Pan Africa NewsThe Newsletter of the Committee for the Careand Conservation of Chimpanzees, and the Mahale
Wildlife Conservation Society
ISSN 1884-751X (print), 1884-7528 (online) mahale.main.jp/PAN/

JUNE 2014
P. A. N. EDITORIAL STAFF
Chief Editor:
Kazuhiko Hosaka, Kamakura Womenʼs University, Japan
Deputy Chief Editor:
Michio Nakamura, Kyoto University, Japan
Associate Editors:
Christophe Boesch, Max-Planck Institute, Germany
Jane Goodall, Jane Goodall Institute, USA
Tetsuro Matsuzawa, Kyoto University, Japan
William C. McGrew, University of Cambridge, UKJohn C. Mitani, University of Michigan, USA
Vernon Reynolds, Budongo Forest Project, UK
Yukimaru Sugiyama, Kyoto University, Japan
Richard W. Wrangham, Harvard University, USA
Takeshi Furuichi, Kyoto University, Japan

VOL. 21, NO. 1

Contents<NOTE>
Encountering Crocodiles while Chasing Chimpanzees
William C. McGrew
1<NOTE>
Discriminating Saba and Landolphia Seeds in
Chimpanzee Feces at Mahale
Michio Nakamura
3<NOTE>
Homosexual Interactions among Young Female Wild
Chimpanzees: An Example of Social Pretend Play?
Masaki Shimada
6

Editorial Secretaries:
Noriko Itoh, Kyoto University, Japan
Koichiro Zamma, Kyoto University, Japan
Agumi Inaba, Mahale Mts. Chimpanzee Research Project
Eiji Inoue, Kyoto University, Japan
Instructions for Authors:
Pan Africa News publishes articles, notes, reviews,
forums, news, essays, book reviews, letters to editor, andclassified ads (restricted to non-profit organizations) on any
aspect of conservation and research regarding chimpanzees(Pan troglodytes) and bilias (Pan paniscus). Contributors arerequested to write in English and the papers except forums,
reviews and essays should usually be 1,500 words or less.
Articles, notes and reviews will be peer-reviewed by at least one
appropriate expert on request of the PAN editorial staff.
PAN is published twice a year in June and December.
Deadline for manuscripts is two months before publication (i.e.
the ends of April and October). Submit your manuscripts via
email to pan.editor@gmail.com.
• Manuscripts: Format as DOC or RTF files• Photos and figures: Format as JPEG or GIF files. Do NOT
paste on Word files or create as PDF files. Figures could besent as excel files also.
• Audiovisual data: Authors could include audiovisual data toenhance their papers, although they will be included in the
online version only. Sound or video files should be sent only
after communicating with the editor to obtain more detailed
instructions.
• Send these separately by e-mail attachments.
©Michio Nakamura

See also http://mahale.main.jp/PAN/instruction.html.
Deadline of the next issue is October 2014!

1

Pan Africa News, 21(1), June 2014

2

<NOTES>

Encountering Crocodiles while
Chasing Chimpanzees
William C. McGrew
Department of Archaeology & Anthropology, University ofCambridge, U.K.
(E-mail: wcm21@cam.ac.uk)

T he Nile crocod ile (Crocod ylu s nilot ic u s) is
sympatric with the chimpanzee (Pan troglodytes) acrossmost or all of its range (IUCN 2013). The crocodile is a
top predator and even when not fully grown is big enoughto prey upon a chimpanzee. Crocodiles prey on smallerbodied monkeys (Galdikas & Yeager 1984; Galdikas
1985; Cheney & Seyfarth 2007). Njau and Blumenschine(2012) showed that crocodiles of only 1–2 m bodylength preyed on Homo habilis, whose body-size waschimpanzee-like. However, crocodiles are rarely listed asa potential predator in studies of wild chimpanzees.
In reports of the ethology and ecology of chimpanzees, crocodiles figure, if at all, only in passing comments(McGrew et al. 1996, p. 318). Risk of predation by aquatic
predators, including crocodiles, is not mentioned inpublished accounts of chimpanzees’ avoidance of water(hydrophobia) (Angus 1971; McGrew 1977; Nishida1980). There seem to be no detailed published data on
interactions between the apes and these reptiles, nor any
data on potential encounter rates between the two species.
The aim of this short report is to present some data on
chimpanzees and crocodiles in close sympatry, with thetwo species making use of the same watercourses at asingle study site.

