Behavioral Genetics and Animal Science - Genetics and the Behavior of Domestic Animals - page 29

fat. Separating single animals from the group is also more difficult. Research
shows that cattle with an excitable temperament have lower weight gains
and more meat quality problems ( Caf´ et al., 2011; Silveira et al., 2012;
Voisinet et al., 1997a, b ). This research illustrates that selection away from a
very excitable temperament would be beneficial. However, ranchers report
that mothering ability is reduced by overselecting for extremely calm tem-
peraments. There are always trade offs.
Links Between Different Traits
Casual observations by the first author indicate that the most excitable pigs
and cattle have long slender bodies and fine bones. Some lean hybrid pigs
have weak legs and normally brown-eyed pigs now have blue eyes. Blue
eyes are often associated with neurological problems ( Bergsma and Brown,
1971; Schaible, 1963 ). Furthermore, pigs and cattle with large bulging
muscles often have calmer temperaments compared to lean animals with less
muscle definition. However, animals with double muscling (hypertrophy)
have more excitable temperaments ( Holmes et al., 1972 ). Double muscling
is abnormal and may have opposite effects on temperament compared to
normal muscling.
Deafness in pointer dogs selected for nervousness is another example of
apparently unrelated traits being linked ( Klein et al., 1987; 1988 ). There
appears to be a relationship between thermoregulation and aggressiveness.
Sluyter et al. (1995) found that wild mice selected for aggressiveness used
larger amounts of cotton to build their nests than mice selected for low
aggression. This effect occurred in both laboratory and wild strains of mice.
The complexity of genetic interactions continues to frustrate researchers
using high-tech “knockout” gene procedures. Genes are knocked out in a
gene-targeting procedure, and prevented from performing their normal
functions. Knockout experiments show that blocking different genes can have
unexpected effects on behavior. In one experiment, super-aggressive mice
were created when genes involved with learning were inactivated ( Chen et al.,
1994 ). The mutant mice had little or no fear and fought until they broke their
backs. In another experiment, knockout mutants demonstrated normal behavior
until they had pups, which they failed to care for ( Brown et al., 1996 ). In still
another experiment, Konig et al.(1996) disabled the gene that produces
encephalin (a brain opioid substance) and found unexpected results.
Enkephalin is a substance normally involved in pain perception. Mice deficient
in this substance were very nervous and anxious. They ran frantically around
their cages in response to noise. Traits are linked in unexpected ways and
completely isolating single gene effects may be impossible. Researchers sug-
gest using caution and being careful not to jump to conclusions about claims
by those who say they found an “aggression gene” or a “maternal gene” or
an “anxiety gene.” To use an engineering analogy, one cannot conclude the