Growth and developmental instability.

Developmental stability reflects the ability of an individual to develop a regular phenotype under given environmental and genetic conditions. Measures of developmental instability include the degree of fluctuating asymmetry and the frequency of phenodeviants. Endocrine, neural and circulatory mechanisms that control similar development of morphological characters on the two sides of the body are also involved in controlling overall development. Cross-validation studies have shown that measures of developmental instability are positively correlated with other measures of welfare such as tonic immobility in poultry. Asymmetric animals grow less rapidly than symmetric individuals. Eleven studies have investigated the relationship between growth rate and developmental instability, and the observed effect size (Pearson correlation coefficient adjusted for sample size) is -0.15. Studies of chickens have shown that asymmetry increases as a response to selection for increased growth rate. As conditions for rearing deteriorate by higher density, fluctuating asymmetry increases and growth rate decreases, both within and among farms. Fluctuating asymmetry can be considered a measure of animal welfare since larger values reflect worse environmental conditions as experienced by the individual animal itself. Since growth rate and fluctuating asymmetry are negatively correlated, we can infer that improvement of rearing conditions leading to reduced asymmetry will both benefit the producer (in terms of increased growth), but also the animals in terms of better conditions for rearing.