Behavioral Genetics in Pigs and Relations to Welfare - Genetics and the Behavior of Domestic Animals - page 415

difference, stillbirth and piglet weight were both higher in modern-type lit-
ters ( Canario et al., 2007 ).
A complicating factor when selecting for improved maternal capacity is
that cross-fostering is widely practiced, even in nucleus herds. The breeding
value for maternal capacity can be calculated as the maternal genetic effect
on piglet survival related to the sow that fosters the piglets, regardless of
whether it is the biological mother or not. It is, however, not obvious
whether piglets dying shortly after being moved should be regarded as a
result of the biological or the foster mother. An advanced model can include
three genetic effects; the maternal effects of the biological and the foster
mother and the direct effect of the piglet. Such a model puts very high
demands on data structure and computer capacity and is difficult to use in
routine genetic evaluation.
FREEDOM FROM FEAR AND DISTRESS
High levels of fear and anxiety are disadvantageous for pigs as well as farm-
ers. Fearful and aggressive animals are more difficult to handle and they can
be a risk for the handler. Fortunately, pigs are not very aggressive towards
humans. All caretakers know they should never turn their back to a boar, but
we are not aware of any genetic studies on human-directed aggressive behav-
ior of boars. We have, however, noted that the word “aggressive” is used in
the marketing of sire lines. A sire line can, e.g., be described as “aggressive
AI boar excellent for semen collection”, which could reflect behavioral dif-
ferences between lines.
Fearfulness
Domestic pigs meet humans every day, and there is usually no escape.
Fearfulness thus could have a large effect on pig welfare, but in general pigs
seek contact with the caretaker rather than avoid him/her, unless the care-
taker behaves “badly”. There is an ongoing threshold selection against fear
at the herd level which is partly a natural selection and partly an active selec-
tion performed by the breeder. This selection has always been an important
part of the domestication process. In case of unfavorable genetic correlations
between fear and production traits, natural selection may however not be
enough to hinder a negative development. There are indications that selec-
tion for lean growth rate results in a more excitable temperament in pigs
( Grandin, 1992 ). Leaner pigs are more stressed at transportation, balk more,
and are more difficult to drive through races at the slaughter plant than
fatter pigs ( Grandin, 1998 ). Scott et al. (2000) compared a lean and a fat
Landrace line. The lean animals were more reluctant to voluntarily approach
a human.