An ethological study of pygmy chimpanzees in Wamba, Zaïre: A comparison with chimpanzees

PRIMATES, 25(3): 255-278, July 1984 255

An Ethological Study of Pygmy Chimpanzees in Wamba, Zaire: A Comparison with Chimpanzees*

AKIO MORI

Kyoto University

ABSTRACT. An ethological study was conducted on a provisioned pygmy chimpanzee group in Wamba, Zaire and was compared with the author's previous study on common chimpanzees, in Mahale Mountains with special reference to the evolution of behavior systems. The relationships between behavior patterns were investigated by analyses of the intra- or inter-individual sequential behaviors. Analysis of behavior pathways showed that "bipedal" was not found (i.e., observed frequency lower than expected value) in the course of the charging display in pygmy chimps, though it was an important element of the charging display in common chimps. Additionally, "bipedal" was found to be an initiating behavior pattern in sexual behavior or dominant-subordinate behavior in pygmy chimps. These differences were related to the decrease of the role of charging display and to the in- crease of the roles of sexual and dominant-subordinate behaviors in pygmy chimps. Sexual behavior, "female genito-genital rubbing," "mutual present" and frequent mounting and presenting between males were suggested to have evolved together as a system. These characteristics of behaviors correspond to the fact that pygmy chimps form more stable and larger temporary groups than common chimps.

I N T R O D U C T I O N

Recently, the social structure and social behavior of pygmy chimpanzees (Pan paniscus) have been described (KURODA, 1979, 1980; KANO, 1980, 1982; KITAMUI~, 1983). These studies tried to discern common and different characters of pygmy chimpanzees and chimpanzees (Pan troglodytes) (hereafter, referred to as common chimps).

Sociologically, both species have common characteristics; multiple male unit groups and joining and part ing of group members in temporary groups. Pygmy chimps have larger temporary mixed groups with a 1 : 1 sex ratio (KuRoDA, 1979; KANO, 1982), while common chimps have smaller temporary groups which tend to have varying functions (HALPERIN, 1979).

Many behavior patterns are common to the two species, though some, such as female genito-genital rubbing, differ (KURODA, 1980). Behavior of pygmy chimps was analyzed from an ethological point of view in this report. Relationships between various behavior patterns were studied, and they are examined in order to understand them as a system of behaviors. This kind of analysis has been done for common chimps (VAN-HOOFF, 1971; MORI, 1982). The characteristics of the behavior system of chimpanzees corresponded to those of their social structure (Mol~I, 1982). In this report, differences of behaviors between pygmy chimps and common chimps were studied with particular attention to the differences of behavior systems between the two species.

*This study was carried out as part of the academic cooperation between l'Institut de Recherche Scientifique (I.R.S.) du ZaYre and Kyoto University.

256 A. MORI

The difficulty in reconstructing the social evolution of apes arises from the fact that con- temporary apes have very varied social structures. WRANGHAM (1979) suggested a continuity of social structure between common chimps and orang-utan. ITANI (1977) suggested that the original social unit of apes was the pair-type, like gibbons, and that the large unit group of present chimpanzees developed through association of the original units which lost their pair bonds. Naturally, the same kind of problems in reconstructing the social evolution of apes are found in tracing behavioral evolution.

The methods used in the present study of pygmy chimps are similar to those of the author's previous study of common chimps (MoRI, 1982); the findings of the two species' behavior were thus comparable.

MATERIALS AND METHODS

SUBJECTS AND OBSERVATIONS

A provisioned pygmy chimp group was observed around Wamba district, Zone de Djolu, Equateur, Republique du Za'ire. These groups of pygmy chimps have been studied since 1974 by KURODA, KANO and KITAMURA, and one, the Elanga group, was provisioned in 1976 by KANO. The group has been periodically provisioned since then and they came to the feeding place only when fruit was not abundant in the forest.

The whole study period was from September 1979 to May 1980. The data used in this report were obtained at the artificial feeding site between December 21, 1979 and April 16, 1980.

The artificial feeding site is an area (40 m • 35 m) where the forest was cleared. The fewer the individuals to observe, the easier it is to check every behavior pattern in a bout of interactions. The data treated in this report consist of those which were taken when only part of the Elanga group came to the artificial feeding site. Relatively stable subgroups are found in the Elanga group (KANo, 1982; KITAMUV, A, 1983). The subjects of the study are members of the K a r n e - I f a k e subgroup (2AM, 5AF, 3ADOLM, 2JM, 1JF and 21 at the beginning of the present study), three adult males (3AM) which frequently associate with the . K a m e - K a k e

subgroup, and an adolescent female (1NF) which newly transferred into the l ( a m e - K a k e

subgroup (KuRODA, pers. comm.). Two adult females gave birth in the latter part of the study period; 1fame on March 16 and Sen on April 16. Days and hours of observation are

Table 1. Days and hours of observation.

Adult Adolescent Juvenile Males' name Kuro Ika Hata Kake Ibo Goro Mon Ten Tawa Kano

Days of 25 13 25 33 19 22 30 17 38 24 observation Hours of 67 26 70 88 49 60 83 45 102 70 observation

Adult Adolescent (NF) Juvenile Females' name Kame Sen Mitsu Shiro Haru Aei Junko

Days of 31 16 24 20 33 6 17 observation Hours of 87 44 70 57 91 17 51 observation

Ethological Study of Pygmy Chimps 257

given in Table 1. The observa t ion hour was recorded f rom when the individual first came

to the feeding site unt i l it left wi th some others. The observa t ion hours include bo th direct

observa t ions and indirect observat ions where the indiv idual s tayed in the forest a r o u n d the

art if icial feeding site while some others remained at the feeding site. M o s t of the individuals

r ema ined in and could be well observed in the art if icial feeding site except an old female,

Sen, which came to the feeding site only to t ake sugar canes. Observa t ion was conduc ted f rom above an a b a n d o n e d te rmi te m o u n d (2 m high) at one end of the feeding site. A b o u t

100 pieces o f sugar carte o f 20-40 cm long were p laced on four fal len tree t runks which

lay 20-30 m in f ront o f the te rmi te mound . Sugar cane was set out before the pygmy chimps

came to the feeding site, and was replenished a b o u t every one and a ha l f hours after their arr ival . A n aud io - t ape was used to record all the interact ions tha t occurred.

BEHAVIOR PATTERNS

Vocal iza t ions and behaviors were classified into 14 vocal iza t ion pa t te rns and 47 behav ior

pa t t e rns which were easy to dis t inguish f rom a dis tance (Table 2). A shor t descr ip t ion o f

some behaviors is given be low: (1) R u n : bo th quad rupeda l and b ipeda l running. Pygmy

ch imps ' runn ing was most ly quad rupeda l in cont ras t to tha t o f c o m m o n chimps; (6) Erect

Table 2. Frequencies of each behavior pattern.