Nile crocodiles (Photo by M. Nakamura)

If we have not yet seen crocodiles and chimpanzees
encounter one another, then the next best source of data
may be primatologists searching for or tracking unhabituated chimpanzees. Because such researchers seek tomaximise the chances of contacting wild chimpanzees intheir daily activities, the humans’ encounter rate may be aproxy for that of the apes.
As Mt. Assirik, in the Parc National du NiokoloKoba, Republic of Senegal, we studied chimpanzees(P. t. verus) over almost four years, from February
1976 –December 1979. We spent each working day
walking through various types of habitat, in searchof chimpanzees. We focussed on gallery forest alongseasonal watercourses, which, although comprising only
3% of the surface area of the 50 km², yielded most of
the chimpanzee contacts. Assirik is a hot, dry, and open
habitat dominated by grassland and open, deciduous
woodland (McGrew et al. 1981). The dry season lasts for7 months (November–May), during which time almost allwatercourses either dry up or cease flowing. Assirik has
no rivers but instead tributary streams of varying size,
none more than 2 m in width. Chimpanzees drink daily
and prefer running water; as the dry season progresses,
they must make use of water in gallery forest strips.
We encountered crocodiles nine times during thestudy (see Table 1). This equates to one encounter every
5.2 months (when the median number of days per spent in
the field per month was 27). All but one of the encounters
took place in the dry season, when the reptiles wereconfined to pools in watercourses. The exception (10 June
1976) was a dead individual in a shallow pool, riddenwith maggots. Only half of the encounters were in closedcanopy gallery forest; the others were in more open

Pan Africa News, 21(1), June 2014

3

Table 1. Details of encounters with crocodiles, Mt. Assirik, Senegal.

Date

Habitat type

Place

Length (m)Comments

28-Apr-76??

Hidden Valley

1.2–1.5

10-Jun-76??

Stella’s Valley??

In stream

14-Feb-77

W

Lower Lion Valley

1.5

Flees from bank into pool

07-Mar-77

G

Tsetse Plains

small

Only intermittent pools but some running water

04-Jan-78

F

Elephant Rib Valley

1.5

Slides into pool

02-Nov-78

F

Elephant Rib Valley

0.9–1.2

13-Feb-79

F

Middle Lion Valley

1.2

Stream; upstream from bathing pool

14-Mar-79

G

Tsetse Plains

0.6

Pool

25-Apr-79

F

Middle Lion Valley??

Old Camp bathing pool

Dead in pool; in steep-sided bat grotto; maggots

Beginning of forest

Habitat type: F = forest, G = grassland, W = woodland

habitat types. All crocodiles were immature, ranging
from 1–1.5 m long. All retreated upon encountering us,
into whatever water was available.
Lacking permanent surface water, Assirik is probably
too dry to maintain a population of crocodiles, but it
seems likely that young crocodiles disperse in the rainyseason up the rain-swollen tributaries of the Gambia
River, bringing some of them into the main valleys of thestudy area. Those that did not retreat with the advancing
dry season were probably trapped in diminishing pools,
where they were temporarily more exposed and observable, but ultimately perished.
Are crocodiles a predatory threat to the Assirik
chimpanzees? Perhaps.

ACKNOWLEDGEMENTS
I thank the 15 other researchers who took data on animal
encounters during the Assirik project, especially Caroline Tutin
and the late Pamela Baldwin; Kazuhiko Hosaka and Michio
Nakamura for helpful comments and suggestions on the manuscript. Writing of this report was supported by an Emeritus
Fellowship from The Leverhulme Trust.

