Handclasp Grooming Among Captive Chimpanzees

Primates (2006) 47: 27–34 DOI 10.1007/s10329-005-0141-0 ORIGINAL ARTICLE Kristin E. Bonnie Æ Frans B. M. de Waal Affiliation promotes the transmission of a social custom: handclasp grooming among captive chimpanzees Received: 23 September 2004 / Accepted: 22 January 2005 / Published online: 3 September 2005 Ó Japan Monkey Centre and Springer-Verlag 2005 Abstract Handclasp grooming is a unique social cus- Keywords Chimpanzees Æ Culture Æ Handclasp tom, known to occur regularly among some, but not all grooming Æ Social learning populations of chimpanzees (Pan troglodytes). As with other cultural behaviors, it is assumed that this distinc- tive grooming posture is learned socially by one individual from another. However, statistical comparisons Introduction among factors thought to influence how a behavior spreads within a group have never, to our knowledge, Most of the socially learned behavior that has been been conducted. In the present study, the origination documented in nonhuman primates relates to interac- and spread of handclasp grooming in a group of captive tions with the physical environment, such as tool use, chimpanzees was followed throughout more than foraging techniques, and other behaviors often labeled 1,500 h of observation over a period of 12 years. We ‘‘material culture’’ (Nakamura 2002). However, a small report on the frequency, bout duration, and number and number of ‘‘social conventions’’ (cf. Perry et al. 2003b) demography of performers throughout the study period, also exist. These include unique grooming patterns such and compare these findings to those reported for wild as social scratching (Nakamura et al. 2000) or leaf- populations. We predicted that dyads with strong affi- clipping in chimpanzees (Pan troglodytes; Boesch 1996; liative ties, measured by time spent in proximity to and Nishida 1987; Whiten et al. 1999), and hand-sniffing and grooming one another, were likely to develop a hand- various ‘‘games’’ among wild white-faced capuchin clasp grooming partnership during the study period. A monkeys (Cebus capucinus; Perry et al. 2003a, b). Apart quadratic assignment procedure was used to compare from these social conventions, the transmission of social correlations among observed frequencies of grooming ‘‘attitudes’’ such as peacefulness (e.g., de Waal and and proximity with handclasp grooming in all possible Johanowicz 1993; Sapolsky and Share 2004), and social dyads within the group. As predicted, the formation of positions, such as the inheritance of social rank in cer- new handclasp grooming dyads was positively correlated copithecine monkeys (e.g., Kawai 1958; Kawamura with the rate of overall grooming and proximity within a 1958) and the transmission of preferred social partners dyad. In addition, in nearly all dyads formed, at least from mothers to daughters (de Waal 1990, 1996), have one individual had been previously observed to hand- also been reported. clasp groom. We concluded that affiliation and indi- One social custom that has attracted significant vidual experience determines the transmission of attention is handclasp grooming in chimpanzees. First handclasp grooming among captive chimpanzees. reported from Mahale Mountains National Park, Tan- zania, by McGrew and Tutin (1978) this striking behavior is known to be customary in some, but cer- K.E. Bonnie (&) Æ F.B.M. de Waal tainly not all, wild populations (McGrew et al. 2001; Department of Psychology, Emory University, Nakamura 2002; Whiten et al. 1999). The handclasp and Living Links Center, Yerkes National Primate Research Center, GA 30329, USA grooming posture has also been well documented in one E-mail: [email protected] captive chimpanzee group housed at the Field Station of Tel.: +1-404-7279071 the Yerkes National Primate Research Center, near Fax: +1-404-7273270 Atlanta, Georgia (de Waal and Seres 1997). A second K.E. Bonnie Æ F.B.M. de Waal group at the Yerkes Field Station, kept under almost Living Links, Yerkes National Primate Center, identical conditions as the first, has never been observed 954 N. Gatewood Road, Atlanta, GA 30329, USA to handclasp groom despite equally intensive 28 observation. The chimpanzees in each group must be could be closely followed in a contained group for many equally able to physically perform the posture, but years. ecological, functional, or methodological variants can- There are a number of reasons why affiliation might not explain with any certainty why handclasp grooming be an important factor in the development of handclasp is present in one group, but absent in another. grooming within a dyad. First, social grooming in gen- The widely accepted explanation for the occurrence eral represents an intimate affiliative interaction between of handclasp grooming and other cultural behaviors is two individuals (McGrew and Tutin 1978). Moreover, that individuals within a population adopt new social handclasp grooming requires at least some degree of customs by watching and interacting with members of cooperation, coordination, and trust among the their group. The question of how behaviors are