Morphological asymmetry in mammals: genetics and homeostasis reconsidered
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AbstractIt has been hypothesized that developmental stability is increased at higher levels of genetic variability (heterozygosity) in animals. However, the existence of this relationship is questionable for homeotherms in general and mammals in particular. The difference between the sides of a bilateral character in an individual is a measure of fluctuating asymmetry that can be used as a measure of the developmental stability of mammals. Increased developmental stability should result in a greater degree of similarity between the right and left side of the body even though environmental variability would tend to increase the differences between right and left sides of the body. It is necessary to separate the effects of the three types of asymmetry so that an accurate estim.ate of the variance attributable to fluctuating asymmetry can be made. In addition, many early studies of asymmetry in poikilotherms used meristic characters (such as scale counts), and these types of characters are not easily studied in mammals. Mammals, because of their precise regulation of body temperature show little phenotypic effect of environmental variability, and thus may exhibit low absolute levels of asymmetry. Mammals may also be able to reduce the level of asymmetry during their prolonged intrauterine development and juvenile growth period. The literature is reviewed relative to relationships between genetic variation and asymmetry in mammals. Hypotheses are reviewed as they relate to the relationship between fluctuating asymmetry and heterozygosity observed in previous studies. Finally, recommendations are put forth regarding the design and interpretation of future research into the relationship between developmental homeostasis and genetic variability.