NOTICE: This is the author’s version of the following article.
Nakamura M, McGrew WC, Marchant LF, Nishida T (2000) Social scratch: another custom in wild chimpanzees? Primates
41:237–248. DOI 10.1007/BF02557594© Japan Monkey Centre and Springer-Verlag. The original publication is available on SpringerLink at http://link.springer.deSocial scratch: Another custom in wild chimpanzees?
Michio Nakamura1, William C. McGrew2,3, Linda F. Marchant2,
and Toshisada Nishida1
1 Department of Zoology, Graduate School of Science, Kyoto University, Kyoto, 606-8502, Japan
2 Department of Sociology, Gerontology, and Anthropology, Miami University, Oxford, OH 45056, USA
3 Department of Zoology, Miami University, Oxford, OH 45056, USA

ABSTRACT
Chimpanzees (Pan troglodytes schweinfurthii) in the Mahale Mountains National Park, Tanzania, scratch other individuals’ bodies while they groom them. This behavioral pattern of “social scratch” is another example of locality-specific social behavior, or custom, as it is not found in the Gombe National Park, Tanzania, about 150 km northof Mahale, nor has it been reported from any other sites of chimpanzee study. Frequency of social scratch was correlated with frequency of social grooming, but not with frequency of self-scratch. Frequencies of social scratch per
grooming bout among adult and adolescent males, and from lactating females to infants or juveniles, were high, andamong males, higher-ranking males especially received more. These facts indicate some social function of the behavior. Social scratch was directed mostly to the dorsal side of the body. However, when lactating females social
scratched to infants or juveniles, they scratched other body-parts. Social scratch was not lateralized to left or right.
We present four hypotheses on the functional origin and on the learning process of this cultural behavioral pattern.
Key Words: Chimpanzees; Pan troglodytes schweinfurthii; social scratch; social groom; self-scratch; custom; culture.

INTRODUCTION
When Dawkins (1976) chose “You scratch my back,I’ll ride on yours” as the title of Chapter 10 of The
Selfish Gene, he likely did not know that scratchingother individuals is uncommon in most chimpanzee
populations. Although self-scratching is observed
throughout primate taxa including chimpanzees (forreview, see Maestripieri et al., 1992), to scratch sociallyis rare.
The chimpanzees of Mahale scratch other individuals’ bodies while they groom them, hence the term“social scratch”. Nishida (1983) briefly described thepattern as “scratch and rub” in the context of alloparental care among Mahale K-group chimpanzees. Mothers
and allomothers scratch infants as part of their maternal
behavior. Nishida did not publish a detailed report ofthe pattern because it was so commonplace at Mahale
that he did not consider that it might be absent in otherpopulations.
There are many locality-specific behavioral patterns

in chimpanzees, and these fuel an ongoing debate as towhether chimpanzees have “culture” (e.g. Nishida,
1987; Tomasello & Call, 1997; Boesch & Tomasello,
1998; McGrew, 1998; Whiten et al., 1999). Local differences such as tool-use or food preferences (for review, see McGrew, 1992) can be characterized as therelationships between an individual and inanimate objects. On the other hand, locality-specific social behavioral patterns are used in relationships among individuals. For example, a variant of social grooming called
the grooming-hand-clasp occurs in a few populations:
Mahale (McGrew & Tutin, 1978), Kibale Forest(Ghiglieri, 1984), Kalinzu Forest, Uganda (Hashimoto,pers. comm.), Lope (Tutin, pers. comm.) and Tai(Boesch & Boesch, in press). It also occurs in the captive colony of the Yerkes Regional Primate ResearchCenter (de Waal & Seres, 1997). However, the pattern
has never been recorded at Gombe despite more than
37 years of research, nor at other long-term sites suchas Bossou or Budongo. Likewise, the leaf-clipping
courtship display (Nishida, 1980) has been observed at

Mahale but not at Gombe. This also regularly occurs at
Bossou (Sugiyama, 1981) and Tai (Boesch, 1995), but
the contexts vary from place to place. Some researchers(e.g. Nishida, 1987; Boesch, 1996; McGrew, 1998;
Whiten et al., 1999) argue that these social patterns
should be regarded as cultural. However, Tomasello
and Call (1997) assert that most, or all, of such patterns
could be explained by ontogenetic ritualization or conventionalization, in which two organisms essentiallyshape one another’s behavior in repeated instances ofsocial interaction. As social scratch has not been seen atGombe or other sites, it may be another example of a
locality-specific social behavioral pattern that adds
more information to the debate.
Here, we report the details of this behavior anddiscuss its hypothetical origins and function of thisbehavioral pattern and the mechanisms that may facilitate the transmission of this custom among groupmembers.

