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

Abstract

Summary 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 (R i ), and the scaled mass index (M i ), which was recently proposed as a more appropriate method. 3. Linear, quadratic and correlational selection gradients obtained using R i and M i 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 favoured 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.

Condition dependence of shared traits differs between sympatricAnolislizards

“…Our results agree with previous research demonstrating that alternative indices of body condition can produce highly congruent results with regard to inferences about selection and fitness (Cox & Calsbeek, ). The residual index ( R i ) has been used historically, but the scaled mass index ( M i ) has recently emerged as a purportedly superior measurement (Cox & Calsbeek, ; Peig & Green, , ). We recovered the same statistical result using either index in seven out of eight analyses, suggesting that these indices are largely interchangeable in these two species of anole.…”

“…We recovered the same statistical result using either index in seven out of eight analyses, suggesting that these indices are largely interchangeable in these two species of anole. A broader critique of using mass‐SVL regressions (or related techniques) is that they do not necessarily reflect the energetic condition of individuals (Vervust, Lailvaux, Grbac, & Van Damme, ) and can be easily confounded by variation in body shape (Cox & Calsbeek, ). While we agree that energetic condition can be most accurately assessed by dissecting and quantifying total organism lipid stores, there is a tradeoff in that the study organisms must be sacrificed (Blanckenhorn et al., , ).…”

“…A third method of assessing condition is using quantitative magnetic resonance, which generally produces highly accurate results (Warner, Johnson, & Nagy, ), but requires specialized equipment that is not practical for field research. Finally, condition is often measured using indices obtained by analyzing residuals of a regression of body mass on some linear index of body size, such as length (Cox & Calsbeek, ; Schulte‐Hostedde et al., ). The principal advantage to these approaches is that they can be easily measured in the field with no specialized equipment, and they can be measured while only minimally disturbing the animal.…”

A validation of 11 body-condition indices in a giant snake species that exhibits positive allometry

“…Also, and unlike the ratio BCIs, estimates from regression BCIs cannot be compared among individuals from different statistical populations because the regression slopes among populations may be different [18, 19]. Not surprisingly, choosing which BCI to use is not straightforward, and different studies have championed different BCIs as most appropriate [13, 18, 20–23]. …”

The Diversity, Plasticity, and Evolution of Sperm Morphology in Anolis Lizards

“…Therefore, we also performed multiple regressions with log 10 mass and log 10 SVL as independent variables in models with individual means or CVs of sperm head, midpiece, and tail as response variables. In brown anoles, these three approaches (R i , M i , multiple regression) tend to generate similar results in subsequent analyses (Cox and Calsbeek 2014). All statistical analyses were performed using JMP (SAS Institute Inc., Cary NC, Version 9).…”