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AGGRESSIVE BEHAVIOR Volume 14, pages 155-167 Assessment of Fighting Ability or Simple Habituation: What Causes Young Pigs (Sus scrofa) to Stop Fighting? Jeffrey Rushen Animal Research Centre, Ottawa, Ontario, Canada .................................................... .................................................... To determine how pigs assess their relative fighting ability, I observed paired contests between young pigs that were either of similar weight or of different weight. On the first day the incidence of fighting was high, but this was not affected by the size of the weight difference. Fights between pigs of different weight were shorter than those between pigs of similar weight. When the pigs were of different weight, the eventual winner bit more frequently and spent more time attacking than did the eventual loser. This difference was not apparent when the pigs were of similar weight. On the first day, winners and losers were equally likely to initiate fights. The incidence of fighting was lower on the second day, although this was unaffected by the weight difference. Losers were less likely to initiate fights on the second day, and fights were shorter than on the first day. However, for the time that they did fight, the losers fought as vigorously (based on the rate of biting) on the second day as on the fist. Seventy-two hours of visual and olfactory contact before the fight had no effect on the incidence of fighting and minimal effect on its duration. I suggest a young pig cannot assess its chance of success prior to the first encounter but only during the course of the fight. This assessment is reduced by attacks from the opponent until it drops below a critical threshold, whereupon the pig stops fighting. As a result of a previous defeat, a pig will lower its assessment and will be less likely to fight or to initiate fights on subsequent encounters. It is this change in assessment rather than habituation alone that makes pigs less likely to fight as they become acquainted. .................................................... .................................................... Key words: agonistic behavior, swine, social dominance, familiarity INTRODUCTION What motivates the fighting that occurs when unacquainted animals first meet? To answer this question we must understand the cause of the decline in aggression that occurs as animals become familiar. A reduction in the aggression that occurs as two Received for publication June 26, 1987; accepted December 11, 1987. Address reprint requests to Dr. Jeffrey Rushen, Animal Research Centre, Ottawa, Ontario, Canada KIA OC6. 0 1988 Alan R. Liss, Inc.

Assessment of fighting ability or simple habituation: What causes young pigs (Sus scrofa) to stop fighting?

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AGGRESSIVE BEHAVIOR Volume 14, pages 155-167

Assessment of Fighting Ability or Simple Habituation: What Causes Young Pigs (Sus scrofa) to Stop Fighting? Jeffrey Rushen

Animal Research Centre, Ottawa, Ontario, Canada

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . To determine how pigs assess their relative fighting ability, I observed paired contests between young pigs that were either of similar weight or of different weight. On the first day the incidence of fighting was high, but this was not affected by the size of the weight difference. Fights between pigs of different weight were shorter than those between pigs of similar weight. When the pigs were of different weight, the eventual winner bit more frequently and spent more time attacking than did the eventual loser. This difference was not apparent when the pigs were of similar weight. On the first day, winners and losers were equally likely to initiate fights. The incidence of fighting was lower on the second day, although this was unaffected by the weight difference. Losers were less likely to initiate fights on the second day, and fights were shorter than on the first day. However, for the time that they did fight, the losers fought as vigorously (based on the rate of biting) on the second day as on the fist. Seventy-two hours of visual and olfactory contact before the fight had no effect on the incidence of fighting and minimal effect on its duration. I suggest a young pig cannot assess its chance of success prior to the first encounter but only during the course of the fight. This assessment is reduced by attacks from the opponent until it drops below a critical threshold, whereupon the pig stops fighting. As a result of a previous defeat, a pig will lower its assessment and will be less likely to fight or to initiate fights on subsequent encounters. It is this change in assessment rather than habituation alone that makes pigs less likely to fight as they become acquainted.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Key words: agonistic behavior, swine, social dominance, familiarity

INTRODUCTION What motivates the fighting that occurs when unacquainted animals first meet? To

answer this question we must understand the cause of the decline in aggression that occurs as animals become familiar. A reduction in the aggression that occurs as two

Received for publication June 26, 1987; accepted December 11, 1987.

Address reprint requests to Dr. Jeffrey Rushen, Animal Research Centre, Ottawa, Ontario, Canada KIA OC6.