METHODSWe observed 53 chimpanzees of M group in the
Mahale Mountains National Park, Tanzania. For detailed information about the research site, see Nishida(1990).
Methods of MN (first author)
From July 1996 to May 1997, MN followed 10
males and 10 females as focal target individuals. Eachday MN followed one target as long as possible andrecorded all the grooming behavior during the follow.
This sampling method primarily aimed to record structure and membership of grooming clusters in which
focal individuals engaged. Therefore, grooming behavior was recorded not only on focal individuals but also
on non-focal individuals within the grooming cluster,even when they were not directly grooming with focal
individuals. Total duration of follows was 480 hours,
during which 137 hours of grooming (total accumulation of individual grooming) was recorded. Thoughvisibility was poor in some places, most grooming occurred in relatively open spaces, so that grooming
within 10 m around the target was recordable. Whenmultiple individuals were grooming at the same time,
the start and end time of each individual’s groom or
scratch and changes in partner were recorded on a taperecorder. This sampling method enabled wider observation than that of ad libitum sampling on various individuals by minimizing the possibility that observations
might be biased toward those individuals who tended tocongregate. Unless otherwise stated, MN’s data include
all grooming recorded during the follow, not just the

focal subjects’. Only data after November 1996 wereused in calculating a focal subject’s frequency of behavior per follow, as there was some bias in the focal
data before then. Only 9 focal males were available
after November 1996.
Methods of WCM (second author) and LFM (third
author)
Between 12 September and 18 December 1996,
WCM and LFM collected data on laterality of handfunction on 44 members of M group, ranging in age
from 3–41 years (only babes in arms were excluded).
We sought comprehensive and balanced coverage ofthe group and so used a scheme of switching focal
sampling. When a party was encountered, the observer
chose as targets the individuals with the least data accumulated up to the point. If a party split up, or fused,
the same criterion was used to change subjects. Lengthof observation session was proportional to data accumulated, that is, when shy or peripheral individuals
were present, they were given priority, opportunistically. To economize on effort, an arbitrary ceiling of
100 data-points (bouts) per behavioral category per
individual was imposed.
We defined a bout of social scratch (see Results) tobe separated from another by other elements of grooming (e.g. stroke, pick). For age-sex classes, we defined“mature males” as males after puberty (over 9 years old,
the reported age of first ejaculation), “cycling females”
as those who showed estrous swelling cycles, and “lactating females” as those who suckled infants (therefore
not cycling) during the period of observation. “Youngsters” were subjects below the age of sexual maturity(infants and juveniles).
For comparison, we used the chi-square test,
Spearman’s rank correlation coefficient test,
Mann-Whitney’s U-test, and the Binomial test. All
p-values presented are two-tailed.

RESULTS
Observations
In social scratch, one individual rakes the handback and forth across the body of another, usually with
the nails but sometimes with the distal finger pads ofthe four fingers (Fig. 1). This manual motor pattern isthe same as that of self-scratch. Social scratches always
occurred during sessions of social grooming, none occurred separately. Recipients of social scratch showedno specific reaction to it; instead they just continued tosit or lie still while being groomed.