Behavior pattern Frequency Behavior pattern Frequency 1. Run 462 2. Drag branch 363 3. Bipedal 268 4. Present 139 5. Flight 138 6. Erect sit 112 7. Mount 107 8. Penile erection 80 9. Attack 78

10. Groom 66 11. Lie on back 61 12. Mate (ventral-dorsal) 59 13. Female genito-genital rubbing 56 14. Walk backwards 53 15. Defensive face 53 16. Hold out hand 53 17. Mate (ventral-ventral) 51 18. Crouch 48 19. Sway side to side 46 20. Fake run 46 21. Light attack 43 22. Play 42 23. Sway backwards 35 24. Thrust 35 25. Chase 34 26. Follow 32 27. Face to face 31 28. Penile erection (half erection) 27 29. Embrace 24 30. Raise arm 23 31. Hair erection 20 32. Stamp walk 17

33. Mutual present 16 34. Look 16 35. Mount (inter) 15 36. Throw branch 13 37. Mouth to mouth 12 38. Arm shake towards ground 8 39. Hide 7 40. Push 7 41. Show back 5 42. Beat ground 5 43. Grasp 4 44. Open mouth 4 45. Male ventro-ventral

genito-genital rubbing 3 46. Touch 3 47. Give 3

Vocalization pattern Frequency

48. V (hii) 160 49. V (nbiin) 146 50. V (howa) 104 51. V (kii) 99 52. V (huff) 86 53. V (ka) 29 54. V (wraah) 26 55. V (hoi) 26 56. V (kui) 14 57. V (huhuhu) 7 58. V (hog) 6 59. V (ga) 3 60. V (go) 2 61. V (gu) 2

258 A. MORI

sit: sitting hunch with the buttocks slightly elevated; (8) Penile erection: difficult to observe from some directions and from a distant place. Penile erection during mating was excluded; (9) Attack: violent bodily contact, such as bumping, kicking and biting, etc. ; (10) Groom: underestimated, as some of the females stayed and groomed in the forest around the feeding place; (11) Lie on back: noted when it appeared in the course of inter-individual interactions; (13) Female genito-genital rubbing: a characteristic behavior pattern in pygmy chimps. "Two females hold each other and swing their hips laterally while keeping the front tips of vulvae, where the clitorises protrude, in touch with each other" (Ku~oDA, 1980); (14) Walk backwards: mostly in bipedal posture; (15) Defensive face: difficult to observe from some directions and from a distance; (16) Hold out a hand: this pattern is frequently observed among common chimps as an appeasement behavior. This pattern occurring between a mother and her offspring is excluded in counting in order to compare the frequency with that of common chimps. This behavior pattern was most frequently observed between adults and juveniles in pygmy chimps, and rarely observed between adults; (17) Mate (V-V): ventro- ventral mating. In trees both a male and a female were frequently observed to hang from branches and to mate ventro-ventrally in an upright posture. As observations were conducted in the feeding site, this behavior pattern was frequently observed on the ground; (20) Fake run: quadrupedal standing, and throwing out its shoulder. As the pygmy chimps frequently run toward another individual, "fake run" was art effective threat behavior; (21) Light attack: aggressive interactions, but not so violent as "attack"; i.e., to grasp, or to thrust out a hand when another was passing in front, to pull, lightly tapping and pushing; (22) Play: only initiation of play is counted. As play frequently occurred on trees around the feeding place, its frequency was underestimated; (24) Thrust: this pattern was recorded when it accompanied "mounting," and "mutual presenting," but not recorded in the cases of "mating" and "female genito-genital rubbing" which were always accompanied by thrusting. The observed frequency was underestimated due to difficulty in observing from a distance; (29) Embrace: embracing ventro-ventrally. Besides mothers and their offspring, pygmy chimps rarely embraced except during sexual behavior. This behavior was usually observed between juveniles and adults; (30) Raise an arm: stretching an arm at eye level or above the head, or raising both arms sideways; (31) Hair erection: hair erection of pygmy chimps was not so conspicuous as that of common chimps. Underestimated as it was difficult to recognize; (33) Mutual present: KURODA (1980) described it as rump-rump touching. Both participants presented, and their rumps touched each other while thrusting; (34) Look: recorded when it was accompanied by "bipedal" in the case of an alert behavior; (35) Mount (inter): when two pygmy chimps were mating or two females were conducting genito-genital rubbing, a juvenile or an infant (mostly the son of the participating females) rushed to the pair and mounted between the participants or on the back of the upper individual of the pair. The juvenile's behavior patterns were mounting, male genito-genital rubbing in ventro-ventral position (described later), or embracing the participating female ventro-ventrally as mating;

(37) Mouth to mouth: mouth to mouth contact. This pattern mostly occurred as a food begging behavior. The cases between a mother and her offspring were excluded. This behavior was not observed between adults, though it was common among common chimps as an appeasement behavior; (38) Arm shake towards ground: mostly in sitting position. Some- times shaking small branch in standing posture; (39) Hide: adolescent or subadult male was frequently threatened. When a potential aggressor approached, it hid behind a third individ-


ual (usually its mother or adult females); (40) Push: recorded when it occurred in non-aggressive contexts; (41) Show back: recorded when it was followed by grooming; (43) Grasp: in non-aggressive contexts; (44) Open mouth: a threat face. Underestimated, as it was sometimes impossible to see the face from the observation point; (45) Male genito-genital rubbing: observed between males with a large age difference. The younger one clinged to the elder which held him in a face-to-face position. The genital area of both participants touched and rubbed when they thrusted; (46) Touch: excluding the cases between a mother and her offspring; (47) Give: giving food. Cases between a mother and her offspring are excluded; (48) V (hii): hissing. This sounded like a defensive vocalization, but it occurred in wider contexts, especially in tense situations; (49) V (nbiin): defensive vocalization which was uttered when an individual was attacked, etc. It was also uttered in a tense situation, such as by a female during mating; (50) V (howa): when this vocalization is compared to that of common chimps, it corresponds to the latter part of the pant hoot; (51) V (kii): defensive vocalization, which is not as tense as V (nbiin); (52) V (huii): corresponding to pant hoot of common chimps, but which followed initial slow panting. However, this vocalization also involved a defensive element in pygmy chimps; (53) V (ka): threat vocalization, corresponding to the same kind of vocalization among common chimps; (54) V (wraah): corresponding to the wraah call (GoODALL, 1968); (55) V (hoi): a variation of pant hoot, but is uttered as a vocalization of group coordination; (56) V (kui): involving"kui" and "ku." A small vocalization and difficult to locate. In this report, the sound was recorded when it appeared in the course of an interaction; (57) V (huhuhu): a vocalization from an infant to its mother, frequently accompanying a temper tantrum; (58) V (hog): a peculiar and infrequent vocalization made by an adult and seemingly related to group movement; (59) V (ga): play vocalization.

SEQUENTIAL ANALYSIS OF BOUTS OF BEHAVIORS

The sequential analysis employed in the present study follows that of the author's previous study (MoRJ, 1982), and is a little different from that used formerly (VAN-HOOFF, 1971). Series of social behaviors appeared together as a bout which lasted 1 to 3 min in the feeding site. The analyses were conducted on the basis of the assumption that the behaviors which appeared in a bout were related by a series of motivations. The behaviors in the bout were analyzed as a chain of behaviors. For this reason, behavior patterns which did not appear to have much meaning as social behavior, such as mere locomotion and posture, were not adopted. In addition, when the same behavior pattern occurred consecutively, the consecu- tive cases were grouped together and regarded as one.

COMPARISON WITH COMMON CHIMPANZEES

In this study, 17 independent individuals and 2 infants were observed. As this does not differ much from the number of common chimps (26 independent individuals and 3 infants) of K-group which the author studied previously in the Mahale Mountains, Tanzania (1VIORI, 1982) and as study methods were similar, it is easy to compare these two studies. In this report uncited material on common chimps is from the author's previous study (MoRI, 1982).

260 A. MORI

RESULTS

The characteristic feature of behaviors in common chimps is that a rather long series of behaviors appears as a single bout (MoRI, 1982). This was so also among pygmy chimps. Bouts of interactions occurred especially when pygmy chimps arrived at the feeding site, or while we prepared the sugar cane, carrying and cutting it. Bouts of interactions occurred more frequently when more animals were present.

The mean number of behavior patterns per bout was 2.92 (Table 3), and this figure is not very different from the 2.83 obtained for common chimps (MoRI, 1982). Behaviors of pygmy chimps occurred in a sequence as a bout as in common chimps.

Table 3. Parameters which show the occurrence of behavior as a bout.*

Pygmy Common chimp chimp

1. Number of bouts (of interactions) 1,208 2. Total number of behaviors in the whole situation 3,533 3. Total number of successive behaviors in the same individuals 1,168 4. Number of responses by signalees 680 5. Number of participative interactions by an individual beside

interacting chimpanzees 421 6. Ratio of succession of behavior in the same individuals: pair

of succession per behavior pattern

7. Average number of behaviors per bout

8. Average number of responses per bout

9. Average number of participations per bout

*Excluding the interaction of food sharing.