0 1988 Alan R. Liss, Inc.

156 Rushen

individuals become acquainted is a common phenomenon in the animal kingdom, underlying the “dear enemy” effect in territorial species [e.g., Wilson, 19751 and the ability of nonkin to live in groups. Most often this is thought to result from a process of habituation as animals come to recognize one another. For example, Archer [1976] argues that nonrecognition per se of an opponent is what elicits both aggression and fear; there is much evidence that habituation resulting from passive exposure to an opponent without interaction can lead to a reduction in aggression [e.g., Peeke, 1969; Peeke and Veno, 19761. However, if animals are able to assess relative fighting ability of an opponent and can avoid fights when they have little chance of success [Parker, 19741, then, when fighting does occur, it may, in part, be motivated by uncertainty about relative fighting ability [Enquist and Leimar, 19831 rather than nonrecognition alone. The decline in fighting with increasing familiarity may result from the reduc- tion of this uncertainty [Rushen, 19851.

The idea that animals have some means of judging the probability of winning a fight, and do not fight unless they have a reasonable chance of success, has been articulated many times with varying degrees of formalism [Ewer, 1968; Parker, 1974; Maynard Smith and Parker, 1976; Barnard and Burke, 1979; Popp and DeVore, 1979; van Rhijn and Vodegel, 1980; Parker and Rubenstein, 1981; Bekoff and Byers, 1985; Beaugrand and Zayan, 1985; Rushen, 19851. Most commonly, this idea has been used to try to understand the evolution of ritualized fighting, but there is interest in its use to understand the causal bases of fights [Enquist and Leimar, 1983; Beaugrand and Zayan, 1985; Rushen, 19851.

It has long been known that animals need not fight in order to settle a contest but instead can reach a decision on the basis of certain anatomic cues, such as relative antler size in deer, or through certain displays, such as pushing contests. The idea that this occurs because animals make some assessment of relative fighting ability prior to fighting is supported by four main lines of evidence: 1) the individual with the greater chance of winning is more likely to initiate fighting [e.g., Wachtel et al., 1978; Caldwell and Dingle, 1979; Berger, 1981; Elwood and Glass, 1981; Robertson, 19861; 2) the anatomic cues or behavior displays that are used to settle contests are accurate predictors of actual fighting ability [e.g., Davies and Halliday, 1978; Clutton- Brock and Albon, 1979; Robertson, 19861; 3) experimental changes to these cues result in predictable changes in the fights [e.g., Guhl and Ortman, 1953; Davies and Halliday, 19781; and 4) the use of displays is less successful and takes longer, and fights are more likely to occur when the individuals are evenly matched for fighting ability [e.g., Geist, 1971; Wachtel et al., 1978; Caldwell and Dingle, 1979; Robert- son, 19861.

Much of the interest in the concept of assessment has centered on animals’ use of cues before initial fights and the possibility that fighting can be avoided altogether. The possibility that animals will update their assessment of relative fighting ability as a result of events that occur during a fight has been raised many times [Maynard Smith and Parker, 1976; Bekoff et al., 1981; Parker and Rubenstein, 1981; Enquist and Leimar, 19831 but has been subject to relatively little detailed empirical investi- gation [e.g., Crespi, 1986; Enquist and Jakobsson, 1986; Turner and Huntingford, 19861. If assessment does occur during a fight, then fights should be shorter (that is, the loser should give up sooner) when there is a large difference in fighting ability between the two contestants. Furthermore, there should be some observable behav-

Assessment of Fighting Ability in Pigs 157

ioral differences between ultimate winners and losers that could serve as a source of information for the contestants to update their assessment of fighting ability [Enquist and Leimar, 1983; Enquist and Jakobsson, 19861.

That animals might use the outcome of previous encounters as a basis for assessing relative fighting ability has also been much discussed [Barnard and Burke, 1979; Popp and DeVore, 1979; van Rhijn and Vodegel, 1980; Beaugrand and Zayan, 1985; Rushen, 19851 but little investigated. In this case, losers of the first encounter should be less likely to initiate subsequent encounters than should the winners, and should give up sooner in later fights.

