Data from: Survival of the fattest? Indices of body condition do not predict viability in the brown anole (Anolis sagrei)

1. Measures of body mass and length are commonly used to derive indices of condition, which are often assumed to reflect the energetic state of an animal and, by extension, to predict its fitness. However, the relationship between condition and fitness is rarely quantified, and the appropriate method(s) for deriving indices of condition are frequently debated. 2. Data from a decade of mark-recapture studies involving over 4,600 individual lizards (Anolis sagrei) and 41 replicates of selection across seven populations were used to test the common assumption that condition, as estimated from body mass and length, is a strong predictor of adult survival, an important component of fitness. Inferences about natural selection were compared between two alternative indices of condition: the popular residual index (Ri), and the scaled mass index (Mi), which was recently proposed as a more appropriate method. 3. Linear, quadratic, and correlational selection gradients obtained using Ri and Mi were highly correlated with one another. Relative to variance in selection among replicates and to error associated with the estimation of selection gradients, variance due to the use of alternative condition indices was minor and effectively negligible. 4. Contrary to the intuitive prediction that individuals in better condition should exhibit higher survival, there was no evidence for strong or consistent linear (directional) selection for higher condition indices in either males or females. Significant quadratic (stabilizing or disruptive) selection on condition was similarly rare in both sexes. Correlational selection favored combinations of large size and high condition in males, but not females. 5. Collectively, these results indicate that inferences about natural selection may be robust to the choice between indices of condition, but that indices of condition can be unreliable as proxies for fitness, particularly when relationships between fitness components and condition are contingent upon interactions with other traits.

1. 体重与体长的测量值常被用于构建身体状况指数（condition indices），这类指数通常被认为可反映动物的能量状态，并由此延伸用于预测其适合度（fitness）。然而，身体状况与适合度之间的关联极少被量化，且构建身体状况指数的适宜方法也常引发争议。2. 本研究依托一项长达十年的标记重捕研究（mark-recapture studies）数据，该数据涵盖了4600余只安乐蜥（Anolis sagrei）个体，以及7个种群共41次选择重复实验，用于验证一项普遍假设：基于体重与体长估算的身体状况指数可有效预测成体存活率——而存活率是适合度的重要组成部分。研究对比了两种备选身体状况指数用于自然选择推断的差异：一是应用广泛的残差指数（residual index，Ri），二是近年提出的更具合理性的标准化体重指数（scaled mass index，Mi）。3. 使用Ri与Mi得到的线性、二次及相关选择梯度彼此间均呈现高度相关性。相较于重复实验间的选择方差以及选择梯度估算所伴随的误差，使用两种不同身体状况指数所带来的方差极小，实际上可忽略不计。4. 与“身体状况更佳的个体存活率更高”这一直观预测相悖的是，无论是雄性还是雌性，均未发现存在针对更高身体状况指数的强烈且一致的线性（定向）选择。在两性中，针对身体状况的显著二次（稳定或分裂）选择同样罕见。相关选择在雄性中青睐大体型与高身体状况的组合，但在雌性中并无此现象。5. 综合来看，这些结果表明：针对自然选择的推断或许对身体状况指数的选择具有鲁棒性，但身体状况指数作为适合度的替代指标并不可靠，尤其当适合度组分与身体状况之间的关联取决于与其他性状的互作时。