental Sciences and Natural Resource Management, Norwegian University of Life Sciences, Ås, Norway.
The James Hutton Institute, Aberdeen, UK.
J Anim Ecol. 2022 Jan;91(1):61-73. doi: 10.1111/1365-2656.13593. Epub 2021 Oct 3.
The cost of reproduction on demographic rates is often assumed to operate through changing body condition. Several studies have found that reproduction depresses body mass more if the current conditions are severe, such as high population densities or adverse weather, than under benign environmental conditions. However, few studies have investigated the association between the fitness components and body mass costs of reproduction. Using 25 years of individual-based capture-recapture data from Svalbard reindeer Rangifer tarandus platyrhynchus, we built a novel Bayesian state-space model that jointly estimated interannual change in mass, annual reproductive success and survival, while accounting for incomplete observations. The model allowed us to partition the differential effects of intrinsic and extrinsic factors on both non-reproductive mass change and the body mass cost of reproduction, and to quantify their consequences on demographic rates. Contrary to our expectation, the body mass cost of reproduction (mean = -5.8 kg) varied little between years (CV = 0.08), whereas the between-year variation in body mass changes, that were independent of the previous year's reproductive state, varied substantially (CV = 0.4) in relation to autumn temperature and the amount of rain-on-snow in winter. This body mass loss led to a cost of reproduction on the next reproduction, which was amplified by the same environmental covariates, from a 10% reduction in reproductive success in benign years, to a 50% reduction in harsh years. The reproductive mass loss also resulted in a small reduction in survival. Our results show how demographic costs of reproduction, driven by interannual fluctuations in individual body condition, result from the balance between body mass costs of reproduction and body mass changes that are independent of previous reproductive state. We illustrate how a strong context-dependent fitness cost of reproduction can occur, despite a relatively fixed body mass cost of reproduction. This suggests that female reindeer display a very conservative energy allocation strategy, either aborting their reproductive attempt at an early stage or weaning at a relatively constant cost. Such a strategy might be common in species living in a highly stochastic and food limited environment.
繁殖对种群增长率的成本通常被认为是通过改变身体状况来发挥作用的。有几项研究发现，如果当前条件恶劣，如种群密度高或天气恶劣，繁殖会比在良性环境条件下更能降低体重。然而，很少有研究调查繁殖的适应性成分和体重成本之间的关联。本研究利用斯瓦尔巴驯鹿 Rangifer tarandus platyrhynchus 25 年的个体基础捕获-再捕获数据，构建了一个新颖的贝叶斯状态空间模型，该模型联合估计了体重的年际变化、年度繁殖成功率和存活率，同时考虑了不完全观测的情况。该模型使我们能够将内在和外在因素对非繁殖体重变化和繁殖体重成本的差异影响进行区分，并量化它们对种群增长率的影响。与我们的预期相反，繁殖的体重成本（均值=-5.8kg）在年份之间变化很小（变异系数=0.08），而与前一年繁殖状态无关的体重变化的年际变化则与秋季温度和冬季的融雪量有很大的不同（变异系数=0.4）。这种体重下降导致下一次繁殖的繁殖成本增加，而且同样的环境协变量也会放大这种成本，从良性年份繁殖成功率降低 10%，到恶劣年份降低 50%。繁殖的体重损失也导致了生存的小幅度降低。本研究结果表明，繁殖的种群增长率成本是如何由个体身体状况的年际波动驱动的，这是繁殖的体重成本和与前一年繁殖状态无关的体重变化之间的平衡所导致的。我们说明了尽管繁殖的体重成本相对固定，但如何会产生强烈的依赖于环境的繁殖适应性成本。这表明驯鹿表现出非常保守的能量分配策略，要么在早期就放弃繁殖尝试，要么以相对固定的成本进行断奶。这种策略可能在生活在高度随机和食物有限的环境中的物种中很常见。
