How to estimate body condition in large lizards? Argentine black and white tegu (Salvator merianae, Duméril and Bibron, 1839) as a case study

Body condition is a measure of the health and fitness of an organism
represented by available energy stores, typically fat. Direct measurements
of fat are difficult to obtain non-invasively, thus body condition is
usually estimated by calculating body condition indices (BCIs) using mass
and length. The utility of BCIs is contingent on the relationship of BCIs
and fat, thereby validation studies should be performed to select the
best-performing BCI before application in ecological investigations. We
evaluated 11 BCIs in 883 Argentine black and white tegus (Salvator
merianae) removed from their non-native range in South Florida, United
States. Because the length-mass relationship in tegus is allometric, a
segmented linear regression model was fit to the relationship between mass
and length to define size classes. We evaluated percent, residual, and
scaled fat and determined percent fat was the best measure of fat because
it was the least associated with snout-vent length (SVL). We evaluated
performance of BCIs with the full dataset and within size classes and
identified Fulton’s K as the best-performing BCI for our sampled
population, explaining up to 19% of the variation in fat content. Overall,
we found that BCIs: 1) maintained relatively weak relationships with
measures of fat and 2) splitting data into size classes reduced the
strength of the relationship (i.e., bias) between percent fat and SVL but
did not improve the performance of BCIs. We postulate that the weak
performance of BCIs in our dataset was likely due to the weak association
of fat with SVL, the body plan and life-history traits of tegus, and
potentially inadequate accounting of available energy resources. We
caution against assuming that BCIs are strong indicators of body condition
across species and suggest that validation studies be implemented, or that
alternative or complementary measures of health or fitness should be
considered.

身体状况（body condition）是衡量生物体健康与体能状态的指标，通常以可利用的能量储备——尤以脂肪为主——作为表征。脂肪的无创直接测量难度较大，因此研究人员通常通过体重与体长数据计算身体状况指数（body condition indices, BCIs）来估算机体的身体状况。身体状况指数的应用效用取决于其与脂肪含量的相关性，因此在开展生态学研究前，需先通过验证性研究筛选出表现最优的身体状况指数，方可投入使用。本研究针对883只采自美国南佛罗里达非原生分布区的阿根廷黑白泰加蜥（Salvator merianae），对11种身体状况指数进行了评估。鉴于泰加蜥的体长-体重关系呈异速生长模式，本研究通过分段线性回归模型拟合体重与体长的关联，以此划定体型等级。本研究对脂肪百分比、残差脂肪、标准化脂肪三种脂肪表征方式进行了评估，最终确定脂肪百分比为最优脂肪测量指标，因其与吻肛长（snout-vent length, SVL）的相关性最弱。本研究基于完整数据集及各体型等级分组分别评估了身体状况指数的表现，结果显示Fulton氏K指数为本次采样种群中表现最优的身体状况指数，可解释高达19%的脂肪含量变异。综合来看，本研究得出两点核心结论：其一，身体状况指数与脂肪测量指标间的相关性普遍偏弱；其二，将数据按体型等级分组虽削弱了脂肪百分比与吻肛长之间的相关性（即偏差），但并未提升身体状况指数的整体表现效果。本研究推测，本次数据集内身体状况指数表现欠佳的原因可能包括：脂肪含量与吻肛长的相关性较弱、泰加蜥的躯体构型与生活史特征，以及现有能量储备的核算方式或存在不足。本研究提醒学界，不应默认身体状况指数可跨物种作为机体身体状况的可靠指标，并建议开展针对性的验证研究，或考虑采用替代或补充性的健康与体能表征手段。