Agonistic interactions during competition for different resources in captive European wild pigs (Sus scrofa)

Applied Animal Ethology,

10 (1983) 291-300 Elaevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

291

AGONISTIC INTERACTIONS DURING COMPETITION FOR DIFFERENT RESOURCES IN CAPTIVE EUROPEAN WILD PIGS (SUS SCROFA)

EDGAR M. SCHNEBEL

and JOSEPH G. GRISWOLD

Department of Biology, The City College of New York, and The American Museum of Natunal History, New York (U.S.A.)

(Accepted for publication

28 July 1982)

ABSTRACT Schnebel, E.M. and Griswold, J.G., 1983. Agonistic interactions during competition for different resources in captive European wild pigs (Sus scrofa). Appl. Anim. Ethol., 10: 291-300. We observed a group of captive wild pigs to determine how agonistic encounters vary aa a function of preceding activities and with the availability and distribution of resources. Data are presented supporting previous competition hypotheses. The frequency and intensity of encounters was highest during competition for resources that were limited and defendable. Activities associated with limited but undefendable resources were followed by fewer encounters which were of lower intensity. When unlimited and undefendable resources were involved, agoniatic encounters were infrequent. Comparisons with earlier work show that patterns of agonistic behavior in groups of Sus scrofa, regardleas of the environmental setting, are not simply related to the preceding activity per se, but are related systematically to the availability and distribution of resources associated with that activity. A short-term decrease in the tendency of one male to displace its rank neighbor, not encompassed by familiar notions of rank-neighbor relations, was discovered. A previously undescribed visual display used to prevent or inhibit aggression was observed.

INTRODUCTION Social behavior is flexible in many organisms. Food availability, predation pressure, group composition and other factors are known to influence social behavior (Southwick, 1967; Wilson, 1975; Barash, 1977). Agonistic behavior, in particular, changes with major alterations in the environment. In swine, when new groups are formed of previously unfamiliar individuals, the frequency and intensity of agonistic behavior is high initially and declines after 24 or 48 h (Meese and Ewbank, 1973). Similarly, agonistic behavior rises as more swine are put in pens with a limited number of feeders (Ewbank and Bryant, 1972). It is not clear from these studies how agonistic behavior of swine varies in the absence of major alterations in the environment. Competition hypotheses

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(Wilson, 1975; Barash, 1977) predict that even in the absence of such major changes, agonistic behavior during competition for resources would vary as a function of their relative abundance and distribution. For most species, however, data supporting or disproving such hypotheses are surprisingly limited. To test this prediction, we studied agonistic behavior during the normal daily activities of an established group of wild pigs kept in a zoo enclosure. Specifically, we sought to determine how the frequency and intensity of agonistic encounters, and the proportion of encounters that result in displacements, vary as a function of preceding activities and the resources associated with these activities. METHODS

Animals

AND

MATERIALS

and housing

A group of seven wild pigs (European Wild Boar: Sus scrofa) was studied at the New York Zoological Park (Bronx Zoo). The group originally consisted of 2 mature adult males (M), one of which (Ml) was considerably larger than the other (M2), 3 mature adult females (Fl, F2, F3) and 2 young females (Yl, Y2). Periodically, litters were born to the mature females and the surviving young were raised in the enclosure for short periods before being sold to animal dealers. Except when piglets were nursing, the group was maintained at 7 individuals. The 5 adults were present for the entire observation period. Each animal could be identified by its distinct physical characteristics, such as body shape, hair tufts on the ears, and patterns of white hair on the face. The animals were kept outdoors in a large semi-circular enclosure approximately 10 m in diameter, surrounded on its circumference by concrete walls 2 m high. To the front, along the diameter, visitors were separated from the group by a short fence, a small area of vegetation and a deep trench, 1.5 m wide. Built into the floor of the enclosure was a waterhole, approximately 2 X 3 m, where the pigs could drink and wade. Two small areas of the asphalt floor were rooted up by the animals during the study, and exposed areas were used for wallowing and foraging. The trees present in the enclosure were used for rubbing, scratching and tusk-sharpening. Once each day, zoo attendants swept the enclosure clean and fed the pigs. Purina Macromix, pig mash, and hydroponic grasses were mixed and spread out on the floor of the enclosure in a 2 X 3-m oval, providing ample room for all pigs to feed simultaneously. Observations

and behavioral

categories

0 bservations

Data were recorded for a total of 230 h over a 2-year period. Observations ranged from 1 to 6.5 h per day. The objective in sampling was, for each