MN observed 391 bouts of social scratch, WCM
and LFM observed 139 (Table 1). Thirty-one (27 in
MN’s data, 26 in WCM and LFM’s data) of 53 individuals of M group’s members were seen to performthe pattern. Thirty-eight individuals received social
scratch (MN’s data). Using the same methods, WCM
and LFM studied laterality of hand function in Gombe
National Park from September to December 1992 but
never observed social scratch there.
Concordance of the Two Data Sets
For the 31 individuals who were observed at least
once to do social scratch, the relative frequency of social scratch across individuals in MN’s data is significantly correlated with that in WCM and LFM’s data
(SS vs. SSg in Table 1, rs=0.43, p<0.05, N=31). The
concordance of the two data sets is impressive, despite
their having been obtained by different sampling
methods.
Frequency of Social Scratch
Duration of follows (observation hours), number ofsocial scratches, and duration of grooming bouts of
MN’s focal targets between November 1996 and May
1997 are shown in Table 2. The 19 focal individuals
averaged 0.31±0.09 (mean±SE) social scratches per
observation hour and 1.99±0.57 social scratches per
grooming hour. They received an average of 0.39±0.13
social scratches per observation hour and 2.53±0.64 pergrooming hour. With the inclusion of MN’s data on
non-focal individuals, social scratch was seen 0.81
times in every observation hour, and 2.84 times inevery grooming hour.
For the 24 individuals who both gave and received
social scratch, the frequency of giving social scratch
was not correlated with the frequency of receiving social scratch (SSg vs. SSr in Table 1, rs=0.36, p=0.09,
N=24).
There was no sex difference in giving social scratch(SSg in Table 1, U=80, N[male]=12, N[female]=15,
p=0.62; SS in Table 1, U=75.5, N[male]=12,N[female]=14, p=0.66) nor in receiving social scratch(SSr in Table 1, U=129.5, N[male]=16, N[female]=22,
p=0.17).
Comparisons of Social Scratch with Social Groomand Self-Scratch
For those subjects who exhibited social scratch, its
frequency was correlated with the duration of theirsocial grooming in MN’s data (SSg vs. GRg(h) in Table 1, rs=0.65, p<0.001, N=27) as well as in WCM and
LFM’s data (SS vs. GR in Table 1, rs=0.43, p<0.05,
N=26). Those who groom others longer tend to scratch

others more often. The correlation was the same for thefrequency of received social scratch and the duration of
being groomed (SSr vs. GRr(h) in Table 1, rs=0.61,
p<0.001, N=38). The frequency of social scratch also
correlated with that of self-grooming (SS vs. SG in
Table 1., rs=0.42, p<0.05, N=26). The latter correlation
may reflect a connection between social andself-grooming (GR vs. SG in Table 1, rs=0.62, p<0.001,
N=42).
The frequency of social scratch did not correlatewith that of self-scratch (SS vs. SC in Table 1, rs=0.43,
p=0.11, N=15). Those who often scratch themselves do
not scratch others more often.
Distribution of Social Scratch by Age-Sex Class
More than half (220/391) of social scratches were
given in only two combinations of age-sex classes:
mature males to mature males (MM in Fig. 2) and lactating females to youngsters (LY in Fig. 2). Although
the duration of grooming was also long in these two
combinations, the number of social scratches was larger
than expected from duration of grooming (the line in
Fig. 2). Cycling females groom others as often as lactating females groom youngsters but they seem toscratch others less than expected from grooming. The
observed distribution of social scratch was significantly
different from that expected from grooming distribution(χ2=220.86, df=15, p<0.001).
Among mature males, the frequency of received
social scratch per grooming time was correlated with
their dominance rank (rs=-0.84, p<0.01, N=12, see Table 1). This means that higher-ranking males received
more social scratches than expected from the durationof the grooming they received. Social scratch was also
positively correlated with age (rs=0.85, p<0.01, N=12,
see Table 1). However, the frequency of giving social
scratch per grooming by mature males was neither correlated with rank (rs=-0.51, p=0.08, N=12) nor with age(rs=0.48, p=0.47, N=12).
Body Parts Scratched
Body parts to which each class gave and received
social scratch are shown in Fig. 3 and Fig. 4, respectively. In total, the back was most often scratched andaccounted for 65% (256/391) of bouts. However, lactating females showed a different tendency, in that theyscratched the back less than did other age-sex classes(the back and other body parts were not evenly
scratched in different age-sex classes, χ2=44.57, df=3,
p<0.01). The same tendency was seen in the parts
where youngsters received social scratches (χ2=24.76,
df=3, p<0.01). This is because the lactating females
who frequently scratched youngsters were mostly
mothers scratching their offspring. They usuallygroomed infants in their lap and often placed infants
horizontally, holding their limbs. This enabled mothers
to scratch all parts of the infant’s body more freely thanwhen others scratched adult conspecifics.
Laterality of Social Scratch
For 19 individuals who were observed (in both datasets) often enough for statistical testing (Binomial test,
N>6), two (DG and NK) were significantly
left-preferent, three (FT, HB and NS) were significantly
right-preferent, and the other 14 were ambidextrous(Fig. 5). This is level 1 in the 5-tier framework of laterality proposed by McGrew and Marchant (1996).

