EXPERIMENTAL INTERGROUP ENCOUNTERS IN ...

D. Zinner et al.: Group Enconters in Lion-tailed Macaques EXPERIMENTAL INTERGROUP ENCOUNTERS IN LION-TAILED MACAQUES (MACACA SILENUS). ZINNER, D., HINDAHL, J. AND KAUMANNS, W.

Key words: intergroup encounter, Macaca silenus, male-female differences, matedefense, resource defense, infanticide avoidance

Abstract We studied the behavior of two small one male-groups of lion-tailed macaques (Macaca silenus) during experimental intergroup encounters. Each group was confronted 10 times with the members of the other group for 30 minutes. Control observations under the same conditions in the absence of the second group were conducted. We compared the male and female behavior during encounters and in control situations. Males and females participated actively in the encounters. Members of the larger group showed aggression towards the other group, with females being as aggressive as the male. Both sexes were targets of aggression. Males of both groups directed more friendly behavior towards the members of the other group than did females. Mainly the male of the respective other group was target of friendly behavior. During intergroup encounters, males showed herding behavior towards their own females, as reflected by increased aggression and mounting rates compared to control situations. The results of this study suggest that a mixture of resource and mate defense strategies and infanticide avoidance influence intergroup encounters in liontailed macaques.

Introduction In primates, intergroup encounters are relatively frequent and intergroup interactions are highly variable, from friendly interaction to aggressive confrontations (CHENEY, 1987). Males and females should respond differently towards strange or extra-group conspecifics, because of their divergent reproductive strategies (TRIVERS, 1972). Three main hypotheses have been proposed to explain primate behavior during intergroup encounters, (1) resource defense hypothesis (WRANGHAM, 1980; CHENEY, 1987), (2) mate defense hypothesis (CHENEY, 1987), and (3) infanticide avoidance (VAN SCHAIK and DUNBAR, 1990). These hypotheses are not mutually exclusive, since they reflect strategies of different sexes. In relation to resource defense, an additional aspect may occur. Larger groups or groups with more males may have an advantage during intergroup conflicts (dominant group concept: MITANI and RODMAN, 1979; CHENEY, 1987; ‘hired guns’: WRANGHAM, 1980; RUBENSTEIN, 1986). WRANGHAM (1980) suggested that in female-bonded species, such as macaques, between-group competition should occur over food resources. Females are expected to be hostile towards females in other groups, because their reproductive success is potentially reduced by between group competition (resource defense hypothesis). The behavior of females towards males should be more ambiguous, depending on their reproductive state (LINDBURG, 1971; HAUSFATER, 1972; CHENEY, 1983; RUBENSTEIN, 1986) and by the degree of sexual dimorphism in size and fighting