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month of the year, to make observations during as many daylight hours as possible when activity in the group was high. Sample observations before 10.30 h, after 16.00 h and on days of severe cold, revealed relatively little activity. Observations were made from a position at the front of the enclosure. Central to the study were agonistic interactions among the 5 adult pigs. For each agonistic interaction or encounter, at least 4 types of information were recorded: the identities of the individuals involved; the type of encounter; the intensity of the encounter; and non-agonistic activities of the participants before the encounter. Specific categories for the last 3 types are described below. Types of encounters Encounters were classified in one of three categories. (1) Displacement. One individual is physically replaced by another animal (Meese and Ewbank, 1973) or moves away from an area with the approach of another individual. (2) Rebound displacement. Same as (l), except that one animal displaces another within 30 s after being displaced itself. A 30-s time interval was chosen because this is about the time needed for a walking animal to come in contact with other individuals anywhere in the enclosure. For both regular and rebound displacements, dominant/subordinate relationships exist. (3) Non-displacement. Neither animal is displaced after an interaction that is of Moderate or High intensity. Intensity of encounters The intensity of each encounter was recorded using a scale of categories similar to that of Ewbank and Bryant, 1972. In all cases, the displaced individual is referred to as the subordinate animal. (1) Low. Displacement without resistance from the subordinate animal after a simple approach or slight physical contact by the dominant animal. Displays were not apparent. (2) Moderate. Displacement after auditory retaliation (loud high-pitched squeals) and a clear visual display by the subordinate animal (arched back, head and snout extended toward the displacer) in response to orientation or approach by the dominant individual. (3) High. Displacement after longer retaliation with more visual display by the subordinate animal in response to orientation or approach by the dominant individual. Hard and frequent snout and head thrusts contact the displacer. Biting and chasing may occur. Behavioral contexts The contexts during which agonistic encounters occurred are as follows: (1) Feeding (F). The animal feeds, either singly or as a member of a group, from a large oval area on the floor of the enclosure.

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(2) Sleeping/Resting (S). The animal lies on the ground with its eyes open or closed anywhere in the enclosure, singly or in a group. (3) Rooting (R). The animal turns up the ground or overturns rocks with upward thrusts of the snout. Rooting occurred over 2 small areas. (4) Exploring (E). The animal moves slowly, with its head down and eyes on the ground, pausing to sniff as it moves along. (5) Nuzzling (N). A male sniffs and rhythmically prods a female with his snout, in the area of the hind quarters, genitals, flanks and behind the forelimbs. This behavior is usually a prelude to attempted matings. (6) Passing (P). An animal moves with its head raised, looking about, without performing any of the above-mentioned activities (in between activities). Analysis of data

Adults were ranked by dominance according to the frequency of displacements by each animal over all others. Each agonistic interaction was scored by its intensity and by the preceding activities of the participants. To do this, a value of 0.5 was assigned to each individual involved in the encounter, and that score was entered into the appropriate behavioral category at the relevant intensity level. For example, if both individuals were feeding at the time of the encounter, the observer would enter 2 counts of 0.5 to the category of Feeding (F) at the relevant intensity-level. If one individual was feeding and the other resting, one count of 0.5 would be entered for Feeding (F) and one for Sleeping/Resting (S) at the proper intensity-level. Based on an ethogram conducted during the first 32 h of the study, we found that the pigs spent different amounts of time engaged in the various activities. To obtain total frequency-scores corrected for the unequal time spent at each activity, the raw frequencies were multiplied by the reciprocal of the percent time spent at that activity (see Table II). For example, if, on the average, pigs spent 10% of the observed time rooting, the raw frequencies for Rooting (R) were multiplied by 10. If the group had spent 50% of the time resting, the raw scores for Sleeping/Resting (S) would be multiplied by 2, etc. Using this method, all counts reflect a time-balanced picture of interactions during each activity. A potential limitation is that there may be seasonal fluctuations in the exact percentages of time spent at each activity. RESULTS

Social strut ture

Swine at the Bronx Zoo have a linear dominance hierarchy, as described for other groups of pigs (Beilharz and Cox, 1967; Ewbank and Bryant, 1972; Meese and Ewbank, 1973). For most of the study, the large male (Ml) was