DISCUSSION
Although there were large individual differences insocial scratch (for example, MJ and DG gave many andDE received many), this pattern was not restricted to a
few individuals but occurred widely among members of
M group. Eighty-one percent (43/53) of M groupmembers either gave or received social scratch; of the
10 who did not, none was older than 9 years. Infants
seldom showed social scratch, but this may be because
they also seldom groomed others until about 3 years old(Nishida, 1988), while most adult individuals showed
this behavior. The frequency of occurrence was highenough to conclude that social scratch was neither anecdotal, idiosyncratic, nor habitual, but was customary(McGrew & Marchant, 1997). Social scratch has notbeen seen at other sites of chimpanzee studies, such as
Gombe (Goodall, pers. comm.), Taï (Boesch, pers.
comm), Bossou (Sugiyama, pers. comm.; Matsuzawa,
pers. comm.; Yamakoshi, pers. comm.), or Ndoki (Kuroda, pers. comm.). Plooij (1984, p. 173) listed a general category of “SCR (scratch)”, but he did not specify
its form or its context.
Function and Origin of Social Scratch
There are several hypotheses on the function andorigin of social scratch:(1) Effective Way of Grooming: Scratching makes thegroomee’s hair erect, which enables the groomer to findmore easily ectoparasites or other materials in the hairor on the skin. Or, scratching may remove ectoparasites(Tanaka, 1998), such as ticks, or sticky fruit sap, suchas of Saba, from the hair. Such substances seem lesseasy to remove by conventional grooming.
(2) Extension of Grooming Context: Grooming is often
regarded as an expression of an intimate relationshipbetween participants. However, grooming is tedious
because it requires taking care to pick at tiny objects.
Those who seek to service intimate relationships but are
reluctant to engage in bothersome behavior may use
social scratch as an easier alternative to keep grooming
contact. Social scratches are brief, but they may be used
to fill gaps between bouts of grooming.
(3) Relief of Tension: Self-scratch can be related tosocial tension or stress in chimpanzees (te Boekhorst etal., 1991; Aureli & de Waal, 1997). Thus, social scratch
may emerge for displacement or release of tension or
stress for the giver, as it does for self-scratch.
(4) Reducing Itchiness of the Recipient: Chimpanzees
may scratch others in order to reduce the recipients’itchiness. It is easy to imagine that chimpanzees (likehumans) feel pleasure when an itch is scratched. They
may scratch others in order to get scratched in return, or
to make others more comfortable, if making them feel
good would make them less likely to be aggressive.
Even if these hypotheses explain social scratch at
Mahale, they also must explain why social scratch does
not occur at other localities. Hypothesis (1) is plausible,
given that social scratch is mostly given to the dorsumof the body (where scratching oneself is difficult), andthat it is often given from mothers to their infant orjuvenile offspring. If there were more detritus or ectoparasites at Mahale than at other sites, this could explain why social scratch occurs only in Mahale. For
example, in Mahale, there are three species of buffalo
bean (Mucuna spp., Papilionaceae) (Nishida & Uehara,
1981) whose pods are covered with many transparent
tiny filaments. In the dry season the needle-like filaments are dispersed by wind and stick to everythingfrom soil and rocks to the trunk, branches and leaves oftrees and woody vines. If you touch such a substratum
coated with the needles, you will itch and scratch. At
Gombe and Taï, there are no buffalo beans (Nishida,personal observation). This abundance of buffalo beans
also supports hypothesis (4). These hypotheses can betested by systematically comparing the frequency ofself-scratch of Mahale and other localities lacking theseplants. WCM and LFM’s frequency data on self-scratch
at Gombe and Mahale do not differ.
The fact that higher-ranking males receive moresocial scratch implies that there are social factors involved in this behavior, which suggests that hypotheses(2) and (3) have merit. Lower-ranking males always
seek to better their relationships (e.g. by grooming)with higher-ranking males, but the latter seldom groom
the former. As a result, lower-ranking males have togroom one-sidedly in order to extend grooming contacts, so they more often social scratch as a function of

