Genetics and Animal Welfare - Genetics and the Behavior of Domestic Animals - page 450

of an animal's body ( Markow, 1992 ). Fluctuating symmetry (FA) refers to
random deviations from bilateral asymmetry, and can be measured in the
body—as in bilateral symmetry of finger or foot lengths—or in a particular
organ. FA is a key concept in evolution and development and underlies the
theory of developmental stability—the ability to maintain normal develop-
ment under stress. Animals with high levels of FA have reduced develop-
mental stability. Deviations from perfect bilateral symmetry can be caused
by environmental stress, or genetic problems during development, leading to
developmental instability ( Graham et al., 2010 ). Conservation biologists use
measurements of body symmetry to assess detrimental effects of environ-
mental pollution in rainbow trout ( Leary and Allendorf, 1989 ), chickens
( Buijs et al., 2012; Erikson et al., 2003; Moller et al., 1995 ), and horses
( Manning and Ockenden, 1994 ). Graham et al. (2010) discuss the importance
of taking body measurements on several different body locations to get an
accurate measure.
Fluctuating asymmetry is also affected by purely genetic causes.
Parsons (1990) states that inbreeding increases the degree of asymmetry in
a variety of animals. The teeth of hybrid mice are more symmetrical com-
pared to inbred mice ( Bader, 1965 ). Symmetrical trout have greater het-
erozygosity and gene variation ( Leary and Allendorf, 1989 ). Asymmetry
also affects an animal's performance and adaptability. Manning and
Ockenden (1994) measured the differences in the width and length of
knees, teeth, ears, coronet bands, upper legs, and six measurements of the
heads of racehorses. More symmetrical horses performed better on the
racetrack. Symmetry is also involved in sexual selection. Moller (1992,
1993) found that female swallows preferred males with long, symmetrical
tails.
There also appears to be a relationship between domestication and fluctu-
ating asymmetry. Domestic animals tend to be more asymmetrical than wild
animals ( Parsons, 1990 ). Fluctuating asymmetry and minor physical abnor-
malities are signs of greater problems. Human children with developmental
abnormalities such as mental retardation and autism have a greater percent-
age of minor physical abnormalities than the general population ( Links
et al., 1980; Steg and Rapport, 1975 ). They are also more likely to have
greater asymmetry ( Malina and Buschang, 1984 ). Assessments of body
asymmetry may be one simple way to provide an early warning of potential
problems with a selective breeding program. Intuitively, dairy cow breeders
for years have evaluated various body parts for symmetry. For example:
dairymen routinely look at traits such as a well-formed, symmetrical udder.
All major dairy bull studs do this because they intuitively know that symme-
try is a good trait.
An important conclusion can be made. Over-selection for a single trait
ruins the animal. In breeding animals, people must be aware of the complex
interaction between traits that do not appear to be related.