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the dominant animal, followed in order by M2, Fl, F2, F3 and Yl and Y2. The only change in dominance relations occurred during the first 10 weeks, when Fl was in the latter months of pregnancy and clearly the dominant individual in the group. After parturition, she never again displaced the large male, Ml, and instead was always displaced by him. A change in rank did not re-occur during Fl’s next pregnancy, nor during the pregnancies of other adult females. As found for domestic swine (Meese and Ewbank, 1973) and other species (cattle: Hafez et al., 1969; shelduck: Patterson, 1977), each individual, with one exception (M2), most often displaced its rank neighbor. Interactions between M2 and his subordinates seem an exception to the rank-neighbor generalization. Further analysis of the data shows that ,M2’s tendency to displace other individuals was in part a function of immediate past encounters with his dominant rank-neighbor, Ml (Table I). The distribution of regular displacements by M2 is consistent with the rank-neighbor phenomenon, in that Fl is displaced significantly more often than lower-ranking animals. By contrast, in rebound encounters (those immediately following a displacement by Ml), the subordinate rank-neighbor was displaced significantly less often than all other lower-ranking animals. TABLE I Regular and rebound displacements by the beta-male, M2 Animal and relation to M2

Fl, F2, F3, Yl,

rank neighbor non-rank neighbor non-rank neighbor Y2, non-rank neighbor

Displacements Regular

Rebound

47a 28 21 28

2b 15 29 26

aSignificantly more regular displacements of Fl (rank neighbor) than other individuals (x’ = 8.64, P < 0.05). bSignificantly fewer rebound displacements of Fl than other individuals (x’ = 25, P < 0.001).

The highest proportion of displacements between rank-neighbors occurred during passing encounters. Out of 132 encounters during Passing (P), 102 (77.3%) occurred between rank-neighbors. By comparison, out of 823 encounters during other activities, only 352 (42.8%) involved rank-neighbors. During Passing, a pig frequently deviated from its direction of movement, bringing it into the proximity of its subordinate rank-neighbor and precipitating an encounter. The highest time-corrected frequency of encounters took place during Nuzzling (N), in which Ml and M2 competed for access to estrus females (Table II). Encounters occurred with a moderate frequency in contexts in-

82.5 48 132 162 442 88.5 955.0

Sleeping/Resting (S) Exploring (E) Passing (P ) Rooting (R) Feeding (F) Nuzzling (N) Total

42.0 18.1 10.1 10.7 13.3 0.9 95.ld

Percent time observed

196.4 265.2 1306.9 1514.0 3323.3 9833.3

Time-corrected scores for frequency of displacementsa

87.9 85.4 80.3 78.4 81.8 24.9 77.4

Low 12.1 14.6 11.4 21.6 15.9 33.3b 17.5

Moderate 0.0 0.0 8.3 0.0 2.3 41.Sb 5.1

High

Percentage of displacements at each intensity level

aTime-corrected scores (see text). bSignificantly higher intensity during (N) than all other activities (x” = 191, P < 0.001). ‘Significantly greater proportion of encounters resulting in displacements (P < 0.001). d4 .9% miscellaneous activities.

Total frequency of displacements for each activity

Prior activity

17 10 18 10 25 4 84

Frequency of nondisplacements

82.9 82.8 88.0 94.2c 94.7c 95.7c

Percentage of encounters resulting in displacements

Frequency and intensity of enconnters and percentage of encounters resulting in displacements during various behavioral activities

