asymmetry, the largest pig won around 50 of the fights, and 25 of the variation in the percentage of fights won was explained by the weight asymmetry.
In groups with small weight asymmetry, less than 10 of the variation in the percentage of fights won could be explained by weight asymmetry. The two largest pigs delivered significantly
more bites and spent significantly more time fighting than the smallest pig. The second largest pig received significantly more bites and body lesions than the smallest pig in the groups. Fights
between the two largest pigs had a significantly longer duration than when other pigs were involved. The results were discussed in relation to sequential assessment theory and resource
defence theory. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Aggression; Weight asymmetry; Heterogeneous; Homogenous; Resource; Pigs
1. Introduction
Much effort has been put into identifying the causal factors that explain aggressive Ž
behaviour in pigs Graves et al., 1978; Friend et al., 1983; Tan and Shackleton, 1990; .
Barnett et al., 1993, 1994; Arey and Franklin, 1995 , but only a few authors have Ž
focused on the functional aspects of fighting Fraser et al., 1995; Jensen and Yngvesson, .
1998 . A behavioural ecology approach may complement the more traditional ap- proaches in explaining the underlying causes of the fighting and its consequences.
Ž .
According to game theory models, evenly matched i.e. size, weapons opponents have Ž
difficulties with determining relative strength or fighting ability Enquist and Leimar, .
1983 . Because of this uncertainty in assessment, the contestants are unwilling to give Ž
. up early Enquist and Leimar, 1983 , and the fighting will be prolonged compared to a
Ž .
situation with unevenly matched opponents Enquist et al., 1990 . To mix unfamiliar pigs in uniform weight groups after weaning is common practice
in pig production. This is usually followed by vigorous fighting where some pigs get Ž
. badly injured McGlone and Curtis, 1985; Moore et al., 1994 , and may suffer from
Ž health problems and growth retardation Graves et al., 1978; Tan and Shackleton, 1990;
. Stookey and Gonyou, 1994 . For several species across different taxanomic groups,
Ž .
including the pig Sus scrofa , body weight is probably the best known indicator of Ž
. Ž resource holding potential RHP
spider: Austad, 1983; fish: Bronstein, 1984; rodents: Robitaille and Bovet, 1976; Marques and Valenstein, 1977; birds: Robinson, 1986; pigs:
. Rushen, 1987 . Where weight differences are small between pigs, fighting is seen to last
Ž longer and biting is more frequent than if the weight difference is large Rushen, 1987;
. Francis et al., 1996 . Larger weight asymmetry will also make the ultimate loser cease
Ž .
fighting sooner Rushen, 1988 . This corresponds well with results for other species, Ž
which show that the larger animals possess an advantage in settling disputes Reichert, .
1978; Siguronsdottir and Parker, 1981; Leimar et al., 1991 . Therefore, the probability
` `
Ž of victory for the larger animal will increase with the size difference Leimar et al.,
. 1991; Rosenberg and Enquist, 1991 .
Clumping of resource items in space increases the defensibility and thereby the Ž
aggressive competition between individuals Kennerly and Lean, 1983; Monaghan and Metcalfe, 1985; Milinski and Parker, 1991; Grant and Guha, 1993; Ryer and Olla,
. 1995 . Feral swine seldom interact aggressively during summer months when food is
plentiful and evenly dispersed. However, during winter when food is scarce and clumped, dominant animals actively defend the feeding areas, resulting in a higher
Ž frequency and intensity of encounters Graves et al., 1975; Schnebel and Griswold,
. 1983 . For pigs kept in intensive production systems, straw is a preferred bedding
Ž .
material and an important rooting stimulant Fraser, 1985; Fraser et al., 1991 . The presence of straw in itself does not appear to have any major effect on the amount of
Ž aggression after mixing Kelley et al., 1980; Fraser et al., 1991; Arey and Franklin,
. 1995 . However, to the best of our knowledge, the effect of changing the distribution of
straw has yet not been investigated. The aim of this experiment was to investigate the relationship between weight
asymmetry and aggression when mixing groups of unacquainted pigs in environments with a heterogeneous vs. homogenous distribution of straw. Some of the predictions
Ž from sequential assessment theory were used Enquist and Leimar 1983, 1987; Enquist
. et al., 1990; Leimar et al., 1991 . From this theory, it was predicted that the number of
bites delivered during the fights and the fighting duration should be negatively related to Ž
. the weight difference, the total cost of fighting measured as number of body lesions
should increase when the weight asymmetry decreased, the probability of winning for a single pig should increase when its weight was increased relative to that of the
opponents.
Furthermore, if straw bedding is strongly preferred by the pigs, it would be predicted Ž
. that an environment with straw in a limited and defensible area heterogeneous would
increase the amount of aggression and body lesions compared to an environment with Ž
. straw spread all over homogenous . Finally, we predicted that the effect of a small
Ž .
weight asymmetry would be stronger when the straw area was limited heterogeneous , and thereby increase the amount of aggression even more.
2. Materials and methods