REFERENCES

Angus S 1971. Water-contact behavior of chimpanzees. Folia
Primatol 14:51–58.
Cheney DL, Seyfar th R M 2007. Baboon Metaphysics.
University of Chicago Press, Chicago.
Galdikas BMF, Yeager CP 1984. Crocodile predation on a crabeating macaque in Borneo. Am J Primatol 6:49–51.
Galdikas BMF 1985. Crocodile predation on a proboscis monkey in Borneo. Primates 26:495–496.
IUCN 2013. IUCN Red List of Threatened Species. http://www.
iucnredlist.org/details/46590/0
McGrew WC 1977. Socialization and object manipulation ofwild chimpanzees. In: Primate Bio-social Development.
Chevalier-Skolnikoff S, Poirier FE (eds), Garland, New
York, pp. 261–288.
McGrew WC, Baldwin PJ, Tutin CEG 1981. Chimpanzees ina hot, dry and open habitat: Mt. Assirik, Senegal, West
Africa. J Hum Evol 10:227–244.
McGrew WC, Marchant LF, Nishida T (eds.) 1996. Great Ape
Societies. Cambridge University Press, Cambridge.
Nishida T 1980. Local differences in responses to water amongwild chimpanzees. Folia Primatol 33:189–209.
Njau JK, Blumenschine RJ 2012. Crocodylian and mammalian carnivore feeding traces on hominid fossils from
FLK22 and FLK NN 3, Plio-Pleistocene, Olduvai Gorge,
Tanzania. J Hum Evol 63:408–417.

<NOTE>

Discriminating Saba andLandolphia Seeds in Chimpanzee
Feces at Mahale
Michio Nakamura
Wildlife Research Center, Kyoto University, Japan(E-mail: nakamura@wrc.kyoto-u.ac.jp)

INTRODUCTION

The fruits of Saba comorensis (local name, ilombo)
and Landolphia owariensis (local name, mpila) are often
enthusiastically eaten by chimpanzees at Mahale, andthey have been classified as a “major food” and “importantfood,” respectively, by Nishida (1991). Both of them belong to the family Apocynaceae with liana life forms, andthey share some common characteristics. However, the4

Pan Africa News, 21(1), June 2014

Figure 1. Seeds of Saba (above) and Landolphia (below). Although the former is slightly larger than the latter, it is
not easy to distinguish between the seeds in chimpanzee feces.

fruits of Saba and Landolphia are easily distinguishable
by their color and size: the ripe fruit-shell color of Saba
is yellow, while that of Landolphia is orange; the colorof the fruit pulp is yellow in the former and white in
the latter, and the fruit size of the former (typically ca.
7–10 cm in fruit-shell diameter) is about twice as large asthat of the latter (ca. 3.5–5 cm). Therefore, no researchers
would confuse these species when they actually observe
chimpanzees eating these fruits.
However, it is very difficult to identify the seeds
found in chimpanzee feces (see McGrew et al. 2009 forthe standard protocol for fecal analysis). Chimpanzees
typically swallow seeds of both species together with thefruit pulp. Seeds are then defecated intact in their feces.
The shapes and colors of their seeds resemble each otherand are thus not easily distinguishable (Figure 1). Because
these fruits are available in the same season, it is also
not possible to discriminate them by season. Therefore,
Saba and Landolphia were clumped together in mostprevious studies on seed dispersal and fecal analyses at
Mahale (Nishida & Norikoshi n.d.1, n.d.2; Takasaki 1983;
Takasaki & Uehara 1984). However, because Saba is themost important fruit species for Mahale chimpanzees,
it may be useful to develop some simple criteria to
distinguish its seeds from those of Landolphia. The SabaLandolphia problem in fecal analyses may go beyond
Mahale, because different species of these two genera are
known to coexist in some other chimpanzee study sites(e.g., Saba senegalensis and Landolphia heudelotii areeaten by Assirik chimpanzees in Senegal: McGrew et al.
1988).

seeds by using a vernier caliper to the nearest 0.05 mm.
Linear discriminant analyses were then conducted using
R software (version 2.12.1) with these length and width
measures.
I also collected 24 Saba/Landolphia-shaped seeds
from a chimpanzee fecal clump found during the same
study period. After washing and air-drying, their length
and width were measured in the same way. The size
data for these seeds were used in order to trial-run theapplicability of the analysis.