trans- grooming partners (de Waal and Seres 1997). Therefore, mitted from one individual to another can be addressed we would expect that only pairs with strong affiliative on at least two different levels. One method is concerned ties would establish a handclasp grooming partnership. with identifying the particular learning mechanism Affiliation can be defined on a number of levels, from through which behaviors are transmitted, such as imi- genetic relatedness to complex political alliances. It can tation, emulation, local, and stimulus enhancement (e.g., also be measured in terms of the amount of time two Whiten 2001; Whiten and Ham 1992). Although this individuals spend in proximity to one another or in approach is widely applied in a range of taxa, estab- engaged in positive interactions such as grooming. lishing the mechanism via which learning takes place has The purpose of the present study was twofold. Our historically proven to be an arduous task requiring first goal was to document the continued spread of controlled experimental settings. A complementary, handclasp grooming throughout the Yerkes group in equally important, issue is not how, but from whom a terms of the number of individuals performing it, the behavior is learned. rate at which it occurs, the duration of handclasp Traditionally, this issue has been addressed with grooming bouts, and the variety and composition of observational methods. Among Japanese macaques handclasp grooming dyads. This goal expands upon the (Macaca fuscata), for example, the practice of sweet- findings reported by de Waal and Seres (1997). Sec- potato washing spread first within the innovator’s family ondly, we sought to evaluate retrospectively which fac- and to her peers, and was only later adopted by others tors may have promoted the spread of this social (Hirata et al. 2001; Kawai 1965). Similarly, Perry et al. custom. Specifically, we investigated how two vari- (2003a, b) reported that among wild white-faced capu- ables—previous experience with the behavior and affili- chin monkeys, games and other social customs appeared ation—might have affected the formation of new to spread along affiliative lines. Nevertheless, reports handclasp grooming dyads. We were able to conduct rigorously examining the spread of behavioral variations such an analysis thanks to more than a decade of con- within a population are rare. There are a few notable tinued data collection on behavior in this particular exceptions in which the transmission pattern of a group with unchanged methods of observation. cultural behavior has been illustrated and/or described (e.g., Hirata et al. 2001; Lonsdorf et al. 2004; Matsuzawa et al. 2001; Perry et al. 2003b), but thus far Methods all such reports lack statistical evaluation of the data against a random model. As a result, questions about For the past 12 years (1992–2003), handclasp grooming the probability that some individuals, but not has been observed in one of the two socially housed others, acquired a particular new behavior remained groups of chimpanzees at the Yerkes Field Station in unanswered. Lawrenceville, Georgia. During this period, the colony The relative absence of rigorous analysis may be was comprised of between 16 and 19 individuals. A attributed in part to challenges of establishing the description of all subjects can be found in Table 1. innovation and diffusion of a particular behavior within Group composition varied slightly during the study due a population (McGrew 1998). Doing so is difficult in to the birth of six infants, several temporary removals natural settings, as behavioral traditions are often for veterinary reasons, and the permanent removal of noticed long after the tradition has been established. four individuals for management purposes. With the Moreover, rough terrain, thick foliage, and other envi- exception of an adult male who was introduced into the ronmental challenges inherently limit when and how colony in 1991, all individuals had been living together these behaviors are observed by human researchers. for up to 15 years prior to the study’s onset or were born Finally, chimpanzee communities have a fluid compo- into the group (see de Waal and Seres 1997, for more sition, such that as individuals immigrate and emigrate details). and groups fissure and fuse the diffusion of a behavior The chimpanzees have access to indoor sleeping within a particular community can be difficult to follow. quarters and a large outdoor compound, measuring Therefore, the spontaneous emergence of handclasp 750 m2. The compound is equipped with visual barriers, grooming within the Yerkes group provided a unique vertical climbing structures, plastic barrels, large tires, opportunity in that the transmission of the behavior and an assortment of toys and other objects with which from one individual to another, since its origination, the chimpanzees can interact. 29 Table 1 Composition of the study group of chimpanzees (Pan any one year the number of total observation hours troglodytes) at the Yerkes Field Station ranged from 48 to 348.

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