997 2,825

850 407

328

0.330 0.301 (1,168/3,533) (850/2,825)

2.92 2.83 (3,533/1,208) (2,825/997)

0.562 0.408 (680/1,208) (407/997)

0.348 0.328 (421/1,208) (328/997)

INITIATION OF INTERACTIONS

As it is sequences of behaviors that are treated in this report, the first question that arises is which behavior pattern is the initiating behavior of interactions. Frequencies of initiative behavior for each age-sex class are given (Table 4). Cases of initial interactions which were observed more than five times in respective age-sex classes are shown in the table in order to avoid the table becoming too large by showing all cases. As a result, some behavior patterns and cases of low frequencies (less than five times) do not appear in Table 4. V (hii), V(kii) and V (nbiin) are included in one category, V (defensive). Though numbers of members of each age-sex class differ, some tendencies can be discerned. Most of the initiators were adult males, which performed "drag branch," or uttered V(huii) (frequently followed by "drag branch"). The "drag branch" display of adult males was not directed primarily to a specific individual and this is so for both pygmy and common chimps. However, almost half of all "drag branch" cases were directed to specific individuals among male pygmy chimps in contrast to a low frequency of specifically directed behavior in common chimps. Adult females initiated interactions relatively infrequently among pygmy chimps, and this is also the tendency in common chimps. However, female pygmy chimps initiate "drag branch," and this is in sharp contrast to common chimps. Furthermore, sub-adult male pygmy chimps


display "drag branch," whereas even lower rank adult males infrequently conducted charging display among common chimps. Female pygmy chimps initiated "bipedal" arid "lie on back" which resulted in "genito-genital rubbing"; this behavior sometimes spread independently with other interactions as a regulating behavior among females.

SEQUENTIAL BEHAVIOR

Intra-individual Sequences

Intra-individual successive transition of behavior patterns were observed in 1,168 cases (Table 3). The ratio of successive behavior patterns (1,168) to total behavior patterns (3,533) was 0.33, and it is close to the ratio in common chimps (0.301). Pairs of successive behavior patterns which were observed in more than four cases, an arbitrary criterion, were selected and the ratio of the observed to the expected frequency was calculated (Table 5; Fig. 1). Two kinds of expected frequencies were estimated as follows: First, the total frequencies of each behavior pattern (Table 2) was used to calculate the expected value (Eij) 1) of a combination of behavior patterns. The co-occurrence ratio was calculated as Cij /E, j . 2) The second expected value of combination ( F i y ) was calculated using the frequencies of behavior patterns which occurred only in cases of successive behavior patterns (in the transition matrix of successive behavior patterns). In the course of an interaction the addressed individual re- sponds to successive behavior patterns of the addressing individual; then, succession of the behavior patterns of the addressing individual was regarded to have ended by the interven- tion of the responding individual in the definition of the above intra-individual succession even if a behavior pattern of the first addressor follows again after the response. The intra- individual succession of two behavior patterns which involves response of the addressed individual in between is called "skipping association" here. The frequency of the skipping association and its ratio to the expected value are given (Table 6). The expected values are calculated by the total frequency of each behavior pattern (Table 2).

Figure 1A shows behavior pathways which were obtained with the ratio of observed to the second expected value (Table 5) and with skipping association. Behavior pathways of common chimps (using second type of expected value) are given for comparison (Fig. 1B) (MoRI, 1982). In calculating behavior pathways of common chimps, "direct" and "skipping" intra-individual association of behavior patterns were not distinguished. A characteristic feature can be observed in sexual behaviors of pygmy chimps as has been reported in a previous report (KURODA, 1980), which is in contrast to the variety of appeasement behaviors in common chimps (GooDALL, 1968; NISHIDA, 1970; MORI, 1982). The first characteristic difference between the behavior pathways of pygmy chimps and common chimps is the position of "bipedal." In common chimps "bipedal" is found in the course of charging

1) If a behavior pattern, i, is followed by a behavior pattern, j, Eij = 1168BiBj/3533(3533--BO. B~ is the total frequency of the behavior pattern, i (Table 2). A behavior pattern is not followed by the same pattern in accordance with the definition mentioned in the methods. 2) C,j is the observed frequency of a transition pair, where the behavior pattern, i, is followed byj. 3) F~j = C~. • C./(1168-- C~.). C~. is the total frequency of the pair of behavior patterns, in which the behavior pattern, i, precedes any of the other behavior patterns in the transition matrix of behavior patterns. C.j is the total frequency of pairs in which behavior pattern,L follows any of the other behavior patterns.

O

t~

t~

X

o~

Initiator

Recipient

V (huii)

V (howa), V (hoi)

Drag branch

Run

Attack

Chase

Fake run

V (threat)

Flight

V (defensive)

Bipedal

Penile erection

Erect sitting

Walk backwards

Present

Genito-genital rubbing

Lie on back

Hold out hand

Groom

Face to face

Total

t~

B' ,-.v

O ~

O

0~

iao~ "V ~9~

o~ o

~-~.. ~ ==~ I~

~. �9

~ N

~.~

~,~'~

~'~_~.

tOl

~D

~D

to o0

4~

O to

Initiator

Recipient

V (huii)

V (howa), V (hoi)

Drag branch

Run ~, 0

Attack

Chase

Fake run

V (threat)

Flight

V (defensive)

Bipedal

Penile erection

Erect sitting

Walk backwards

Present

Genito-genital rubbing

Lie on back

Hold out hand

Groom

Face to face

Total

s sdtu!q~) .~tU~gd jo .~pn~ S i~o!~o[oqH

264 A. MoRI

display which is followed by "drag b r anch" and " r u n . " However, in pygmy chimps "bi-

pedal" is not found in the course of "d rag b r a n c h " - - " r u n , " and it is the start ing point of the

complex behavior pathways which involve "erect sit," "walk backwards ," "penile erection,"

"sway backwards ," "sway side to side" and "raise an a rm," and lead to sexual or submissive

behaviors. A n interest ing p h e n o m e n o n which should be connected with the change of the

posi t ion of "b ipedal" is "hai r erection." "Ha i r erect ion" usually accompanied "b ipedal"

and was very conspicuous on shoulders, hands, arms and legs in c o m m o n chimps, while it

Table 5. Succession of behavior patterns in the same individuals.

First behavior Following behavior Observed Observed/E(I) Observed/E(lI) Drag branch run 194 11.09 2.98 Erect sit sway backwards 24 63.35 12.33 V (howa) run 20 4.31 2.41 V (huii) drag branch 19 6.34 5.39 V (huii) run 19 4.98 1.23 Bipedal sway side to side 18 14.41 3.07 Run attack 18 4.63 6.7 l Mount thrust 17 47.04 22.48 V (hii) run 16 2.20 0.68 Bipedal look 15 34.54 6.56 Run chase 15 8.87 11.55 V (hii) drag branch 14 2.45 2.64 Bipedal run 14 1.11 0.34 V (nbiin) flight 11 5.59 8.02 V (kii) defensive face 10 19.79 8.03 Present crouch 10 15.38 22.61 Bipedal walk backwards 10 6.95 2.41 Bipedal drag branch 10 1.01 1.07 Bipedal mount 9 3.09 2.17 V (hii) defensive face 8 9.62 2.65 Bipedal hold out hand 8 5.56 2.94 Bipedal penile erection 8 3.68 1.36 Erect sit bipedal 8 2.75 1.69 V (hii) bipedal 8 1.90 1.30 Drag branch bipedal 8 0.78 0.46 Present mutual present 7 32.31 35.62 Erect sit sway side to side 7 14.05 2.71 Walk backwards erect sit 7 12.41 6.70 Bipedal raise arm 7 11.21 2.72 Run throw 7 10.82 7.35 Erect sit raise arm 6 24.10 5.30 Crouch lie on back 6 21.60 23.55 V (nbiin) defensive face 6 7.94 4.14 Penile erection erect sit 6 6.99 5.95 Erect sit penile erection 6 6.92 2.33 V (kii) flight 6 4.56 5.09 V (nbiin) mount 6 3.93 5.28 V (huii) bipedal 6 2.71 1.47 Run V (howa) 6 1.15 4.95 V (nbiin) run 6 0.91 0.53 Run bipedal 6 0.45 0.92 Mutual present thrust 5 94.98 31.89 Penile erection sway side to side 5 14.19 4.96 Bipedal stamp walk 5 10.83 5.00 Defensive face V (kii) 5 10.03 11.83