METHODS

I collected 25 Saba seeds and 28 Landolphia seeds
from ripe fruits (five to six fruits each) of these plants in
December 2013 at the Mahale Mountains National Park,
Tanzania. The fruits were collected from the middle ofthe home range of the habituated M group. The sampling
was performed opportunistically because of the difficultyin randomly collecting fruits from the high canopy wherethese fruits are typically seen. After removing the fruit
pulp and air-drying the seeds, I measured the longest axis
(hereafter “length”) and the second longest axis (orthographical to the longest axis, hereafter “width”) of each

A Mahale chimpanzee feeding on a Saba fruit.

Pan Africa News, 21(1), June 2014

5

Figure 2. Length-width plots of known
Saba and Landolphia seeds. The lineis the discrimination line between thetwo species (i.e., D sp = 0). Althougha few seeds of each species areinaccurately judged as the other,
88.68% of the data points are plottedon the correct sides of the line.

Figure 3. Length-width plots of
unknown Saba/Landolphia-shaped
seeds from chimpanzee feces. The lineis the same discrimination line (i.e.,
Dsp = 0) as that in Figure 2. Almost all
seeds are plotted on the side of Saba.

RESULTS AND DISCUSSION

T h e le ng t h a n d w id t h d a t a fo r k n ow n S a b a
and Landolphia seeds are plotted in Figure 2. Saba
seeds averaged 19.06 (± 1.81 SD) mm in length and9.97 (± 1.05) mm in width, while Landolphia seeds
averaged 14.64 (± 1.92) mm by 8.49 (± 1.05) mm. Saba
tends to have a longer and wider shape than Landolphia,
but there is still some overlap in their sizes. It seems that
length has less overlap than width (thus, Saba is relatively
longer and ovoid than Landolphia).
Linear discriminant analysis yielded the discriminating function of the two species (D sp) in the following
formula:Dsp (L,W) = -0.455×L -0.367×W + 11.06
where L is the length and W is the width of a seed inmillimeters. When the Dsp value is negative, the seed isjudged to be that of Saba, and when it is positive, judged
as Landolphia. Re-evaluation of the original data set inthis formula resulted in the misidentification of three
Saba seeds as Landolphia and three Landolphia as Saba.
Thus, discrimination accuracy was 88.68% (47/53).
When length/width values of the 24 unidentified
Saba/Landolphia-shaped seeds from a chimpanzee
fecal clump were assigned to this formula, all but one
was judged as Saba (Figure 3). I assume that the fecal

6clump actually contained only Saba seeds (i.e., one was
misjudged as Landolphia) for the following reasons. First,
the one judged as Landolphia was almost on the borderof these two species (D sp = 0.002), and the proportionof seeds judged as Saba (95.83% = 23/24) was higher
than the discrimination accuracy of 88.68% (thus, it isvery likely that a small portion was misjudged). Second,
because chimpanzees usually swallow multiple seeds ofa Saba/Landolphia fruit at a time, that is, a fecal clump
is expected to contain multiple seeds from one fruit, it is
not very likely (though not completely impossible) that
only one Landolphia seed was contained in a fecal clump
with many Saba seeds. In addition, direct observationsof feeding at the time showed that the chimpanzees more
often fed on Saba than Landolphia.
Although this result is preliminary and we may needmore data by performing additional random samplingof fruits, simple measurements of the length and widthof a seed seem to distinguish these two species with anaccuracy of close to 90%. There are, of course, moreaccurate ways to discriminate the species, for example, by
germination tests to confirm the species by the characteristics of their seedlings or by DNA analyses to genetically
identify species. However, measuring sizes is far easierfor field researchers. Thus, this may be useful for futurestudies of chimpanzee seed dispersal and /or fecal
analyses at and around Mahale, especially on unhabituated chimpanzee groups whose direct observation is often
difficult but who may share similar food items with the M
group. A similar method may also be applicable to otherstudy sites where Saba-Landolphia (and other confusing
sets of species or genera) coexist, with modifications of
the formula with their own data sets.

ACKNOWLEDGEMENTS
I would like to thank COSTECH, TAWIRI, and
TANAPA for their permission to conduct long-term
research at Mahale. I would also like to thank Dr. William
C. McGrew for useful suggestions on the manuscript.
This study was financially supported by the Toyota
Foundation.