(2). For hypothesis (3), it is reasonable to assume thatlower-ranking males are tense when they groom
higher-ranking males, so that social scratch appears
more often. However, hypotheses (2) and (3) cannot
explain why it does not occur in other localities. These
hypotheses also cannot explain the frequent social
scratches from lactating females to youngsters. Perhaps
social scratch originated as (1) or (4) and was given
mainly from mothers to offspring, then it acquired an
added function of (2) or (3) which is now often used bymales.
Learning Process of Social Scratch
Because it is a social behavior, the process oflearning social scratch might be an example of ontogenetic ritualization, as suggested by Tomasello and Call
(1997). According to them, ontogenetic ritualization iswhen individual A performs behavior X, and individual
B reacts consistently with behavior Y, so this repetition
causes A to ritualize behavior X in order to elicit Y
from B. Here if behavior X is social scratch, then whatcorresponds to behavior Y? In social scratch, the recipient shows no specific reaction, so the typical reaction that corresponds to behavior Y, is “no-response”.
For example, recipients do not react negatively byleaving or by aggressing, but instead allow thescratcher to go on scratching. Moreover, the scratcher
need not ritualize social scratch in order to elicit“no-response”, which can be most easily evoked bydoing nothing! Therefore, it is difficult to explain social
scratch as ontogenetic ritualization.
What learning processes may be involved (Zentall,1996)? First, it would seem to be easy for a chimpanzee
to acquire this behavior by individual learning, because
the motor pattern is just self-scratch redeployed. If so,
why do not individual chimpanzees in any other locality learn it as at Mahale? Perhaps the environmental
causes discussed in the first hypothesis enhance individual learning by local enhancement (Zentall, 1996),
but this seems unlikely to explain the differences at
Mahale across ranks or age-sex classes. This would
require (for example) more dominant individuals tohave dirtier hair.
Second, if the fourth hypothesis is true, a chimpanzee could learn to social scratch from the experience ofbeing scratched by others. Chimpanzees may be intelligent enough to remember the pleasurable feeling of
being scratched when they are itchy. It may be that they
can take the groomee’s perspective when they groom,
given their ability to reciprocate, but why should they?
Local differences across populations could also be explained (for example) by environmental factors thatcause itchiness, but this fails to account for the rank andclass differences within a population.
Third, chimpanzees may learn from watching otherindividuals engage in social scratch (Zentall, 1996).
Social scratch sometimes is noisy enough to be an attention-getter. This third type of learning could be program level imitation, in which the goal of the behavior
is learned, or action level imitation, in which the behavioral sequence is copied (Byrne & Russon, 1998).
Either is a reasonable explanation for skillful tool use
such as nut cracking, given that these complex patterns
have a beneficial payoff to the performer (Whiten,
1998). Social scratch, on the other hand, achieves no
obvious benefit for the scratcher, but instead is beneficial to the recipient. Thus, it is hard to see a goal foremulation. Social scratch seems to be a custom of theMahale population, the origin and dissemination ofwhich requires further study.

ACKNOWLEDGEMENTS
MN’s fieldwork was supported by a grant from theInternational Scientific Research Program (07041138 toT. Nishida) of the Monbusho (Japanese Ministry of
Education, Science, Sports, and Culture). WCM and
LFM’s fieldwork was supported by a grant from thePhilip and Elaina Hampton Fund of Miami University.
We thank: Tanzania Commission for Science andTechnology, Serengeti Wildlife Research Institute,Tanzania National Parks, Mahale Mountains Wildlife
Research Centre, and Mahale Mountains National Parkfor permission to do the research; K. Kawanaka, S.
Uehara, H. Sasaki, R. Kitopeni, M. Bunengwa, M. Matumula, and K. Athumani for cooperation in the field.
MN thanks F. Fukuda, T. Nemoto, and H. Kayunbo forhelp in various ways when in Tanzania. We also thank
H. Ihobe, N. Itoh who read earlier drafts of the paper,and other members of the Laboratory of Human Evolution Studies, Kyoto University for useful comments andsuggestions.