(continued)

Ethological Study of Pygmy Chimps

Table 5. (continued)

265

First behavior Following behavior Observed ObservedfE(I) Observed/E(II) Raise arm bipedal 5 8.61 5.12 Attack V (nbiin) 5 4.58 8.37 Penile erection bipedal 5 2.43 2.71 V (hii) flight 5 2.31 1.75 V (hii) V (nbiin) 5 2.18 1.42 Bipedal erect sit 5 1.64 0.85 V (howa) drag branch 5 1.37 2.64 Bipedal present 5 1.32 1.20 Penile erection walk backwards 4 9.85 5.61 Crouch V (kii) 4 8.87 12.21 Erect sit walk backwards 4 6.97 2.19 V (wraah) run 4 3.53 2.03 Walk backwards bipedal 4 2.96 2.09 Mount V (hii) 4 2.42 5.09 Run play 4 1.91 3.55 Flight V (hii) 4 1.86 7.37 Bipedal V (hii) 4 0.92 1.03 Run V (nbiin) 4 0.55 1.07

E(I) = Eiy, E(II) =Fty in the text. See text for details.

was inconspicuous and infrequent (Table 2) in pygmy chimps. These facts indicate that the character of antagonistic display decreased in "drag branch" of pygmy chimps as compared with charging display of common chimps.

On the other hand, "bipedal" also appears in situations other than "charging display" among common chimps; "bipedal swagger" in threat and sexual behavior (GoODALL, 1968). Pygmy chimps' "bipedal" rarely accompanied "swagger." They stayed in the same position without "swagger," or "walked backwards," or "swayed side to side" and "moved sideways," when they behaved "bipedally." Female pygmy chimps also behaved "bipedally" to invite a partner to "genito-genital rubbing." In common chimps, the author observed "erect sitting" ( = "sitting hunch") (GoODALL, 1968) to invite a sexual partner, and "bipedal" appeared only in the course of charging display (MoRI, 1982). In summary, "bipedal" of pygmy chimps had characteristics distinct from those of common chimps.

"Hold out a hand" appeared before mating or before female "genito-genital rubbing" in pygmy chimps, and it was accompanied by "bipedal" and sometimes "touching" on a shoulder of the partner. Although "hold out a hand" and "mouth contact" were important as appeasement behaviors among common chimps, these behavior patterns were rarely observed in similar functions among adult pygmy chimps.

The second difference between pygmy chimps and common chimps is that "mount" and "present" were frequently observed as dominant and subordinate behavior among pygmy chimps, while they were rarely observed among common chimps. Frequency of "mount" to total frequency of all behavior patterns was 3.0 ~ in pygmy chimps (Table 2), while it was 0 . 4 ~ in common chimps (MoR~, 1982). "Mount" occurs in behavioral sequences which start from "bipedal." Thus, the sequences which start from "bipedal" lead in two directions, one to sexual and the other to dominance and subordinate behavior.

The third characteristic feature among pygmy chimps was that defensive vocalizations, V (hii) and V (nbiin) accompanied aggressive behavior, such as "drag branch," " run" and "at tack." As V (nbiin) and V (hii) were connected with "flight" and "defensive face" in the behavior pathways, they can be regarded as defensive vocalization. However, the animal

266 A. MORI

which "a t tack" or "m oun t " uttered V (nbiin), and a mounter also uttered V (hii). V (hii) was not strongly connected with a specific behavior pattern, and co-occurrence ratios indicate

/ . ~

)

i Z,'E~!~ ~ i

J . ~ i ~ ~ . ~ ~ . ~Sway backwe rds ] -~ ~ ~

1 \ [ I : b ~ ' ~ - . I I - , , t . ~ ' ~ / I . / , ~' , "'[.[Penile erectiont--'-)JSway side to side] ,~

lJHol I out handJ %]Genito-genital [rubbing

(A) jDefensive face/

~ ~ ~ - - ' ~ ~ - - - ~ TS ,~ Iv (defensi v e ) J - ' - ) ~ i ! : r : - ' . ' - " "- " - : - - ~,',, $

. . . . . . . . . . . . . . . . . . . . .

, . .- . . . _ . ~ i 1:1: : _ . - _ - . - : : ? : ~ , ' " ....... ~ ~ . " . r~-~-;... . -

i / / " ' - ' i i " ' ~ ~ " .v ,." ~, ' .'Ylhand ] ~ J g e n i t a l l

(a)

Times of the expected values

. . . . ~2 ~ X < 3 ~,I0 =< X< 15 )Skipping association . . . . -)3 = X < 5 ~15 -< X < 20

~5--< x <]o :~::~2o ~x

Fig. 1. Behavio r pathways in the transition of behavior patterns in the same individual. (A) Pygmy chimpanzees: direct intra-individual succession [observed/E(II)] together with skipping association (see text). (B) Common chimpanzees.

Ethological Study of Pygmy Chimps

Table 6. Skipping succession* of behavior patterns in the same individuals.

267

First behavior Skipping following behavior O b s e r v e d Observed/E(I) Present V (hii) 15 29.64 Bipedal mount 11 16.85 Erect sit mate (ventral-dorsal) 8 53.20 Present crouch 7 46.11 Bipedal genito-genital rubbing 7 20.49 Present V (kii) 6 19.16 Present V (nbiin) 6 12.99 Walk backwards mate (ventral-dorsal) 5 70.27 Penile erection mate (ventral-dorsal) 5 46.55 Bipedal mate (ventral-dorsal) 5 13.89 Sway backwards mate (ventral-ventral) 4 98.48 Lie on back thrust 4 82.34 Present mutual present 4 79.04 Erect sit mate (ventral-ventral) 4 30.77 Crouch V (hii) 4 22.89 Bipedal mate (ventral-ventral) 4 12.86 V (nbiin) mount 4 11.25 Mount penile erection; half 3 45.63 Follow present 3 29.64 Sway side to side mount 3 26.78 Present defensive face 3 17.89 Mount penile erection 3 15.40 V (kii) V (kii) 3 13.45

*The intra-individual succession ot two behavior patterns which involves response of the addressed individual in between. See text for details.

that V (hii) preceded "drag branch" and "run." Aggressively or dominantly behaving pygmy chimps frequently uttered defensive vocalizations at the same time. The above characteristic is distinct from common chimps, and this might be important to understanding the social characteristics of pygmy chimps. The problem will be discussed again in the section on "Dominance."

Signal Behavior

A chain of directed behavior patterns of different individuals in a interaction can be regarded as a communication process, signal and response. A total of 1,208 bouts of interactions were observed, and 680 responses to directed signals were observed (Table 3). The response ratio for a bout was 0.56, being a little higher than 0.408 in common chimps.

Signal-response cases, which were observed in over four cases, were selected and compared with the expected value (Gij) (Table 7). The expected values were calculated by combination of the total frequency of each behavior pattern (Table 2): G~j = 680 :< B~Bj/(3533) 2. Viewing the correspondence as a signal response, the highly responded to behavior patterns were, "present," "lie on back" and "crouch" except for "show back" responded to by "grooming" which should be excluded by definition; "show back" was recorded only when it accompanied "grooming." "Present" was responded to by "mount ," and "mutual present" between the same sex (males), and was responded to by "mate (V-D)" between different sexes. Females' "lie on back" was responded to by "female genito-genital rubbing" by another female, and by "mate (V-V)" by a male. "Crouch" was responded to by "mount" between males, and was responded to by "mate (V-D)" between different sexes.

Looking over the table of signal-response together with the behavior pathways mentioned

268

Table 7. Relationships between behavior patterns as signal responses.