MATERIALS AND METHODS

To investigate the ability of animals to make assessments of relative fighting ability, it is necessary to observe fights between animals of known fighting ability. In many species, the larger animal tends to win in a fight, and fights are most intense between individuals matched for weight [e.g., Clutton-Brock and Albon, 19791.

Repeated paired contests were arranged by placing two unacquainted pigs into a small pen for 1 hour each day on two consecutive days and videotaping the fights that occurred. Half of the contests were between pigs that differed in weight by no more than 0.5 kg, and half were between pigs that differed in weight by no less than 3 kg. Larger pigs tend to dominate smaller ones [McBride et al., 1984; Beilharz and Cox, 1967; James, 1967; Scheel et al., 1977; Hansen et al., 1982; Tindsley and Lean, 19841, although the relationship between body weight and success in fights can be highly variable [Rasmussen et al., 1962; Meese and Ewbank, 19731. There is some evidence that fights are more common between evenly matched pigs [Tindsley and Lean, 19841.

There are many advantages to using domestic pigs as subjects in studies of fighting behavior. First, there is available a large number of animals already adapted to the conditions of close confinement needed for detailed observations. In addition, a large body of literature exists describing the basic motor patterns used in fights and exploring the causal basis of porcine aggression [reviewed in Signoret et al., 1975; Fraser, 1984; Fraser and Rushen, 19871. Most important, however, is that the mixing of different litters of pigs after weaning is common farm practice, and the fighting that results has led to concern about the animals’ welfare [Fraser, 1984; Fraser and Rushen, 19871. Research into the motivation of aggression may provide some clues as to how this aggression can be controlled. The results presented here come from a larger research project, which investigated means of controlling fighting between young pigs. Hence the procedures chosen approximate the conditions under which many pigs, in commercial conditions, live. In this paper, however, only results that are of general theoretical interest for the study of the motivation of aggression are presented. The practical aspect of the results for the control of fighting are presented elsewhere.

Paired contests were observed rather than the routine mixing of the whole litters because in the latter case it proved difficult to obtain records of behavior with sufficient detail. Since it was unclear whether the fights that occurred would be more or less stressful than the fights occurring during normal mixing, the number of animals used was kept to a minimum. The experiment was terminated once the major statistical differences became apparent.

158 Rushen

Subjects Fifty-six Yorkshire pigs were chosen from the specific-pathogen-free herd of the

Animal Research Centre, Ottawa. The pigs had been born in conventional farrowing crates that held one sow and her litter. Eye-teeth were clipped on day 1 and males castrated on day 10. These are routine farm practices and not experimental treatments. Weaning took place when the pigs were about 4 weeks old, the pigs being placed in litter-mate groups with three to six pigs per pen. The pigs chosen for this study had been weaned at least 7 days before but had not previously met any pigs other than litter-mates. The pigs were taken from litters that were about to be mixed together.

Paired contests were arranged between 28 pairs of pigs taken from different litters. For half of the pairs, the pigs were of similar weight (liveweight: X = 10.00 kg, SD = 1.04; difference in liveweight; X = 0.17 kg, SD = 0.17, range 0 to 0.5 kg), while in the other pairs, the pigs differed in weight (liveweight: X = 10.09 kg, SD = 2.38; difference in liveweight: X = 4.05 kg, SD = 0.62, range 3.0 to 5.0 kg). For each pair of pigs of similar weight, an equivalent pair of pigs of different weight was taken from the same two litters. Ten pairs consisted of females, and 18 pairs consisted of castrated males. The distribution reflected the availability of animals. However, sex differences were minor, and so all results were pooled.

Apparatus

The fights took place in a 1.2 x 1.1-m pen with three sheet metal sides, a fourth side of metal bars, and a plastic coated, expanded metal floor. It was not possible to obtain observations with sufficient detail in a larger pen. No food or water was available in the pen. The pen was in a small, temperature-controlled room in which there were no other pigs. A video camera was mounted on a tripod 1-2 m from the front of the pen and angled downward into the pen at about 45 O . A wide-angle lens enabled the whole pen to be seen through the video camera.