REFERENCES
McGrew WC, Baldwin PJ, Tutin CEG 1988. Diet of wild
chimpanzees (Pan troglodytes verus) at Mt. Assirik,
Senegal: I. Composition. Am J Primatol 16:213–226.
McGrew WC, Marchant LF, Phillips CA 2009. Standardised
protocol for primate faecal analysis. Primates 590:363–366.
Nishida T 1991. Primate gastronomy: cultural food preferences in nonhuman primate and origins of cuisine. In:
Chemical Senses. Vol. 4. Appetite and Nutrition, Friedman
MI, Tordoff MG, Kare MR (eds), Marcel Dekker, Inc, New
York, pp. 195–209.
Nishida T, Norikoshi K n.d.1. The study of food on a basisof fecal analysis of the chimpanzees of the K-group andthe M-group, June–November, 1975. Mahale Mountains
Chimpanzee Research Project Ecological Report 1 (unpublished report).
Nishida T, Norikoshi K n.d.2. The study of food on fecal analysis of the chimpanzees of the K-group and the
M-group, December 1975–May 1976. Mahale Mountains
Chimpanzee Research Project Ecological Report 2 (unpublished report).
Takasaki H 1983. Seed dispersal by chimpanzees: a preliminary note. Afr Stud Monogr 3:105–108.

Pan Africa News, 21(1), June 2014
Takasaki H, Uehara S 1984. Seed dispersal by chimpanzees:
supplementary note 1. Afr Stud Monogr 5:91–92.

<NOTE>

Homosexual Interactions
among Young Female Wild
Chimpanzees: An Example ofSocial Pretend Play?
Masaki Shimada
Teikyo University of Science, Japan(E-mail: masakishimada@japan.email.ne.jp)

INTRODUCTION
Homosexual behavior among animals is ethologicallydefined as a display of courtship that involves mounting
and/or genital contact and stimulation between samesex individuals (Vasey & Sommer 2006, p10). Several
studies have examined homosexual behaviors between
two females of the genus Pan. Wild female bonobos (P.
paniscus) frequently engage in homosexual interactions
such as genito-genital rubbing (GG-rubbing; e.g., Fruth &Hohmann 2006). Conversely, wild female chimpanzees(P. troglodytes) rarely engage in homosexual interactions(Zamma & Fujita 2004), even though young captive
female chimpanzees in a group engaged in GG-rubbing
as frequently as did wild bonobos (Anestis 2004). Duringhomosexual interactions, it has been observed that mostwild female bonobos rub their genital regions on a lateralaxis in the ventroventral position, and rarely engage inpelvic thrusts (Fruth & Hohmann 2006). Captive female
chimpanzees, in contrast, rub their genital regions ona front-back axis in the dorsoventral position (Anestis2004). Homosexual interactions between female bonobos
frequently occur during feeding, when group excitement
is high (Fruth & Hohmann 2006). In captive chimpanzees, however, most interactions occur during group rest
periods (Anestis 2004). Only one previously observedcase of homosexual interaction between chimpanzee
females of the Mahale M group occurred during a relaxedsituation (Zamma & Fujita 2004).
Several hypotheses have been proposed and tested
regarding the function of homosexual behavior between
Pan females (Anestis 2004; Fruth & Hohmann 2006).
However, in other animal species, such as Americanbison (Bison bison) (Vervaecke & Roden 2006) or cervid
species (Bartoš & Holečková 2006), some homosexualinteractions between young females were interpreted
as social play or biologically functionless byproductsof adaptations (Vasey & Sommer 2006). Although
Anestis (2004, p481) reported that one captive female
chimpanzee vocalized play-panting with play-face during
a homosexual interaction, researchers have not discussed
the possibility that homosexual interactions between wild
Pan females are a pattern of social play. In this article, I
describe an observed case of homosexual interaction between wild female chimpanzees, and discuss the property

Pan Africa News, 21(1), June 2014

Figure 1. XP (left) is mounting EJ (right) dorsoventrally
during the first bout (image traced from video footage).

of the interaction as social pretend play (Bjorklund &Pellegrini 2002).

