www.elsevier.comrlocaterapplanim
The effects of weight asymmetry and resource
distribution on aggression in groups of
unacquainted pigs
Inger Lise Andersen
a,), Hilde Andenæs
a, Knut Egil Bøe
a,
Per Jensen
b, Morten Bakken
ca
Department of Agricultural Engineering, Agricultural UniÕersity of Norway, P.O. Box 5065,
1432 Aas, Norway
b
Department of Animal EnÕironment and Health, Swedish UniÕersity of Agricultural Sciences, P.O. Box 234,
SE-53223 Skara, Sweden
c
Department of Animal Science, Agricultural UniÕersity of Norway, P.O. Box 5025, 1432 Aas, Norway
Accepted 5 January 2000
Abstract
The relationship between weight asymmetry and aggression when mixing groups of
unac-Ž
quainted pigs in two different environments was investigated. Ten groups of 4 female Landrace
. Ž
=Yorkshire , unacquainted pigs with a weight asymmetry of 3.1"0.2 kg mean weight:
.
16.6"0.6 kg between each pig, and another 10 groups with a weight asymmetry of 1.2"0.1 kg
Žmean weight: 14.1"0.1 kg , were mixed at the age of 7 weeks. Ten of the groups five of each. Ž .
were mixed in an experimental pen with a heterogeneous distribution of straw. The pen consisted of two main compartments with straw in one of them, and a passage area with concrete floor in between. The other 10 groups were mixed in the same pen, but with a homogenous distribution of
Ž .
straw straw spread all over the pen .
The results showed that fighting duration was significantly shorter in groups with large weight asymmetry than in groups with small weight asymmetry irrespective of the environment. The number of bites delivered during the fights in the heterogeneous environment was lower in groups with large weight asymmetry than in groups with small weight asymmetry. In the homogenous environment, however, there was no significant difference between groups with large and small weight asymmetries regarding the number of bites. The combination of a limited straw area and a small weight asymmetry resulted in the greatest number of bites. In groups with large weight
)Corresponding author. Tel.:q47-6494-8700; fax:q47-6494-8810.
Ž .
E-mail address: [email protected] I.L. Andersen .
0168-1591r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved.
Ž .
( ) I.L. Andersen et al.rApplied Animal BehaÕiour Science 68 2000 107–120
108
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.q2000 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
`
`
Žof 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,
.
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
2.1. Experimental set-up and animals
Ž .
A 2=2 factorial experiment with weight asymmetry small vs. large and resource
Ž .
distribution heterogeneous vs. homogenous as the main factors was conducted. Each of the four treatments consisted of five experimental groups. Twenty groups of four female,
Ž .
unacquainted pigs Landrace=Yorkshire, mean weight: 16.6"0.6 were mixed at the
Ž .
age of 7 weeks. Ten groups had a weight asymmetry of 3.1"0.2 kg large between each pig, which adds up to 9.3 kg between the largest and smallest pig in the group.
Ž .
Another 10 groups mean weight: 14.1"0.1 had a weight asymmetry of 1.2"0.1 kg Žsmall between each pig, and the weight difference between the largest and smallest pig.
Ž .
was 3.6 kg. Ten of the groups five of each were mixed in an experimental pen with a heterogeneous distribution of straw. The pen consisted of two main compartments with
Ž .
straw in one of them, and a passage area with concrete floor in between Fig. 1 . The other 10 groups were mixed in the same pen, but with a homogenous distribution of
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Fig. 1. The experimental pen showing a homogenous vs. heterogeneous distribution of straw.
between the two compartments in the pen so that it should be possible for a single pig to prevent other pigs from entering the straw area.
2.2. Housing
Ž .
Litters were randomly chosen at weaning 5 weeks of age and moved to the research
Ž 2 .
building. They were kept in pens one litterrpen, 0.69 m per pig located next to each
Ž .
diet. The mean temperature in the room was kept within 18–208C, and artificial light was kept on between 0800 and 1500 h.
2.3. Handling procedures
Ž .
On the day of mixing 7 weeks of age , one female pig from each of four litters was selected. They were given numbers from 1–4 according to weight, where 1 identified the largest pig and 4 was the smallest. Only pigs with good health and no movement disorders were used in the experiment. The pigs were taken out of the pens, one at a
Ž .
time, through small doors and further into individual starting boxes Fig. 1 . After marking all the four individuals in the starting boxes, the animals were let into the experimental pen through a passage. The handling facilities were made in order to minimise the physical contact with humans before the experiment. During the experi-ment, the two persons involved left the room. After 1 h, the pigs were returned to the home pens in the same manner as they arrived. There was no presence food or water in the experimental pen, and it was thoroughly cleaned between tests.