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D. Zinner et al.: Group Enconters in Lion-tailed Macaques ability (CHENEY, 1983; 1987). Extra-group males may have some advantages for females, such as higher reproductive quality (CLUTTON-BROCK and HARVEY, 1976; HRDY and WHITTEN, 1987). Females may therefore solicit extra-group copulations or even immigration of new males. On the contrary, extra-group males may increase the risk of infanticide (SOMMER, 1994). Females may therefore prevent male immigration, avoid strange males or practice copulations with extra-group males thus confusing paternity (VAN SCHAIK and DUNBAR, 1990; REICHARD, 1995). These precautions would lower the risk of infanticide. The reproductive success of males depends largely on their access to fertile females and their ability to monopolize them. During intergroup-encounters, males may try either to gain access to females of the other group, to exclude other males from access to females of their own group (mate defense hypothesis) or both. The behavior of males is strongly influenced by whether or not an individual has already transferred from his natal group (CHENEY, 1983). In many species, males show more aggression during intergroup encounters than females (CHENEY, 1987), mainly towards non-group males. They also may herd females and infants of their own group away from extra-group males, thus monopolizing their reproductive resources and protecting their offspring (HAMILTON et al., 1975; PACKER, 1979; BYRNE et al., 1987). Natal males of female philopatric species have to find another group to reproduce. They often provoke intergroup encounters and approach members of neighboring groups, attempting to initiate affiliative interactions (SUGIYAMA, 1976; PACKER, 1979; CHENEY, 1981). Intergroup encounters provide opportunities to gather information about potential competitors or reproductive partners. Both sexes may be attracted by another group. Furthermore, the reaction towards other groups must be influenced by the group's history. If groups are the result of a recent fission of a larger group or if closely related males are in the two groups, individual reactions may be affected by former relationships (GABOW, 1972; STRUHSAKER and LELAND, 1979; CHENEY, 1983; RON, 1996). Systematic studies of intergroup encounters are lacking for most primate species, often due to methodological difficulties (CHENEY, 1987). In most cases, the simultaneous study of social behavior in more than one group is difficult to achieve, particularly if the groups are large or the habitat limits observations. The interactions are often fast and chaotic, which makes it almost impossible for observers to identify all participants and to observe their relevant social interactions. The presence of the observer may also have an effect by altering the behavior of habituated focus groups and unhabituated strange groups. As a result, most data on behavior during group encounters are anecdotal. Also under captive conditions there is very rarely a possibility to investigate systematically intergroup encounters of nonhuman primates. Therefore, we took advantage of the situation at the German Primate Center, where two lion-tailed macaque groups had alternately access to a large outdoor enclosure. Lion-tailed macaques (Macaca silenus) are unique among macaques, because they are the only obligate rain forest dwellers in this genus (GREEN and MINKOWSKI, 1977) and because they are truly arboreal (SOUTHWICK et al., 1970). It was therefore questioned, whether these deviations from ‘normal’ macaque ecology have some impact on their social organization, particularly their intertroop relationships. The aim of our study was to analyze the inter- and intragroup behavior of male and

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D. Zinner et al.: Group Enconters in Lion-tailed Macaques female lion-tailed macaques during intergroup encounters. We used an experimental approach to investigate intergroup encounters of this poorly studied species, comparing encounter and control situations. The following questions were addressed: 1) does mate defense, resource defense or infanticide avoidance influence the behavior during intergroup encounters; and 2) does any evidence exist to suggest that the interaction pattern of lion-tailed macaques are an exception to the pattern of other female-bonded macaque species?

Methods We observed two small groups of lion-tailed macaques at the Deutsches Primatenzentrum Göttingen, Germany. Group composition, age and sex of the nine members of the groups and kinship of the study females are shown in Figure 1. Each group consisted of one adult male and one or two adult female, respectively. The two males, J and A, had the same father, but different mothers and spent part of their lives in the same group. Female C was pregnant during the study. None of the females showed estrus swellings during the study. Female E and the juvenile male Me of group 2 were handreared.

Group1 J (adu, m)

C (adu, f) A (adu, f) Am (juv, m) Lu (inf, f)

Group 2 A (adu, m) E (adu, f) He (inf, f) Me (juv, m)

Fig. 1: Group composition and kinship of the studied lion-tailed macaques. Dashed lines indicate maternal relationships within groups. Female A is also mother of female E from Group 2, but female E was separated from her mother and handreared. In group 1, male J is father of juvenile Am and infant Lu, but not of female A. In group 2, male A is father of infant He. Juvenile Me was handreared and introduced into the group. (m = male; f = female; adu = adult; juv = juvenile; inf = infant (age classes according to KUMAR, 1987)). Group 1 was established 3 years prior to the study and was housed in a 27 m2 indoor enclosure. Group 2 was established 2 years before the study and was housed in

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D. Zinner et al.: Group Enconters in Lion-tailed Macaques an adjacent 14 m2 indoor enclosure. Each day, both groups had access alternately to a 1500 m2 outdoor enclosure for half the day, respectively. Indoor and outdoor enclosures were furnished with wooden stems and branches. The macaques were fed indoors twice a day. There was no visual contact between the two groups, but they could hear and smell each other.