A. MORI

Signal Response No. of observations Observed/E Present mount 44 54.30 Run flight 40 11.51 Present mate (ventral-dorsal) 30 67.14 Lie on back genito-genital rubbing 28 150.45 Lie on back mate (ventral-ventral) 18 106.20 Run V (nbiin) 17 4.62 Bipedal present 14 6.89 Mount V (hii) 13 13.93 Run V (kii) 13 5.21 Crouch mount 12 42.88 Attack V (nbiin) 11 17.73 Present present 11 10.45 Crouch mate (ventral-dorsal) 9 58.33 Groom groom 9 37.92 Attack V (kii) 9 21.39 Erect sit present 9 10.61 Mate (ventral-dorsal) V (hii) 8 15.55 Mount V (nbiin) 8 9.40 Penile erection follow 7 50.19 Fake run flight 7 20.24 Bipedal bipedal 7 1.78 Present mutual present 6 49.52 Mount crouch 6 21.44 Mate (ventral-dorsal) V (kii) 6 18.85 Sway side to side present 6 17.22 Bipedal lie on back 6 6.73 Present mate (ventral-ventral) 5 12.94 Walk backwards present 5 12.45 Attack flight 5 8.52 Run V (hii) 5 1.24 Show back groom 4 222.49 Embrace mate (ventral-ventral) 4 59.98 Sway backwards crouch 4 43.70 Genito-genital rubbing thrust 4 37.46 Chase V (nbiin) 4 14.79 Bipedal follow 4 8.56 Mate (ventral-dorsal) V (nbiin) 4 8.52 Bipedal crouch 4 5.70 Lie on back bipedal 4 4.49 V (hii) present 4 3.30

prev ious ly (Fig. 1), the s ignal-response can be under s tood as s ignal-response between groups

o f behav io r pat terns . In te rac t ions p roceeded as fol lows: the first g roup ( "b ipeda l , " "erect

si t ," " w a l k b a c k w a r d s , " " sway backwards , " "peni le e rec t ion" and " s w a y side-to-side") [(signal)]; the second g roup ( "presen t , " " c r o u c h " and " l ie on back" ) [(response ---- signal)];

the th i rd g roup ( " m o u n t , " " m u t u a l p resen t , " " female geni to-geni ta l r ubb ing" and " ma te " ) [(response)].

Aggress ive behaviors , " r u n " and " a t t a c k " were r e sponded to by defensive vocal izat ions and "f l ight ."

Participation

A n in terac t ion tended to spread f rom the init ial in terac t ion to others among pygmy

chimps. This p h e n o m e n o n is analyzed th rough examin ing par t ic ipa t ing behav ior pat terns.


Table 8. Relationships between the initial undirected behavior pattern and the following behavior pattern of the participating individuals.

Initial undirected behavior Behavior of participant No. of observations Observed/E Drag branch drag branch 24 5.40 Run run 22 3.05 V (howa) V (howa) 21 57.56 Run drag branch 17 3.00 Run V (howa) 12 7.40 Attack V (howa) 10 36.54 V (howa) V (wraah) 9 98.68 Run V (ka) 9 19.91 Genito-genital rubbing mount (inter) 8 282.36 V (howa) run 8 4.93 Run V (hoi) 6 14.80 Mount drag branch 6 4.58 Bipedal bipedal 6 2.47 Drag branch run 6 1.06 V (hoi) V (hoi) 5 219.29 Run V (nbiin) 5 2.19 Mount mount 4 10.35 Run V (wraah) 4 9.87 Mate (ventral-ventral) mount (inter) 3 116.26 Mate (ventral-dorsal) mount (inter) 3 100.50 Mate (ventral-dorsal) embrace 3 62.81 Mate (ventral-ventral) mate (ventral-ventral) 3 34.19 V (hoi) V (howa) 3 32.89 Genito-genital rubbing mate (ventral-ventral) 3 31.14 Groom groom 3 20.41 Attack flight 3 8.26 Fake run run 3 4.18 Attack drag branch 3 3.14 Run attack 3 2.46 V (howa) drag branch 3 2.35 Drag branch V (nbiin) 3 1.67 V (nbiin) drag branch 3 1.67 V (nbiin) run 3 1.31 Drag branch bipedal 3 0.91 Run bipedal 3 0.71

W h e n a pygmy chimp displays a behav ior pa t te rn , an individual to whom the behav io r is not

directed, may somet imes r e spond to it. In such cases, the response is directed not only to the

first in teract ing individuals , bu t also to a four th individual . Both types o f pa r t i c ipa t ion were observed in 421 cases out o f 1,208 bouts o f interact ions. The ra t io o f pa r t i c ipa t ion per bou t

was 0.348, and was similar to tha t o f c o m m o n chimps (0.328) (Table 3). In terac t ions tend to

spread as easily among pygmy ch imps as a m o n g c o m m o n chimps. Cases o f t rans i t ion o f the ini t ial behav io r and the behavior o f pa r t i c ipan t are given together

with their ra t io to the expected value (Table 8), when they were observed in more t han three cases. Expected values (Hi j) were ca lcula ted by combina t ion o f the to ta l f requency o f each

~--- 1 i 4) behavior pa t t e rn (Table 2): H~j 421 • BiBJ(3533) 2. Derivat ives of voca l ' za t ons, " p a n t hoo t , " V (hoi), V (howa), were f requent ly pa r t i c ipa ted by the same category o f vocal iza t ion

and V (wraah). " R u n " was often, bu t not as f requent ly as expected, pa r t i c ipa ted b y a mtmber

4) Pygmy chimps' three vocalizations, V (huii), V (hoi) and V (howa), corresponded to "pant hoot" of common chimps.

270 A. MORI

of behavior patterns involving derivatives of vocalization, "hoot" and V (wraah). These characteristics of participation in pygmy chimps corresponded to common chimps' excitement in V (hoot) and charging display. Both species have a common characteristic to make "booming" (SUalYAMA, 1969; MORI, 1982). However, the frequency of V (hoot) (frequency of"pant hoot"/frequency of total behavior patterns = 328/2825 ~ 0.11) in common chimps (MoRt, 1982) was higher than the frequency of "pant hoot" derivatives (ratio = 216/3533 = 0.06) (Table 2) in pygmy chimps. Furthermore, participation of "hoot" in common chimps (hoot--hoot participation/total participation = 70/328 = 0.21) was much higher than participation of hoot derivatives in pygmy chimps (29/421 = 0.06). These facts indicate that excitement of pygmy chimps with "hoot derivatives" and "drag branch display" was not so high as in common chimps.

Though the data concerning "mount (inter)" are not sufficient, the tendencies obtained in the following agree with the overall impression from observations which include data not treated in this report. Considerable frequency of "mount (inter)" was observed in participation to "mate" and to "female genito-genital rubbing." Female "genito-genital rubbing" was frequently participated by "mating" in the whole observation. These may indicate that female "genito-genital rubbing" and "mount (inter)" involve some sexual character, if we take a hypothesis that initial interaction and the participated behavior axe connected with the same motivation. If "mate," "female genito-genital rubbing" and "mount (inter)" are regarded as sexual behavior, participation in sexual behavior was in considerably high frequency among pygmy chimps.

TRIPLET INTERACTIONS

In order to grasp the characteristics of the "bout of behaviors" in pygmy chimps, their triplet interactions are examined and compared with the triplet interactions of common chimps. Triplet interactions are categorized in Table 9. An agonistic interaction, dominant and submissive interaction are grouped together (symbol "--") , while an affinitive and sexual behavior are grouped together (symbol " + " ) in pygmy chimps: the left and right sides of the symbols denote the actor and recipient, respectively. The interaction between the two individuals on the left changed to the right interaction which involved a third individual. The observed frequency of each type of interaction is given in the first column of "number of cases" (1) (Table 9).

