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.