Mass-based condition measures and their relationship with fitness: in what condition is condition?

Abstract

Mass or body-size measures of 'condition' are of central importance to the study of ecology and evolution, and it is often assumed that differences in condition measures are positively and linearly related to fitness. Using examples drawn from ecological studies, we show that indices of condition frequently are unlikely to be related to fitness in a linear fashion. Researchers need to be more explicit in acknowledging the limitations of mass-based condition measures and accept that, under some circumstances, they may not relate to fitness as traditionally assumed. Any relationship between a particular condition measure and fitness should first be empirically validated before condition is used as a proxy for fitness. In the absence of such evidence, researchers should explicitly acknowledge that assuming such a relationship may be unrealistic.

Keywords: birds; condition; fat reserves; fitness; individual phenotypic quality; insects; mammals; mass reserves.

Figure 1

The relationship between condition (e.g., mass) and fitness is generally assumed to be positive and linear (a). However, in cases of highly monogamous species with low rates of extra-pair paternity (b) and polygynous species (c), the relationship may be positive, but non-linear. In some circumstances, the relationship between condition and fitness could be more complicated than in a–c. For example, the trade-off between mass and predation risk may make it costly for birds to gain mass because it increases their chances of being caught by predators (d). The dotted line indicates the optimal fitness yield for condition.