However, some of the dominant and submissive behavior can also be regarded as affiliative behavior. "Mount" and "present" between the same sex are put into the category of affilia- rive behavior ( " + " ) in the second method of categorization. The frequency of each interaction type by the second categorization is given in parentheses at the right of the column of pygmy chimps [column (2)] (Table 9). The results which were obtained in common chimps are given in the column of common chimp (3) (Table 9) (MoRI, 1982). An appeasement or affinifive interaction is indicated by the symbol " + , " while an agonistic interaction is indicated by the symbol " - - " in common chimps. The characteristic feature of pygmy chimps' triplet interaction as compared with common chimps was that an antagonistic interaction tended to spread. The difference arose from the difference of the character of charging display in two species. "Drag branch" and "run" were frequently directed to another individual among pygmy chimps, and they frequently developed into triplet interaction. These triplet interactions were recorded in pygmy chimps. However, "charging display" of common


Table 9. Comparison of triplet interaction types between pygmy chimps and common chimps.

No. of cases Interaction Initial interaction Secondary interaction Pygmy chimp Common chimp No. Actor Recipient Actor Recipient (l) (2) (3)

1. A -t- B B + C 3 (4) 3 2. A -~ B C -t- B 18 (19) 24 3. A -t- B A § C 6 (8) 25 4. A + B C § A 22 (28) 12 5. A + B C + D 14 (19) 4 6. A -- B B -- C 21 (18) 5 7. A -- B C -- B 60 (52) 37 8. A -- B A -- C 45 (35) 6 9. A - B C -- A 68 (59) 7

10. A -- B C -- D 57 (45) 6 11. A -- B B -t- C 4 (3) 8 12. A -- B C + B 1 (3) 3 13. A -- B A + C 2 (7) 5 14. A -- B C + A 5 (11) 4 15. A - B C + D 7 (8) 2 16. A + B A -- C 4 (7) 2 17. A + B C -- A 16 (13) 5 18. A + B B - - C 1 (4) 1 19. A + B C -- B 3 (8) 1 20. A + B C -- D 6 (12) 0

A, B, C and D denote individuals. The initial interaction proceeds to the secondary interaction. The last three columns show numbers of cases of each triplet interaction type depending on different categorization of affiliative (+) and agonistic (--) interaction in initial and secondary interactions. (1) Pygmy chimps + : affinitive and sexual behavior, --: aggressive, dominant and submissive behavior; (2) in parentheses pygmy chimps; + : affinitive and sexual behavior, "mount" and "present" behavior between same sex, -- : aggressive and defensive behavior; (3) Common chimps + : appeasement and affinitive behavior, --: agonistic behavior. Cases of "mount" and "present" between same sex, " - " category in column (1), are put into " + " category in column (2). The triplet interaction type in the first line (interaction No. 1) can be read as follows; A conducted affiliative (+) behavior to B, then B conducted affiliative behavior (+) to C.

chimps was not usually directed to another individual . Even if undirected charging display

was part ic ipated in by a third individual , the case was not be recorded as triplet interaction.

Most of the charging display of c o m m o n chimps dropped f rom the triplet interaction. A n

impor tan t p roblem which is connected with the difference of charging display in two species

was the difference of the freqeuncy of "fake run . " Pygmy chimps frequently behaved "fake run , " while it was infrequent among c o m m o n chimps.

Fur thermore , an impor tan t p roblem which must be connected with the above difference

in two species, is that " m o u n t " and "present" were frequently observed among male pygmy

chimps, while they were rare among c o m m o n chimps. These indicated that pygmy chimps

frequently conducted behavior pat terns which ascertained dominan t - subord ina te relationships between two individuals.

A n impor tan t feature of the tr iplet interact ion of pygmy chimps was that an affiliative

interact ion ( including sexual interactions) tended to spread. Though the characters o f affilia-

tive behaviors were different between c o m m o n chimps ("appeasement behaviors") and pygmy

chimps ("sexual behaviors") , bo th species showed a common tendency to spread an affiliative interaction.

DOMINANCE

U p to now, a few problems of the character of " domi na nc e " in pygmy chimps have been

272 A. MORI

Table 10. Di rec t ion of behav io r pa t t e rn which appea red in in te rac t ions be tween adul t males and adolescent males.

A. M. ~ Adol. M. Adol. M. ~ A . M . Dominant/subordinate 1)

Attack 19 Light attack 7 Walk backwards 1 1 Drag branch 12 10 Bipedal 17 3 Chase 9 Crouch 6 Defensive face 2 9 Erect sit 3 2 Flight 4 41 Face to face 3 Beat ground 3 Hair erection 1 Hold out a hand 1 Look 1 1 Mount 10 1 Open mouth 1 Penile erection (half) 4 3 Mutual present 2 Present 2 7 Fake run 30 Run 81 10 Arm shake towards ground 2 Sway backwards 3 1 Stamp walk 1 Sway side to side 2 Throw 2 2 Thrust 4 V (nbiin) 4 12 V (kii) 1 16 V (huii) 3 V (ga) 2 v (go) 1 V (hii) 7 11 V (howa) 4 3 V (ka) 17 v (ku) 1 V (wraah) 2

§ §

+ +

§

+ +

+

1) Behavior patterns are classified into dominant ( + ) and subordinate ( - - ) behavior in this table.

Table 11. D o m i n a n c e and in te rac t ions be tween males.

Dominant/subor- (a) Kuro Ika dinate 1~ Kuro-~lka Ika--)Kuro Concordance of direction 2)

Drag branch 1 (1) Bipedal § 1 X Defensive face - 1 X Erect sit 1 (1) Mount § 2 O Present - 1 O Run -4- 1 2 (2) X Sway backwards 1 Thrust 1 V (nbiin) -- 1 2 ?

(cont inued)

Ethological Study o f Pygmy Chimps

Table 11. (cont inued)

273

Dominant/subor- (b) Kuro Hata dinate 1~ Kuro~Hata Hata~Kuro Concordance of direction z~

Drag branch 5 (3) 3 Bipedal -? 1 1 ? Chase § l O Crouch -- 1 O Flight -- 1 2 ? Hold out hand 1 Mount § 1 1 ? Penile erection 1 Fake run + 1 (1) O Run -}- 4 4 ? Sway side to side I Thrust 1 1 V (nbiin) -- 2 4 ? V (hii) 3

Dominant/subor- (c) Kuro Kake dinate 1~ Kuro-~Kake Kake~Kuro Concordance of direction 2~

Attack + 1 3 X Walk backwards 3 (1) 1 Drag branch 28 (15) Bipedal + 13 (3) 6 ? Chase + t ? Crouch -- 1 3 ? DefensiVe face -- 4 4 (1) ? Erect sit 1 (1) Flight -- 5 (1) 4 ? Hold out hand I Look 1 Mount q- 17 4 O Penile erection 1 Mutual present 3 3 (1) Present -- 6 14 ? Fake run + 1 X Run 27 (1) 6 Sway backwards 1 1 (1) Sway side to side 3 1 Thrust 3 5 V (nbiin) -- 7 (1) 7 ? V (kii) -- 1 (1) 4 O V (hull) 4 (2) 4 V (hii) 6 (1) 9 (1) V (howa) I 4 V (ka) 1 1 (1) V (ku) 1 V (wraah) 1

Kuro was dominant over Ika and Ham; these relationships were determined from the behavior of access to suger canes. Numbers indicate total frequency of each type of interaction behavior; numbers in parentheses indicate frequency of initial interaction. 1) Symbols, " § and " - - " indicate dominant and subordinate behavior, respectively (see Table 10). 2) Concordance of direction between dominant/subordinate pattern and dominant/subordinate relationships; symbols, "O," " X " and "? , " indicate concordance, reverse and disparity without clear direction, respectively. Concordance concerning Kuro and Kake is examined here on the assumption that Kuro is dominant over Kake. The direction of "drag branch" and " run" were one-sided, Kuro to Kake.

i n d i c a t e d ; " m o u n t , " " p r e s e n t " w e r e f r e q u e n t l y o b s e r v e d b e t w e e n m a l e s , a n d a l m o s t h a l f

o f al l " d r a g b r a n c h " a n d " r u n " b e h a v i o r s we re d i r e c t e d t o a n o t h e r i n d i v i d u a l as a n aggres-

s ive o r a d o m i n a n t b e h a v i o r .

