Mixing together unfamiliar pigs is a common husbandry practice in commercial indoor
piggeries that causes a period of intense fighting and aggression between pigs, resulting in
elevated stress responses and injuries to those involved. Frequency, duration and intensity of
aggressive behaviour have been found to differ between individuals. There is some evidence
that these differences are due to trait-aggressiveness, as consistency of responses to similar
challenges has been demonstrated. This trait-aggressiveness may be linked to other
behavioural (social and non-social) and physiological traits to fonn part of personality. To
understand aggression in the context of behavioural trait the prevalence o f aggression was
studied throughout the lifetime o f a cohort of pigs, examining consistency within and
between situations at numerous points. M ore specifically, the aims o f this thesis were to
investigate: whether the relative magnitude of aggression displayed in response to a repeated
social challenge test (Resident Intruder Test) was consistent over time; if this aggressiveness
changed with age and experience; whether RIT aggressiveness was comparable to, or
predictive o f aggressiveness during social mixing; what strategies were employed by pigs of
differing aggressiveness during mixing; and finally, if aggressiveness was linked to the
response of individual pigs to a challenging environment, unconnected to social
confrontation (maternal behaviour).
Detailed behavioural analysis was performed on the progeny o f 19 gilts. Consistency of
aggressiveness was examined using repeated Resident Intruder Tests at various key stages
during the lifetime o f male and female growing pigs, and a subset of female breeding pigs.
Behaviour during mixing at weaning and as gilts was examined in detail; information was
collected about the way in which these animals behaved in a social context, including nonaggressive social behaviour, and how this related to injuries received (lesions). As social
dominance status is often cited as a parameter related to aggressiveness, this was recorded at
various stages using a group feeding competition test and compared to behavioural measures
from mixing and the RIT. To gain a better understanding of the interaction between
aggressiveness and other traits, the various measures of aggressiveness were compared with
cortisol (as a physiological indicator of the relative stress responses of individuals) and the
behaviour of a subset of female pigs in the period immediately preceding and following
farrowing. Maternal behaviour was chosen for comparison to aggressiveness as it is also
potentially a trait-behaviour, specific to individuals, that is not obviously linked to social
aggressiveness; but also because it is both important commercially and for the welfare of the
gilts and piglets.
Detailed behavioural analysis was performed on the progeny o f 19 gilts. Consistency of
aggressiveness was examined using repeated Resident Intruder Tests at various key stages
during the lifetime o f male and female growing pigs, and a subset of female breeding pigs.
Behaviour during mixing at weaning and as gilts was examined in detail; information was
collected about the way in which these animals behaved in a social context, including nonaggressive social behaviour, and how this related to injuries received (lesions). As social
dominance status is often cited as a parameter related to aggressiveness, this was recorded at
various stages using a group feeding competition test and compared to behavioural measures
from mixing and the RIT. To gain a better understanding of the interaction between
aggressiveness and other traits, the various measures of aggressiveness were compared with
cortisol (as a physiological indicator of the relative stress responses of individuals) and the
behaviour of a subset of female pigs in the period immediately preceding and following
farrowing. Maternal behaviour was chosen for comparison to aggressiveness as it is also
potentially a trait-behaviour, specific to individuals, that is not obviously linked to social
aggressiveness; but also because it is both important commercially and for the welfare of the
gilts and piglets.
Pigs were consistent in their responses to the RIT, but there were differences between sexes.
RIT aggressiveness was consistent over a long period of time in female pigs, even with a gap
of 90 days between tests and the onset o f puberty. M ale pigs showed an unexpectedly high
level of mounting behaviour from a young age, which increased with maturity. Experience of
the RIT improved consistency o f responses, and age at first testing affected both the speed of
attacking and occurrence of attacks: those pigs experiencing the test when younger were
more likely to and quicker to attack. Although aggression in the RIT was consistent, it was
not predictive o f subsequent aggressiveness at mixing.
As with the RIT, there were clear sex-differences observed during mixing at weaning, with
males being more aggressive, more successful in fights, more likely to mount and less likely
to play than females. Pigs employed different strategies during mixing, the extremes of
which were categorised by high-play-low-aggressiveness and vice versa. As expected,
aggressive individuals were involved in more fights and won more fights, but suffered more
skin lesions than non-aggressive individuals. Pigs that engaged in high-playing were
generally the least successful in fights, but suffered fewer lesions and had equal ultimate
dominance rank to aggressive pigs. The behavioural structure of mixing changed between
weaning and puberty, with differences in the occurrence and duration of aggressive and nonaggressive behaviours. Fighting ceased sooner during the gilt mix, but aggression was more
frequent and more severe. Comparisons with maternal behaviour found gilts that reacted
aggressively to their piglets were more aggressive and successful in the mix and more
‘reactive’ during farrowing. There were other links between farrowing and mixing
behaviour, such as more frequent posture changes but less frequent nesting with greater mixaggressiveness; indicating that aggressiveness and maternal behaviour traits are linked
through personality.
The RIT was not a reliable predictor o f aggressiveness, particularly in male pigs.
Aggressiveness did not always equate to status as some individuals managed to attain rank
without severe fighting or receiving excessive numbers of lesions. It may be that aggressive
pigs are more reactive in a number o f situations, reactivity to piglets during farrowing being
one of these situations. The results indicate that aggression should not be viewed in isolation,
but with other traits that make up personality. Rather than being a fixed entity,
aggressiveness develops with age, sex, experience and enviromnental influences.
