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mates; r DS .
0). Pigs with high social breeding value at mixing, referred to
here as social pigs, can be described as follows:
When the social environment induces genetic antagonism, social pigs are
genetically predisposed to lose fights and to be involved in bullying and
being bullied more frequently than others.
When the genetic antagonism is very strong, social pigs initiate and win
fewer fights, probably because the dominant pigs defend their privileged
access to food.
When social interactions are neutral, social pigs lose more fights and
receive more aggression.
When the social environment induces genetic mutualism, social pigs initi-
ate more fights. They actively participate in the establishment of domi-
nance relationships, by being more challenging.
When the genetic mutualism is very strong, social pigs initiate and win
more fights and bully others more. That means they adopt a more aggres-
sive strategy.
Under both genetic antagonism and neutral interactions, social pigs accu-
mulate fewer lesions in the front and more lesions in the rear. Under genetic
mutualism, there is no difference in lesion scores between more or less social
pigs. Three weeks later, social pigs do not differ from others with regard to
lesions in the front but if the social environment induces genetic mutualism
they have more lesions in the rear, which is a sign of involvement in both
reciprocal fighting and receipt of bullying. Social pigs spend less time stand-
ing, which indicates that they are calmer. ( Canario et al., 2012 ).
If pigs are selected for both the direct and social effects on growth rate in
a situation of neutral or positive interactions, social pigs will initiate more
bullying immediately after mixing. When the dominance order has been set,
they will be less willing to fight and at greater risk of being attacked
( Canario et al., 2012 ). Aggressive behavior at mixing is a necessary adapta-
tion to the rearing environment which does not hamper pig result and is
probably not correlated to aggressive behavior later on, in stable groups.
Aggression among pigs is of two broad kinds: intensive fighting during a
brief period when unfamiliar pigs are mixed and longer-term competition
over feed or other limited resources to maintain hierarchy. This stresses the
importance of considering an appropriate time window in pig life if one
intends to select directly on a behavioral trait.
Unfavorable correlations between the direct and the social genetic effect
have been found in trees, quails, and several laying hen populations ( Bijma,
2011 ). Bouwman et al. (2010) , on the contrary, found no direct social corre-
lation on early growth in pigs. The group model assumes that an animal's
social effect is similar on all other pigs in the group, regardless of whether
they are relatives or not. Only cooperation between relatives fits well into
Dawkins' story of The Selfish Gene ( Dawkins, 1976 ), but pigs rely on
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