Behavioral Genetics and Animal Domestication - Genetics and the Behavior of Domestic Animals - page 64

and explorative. This shows a clear pleiotropic effect on behavior of a
specific pigment-related mutation, but it remains unclear which mechanisms
may be involved. Some hypotheses may, however, be discussed.
PMEL17 is a melanocyte-specific protein, which is not directly involved in
the synthesis of melanin, but rather affects the maturation of melanosomes, the
melanocytic vesicles where melanin is deposited. The protein forms amyloid
fibers on the inside of the melanosome membranes, and these serve as the sub-
strate where melanin particles fasten ( Yasumoto et al., 2004 ). The mutation
inhibits the formation of a transmembrane region, and the further development
of the necessary amyloid fibers, and thus leaves the melanosomes without any
substrate for melanin deposition.
Melanin is formed from tyrosine and DOPA as precursors. DOPA is also
a precursor for the synthesis of the potent neurotransmitter dopamine and the
chatecholamines norepinephrine and epinephrine. As a hypothesis, N¨tt and
coworkers suggested that when melanin cannot be deposited, its synthesis
may be down-regulated, and the concentration of DOPA may consequently
increase ( N ¨ tt et al., 2007 ). This could in turn make more DOPA available
for the dopamine synthesis pathway. Although this mechanism is highly
speculative, the fact that there is a close biochemical connection between
melanin synthesis and behaviorally important pathways makes a connection
between pigmentation and stress responsiveness quite logic.

Neurotransmitters and Social Behavior
Among the more prominent behavior changes occurring as a response to
domestication is the modified social behavior. Whereas the wild ancestors of
our domesticates usually live in groups with very little flexibility concerning
size, dynamics, and home ranges, most domestic species tolerate very variable
group sizes and a high degree of dynamic changes in group composition. For
example, wolves live in packs which rarely exceed 15

20 members, and
where all individuals mostly are closely related, whereas free-ranging dogs
form variable-sized groups of up to dozens of dogs where membership of
agroupmaychangefromoneweektothenext( Boitani and Ciucci, 1995 );
Wild boars form tight family groups with little exchange of individuals, while
domestic pigs tolerate being regrouped at regular intervals during rearing
( Gonyou, 2001 ); Red Junglefowl live in small harem groups with strict
territorial boundaries, while chickens under commercial conditions can cope
with thousands of other birds in a limited space ( Mench and Keeling, 2001 ).
Mutations affecting sociality and social tolerance must have been important
selection targets during domestication.
During recent times, there has been a growing interest in the role of
neuropeptides, primarily vasopressin and oxytocin, in shaping the social behav-
ior of an individual ( Donaldson and Young, 2008 ). While the actual hormones
are evolutionary well conserved, variations in the expression of their receptors