TABLE II

P

297

volving Feeding (F), Rooting (R) and Passing (P), as compared to Exploring (E) and Sleeping/Resting (S). A contrasting pattern to that described above is seen in the distribution of 84 non-displacement encounters (Table II). In Feeding (F), Rooting (R) and Nuzzling (N), the incidence of nondisplacements is less than 6%, while during Exploring (E), Sleeping/Resting (S) and Passing (P), the proportion is significantly higher. The data are listed in Table II as the percentage of encounters ending in a displacement. Encounters following most activities are of low intensity, with the exception of Nuzzling (N), in which significantly more encounters were of Moderate or High intensity (Table II). One might assume that the higher intensity of encounters during Nuzzling (N) results from the fact that only males interact in this context. However, male-male encounters in all other contexts are of significantly lower intensity (x2 = 42.3, P < 0.001). Visual signals Agonistic encounter of swine are mediated in part by visual signals (Friidrich, 1965; Gundlach_, 1968; Ewbank and Bryant, 1972; Meese and Ewbank, 1973; Beuerle, 1975). We frequently observed a visual display by the subordinate pig, which included arching of the back, orientation of the head downward with the snout directed toward the ground, and aversion of the eyes from the aggressor. A similar posture is included among Beuerle’s (1976) photographs (Abb. 5C, p. 223), and is characterized as retreat. Our observations indicate that while the subordinate pig sometimes retreats following the display, the signal has the specific effect of preventing or inhibiting aggression by the dominant animal. Use of the signal may be illustrated with 3 examples, each of which was observed many times. If both males were rooting near each other and the dominant male (Ml) looked up from rooting, very often M2 immediately assumed this display configuration. He would not root, but merely keep his head down, eyes averted, etc. When Ml resumed rooting, M2 would straighten up and move about. Similar interactions were frequently observed between a male and female during nuzzling encounters. If the female turned and attacked the male, he would give the display. The female would then stop and resume her activity, only to be followed by the male. This sequence would be repeated frequently. A third context is during the moments after an intense clash. The subordinate animal would give the display for as long as the dominant individual was nearby. DISCUSSION

Our investigation of an established group of wild pigs demonstrates that the expression of agonistic behavior is influenced by several factors. We identified 3 of these as the activity of the participants preceding the encounter, their rank relationships, and their immediate past experience in agonistic interactions.

298 Most important, we found that the frequency and intensity of encounters and the proportion that resulted in displacements depended on the resources being used beforehand by the participants. Four patterns were distinguished. These are noted, along with the nature of associated resources, in Table III. The first 3 patterns appear consistent with predictions of competition hypotheses (Wilson, 1975; Barash, 1977) in that, (a) situations where resources are limited and defendable should lead to agonistic encounters (contests) which determine priority of access to the resources, and (b) situations where resources are plentiful and widespread should lead to less competitive interactions, and individuals should spend more time utilizing as much of the resource as they can. TABLE III Patterns

of interaction

Activity

during agonistic Nature

encounters

of resource

at various

Pattern

behavioral

activities

of interaction

Frequency

Intensity

% Displacements

Limited and defendable

High

High

High

Feeding (F) and Rooting (R)

Limited and undefendable

Moderate

Low

High

Exploring (E) and Sleeping (S)

Unlimited and undefendable

Low

Low

Moderate

Passing (P)

Limited and defendablea

Moderate

Low

Moderate

Nuzzling

(N)

93ee Discussion.

In the first pattern, males attempt to nuzzle a single estrus female, a limited and defendable resource. There are frequent agonistic interactions of high intensity, most of which end in a displacement. In contrast, feeding and rooting areas are spread out, so individuals compete for limited but undefendable resources, Here, agonistic interactions are moderate in number, but low in intensity. Resting and exploring activities occur in all areas of the enclosure, and favored sites are not apparent. Therefore, resources associated with these activities appear relatively unlimited and undefendable. Here, infrequent agonistic interactions of low intensity occur, and a lower proportion of these result in displacements. A fourth pattern occurs during passingactivity. In this case, there are a moderate number of agonistic interactions of low intensity, many of which end without a displacement. The nature of the resource in this context is less obvious and will be discussed below. In comparing our results with previous work, it becomes clear that it is not the activity per se, but rather the quantity and distribution of resources associated with that activity which determine the patterns of interaction.

