Behavioral Genetics in Pigs and Relations to Welfare - Genetics and the Behavior of Domestic Animals - page 396

The Social Pig
The group size is quite similar for wild boars, feral pigs and farmed pigs,
although these animals live in a wide range of habitat types and resource
availability. Under natural conditions, the groups are based on matriarchal
hierarchies ( Stolba and Wood-Gush, 1989 ) and males are loosely associated
with these groups ( Mauget, 1981 ). On the farm, pigs born in different families
are often grouped together in small pens. Whereas aggression is avoided in
the wild, where different groups seldom meet, mixing with unacquainted pigs
is frequent in pig production. When unacquainted pigs meet they fight to
establish dominance. During the first 24 hours after mixing, most pigs are
involved in many fights which leads to energy expenditure and injuries such
as skin lesions. Turner et al. (2006a) counted skin lesions on pigs after mixing
as a measurement of aggressive behavior. Ten per cent of the pigs had more
than 50 skin lesions. The frequency and intensity of aggressive interactions
decline over time after mixing, until social relationships stabilize. However,
an ongoing lower level of aggression persists to maintain social relationships.
Aggressiveness in pigs is known to be repeatable over time and across
different situations and it is partly influenced by the genotype. A pig's deci-
sion to engage in an aggressive interaction or not may be made according to
the relative costs and benefits of the behavior, which will vary depending on
resource scarcity and the other pigs' behavior ( Enquist and Leimar, 1983 ).
Arey and Franklin (1995) studied groups of 15 pigs and found that in 60%
of the dyads (pairs) there were no fights.
Pigs can identify unfamiliar individuals in large groups of up to 80 pigs
( Turner et al., 2001 ). Gilts and sows are able to remember their group mates
and identify them when they meet again after several weeks ( Arey, 1999 ).
Thus fighting can be avoided. The stability of the group of pregnant sows is
maintained by a dominance hierarchy that depends on subordinates avoiding
the dominant sows ( Jensen, 1982 ). The dominance order is, however,
resource relative; group members may have different dominance orders for
different resources ( Lindberg, 2001 ). Whether it is worth it or not to initiate
a fight depends on group size and the predictability of the resource, e.g.
feeding. In large groups, a higher number of competitors dilutes the effec-
tiveness of aggression and increases its energetic cost ( Fraser et al., 1995 ).
Individual differences in aggressiveness can be measured in a
resident intruder test ( R´ale et al., 2007 ). The tested pig, i.e. the resident,
encounters in its home pen an intruder (that should be of slightly smaller
size) and the latency until attack by the resident is recorded. Recording
aggressive behavior among pigs in a group is difficult since the pigs must be
identified individually. Aggressiveness can be investigated by direct or video
observations, either by recording the total number of initiated and received
attacks for each pig or by recording the identity and outcome of each dyadic
encounter.