Procedure

The encounters took place between 0900 and 1400 hours. The two pigs of each pair were taken from the home rooms and placed in the pen. The experimenter immedi- ately left the room, and the video recorder was turned on. The pigs were removed after 1 hour (unless a fight was in progress, in which case the pigs were left until the fight had clearly finished) and returned to their home pens. The procedure was repeated the next day for the same two pigs. By restricting the fighting to l-hour periods, it was hoped to avoid much of the trauma and injury that results from the prolonged fighting that accompanies routine mixing. Previous observations had shown that a second encounter after 24 hours would show many of the changes in fighting that occur when groups are formed. All encounters took place in the one pen, which was thoroughly hosed down after each contest.

The video recordings were played back at one-sixth normal speed, and an electronic event recorder was used to record the frequency and duration of the various behavior patterns.

Behavioral Measurement

A “bout” of fighting was defined as a series of five or more bites in a period of time lasting at least 15 seconds. If the pigs were separated for 60 seconds or more, a new bout of fighting was considered to have begun. These criteria are discussed in

Assessment of Fighting Ability in Pigs 159

TABLE I. Number of Times Winner and Loser Initiated Fights

Winner Loser

Day 1 Same weight 9 Different weight 5 Total 14

Winner vs. loser: x2 = 1.09, d.f. = I , NS Winner vs. loser by weight difference: x2 = 2.11, d.f. = 1, NS

3 6 9

Day 2 Same weight 6 1 Different weight 3 1

Total 9 2 Winner vs. loser: x2 = 4.45, d.f. = 1 , P < .05 Winner vs. loser by weight difference: expected values too low

Rushen and Pajor [1987]. Only fights in which both pigs did some biting were included. A bout of fighting began when the first bite was observed and ended when the loser stopped biting. I deemed the “loser” to be the pig that first stopped biting and did not subsequently resume biting for the rest of the contest.

The duration of each fight was recorded, as was the number of bites by each animal. All bites were recorded regardless of where or whether they made contact, since this sometimes proved difficult to determine. In addition, the amount of time each animal spent attacking from the side was recorded. An attack from the side, with the attacker standing perpendicular to the attacked pig and biting at the ears, face, and neck, is the most effective position for offence. The behavior and the criteria used for scoring are described in detail in Rushen and Pajor [1987]. The pig that first bit or attacked from the side was considered to have initiated the fight.

RESULTS Assessment Prior to First Fight

In all cases where there was a large weight difference, the heavier pig won the fight. If, prior to actually fighting, the pigs are able to make an assessment of relative fighting ability that is adequate to prevent fights, then, on day 1, fights should be less likely to occur where there is a large difference in weight, and the smaller pigs should be less likely to initiate the fighting. However, neither of these predictions was supported. Fighting occurred among 12 of the 14 same-weight pairs, and among 11 of the 14 different-weight pairs, a nonsignificant difference (x2 =0.24 d.f. = 1 NS). On day 1, losers were no less likely to initiate the fights than were the winners (Table I).

Assessment During the Course of the First Fight If the pigs update their assessment of relative fighting ability during the course of

the fight, then ultimate losers should give up sooner where there is a large difference in weight. This prediction was supported. In ten cases, both the same-weight pair and the equivalent different-weight pair (that had been taken from the same two litters) fought on day 1. In these cases, the same-weight pairs fought for longer (X = 500 s, SD = 570 s) before the loser gave up than did the different-weight pairs (X = 107 s,

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Assessment of Fighting Ability in Pigs 161

SD = 116 s) (Wilcoxon matched pairs test: T = 6, N = 10, P < 0.05). Bites and attacks from the side would seem the likely candidates for the behavioral events from which the pigs could draw information about relative fighting ability. During the fights on day 1, the ultimate winners in the different-weight pairs bit more frequently than the losers and spent more time than the losers attacking from the side (Table 11). In the same-weight pairs however, the eventual winners and losers did not differ in either the frequency of bites or in the time spent attacking from the side (Table 11). Furthermore, the winner spent a greater proportion of the time attaclung from the side when the weight difference was large (X = 0.12, SD = 0.08) than when the weight difference was small (X = 0.07, SD = 0.06), a significant difference (Wil- coxon matched-pairs test: T = 7, N = 10, P < 0.05). However, there was no difference in the rate of biting by the winner (Wilcoxon matched-pairs test: T = 10, N = 10, NS).

