It has been hypothesized that developmental stability is increased at
higher levels of genetic variability (heterozygosity) in animals. Howe
ver, the existence of this relationship is questionable for homeotherm
s in general and mammals in particular. The difference between the sid
es of a bilateral character in an individual is a measure of fluctuati
ng asymmetry that can be used as a measure of the developmental stabil
ity of mammals. Increased developmental stability should result in a g
reater degree of similarity between the right and left side of the bod
y even though environmental variability would tend to increase the dif
ferences between right and left sides of the body. It is necessary to
separate the effects of the three types of asymmetry so that an accura
te estimate 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 c
haracters 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 asymmet
ry 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 het
erozygosity 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 va
riability.