2.4. BehaÕioural obserÕations
The pigs were videotaped continuously for 1 h by using two video cameras ŽCCTV-Camera Panasonic WV-BP310 connected to a multiplexer MV 16 and a video. Ž .
Ž .
recorder Panasonic AG 6730 .
The following mutually exclusive behaviours were recorded:
1. Push: pushing another pig by moving the noserhead actively up and down towards it.
2. Bite towards headrshoulders. 3. Bite towards other parts of the body. 4. Chase: moving quickly after another pig.
5. Threat: moving the head andror body quickly towards the head of another pig. 6. Withdraw: moving the head andror body away from another pig that has delivered
nosing, push, bite or threat.
7. Flee: running away from a pig that has delivered nosing, push, bite or threat. 8. Avoid: moving the head andror body away from another pig that has not delivered
nosing, push, bite or threat.
For each behaviour, the receiver was also recorded. The behaviours were scored
Ž .
continuously. The number of fights, the duration of each fight in seconds and the winnerrloser were recorded. Any encounter in which there was at least one bite was
Ž .
called a fight Forkman et al., 1995 . Two fighting bouts were separated by at least 10 s Žsame criteria as used by Stookey, 1991 . The winner was defined as the pig that. delivered the last bite in a fight and forced the other pig to withdraw. A pig withdrawing
Ž .
from or avoiding the opponent after a fight was defined as the loser McGlone, 1985 .
Ž .
The number of body lesions scratches was counted before and immediately after the
Ž .
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Table 1
Ž .
Frequency of behaviours per individual and group means"SE for the two treatments
Behaviour Weight difference Resource distribution P value
Large Small Heterogeneous Homogenous Weight difference Resource distribution Interaction Bite towards headrshoulder 38.8"6.4 57.2"8.1 50.5"8.9 45.6"6.8 0.08 ns 0.06
Bite towards rest of the body 3.3"1.6 2.9"0.8 2.6"0.7 3.6"1.7 ns ns ns
Chase 0.7"0.2 1.3"0.4 0.5"0.1 1.5"0.4 ns -0.05 ns
Flee 4.9"0.9 6.2"1.2 4.1"0.6 7.0"1.2 ns 0.06 ns
Offensive 6.0"1.1 8.6"1.4 7.5"1.3 8.7"1.4 ns ns ns
Defensive 22.2"2.3 25.5"3.3 21.1"2.0 26.6"3.3 ns ns ns
No. of fights 15.0"1.8 17.2"1.7 14.5"1.4 17.6"2.0 ns ns ns
Ž .
Fighting duration s 135.1"23.0 215.9"31.6 181.1"33.2 169.9"27.9 -0.05 ns ns
2.5. Statistics
In the analysis, push, chase and threat were summed into offensive behaviours, whereas avoid, flee and withdraw were categorised as defensive behaviours. An analysis of variance including the class variables weight asymmetry and resource distribution,
Ž .
and the interactions between them GLM-procedure, df models3, df errors16 , was conducted.
Ž .
The relationship between weight rank 1 to 4 and behaviour within groups of a large Ž
vs. small weight asymmetry was tested in a one-way analysis of variance GLM-proce-.
dure, df models3, df errors36 . The Spearman correlation analysis was also used to evaluate the relationship between weight rank and behaviour.
The least-square means were computed for all effects involving the class variables weight difference, resource distribution and weight rank. Differences between means were analysed by using a t-test.
3. Results
3.1. Weight asymmetry, resource distribution and aggression
Approximately 94% of the total bites delivered were directed towards head and
Ž .
shoulders. There was an interaction between weight asymmetry large vs. small and
Ž .
resource distribution heterogeneous vs. homogenous in the number of bites delivered
Ž .
towards headrshoulders Table 1 , and the same tendency appeared for fighting duration Žin seconds . In the heterogeneous environment, more bites tended to be delivered in.
Ž .
groups with small weight asymmetry 1.2 kg than in groups with a large weight
Ž .
asymmetry 3.1 kg , but this difference was not present in the homogenous environment ŽFig. 2 . The number of bites towards head. rshoulders and the fighting duration was longer in groups with a small than with a large weight asymmetry. However, there was
Ž .