Assessment as a Result of Previous Fights

If the pigs update their assessment of relative fighting ability as a result of victories or defeats in previous fights with a given opponent, then fighting should be less likely to occur the next time they meet, and, if fights do occur, the loser of the first fight should be less likely to initiate the fight. These predictions were supported. Twenty- three of the 28 pairs fought on day 1, but only 11 fought on day 2. McNemars test for the significance of changes [Siegel, 19561 showed this to be significant ( x 2 = 7.1 d.f. = 1, P < 0.01). In ten cases, the winner of the fight on day 1 also won the fight on day 2. This did not occur in one evenly matched pair. There was a nonsignificant trend for fights to be less likely on day 2 if the weight difference between the pigs was large. Seven of the 14 pairs of same-weight pigs and only four of the 14 pairs of different-weight pigs fought on day 2 (x2 = 1.35 d.f. = 1 NS). On day 2, the losers of the first fight were less likely to initiate the fight (Table I). However, it was not possible to determine if this effect was influenced by the size of the weight difference, since losers so rarely initiated fights on day 2.

Although a defeat on day 1 did not prevent fighting on day 2, the loser tended to give up fighting sooner on day 2. Eleven pairs of pigs fought on both days: these pairs fought for a longer time on day 1 (x = 461 s, SD = 540 s) than on day 2 (X = 201 s, SD = 378 s) (Wilcoxon matched pairs test: T = 1, N = 10, P < 0.01). The loser did not give up sooner on day 2 because the winner was fighting more vigorously. There was no difference between the days in the proportion of fight time that the winner spent attacking from the side (Wilcoxon matched-pairs test: T = 10, N = 8, NS), and the winner actually bit at a lower rate on day 2 (day 1: X = 0.22, SD = 0.09, day 2: 2 = 0.05, SD = 0.06, Wilcoxon matched-pair test: T = 2, N = 8, P < 0.05). Interestingly, the losers did not fight any less vigorously on day 2 even though they gave up sooner. There was no difference between days in the rate by bites by the loser (Wilcoxon matched-pair test: T = 15, N = 8, NS) or in the proportion of fight time that the loser spent attacking from the side (Wilcoxon matched-pairs test: T = 17, N = 8, NS).

EXPERIMENT 2 I needed to show that the reduction in the incidence of fighting that occurs between

day 1 and day 2 is a result of the reduction in the uncertainty about relative fighting

162 Rushen

Water nipples l . l r n 4

f PENlu<2 !I ‘’ PEN 3 PEN 4 1 . 2 ~ 4 pigs : 2 pigs 2 pigs ‘Arena‘

litter A : litter B litter B . . Meial bars partitions

Fig. 1. Layout of pens and pigs in experiment 2. Fights took place in the “Arena.”

ability as a result of the fights themselves. To do this, I had to separate out the effects of simple habituation. This could result from passive exposure; the animals’ need not interact. Fraser [ 19741 found that young pigs kept in adjacent pens that allowed visual and olfactory but only limited physical contact showed a lower incidence of biting and chasing when they were placed together, compared with pigs that did not have opportunities for such contact. In view of this, the decrease in the incidence and duration of fights noted on the second day in the previous experiment could be a result of habituation rather than to the outcome of the initial fights. The following experiment suggests that this interpretation is unlikely.

MATERIALS AND METHODS

Fights were arranged between pigs that had no prior contact and between pigs that had been in close visual, auditory, and olfactory contact before fighting.

Forty castrated male, Yorkshire pigs were used. Four pigs from each of two litters, matched as closely as possible for age and liveweight, were allocated to the pens shown in Figure 1. Some of the pigs (liveweight: X = 8.71 kg, SD = 1.47) were kept in close contact for 72 hours, separated by metal bars spaced at approximately 10 cm (pens 1 and 2). The others (liveweight: X = 9.54 kg, SD = 1.23) were kept separated (pens 1 and 3). The pens were identical to, and in the same room as, the observation pen used in the previous experiment. Food and water were continuously available. The lights were off for 14 hours per day during the 72 hours.

