Moving beyond body condition indices as an estimate of fitness in ecological and evolutionary studies

1. Body condition indices, measures of body 'plumpness' or mass relative to frame size, are often used as a proxy for lipid reserves or fitness-related traits of animals and assumed to be positively related to fitness. 2. The quantification and analysis of body condition indices has been the subject of debate for decades. Here, we summarize three additional concerns with the use of body condition indices. 3. First, body condition index is often poorly correlated with lipid content in animals. Second, even if body condition index and lipid content are correlated, lipid content of an animal may not be the most important aspect of body composition influencing fitness. Finally, neither body condition index nor lipid reserves are likely to be directly positively related to fitness in animals, as many animals homeostatically regulate intermediate levels of condition index or lipid reserves, with both higher and lower values incurring fitness costs. 4. A wide range of analytical methods, including some relatively inexpensive and simple measures, are available for more detailed measures of animal body composition or fitness-related traits. Replacing body condition indices with more direct measures of body composition - even relatively simple measures - can inform understanding of the physiological mechanisms underlying animal responses in a wide range of behavioural, ecological and evolutionary studies.