274 A. MoRI

Here another problem of the character of"dominance" among pygmy chimps is examined. Aggressively or dominantly behaving pygmy chimps frequently made defensive vocalizations at the same time. Further, male pygmy chimps conducted a peculiar behavior pattern "mutual present," in which both of the participants behaved submissively. The dominance-subordinate relationships of pygmy chimps did not seem to be stable from these facts. This point will be further examined.

Among male pygmy chimps (including adults, adolescents and juveniles) an individual intervened with another to take a sugar cane in the artificial feeding site. Or subordinate male did not dare to take a sugar cane just in front of the dominant. From this respect of access to sugar canes dominant-subordinate relationships between males were rather clear. Adult males were no doubt dominant over adolescent males. The direction of each behavior pattern between an adult male and an adolescent male is examined in order to know which behavior pattern is dominant or submissive one (Table 10). Dominant behaviors are "attack," "light attack," "bipedal," "chase," "mount," "fake run," "run" and V (ka) (= a threat vocalization), and submissive behaviors are "crouch," "defensive face," "flight," "present," V (nbiin) and V (kii). The vocalization, V (hii) is a squeak which sounded like defensive vocalization, but it is not clearly submissive.

The directions of these behavior patterns are examined between three adult males, Kuro, Ika and Hata, that frequently moved together having clear dominant-subordinate relationships among them; Ifuro---,lka---,Hata. Interactions between Kuro and Ika, and between Ifuro and Hata are examined (Table 1 la, b). Some of the dominant behavior which were ascertained between adults and adolescents in the above did not clearly follow the dominant- subordinate relationships among them. Though dominant-subordinate relationships were clear between some adult males, dominant behaviors (or submissive behaviors) were displayed by both dominant and subordinate individuals: dominant behavior or subordinate behavior was not clear among adult male pygmy chimps.

The above three adult males, Kuro, Ika and Hata sometimes moved independently of the _Kame-Kake subgroup, while Kake was always found among the subgroup. The dominant- subordinate relationship between Kuro and Kake was not clear in taking a sugar cane, and the relationship seemed to be changing with time elapse. However, Kuro frequently conducted dominant behaviors such as "run," "mount," "bipedal" against Kake (Table l lc). Kuro frequently conducted "drag branch" against Kake. This indicates that "drag branch" was also a dominant behavior between them, though it was not so between familiar adult males and adolescent males. Kuro conducted dominant behavior, when their dominance relationship was unstable. Dominance behaviors displayed its function, when dominance-subordinate relationships was unstable. Another interesting point is that both Kuro and Kake conducted submissive behaviors between themselves. Submissive behavior functions to regulate tension between male pygmy chimps.

DISCUSSION

The characteristic feature of behavior pathways of intra-individual successive behavior patterns in pygmy chimps was that "bipedal" appeared as a starting behavior of affiliative behavior (sexual behavior, "female genito-genital rubbing," "mounting" between males), and that "bipedal" was not found in the sequence of "drag branch" display, whereas "bipedal" appeared in the course of charging display among common chimps (MoRg 1982).


A phenomenon which should be connected with the change of the character of "bipedal" is that "hair erection" was not conspicuous among pygmy chimps in contrast to its con- spicuousness in common chimps which stood bipedally and behaved charging display. Furthermore, "drag branch" was frequently not accompanied by "run" in pygmy chimps in contrast to common chimps. These facts indicate that the character of charging display differs a great deal between the two species. Common chimps' charging display was mostly not directed to any, particular animal and was performed only as a display in front of others, whereas almost half of all "drag branch" and "run" behaviors were directed to another individual among pygmy chimps. Pygmy chimps frequently "run" to directed individuals especially without "drag branch" display, though common chimps very infrequently run to others. In pygmy chimps "run" was partly independent from "drag branch" display. As compared to common chimps, pygmy chimps' "drag branch" decreased its character of ritualized antagonistic display among males, and showed some characteristics of agonistic or dominant behavior. Another fact which further indicates the change of the character of "drag branch" display of pygmy chimps is that pygmy chimps' "hoot" vocalization, V (huii) which accompanied "drag branch," lacked the characteristic initial slow panting of "pant hoot" of common chimps, adding the defensive character of vocalization "i" (MoRI, 1983); slow panting is common between common chimps and gorillas, and the pant hoot ("hoot series") of gorillas is a specific vocalization in inter-group encounters (FossEv, 1972; MoRI, 1983).

A question to be examined now is which of the two behavior systems, that of pygmy chimps, or that of common chimps, is closer to the behavior system of the common ancestor. The behavior patterns, "present," "mount" and "mate" were commonly found in both species, differing greatly in frequencies. However, "female genito-genital rubbing" and "mutual present" (rump-rump touching) must have developed in pygmy chimps. On the other hand, two appeasement behavior patterns of common chimps, "hold out a hand" and "mouth contact," were not found in pygmy chimps. The two species developed different behavior patterns of affiliative behavior from their common ancestor. We can further suppose that pygmy chimps' affiliative behaviors (including the above "female genito-genital rubbing" and "mutual present") developed as a system after the separation of the two species from their common ancestor. From this point of view, the remarkably high frequency of mating behavior (even involving frequent mating between juveniles and adult females), high frequency of "mount" and "present" between males, "mutual present" and "female genito-genital rubbing," all developed together as a system in pygmy chimps.

As the elements of charging display, V (hoot) --"bipedal"--"run" are common to both common chimps and gorillas, the common ancestor of the African great apes must have had V (hoot) --"bipedal"--"run" (MoRI, 1983): for the above reasoning, it was postulated that separation between Pan troglodytes and Pan paniseus occurred after that of Gorilla and Pan. Consequently, "bipedal" dropped from the pathway of "charging display" after chimps' separation from the common chimps. One important point which should be stressed is that charging display is a most important means to express antagonism between males of gorillas (intergroup encounter) (FossEY, 1972; MORI, 1983) and between males of common chimps (RIss & GOODALL, 1977 ; MORI, 1982, 1983). This important signal changed its form in pygmy chimps.

As "bipedal" occurs as a starting point of courses of affiliative behavior, we can suppose that the antagonistic relationship between adult males which was shown by charging display,

276 A. MORI

was necessarily changed to an affiliative one in the process of speciation of pygmy chimps. The affiliative behavior between adult males here involve some dominant-subordinate behavior, "mount" and "present," and "mutual present" which facilitate in regulating social stability. An interesting point in connection with the above is that the dominant-subordinate relationships were not clear among adult males only from the observation of the frequency of directions of dominant-subordinate behavior in spite of its one sided direction following dominant and subordinate relationships between adults and adolescent males. The function of charging display which appeared in common chimps can be said to be replaced in pygmy chimps with a group of dominant-subordinate behaviors ("mount," "present" and "mutual present") which also involve their modified and milder form of charging display. Neverthe- less, this change of character of behavior was not complete as shown in the inconsistency of the direction of dominant and subordinate behavior between adult males. Male common chimps released their antagonistic tension by frequently conducting undirected charging display, and they prevented the break out of agonistic interaction following charging display by having developed a group of appeasement behavior to calm the displaying male (MoRI, 1982). Pygmy chimps changed the form of the male antagonistic behavior, charging display, and changed its function into the direction of a mere dominant behavior. They also frequently conducted other dominant, subordinate behaviors. Their behavior patterns indicate that their behavior system developed in the direction always to confirm dominant-subordinate relationships by conducting sometimes aggressive behavior and sometimes more regulating dominant-subordinate behavior. The author considers that antagonism between males is stronger in common chimps than in pygmy chimps; common chimps conduct ritualized and undirected display and avoid actual aggression in contrast to pygmy chimps, as actual aggression is a real danger to the social stability of common chimps.