One of the groups of pigs in pens 1 and 2 were videotaped during the 72-hour housing period. The pigs could place their snouts between the bars and sniff each other. Some biting between the bars occurred, but such bites rarely made contact. I timed every time the snout of one of the pigs in pen 1 came within 15 cm of any part of the body of the pigs in pen 2. The total duration, divided by eight, gave an estimate of 2.6-hour “proximity” per pair of pigs during the daylight hours, which is, of course, longer than could occur during experiment 1.

The fights took place between 0800 and 1500 hours. One pig from pen 1 and one pig from either pen 2 or pen 3 were placed together in the “arena” and videotaped for 1 hour. They were then returned to their pens and the procedure repeated until all four pairs had fought. This procedure was replicated five times, until ten familiar and ten unfamiliar pairs of pigs had been tested. The pigs in the familiar pairs had a mean difference in liveweight of 0.85 kg (SD = 0.91), while the pigs in the unfamiliar pairs had a mean difference in liveweight of 0.37 kg (SD = 0.41). Each familiar pair

Assessment of Fighting Ability in Pigs I63

was matched in terms of equivalent body weight with an unfamiliar pair. The pairings were not arranged in any particular order. For technical reasons, the recordings of the interactions between one pair of familiar pigs and one pair of unfamililar pigs could not be used. The duration of each fight was recorded in the manner described for experiment 1.

RESULTS

The incidence of fighting was not affected by prior familiarity between the pigs. Eight of the nine pairs of familiar pigs and eight of the nine pairs of unfamiliar pigs fought when placed together (x2 = 0.00 d.f. = 1, NS). In only seven of the nine cases did both the familiar pair and the equivalent (i.e., matched for weight) unfamil- iar pair fight. There was a tendency for the pigs that had had prior contact to fight for a shorter period of time (familiar: X = 324 s, SD = 250; unfamiliar: X = 566 s, SD = 751). However, this was not significant (Wilcoxon matched-pairs test: T = 12, N = 7, NS). In view of the lack of effect on the incidence of fighting, it is unlikely that habituation alone is responsible for the differences between day 1 and day 2 noted in the previous experiment. Rather, it appears that the animals need to interact physically.

DISCUSSION

Enquist and Leimar [1983] described fights as being driven by the animals’ uncer- tainty about their fighting abilities relative to those of their opponents. If one animal is certain that it can beat its opponent and its opponent is equally certain that it will lose, then fights should be brief; one animal will threaten or attack, and the other will immediately retreat. Prolonged fights will only occur when the animals cannot accurately judge whether or not they are able to beat a given opponent. Fights consist of a series of events from which animals extract information to reduce their uncer- tainty. This paper presents evidence that the cessation of fighting and the reduction in the incidence of fighting that occurs as young pigs become acquainted is due to a reduction in this uncertainty.

The findings suggest that pigs do not, prior to fighting, make an assessment of their chance of success adequate to prevent a fight. The young age of the animals may be relevant; learning which cues to predict success in fights may require more experience in fighting than they had had [Bekoff and Byers, 1985; Rushen, 19851. However, there is evidence that the pigs make an assessment of their chance of success during fights. When the weight difference was large, the ultimate losers ceased fighting sooner. This effect may be due to the losers lowering their assessments of chance of success as a result of receiving attacks from the side and bites from opponents during the fights. The winners spent more time attacking from the side when there was a large weight difference than when there was not. Furthermore, where the weight difference was large, the winners bit more frequently and spent more time attacking from the side than did the losers. Those differences were less apparent when the animals were of similar weight. The number of bites received and attacks from the side would seem to be the specific events from which the animals extract information about relative fighting ability.

The importance of these lateral attacks can be understood in terms of the mechanics of pig fights [Rushen and Pajor, 19871. During fights, most time is spent in the head-

164 Rushen

to-head configuration, with both pigs biting at a similar rate. However, bites are delivered most effectively, and avoided most effectively, by attacking from the side. In order to move from the head-to-head position to attack from the side, the pigs push against each other’s snouts and shoulders in an attempt to lever their opponents around. The opponents, of course, try to do the same. A pig with a marked advantage in weight or strength would be more likely to succeed in this maneuver.