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Table 1 Frequency of Scratching and Grooming by M Group.
Names
Males

Year of
birth

DE
1963?
NS
1973
MA
1977
FN
1978?
HB
1980?
DG
1981?
BB
1981
AL
1982
IW
1982
CL
1985?
CT
1985
SS
1985?
PM
1988
DW
1988
PR
1991
OR
1991
CD
1991
XT's inf. 1995
LD's inf. 1996
MJ's inf. 1996
Females SL
1955?
CA
1960?
WX
1961?
GW
1962?
FT
1963?
IK
1965?
NK
1970
OP
1971?
PI
1972?
JN
1974?
XT
1975?
AA
1977?
LD
1980?
MJ
1980?
AK
1981?
AB
1982
TZ
1982
CY
1982?
RB
1986
SY
1987?
SE
1987
MG
1987
AI
1988
PP
1990
CS
1990
FU
1991
PE
1993
IV
1993
SL's inf. 1994
AA's inf. 1995
JN's inf. 1995
WX's inf. 1996
TZ's inf. 1996
total

Rank of
males
2
1
6
3
6
4
8
6
9
12
10
11

SG

GR

SC

SS

GRg(h) SSg

SSg/GR GRr(h)SSr

SSr/GRr

45
93
92
58
51
62
66
100
60
51
54
43
8
41
6
32
3

100
99
37
52
68
53
32
22
45
63
8
31
26
30
16
15
3

83
100
100
58
100
100
71
100
100
100
100
100
27
91
37
52
17

1
7
3
8
22
1
0
4
23
4
1
0
1
3
0
0
0

1.63
1.02
3.30
4.05
10.30
10.22
0.00
2.24
1.97
0.59
0.85
0.00
1.03
1.15
0.00
0.00
0.00
0.00

1
21
23
57
34
43
32
40
47
34
58
7
37
45
27
32
13
28
23

9
14
71
100
84
94
27
55
89
12
91
3
41
98
35
56
28
7
38

4
33
39
100
53
88
80
100
100
86
100
6
78
44
98
100
55
37
41

0
2
1
0
2
7
2
0
5
1
4
0
0
21
5
6
0
0
2

4
15
10
14
21
45
6
6

9
15
51
2
0
51
3
4

30
95
63
40
59
36
11
19

1
0
0
0
0
2
0
0

11.66
6.83
3.03
4.20
3.69
9.19
2.00
2.68
1.02
3.39
1.18
0.72
2.91
2.61
1.23
1.20
0.04
0.00
0.00
0.00
2.66
1.14
5.12
13.59
6.77
2.50
2.34
1.65
6.90
0.13
6.57
0.53
1.10
3.32
3.30
5.93
6.61
0.44
0.54
0.38
2.68
0.76
2.39
1.26
0.29
0.63
0.39
0.29
0.00
0.00
0.00
0.00
0.00

84
16
7
23
7
19
3
10
0
1
6
1
5
31
8
0
0
1
0
18
1
3
12
6
1
11
8
12
1
0
11
0
2
11
2
29
4
1
3
0
0
0
1
12
2
0
14
4
0
0
0
0
0

8.81
2.89
2.38
2.72
2.44
2.50
2.84
2.33
0.00
0.75
2.08
1.52
2.35
8.70
3.63
0.00
0.00
0.51
0.00
57.24
0.64
2.13
1.21
1.33
0.35
2.37
3.55
3.50
0.26
0.00
2.51
0.00
1.54
2.20
1.02
5.59
0.66
0.50
3.74
0.00
0.00
0.00
0.42
2.90
1.24
0.00
6.48
2.79
0.00
0.00
0.00
0.00
0.00