Behavior of pygmy chimps will be discussed next by examining the behavior of females. Female common chimps showed a tendency to disperse rather than to gather (WRANGHAM, 1979). The author observed that female common chimps interacted infrequently among themselves and females which came to the feeding place were usually only a small part of the unit group (MoRI, 1982). On the other hand, female pygmy chimps usually moved together as members of rather stable subgroups. Furthermore, different subgroups encountered and moved together (KANO, 1982; KITAMURA, 1983), where females of different subgroups mixed. This high tendency of gathering and the tolerance of female pygmy chimps must have developed together with "female genito-genital rubbing," while female common chimps interacted only infrequently among themselves. The emergence of the peculiar "female genito- genital rubbing" in pygmy chimps indicate that gathering of subgroups appeared in the process of speciation of pygmy chimps; it was necessary for females of pygmy chimps to de- velop special affiliative behavior.

It was discussed previously that the high frequencies of "mount" and "present" between males, "mutual present" and "female genito-genital rubbing," all developed together as a system in pygmy chimps. This problem will be examined again from another angle. It was shown in the section on "signal" that a group of behavior patterns which followed "bipedal" in behavior sequences ("bipedal," "erect sit," "walk backwards," "sway backwards," "penile erection" and "sway side to side") not only induced and resulted in sexual behavior, but also induced "female genito-genital rubbing" or submissive behavior, "present" and "crouch," between males and resulted in "mount" and "mutual present." Though the final consequent behavior patterns have different names depending on the combination of gender


of participants, the pathways as signals which led to final results were same. Namely, "mate," "genito-genital rubbing," "mount" and "mutual present" can be said to belong to the same category, the final response of the same signal-response processes. "Bipedal" and behavior patterns which follow "bipedal" can be called common signals regardless of gender.

As observed in the section on "Participation," "mate" and "female genito-genital rubbing" frequently included "mount (inter)" or "mate." These indicate that "genito-genital rubbing" and "mount (inter)" had a sexual character.

As observed in the section on "Triplet interactions," pygmy chimps also showed spreading of affiliative interactions as did common chimps. Affiliative behaviors in common chimps were various appeasement behaviors, while those of pygmy chimps were sexual behavior and "genito-genital rubbing." In this respect, sexual behavior and "genito-genital rubbing" in pygmy chimps played a similar role as appeasement behaviors in common chimps. Sexual behavior of pygmy chimps has a rather important additional role to the reproductive one, that is, an affiliative one, and sexual behavior and "genito-genital rubbing" have a common character as shown upto now, that is, behavior pattern, function, position of behavior pathways, triplet interaction etc.

In summary, "mate," "genito-genital rubbing," "mutual present," "present" and "mount" developed together as a process of signal response to fulfill a social function, affiliation, among pygmy chimps.

Coexistence of plural males and females without agonistic competition in mating could be guaranteed by changing the character of sexual behavior into affiliative behavior in which all individuals can participate, and by decreasing the reproductive meaning. This coexistence is also dependent on the change of character of charging display, or male antagonism. �9 On the other hand, common chimps evolved to lower the threshold value of charging

display, and adult males conducted frequently undirected charging display in front of others (MoRI, 1982). Thus, they succeeded in releasing tension between males. Furthermore, they developed appeasement behavior to calm an excited male.

The fact that pygmy chimps had a social behavior system which weakened male antagonism supports the theory of social evolution that the ancestral social unit of great apes was small, and several small groups gathered into one in the process of evolution into chimpanzees (ITAM, 1977; MORI, 1982, 1983). From this point of view, the society of pygmy chimps is farther advanced in the tendency to gather than that of common chimps.

Pygmy chimps and common chimps have many differences in behavior, and a very different orientation of behavior systems in spite of being closely related species. It is interesting from the point of behavior evolution that so closely related species have such different behavior systems.

A final point may be added. The phenomenon whereby "bipedal" changed from an antagonistic into an affiliative character may be important in considering the problem of bi- pedalism in human evolution from ethological points of view.

Acknowledgements. This work was carried out with the official support and cooperation of l'Institut de Recherche Scientifique du Za'ire. The author expresses his hearty thanks to the staff of I.R.S. He thanks Dr. T. KANO of University of the Ryukyus, the director of the Japanese team for the study of pygmy chimps at Wamba, and who supported his study there. The author thanks Dr. S. KURODA of Kyoto University and Ms. E. VINEBERG of University of California for their help and valuable dis- cussions at Wamba. He is deeply indebted to many helpful assistants at Wamba, Messrs LIKOMBE BATWAFE, NKOY BATOLUMBO, BAFUTUA BOKONDA, IKENGE LOKAKA, BATUAFE MBELE. For accom-

278 A. MORI

modation and travelling, the author and his colleagues were helped by many persons, especially Fa- thers and Sisters of the Catholic Mission Yalisele, Baliko, Boende, and members of the American Peace Corps in Bandaka and Boende. The author expresses his thanks to Dr. PAMELA ASQUITh for revising the English of the provisional draft of this article.

Financial support for this research was obtained from the Grant of the Overseas Special Research Program of the Primate Research Institute, Kyoto University 1979 from the Ministry of Education, Science and Culture, Japan.

REFERENCES

FOSSEY, D., 1972. Vocalizations of the mountain gorilla (Gorilla gorilla beringei). Anita. Behav., 20: 36-53.

GOODALL, J., 1968. A preliminary report on expressive movements and communication in the Gombe Stream chimpanzees. In: Primates, Studies in Adaptation and Variability, P. C. JAY (ed.), Holt, Rinehart & Winston, New York, pp. 313-374.

HALPERIN, S. D., 1979. Temporary association patterns in free-ranging chimpanzees: an assessment of individual grouping preferences. In: The Great Apes, D. A. HAMBURG & E. R. MCCowN (eds.), Benjamin/Cummings, Menlo Park, California, pp. 491-499.

ITANI, J., 1977. Evolution of primate social structure. J. Human Evol., 6: 235-243. KANO, T., 1980. Social behavior of wild pygmy chimpanzees (Pan paniscus) of Wamba: a preliminary

report. J. Human EvoL, 9: 243-260. - - , 1982. The social group of pygmy chimpanzees (Pan paniscus) of Wamba. Primates, 23 :

171-188. KITAMURA, K., 1983. Pygmy chimpanzee association patterns in ranging. Primates, 24: 1-12. KURODA, S., 1979. Grouping of the pygmy chimpanzees. Primates, 20: 161-183. - - , 1980. Social behavior of the pygmy chimpanzees. Primates, 21 : 181-197. MORI, A., 1982. An ethological study on chimpanzees at the artificial feeding place in the Mahale

Mountains, Tanzania. Primates, 23: 45-65. - - , 1983. Comparison of the communicative vocalization and behaviors of group ranging in

mountain gorillas, chimpanzees and pygmy chimpanzees. Primates, 24 : 486-500. NISHIDA, T., 1970. Social behavior and relationship among wild chimpanzees of the Mahali Moun-

tains. Primates, 11 : 47-87. RISS, D. & J. GOODALL, 1977. The recent rise to the alpha-rank in a population of free-living chim-

panzees. Folia Primatol., 27: 1-27. SUGIYAMA, Y., 1969. Social behavior of chimpanzees in the Budongo Forest, Uganda. Primates, 10:

197-225. VArq-HoorF, J. A. R. A. M., 1971. A structural analysis of the social behaviour of a semi-captive group

of chimpanzees. Aspecten Van Het Sociale Gedrag En De Communicatie Bij Humane En Hogere Niet-humane Primaten, Bronder-Offset n.v., Rotterdam, pp. 11-127.

WRANOHAM, R. W., 1979. Sex differences in chimpanzee dispersion. In: The Great Apes, D. A. HAMBURG & E. R. McCowN (eds.), Benjamin/Cummings, Menlo Park, California, pp. 481- 499.

- - R e c e i v e d July 5, 1983; Accepted April 13, 1984

Author's Name and Address: AKIO MORI, Primate Research Institute, Kyoto University, Inuyama, AichL 484 Japan.

Documents

An ethological study of pygmy chimpanzees in Wamba, Zaïre: A comparison with chimpanzees