Previous work has shown that the reduction in fighting that occurs as pigs become acquainted [Meese and Ewbank, 1973; Fraser, 19741 is based on individual recogni- tion [Ewbank and Meese, 1971; Ewbank et al., 1974; Meese and Baldwin, 197.51. The study was not designed to test whether habituation plays any role in the establish- ment of individual recognition. Rather, the aim was to determine whether the reduc- tion in fighting was due in part to a reduction in the animals’ uncertainty about relative fighting ability that resulted from the fight itself and not due solely to habituation, which could result from passive exposure. In this it has partly succeeded, by showing:

1. Keeping the animals in visual, olfactory, and auditory contact had minimal effect on fighting. Therefore, habituation alone is insufficient to account for the changes; a fight seemed necessary.

2. The losers of the first fight gave up fighting sooner on the second encounter not because of any increased vigor in fighting by the winners. This change would seem, therefore, to be due to memory by the loser of the outcome of the first fight with its opponent.

3. The second time the animals met, the losers of the first fight were less likely to initiate fighting than were the winners. No differences were apparent in the tendency to initiate fights on the first encounters, however. A difference in behavior between the winners and losers appeared to have been established as a result of the first fights.

Together, these results suggest that the lower incidence of fighting the second time the pigs met occurred because the losers of the first fight had lowered their estimates of relative fighting ability. A further consequence of this would be that where a large weight difference existed, then the incidence of fighting on the second encounters would be even further reduced, with the initial losers even less likely to initiate fighting the second time. Unfortunately, the data are inadequate to demonstrate this, although the first prediction is supported by a nonsignificant trend for a lower incidence of fighting on the second day, when there was a large weight difference. Losers of first encounters too rarely initiated fights on second encounters for the second prediction to be tested.

A defeat in the first encounter did not always prevent the losers fighting on the second encounter, although they did give up sooner. Possibly there is some threshold above which an initial estimate of fighting ability will lead the animal to fight. As a result of receiving attacks, this estimate will be lowered, with the rate of decline setting the duration of the fight. Once the estimate drops below the threshold, the animal will give up. However, as a result of temporary factors, such as fatigue [Rushen and Pajor, 19871, an animal may stop fighting while its estimate of fighting ability remains above the threshold. Once the fatigue has passed, the animal will resume fighting, but since its estimate of relative fighting ability is closer to the threshold, the second fight should end sooner. Although the idea that animals do

Assessment of Fighting Ability in Pigs 165

estimate their chance of success is a simple one, the precise mechanisms by which this estimate is formed and updated need to be specified.

Maynard Smith [1974] and Caryl [1979] have presented theoretical reasons for believing that an animal engaged in agonistic display should not provide information about the likely duration of its display through the vigor of the display. This paper suggests a similar effect for escalated fights. The losers of the first fights were less likely to start the second fights and gave up sooner. However, once they began to fight the second time, there was no evidence that they fought less vigorously in terms of rate of biting or proportion of time spent in the most effective positions for offence.

Moving the concept of assessment from a context in which functional explanations of behavior predominate [e.g., Parker, 1974; Maynard Smith and Parker, 1976; van Rhijn and Vodegel, 19801 to one where this attractive idea can be given a causal interpretation [Enquist and Leimar, 1983; Beaugrand and Zayan, 1985; Rushen, 19851 may provide the skeleton for a cognitive model of the motivation of aggression that acknowledges that animals have more detailed knowledge about their social compan- ions than simple recognition of them [Barnard and Burke, 19791. Game theorists and observers of fighting behavior have paid too little attention to describing the specific mechanisms by which animals use past experience to update their assessment of fighting ability. For example, none of the strategies examined by van Rhijn and Vodegel [ 19801 describe the behavior of these pigs. The different ways that previous experience with an opponent serves to reduce the uncertainty about the chance of winning a fight may explain some of the properties of dominance relationships and hierarchies [Beaugrand and Zayan, 1985; Rushen, 19851.

ACKNOWLEDGMENTS

I held a Visiting Fellowship awarded by the Natural Sciences and Engineering Research Council of Canada. Thanks to E. Pajor, D. Fraser, and two anonymous referees. I received useful suggestions from participants at an IEC-xx workshop organized by R. Zayan. A.R.C. contribution No. 1429.

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