Encounter procedure and schedule Two weeks before the first encounter experiment we installed small wire mesh cages (100 x 90 x 160 cm) in front of each slider door connecting in- and outdoor enclosures (Fig. 2) and habituated the macaques to these new objects. These cages were called encounter cages. During encounter experiments, one of the two macaque groups had access to the outdoor enclosure, whereas the second group had to stay in its respective indoor area and the adjacent encounter cage. When in the cage, the members of the two groups could see, hear and smell each other, but physical body contact was prevented by the cages.

outdoor

cage

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indoor Control session

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Encounter session

Fig. 2: Possible space use of focal-group members (black dots) and ‘strange’ group members (stars) during control and encounter situation. In both situations, the focal-group was allowed to move freely in the indoor and outdoor enclosure. In control situation, the ‘strange’ group was locked indoors. During encounters, however, members of the ‘strange’ group were allowed to use also the encounter cage.

The group in the outdoor enclosure was called focal-group, the other group was called ‘strange’ group. The members of the focal-group had access to the outdoor enclosure 20 minutes before we started the observation. They were allowed to move freely in the outdoor and indoor enclosure. The ‘strange’ group, respectively, was only allowed to use its indoor enclosure and the encounter cage. The encounter experi-

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D. Zinner et al.: Group Enconters in Lion-tailed Macaques ment started when we opened the door between indoor enclosure and cage. Behavioral data were taken only from the respective focal-groups. Both groups were at differing experimental days used as focal- or ‘strange’ group. KUMAR and KURUP (1985) found that free-ranging lion-tailed macaques had a mean duration of group encounters of 24.7 minutes. We therefore terminated an encounter by confining the ‘strange’ group to its indoor enclosure after 30 minutes by closing the door to the cage. We also observed focal-groups in control situations, where all condition were the same as during encounters with the exception that the ‘strange’ group had no access to the encounter cage, thus the two groups could not see each other. Within four consecutive days, we conducted two encounter and two control sessions, followed by a one day break. We repeated this block of 5 days 10 times. In every block, each group was used in one encounter and one control session as focal-group, the second group was used as the ‘strange’ group, respectively. The sequence within the four days blocks was randomly chosen.

Data sampling We recorded the behavior of all members of the focal-group simultaneously by using video cameras. We also recorded comments of the observers. With a second camera we recorded sequence and duration of cage occupation by members of the ‘strange’ group. The video tapes were analyzed for interactive and spatial behavior, using one-zero and scan sampling methods (ALTMANN, 1974). Interactive behavior was recorded by one-zero sampling with a 15 second interval length. We differentiated aggressive and friendly interactions (DE WAAL, 1989) with group mates and behavior directed towards the ‘strange’ group (encounter cage). Aggressive behavior included threat, attack and chase. Friendly or tension reducing behavior included present, grin, grimace, lipsmack and purse lips (JOHNSON, 1985). The enclosure was divided into a 2 m x 2 m grid system. Every 60 seconds we determined the spatial position of each adult member of the focal-groups in the outdoor enclosure and calculated the distances between individuals and encounter cage.

Statistical analysis Numbers of scans or one-zero intervals in which an animal was at a certain position in the enclosure or showed a certain behavior, respectively, were counted and expressed as a percentage of total number of scans the animal was in sight. We compared control and encounter sessions in course of 10 encounter experiments. These comparisons, as well as comparisons of males and females and between groups were carried out with an ANOVA repeated measure design or Wilcoxon matched pair tests.

Results The respective ‘strange’ group did not avoid the encounter cage. In 94.25 % of the encounter time (group 1: 94.75 %, group 2: 93.75 %), at least one member of the group

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D. Zinner et al.: Group Enconters in Lion-tailed Macaques occupied the cage. During control sessions, no behavior of focal-group members was directed towards the cage. It was therefore concluded that interactions of the focal-group towards the encounter cage were directed towards the individuals inside the cage.

Aggressive behavior towards the ‘strange’ group Both females and the male of group 1 showed aggressive reactions towards the ‘strange’ group, but members of group 2 showed almost no aggression (Fig. 3; ANOVA repeated measures: group F1,3 = 10.72, p