METHODS
Well-habituated wild chimpanzees of the M group in
Mahale Mountains National Park, Tanzania were studied
from December 2013 to January 2014 (nine observation
days and 60.2 observation hours in total) (see Nishida2012 for details of the research site). I use italics toemphasize likely homosexual interactions. The number
and symbol in parentheses after each individual’s namerepresents his/her age and sex, respectively.

7
open mouths, and then began wrestling and tickling eachother. They continued playing until 14:35:08.
At 14:36:49, XP approached the grooming males.
AL first groomed XP, and then XP was groomed by AL,
while EJ groomed WX next to XP. XP stopped grooming
at 14:37:03, and started tickling XP12 with DW. EJ
stopped grooming at 14:38:00, and approached XP. At
14:38:52, XP stopped tickling XP12. EJ presented hergenital region to XP in a crouching posture on the groundat 14:39:03, and XP touched EJ’s genital region with her
right hand. She then sat and mounted EJ at 14:39:06, andthrust her hips subtly until 14:39:09 (Figure 2). Then, XP
began grooming EJ’s back. After XP stopped grooming at
14:39:16, EJ walked away and started grooming WX. DW
and XP began tickling each other, and they continued insocial play with XP12 until 14:44:30. I observed XP until
I lost her at 15:06, and then I observed EJ until 16:00.
Neither of them mated with any males.
I asked several Mahale researchers whether they
had observed similar interactions under similar circumstances. No researchers had made such observations
previously.

DISCUSSION
This observation included two homosexual bouts,
which lasted for 38 and 3 seconds, respectively. EJ’spresenting, and XP’s mounting that followed, were
homosexual behaviors according to Vasey & Sommer’s(2006) definition, even if their genital regions did not
make contact.
According to Burghardt (2005), activities are recognized as play behaviors when they fulfill the following
five criteria: (1) incompletely functional, (2) voluntary,

OBSERVATIONOn January 3, 2014, at 13:47, I found XP (13♀),
without signs of estrous, together with her infant daughter
XP12 (1♀), and began to follow her. At 14:32, XP encountered a party consisting of four adult males (CT, AL, OR,
and DW), an old female WX, and an adolescent female
EJ (estimated 10♀). EJ showed minimal sexual swelling,exclusively in her perineal area. Most party members
calmly engaged in grooming.
At 14:32:33, XP approached CT, and they kissed
each other with open mouths. Then XP turned to EJ, embraced, and kissed her with an open mouth. At 14:33:23,
EJ presented her genital region to XP with a crouching
posture on the ground. Sitting behind EJ, XP began tothrust her hips back and forth toward the genital regionof EJ, and continued thrusting until 14:34:01 (Figure
1; see also Video 1: available online at mahale.main.jp/
PAN/21_1/21(1)_03.html). During the mounting, XP’s
clitoris, or her genital region, faced toward the outside of
the area between XP’s hip and EJ’s hip. Their facial expressions were not visible from my vantage point (Figure1). No clear vocalization accompanied the interaction. At
14:34:08, they investigated each other’s genital regions. EJ
expressed play-face and XP reached out her arm in orderto grasp EJ’s arm. At 14:34:22, XP and EJ kissed with

Figure 2. XP (back) is mounting EJ (front) dorsoventrally
during the second bout (image traced from video
footage).