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no effect of weight asymmetry on the number of fights, body lesions or the other
Ž .
Ž .
Fig. 4. Number of bites received meansqSE towards headrshoulders for pigs ranked 1–4 according to weight in groups with a large vs. small weight asymmetry. Bars with different superscripts differ significantly. Large weight asymmetry: a, b: P-0.05, small weight asymmetry: a, b: P-0.05.
The frequency of fleeing and chasing was greater in the homogenous than the heterogeneous environment, but there was no single effect of resource distribution on the
Ž .
more serious fighting behaviour or the number of body lesions obtained Table 1 .
3.2. Weight asymmetry and aggression within groups
In groups with a large weight asymmetry, there was a significant effect of individual
Ž . Ž
weight rank 1 to 4 on the number of bites delivered towards headrshoulders F3,
. Ž .
s2.9, P-0.05 and the fighting duration F s3.2, P-0.05 . The two largest
36 3, 36
pigs delivered significantly more bites towards headrshoulders and spent more time
Ž .
fighting than the smallest pig Fig. 3a and b . Fights between the largest and second largest pig had a significantly longer duration than when other pigs were involved in
Ž .
large asymmetry groups F3, 36s3.9, P-0.01; Fig. 3c . In groups with a large weight Ž
asymmetry, pigs with weight rank 2 tended to receive more bites F3, 36s2.1,
. Ž .
P-0.11 and obtained more body lesions F3, 36s3.7, P-0.05 than the other pigs ŽFigs. 4 and 5 . No significant differences were found between individuals in groups. with a small weight asymmetry.
Ž . Ž .
Fig. 3. a Number of bites delivered meansqSE towards headrshoulders by pigs ranked 1–4 according to weight in groups with a large vs. small weight asymmetry. Bars with different superscripts differ significantly.
Ž .
Large weight asymmetry: a, b: P-0.05, small weight asymmetry: a, b: P-0.05. b Time spent fighting in
Ž .
seconds meansqSE for pigs ranked 1–4 according to weight in groups with a large vs. small weight asymmetry. Bars with different superscripts differ significantly. Large weight asymmetry: a, b: P-0.05,
Ž . Ž .
small weight asymmetry: a, b: P-0.05. c Fighting duration in seconds meansqSE in pair-wise interactions in groups with a large vs. small weight asymmetry. Large weight asymmetry: bars with different superscripts differ significantly. Large weight asymmetry: a, b: P-0.05, small weight asymmetry: a, b:
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Ž .
Fig. 5. Number of body lesions meansqSE for pigs ranked 1–4 according to weight in groups with a large vs. small weight asymmetry. Bars with different superscripts differ significantly. Large weight asymmetry: a,
b: P-0.05, small weight asymmetry: a, b: P-0.05.
The correlation between weight rank and percentage of fights won was higher in
Ž . Ž
groups with a large rss y0.5, P-0.01 than a small weight asymmetry rss y0.3, .
P-0.05 . This means that in groups with a large weight asymmetry, the weight of the pigs explained 25% of the variation in the fights won whereas in groups with a small weight asymmetry, the corresponding value was less than 10%. Irrespective of weight asymmetry, there was a significant effect of weight rank on the percentage of fights won ŽF3, 76s4.3, P-0.01 , where the largest pigs 52.0. Ž "9.4% won significantly more.
Ž . Ž
fights than second largest pigs 28.8"8.7% and the smallest pigs 12.1"4.6%;
. Ž .
P-0.05 . Pigs with weight rank 3 37.4"8.5% won significantly more fights than the
Ž .
smallest pig P-0.05 .
4. Discussion
This study showed that, in both environments, fighting duration was shorter in groups
Ž .
with a large weight asymmetry 3.1 kg than in groups with small weight asymmetry Ž1.2 kg . In the heterogeneous environment, the number of bites delivered during the. fights was lower in groups with large weight asymmetry than in groups with small weight asymmetry. However, in the homogenous environment there was no significant difference between groups with large or small weight asymmetries. As predicted, the combination of limited straw area and small weight asymmetry resulted in the greatest number of bites. In this situation, the aggressive competition and intensity of the fights
Ž
will increase. Firstly, the attractive area is smaller and more defensible Graves et al., 1975; Kennerly and Lean, 1983; Schnebel and Griswold, 1983; Monaghan and Metcalfe,
.