299

For example, food in the Bronx Zoo enclosure is distributed so that defending it is difficult for one individual. Agonistic interactions in this situation are frequent but of low intensity. For groups of 8 domestic swine studied by Ewbank and Bryant, 1972, however, food was presented in a manner which allowed only 4 pigs to feed at a time. The feeders became the focus of frequent but intense agonistic interactions. A similar contrast is evident in descriptions of free-living swine. When food is plentiful and evenly dispersed during summer months, aggressive contact is rare. When food is scarce and clumped during winter months, dominant animals actively defend feeding areas (Graves et al., 1975). In swine, then, feeding activity per se is not necessarily associated with increases in the probability of high-intensity interactions. Such encounters occur only when food resources are limited and defendable. Further evidence for the same conclusion comes from comparing situations in which intense agonistic behavior has been observed. The only interactions in our group similar to those described for feeding pigs by Ewbank and Bryant (1972) occurred when the 2 males attempted to nuzzle an estrus sow. The common denominator in the 2 situations is not the activity itself, but rather the presence of a limited, defendable resource important to the competitors. An apparent inconsistency with the competition hypothesis is the pattern of agonistic interactions observed during Passing (P). There is a moderate frequency of encounters, yet no visible resource is present. In this situation, pigs might be competing for positions in the dominance hierarchy (Maynard Smith, 1978). Consistent with this suggestion are our data demonstrating that subordinate rank-neighbors are involved in encounters significantly more often during Passing (P) than other activities. Dominant positions in the hierarchy are limited and defendable. If rank is the resource in question, one might expect that encounters would be of higher intensity and that a larger proportion would end with displacements. High-intensity encounters occur frequently in newly formed groups in which ranks are being actively contested (Meese and Ewbank, 1973). However, in our established group, the hierarchy is stable. Simply maintaining rankrelationships apparently does not require the intense interactions and the higher proportion of physical displacements seen during the process of hierarchy formation. The fact that rank neighbors tend to engage one another in agonistic encounters more often than non-rank neighbors has been adequately documented in a number of previous studies (see Results). However, an unexpected finding was that the beta-male’s (M2) normally high tendency to displace his subordinate rank-neighbor (Fl) changed as a result of his being displaced by the alpha-male (Ml). In such rebound encounters, M2 was much more likely to displace lower-ranking animals and avoid Fl (Table I). This dramatic short-term shift in M2’s tendency to displace his rank-neighbor is not encompassed by familiar notions of rank-neighbor relations. How commonly such

300

shifts occur and what impact they have on social structure awaits further investigation.

in animal groups

ACKNOWLEDGEMENTS

The following people gave helpful suggestions on early drafts of the manuscript: S. Anderson, J. Grossfield, L. Powers and C. Simon. Our special thanks go to the New York Zoological Society, and especially to James Doherty, Curator of Mammals, for permitting us to observe the animals over the 2-year period of the study.

REFERENCES Barash, D.P., 1977. Sociobiology and Behavior. Elsevier North-Holland, New York, 378 PP. Beilharz, R.G. and Cox, D.F., 1967. Social dominance in swine. Anim. Behav., 15: 117-122. Beuerle, W., 1975. Freilanduntersuchungen zum Kampf und Sexualverhalten des europaischen Wildschweines (Sus scrofa L.). Z. Tierpsychol., 39: 211-258. Ewbank, R. and Bryant, MS., 1972. Aggressive behaviour amongst groups of domesticated pigs kept at various stocking rates. Anim. Behav., 20: 21-28. Fddrich, H., 1965. Zur Biologie und Ethologie des Warzenschweines (Phacochoerus aethiopieus Pallus), unter Berucksichtigung des Verhaltens anderer Suiden. Z. Tierpsychol., 22: 328-393. Graves, H.B., Wilson, M. and Elicker, J., 1975. Behavior of feral swine on Ossabaw Island, Georgia. Proc. Pa. Livestock Day, AS-SW-75-14; 149-150. Gundlach, H., 1968. Brutfursorge, Brutpflege, Verhaltensontogenese und Tagesperiodik beim Europaischen Wildschwein (Sus scrofa). Z. Tierpsychol., 25: 955-995. Hafez, E.S.E., Schein, M.W. and Ewbank, R., 1969. The behavior of cattle. In: EXE. Hafez (Editor), The Behavior of Domestic Animals. Williams and Wilkins, Baltimore, 532 pp. Maynard Smith, J., 1978. The evolution of behavior. Sci. Am., 239: 176-192. Meese, G.B. and Ewbank, R., 1973. The establishment and nature of the dominance hierarchy in the domesticated pig. Anim. Behav., 21: 326-334. Patterson, I.J., 1977. Aggression and dominance in winter flocks of shelduck (Tadorna tadorna I.). Anim. Behav., 25: 447-459. Southwick, C.H., 1967. An experimental study of intragroup agonistic behavior in Rhesus monkeys (Macaca mulatta). Behaviour, 28: 182-209. Wilson, E.O., 1975. Sociobiology: The New Synthesis. Harvard University Press, Cambridge, 697 pp.