1588 1787 2931 139

19
7
10
17
38
94
0
6
2
2
1
0
3
3
0
0
0
0
0
0
2
0
4
3
19
4
6
0
7
0
4
1
0
88
17
25
7
0
0
0
0
0
0
0
0
1
0
1
0
0
0
0
0

0.75
0.00
0.78
0.22
2.81
1.60
2.57
0.00
1.01
0.00
0.61
1.89
0.00
26.54
5.15
4.22
1.06
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1.59
0.00
3.40

9.54
5.54
2.94
8.46
2.87
7.59
1.06
4.29
1.25
1.34
2.88
0.66
2.13
3.56
2.20
0.90
0.24
1.94
0.10
0.31
1.55
1.41
9.89
4.52
2.88
4.65
2.25
3.43
3.88
0.50
4.38
0.94
1.30
5.01
1.97
5.19
6.10
1.99
0.80
0.24
1.45
0.34
2.37
4.14
1.61
0.76
2.16
1.44
0.39
0.04
0.02
0.37
0.02

Table 2. Observation Hours, Frequency of Social Scratching, and Duration of Grooming of Focal Targets of
MN (Data from Nov.1996-May 1997 shown here.).
Names

SSg*

NS
DE
FN
DG
HB
AL
CT
SS
DW

4
9
13
27
20
0
1
0
0

2
54
12
10
6
2
0
0
20

8.23
23.22
21.07
20.28
17.78
18.00
18.08
8.92
27.07

0.49
0.39
0.62
1.33
1.12
0.00
0.06
0.00
0.00

0.24
2.33
0.57
0.49
0.34
0.11
0.00
0.00
0.74

Females IK
FT
PI
GW
XT
NK
AB
MG
AI
SE
Total

3
4
3
2
1
5
12
0
0
0
104

9
1
0
2
0
8
22
0
1
0
149

15.07
13.15
14.43
21.57
17.80
20.32
16.75
13.50
12.55
12.93
320.72

0.20
0.30
0.21
0.09
0.06
0.25
0.72
0.00
0.00
0.00
0.31

0.60
0.08
0.00
0.09
0.00
0.39
1.31
0.00
0.08
0.00
0.39

Males

SSr* obs.time(h) SSg/obs.

* For abbreviations see Table 1.

Fig. 1 Typical Social Scratch (Photo by L. F. Marchant).

SSr/obs GR* time(h)SSg/GR

SSr/GR

1.52
7.36
3.57
4.57
2.08
0.36
0.71
0.73
2.54

2.63
1.22
3.65
5.91
9.63
0.00
1.41
0.00
0.00

1.32
7.34
3.37
2.19
2.89
5.62
0.00
0.00
7.88

2.08
1.60
1.28
4.03
1.80
2.71
2.88
0.17
0.67
0.24
40.88

1.44
2.50
2.34
0.50
0.56
1.85
4.16
0.00
0.00
0.00
1.99

4.32
0.63
0.00
0.50
0.00
2.96
7.63
0.00
1.49
0.00
2.53

Fig. 2 Total Number of Social Scratches in each Combination of Age-Sex Classes Plotted against Total
Duration of Grooming in the Combination.
Letters in the figure indicate combinations of the classes. The first of the two letters indicates the giver of social
scratches and grooming, and the second indicates the recipient of these behaviors; where M=mature males,
L=lactating females, C=cycling females, Y=youngsters. Therefore LY, for example, means value of social scratches(in y axis) and grooming (in x axis) that lactating females gave to youngsters. The line in the figure indicates
expected number of social scratches when we assume the distribution of them equals to that of grooming duration.

Fig. 3 Percentage of Body Parts by Scratcher.
Body parts combined as follows; back=[back, waist, hip], front=[belly, chest, armpit], shoulder=[shoulder],limb=[arm, hand, leg, thigh, foot], head=[head, face, neck].

Fig. 4 Percentage of Body Parts by Recipient.

Fig. 5 Percentage of Right-Preferent Social Scratch.
Only individuals who social scratched often enough for Binomial testing (N>6) are shown. Number in parenthesis
indicates (right-handed / right-handed + left-handed). “*L” and “*R” indicate individuals who are significantly
left-preferent, and right-preferent respectively, all others are ambi-preferent.