8(3) different structurally or temporally from the related
serious behavior system, (4) expressed repeatedly, and(5) initiated in benign situations. The case reported here
appears to fulfill these criteria. (1′) The direct functionof the observed homosexual interaction is unclear, eventhough some hypotheses have been suggested to explain
the biological function of homosexual interactions
between female bonobos (Fruth & Hohmann 2006). In
addition, if the behavior had a direct function—which isunlikely—many other individuals certainly would have
performed the behavior. (2′) EJ presented her genital
region to XP, and XP mounted EJ independent of any
external factors, as the interactions were observed inrather calm situation. (3′) The ethological pattern of theinteractions was different from heterosexual interactions.
For example, the first case lasted substantially longer (38seconds) than typical heterosexual copulations, whichlast for an average of 7.1 seconds (range 2–13) in the wild
(Nishida 1997), and 6.7 seconds (range 1–30) in captivity(Anestis 2004). (4′) Two bouts of homosexual interactions
were observed; thus, the behavior might have been
repetitive, at least between XP and EJ. (5′) Because these
two bouts were performed directly after encounteringand kissing each other, and socially playing or grooming,
respectively, their contexts were considered calm andbenign. Considering these five factors, it is reasonable
to regard the homosexual interaction between XP and
EJ as a pattern of social play, even though they did notdemonstrate clear signs of play, such as play-face or playpant. This interpretation is not contradictory to the factthat they began tickling and wrestling play directly after
these interactions.
During these interactions, XP and EJ took theroles of male and female, respectively, as they occur innormal heterosexual interactions. Homosexual interactions between female bonobos, which normally involve
their clitoris with lateral movements (Fruth & Hohmann2006), could impart sexual pleasure. However, there
was no evidence that XP and EJ reached orgasm, as they
did not show any distinct facial expressions or vocalizations, and their sexual organs did not touch during thehomosexual interactions due to their body postures. Thissuggests that they might have synchronized their movements only to “pretend as though” they were interacting
heterosexually. Pretense can be recognized if a subject
intentionally performs an idea that he/she knows to be a
fiction, and controls the action (Mitchell 2007). Although
no evidence thus far has conf irmed social pretend
play among play partners in wild non-human animals(Tomasello & Call 1997; Bjorklund & Pellegrini 2002),
the present observation might be interpreted as a rare caseof social pretend play with a division of roles.

ACKNOWLEDGEMENTS
I would like to thank COSTECH, TANAPA, TAWIRI,MMNP, and MMWRC for permission to conduct researchin Mahale. This study was funded by a MEXT Grant-in-Aid(KAKENHI) (#24720399).

REFERENCES
Anestis SF 2004. Female genito-genital rubbing in a group of
captive chimpanzees. Int J Primatol 25:477–488.

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Bartoš L, Holečková J 2006. Exciting ungulates: malemale mounting in fallow, white-tailed and red deer. In:
Homosexual Behaviour in Animals: An Evolutionary
Perspective. Sommer V, Vasey PL (eds), Cambridge
University Press, Cambridge, pp. 154–171.
Bjorklund DF, Pellegrini AD 2002. The Origins of Human
Nature: Evolutionar y Developmental Psycholog y.
American Psychological Association, New York.
Burghardt GM 2005. The Genesis of Animal Play: Testing theLimits. MIT Press, Cambridge.
Fruth B, Hohmann G 2006. Social grease for females? Samesex genital contacts in wild bonobos. In: Homosexual
Behaviour in Animals: An Evolutionary Perspective.
Sommer V, Vasey PL (eds), Cambridge University Press,
Cambridge, pp. 294–315.
Mitchell RW 2007. Pretense in animals: The continuing relevance of children’s pretense. In: Play and Development:
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Göncü A, Gaskins S (eds), Taylor & Francis, New York,
pp. 51–75.
Nishida T 1997. Sexual behavior of adult male chimpanzees ofthe Mahale Mountains National Park, Tanzania. Primates
38:379–398.
Nishida T 2012. Chimpanzees of the Lakeshore: Natural
History and Culture at Mahale. Cambridge University
Press, Cambridge.
Tomasello M, Call J 1997. Primate Cognition. Oxford
University Press, Oxford.
Vasey P, Sommer V 2006. Homosexual behaviour in animals: topics, hypotheses and research trajectories. In:
Homosexual Behaviour in Animals: An Evolutionary
Perspective. Sommer V, Vasey PL (eds), Cambridge
University Press, Cambridge, pp. 3–42.
Vervaecke H, Roden C 2006. Going with the herd: samesex interaction and competition in American bison. In:
Homosexual Behaviour in Animals: An Evolutionary
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Pan Africa News, Vol. 21, No.1
Published in June, 2014
Address: c/o Human Evolution Studies,
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ISSN: 1884-751X (Print), 1884-7528 (Online)