1985; Grant and Guha, 1993; Ryer and Olla, 1995 , and secondly, a smaller weight Ž
. Leimar, 1983; Rushen, 1987; Enquist et al., 1990; Rosenberg and Enquist, 1991 . If, on the other hand, the resources had been extremely clumped and defensible, the amount of aggression would be expected to decline, as more individuals would have ceased to
Ž .
compete for the resource Rob and Grant, 1998 . Contrary to what was predicted, there were no effects of weight asymmetry on aggression in the homogenous environment. When straw is spread all over the pen, it is more difficult for one or two individuals to control the other group members and to monopolise the resource. In this situation, the smaller pigs are likely to have a similar access to straw as the larger pigs. Aggressive interactions will thus become more random and less dependent of weight asymmetry.
Significant differences in aggression and body lesions between pigs ranked 1 to 4 according to weight was only present in groups with a large weight asymmetry. This corresponds well with an earlier study on weight asymmetry in pigs and the results from
Ž
other species Rushen, 1987; Enquist et al., 1990; Leimar et al., 1991; Rosenberg and .
Enquist, 1991; Francis et al., 1996 . Pigs may assess relative strength by comparing body dimensions rather than weight asymmetry. Increased weight is associated with a
Ž
relatively small increase in body dimension i.e. body length and shoulder height, .
Baxter, 1984 . Therefore, the weight asymmetry needs to be large in order to make the pig perceive differences in relative strength, especially when more than two individuals are confronted. Consequentially, the smaller animal may win due to random errors in
Ž .
estimating relative fighting ability, as suggested by Enquist et al. 1990 . The two largest pigs delivered more bites and spent more time fighting than the smallest pig in the group, but there were no differences between pigs with weight ranks 1, 2 and 3. In the pair-wise interactions, fights between the two largest pigs had a longer duration than when other pigs were involved, but again the fighting duration did not differ between the other pairs. Compared to the two largest pigs, which eventually gained control over the resource and the other pigs in most of the groups, the benefit of getting involved in fights was probably much less for the smaller pigs.
As predicted, the probability of winning for the larger animal was greater in groups with a large weight asymmetry. In these groups, the largest pig won more than 50% of the fights, and 25% of the variation in the percentage of fights won was explained by weight asymmetry. Considering that the weight difference was more than 9 kg between the largest and smallest pigs in these groups, however, it was reasonable to expect a higher correlation between weight asymmetry and percentage of fights won. In groups with a small weight asymmetry, the weight of the individuals explained less than 10% of the variation in the percentage of fights won. It is therefore possible that the behavioural
Ž .
strategies offensiverdefensive of the animals may explain more of the individual variation in aggression and the degree of success than the weight asymmetry. In groups with a large weight asymmetry, the smallest pig received significantly less bites and body lesions than the second largest pig. This corresponds with the fact that the smallest pig was least involved in fights whereas the second largest pig spent much time fighting both with no. 1 and 3. The pigs with weight rank no. 2 thus had to suffer the highest costs in terms of time spent fighting and the amount of body lesions received during fights.
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bites or body lesions in this study. On the other hand, a homogenous pen with straw all over resulted in significantly more fleeing and chasing between compartments than the heterogeneous pen with a limited distribution of straw.
To conclude, fighting duration in both environments was shorter in groups with large weight asymmetry than in groups with small weight asymmetry. There was an interac-tion between resource distribuinterac-tion and weight asymmetry in the number of bites delivered during fights, where the combination of a limited straw area and a small weight asymmetry tended to give most aggression. This shows that the distribution of resources in the environment may have some influence on the initial fighting in uniform weight groups of newly mixed pigs. In the groups with a large weight asymmetry, there was a significant relationship between the individual weight rank and the amount of fighting and body lesions received during the fights. When the weight asymmetry was small, however, the competitive relationship between individuals became more difficult to interpret. From this, it can be concluded that the weight differences have to be at least 3 kg to have effect on aggression. Although weight appeared to be a better predictor of RHP in pigs when the asymmetry was large, other factors may be more important to explain individual differences in aggression and the degree of success in social groups.
Acknowledgements
We gratefully acknowledge our research technician, Øyvind Vartdal, for building the pens and for all practical help. We also wish to thank Dr. Jim Greatorex for correcting the language.
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