Original Article
Folia Primatol 2009;80:19–32
DOI: 10.1159/000181187

Received: December 20, 2007
Accepted after revision: August 27, 2008
Published online: December 11, 2008

Skewed Matrilineal Genetic Composition in a
Small Wild Chimpanzee Community
Makoto K. Shimada a–c Sachiko Hayakawa a Shiho Fujita a, d
Yukimaru Sugiyama a, e Naruya Saitou baPrimate Research Institute, Kyoto University, Inuyma, b Division of Population Genetics,
National Institute of Genetics, Mishima, c Institute for Comprehensive Medical Science,
Fujita Health University, Toyoake, d Department of Veterinary Medicine, Faculty ofAgriculture, Yamaguchi University, Yamaguchi, and e Kyoto University, Kyoto, Japan

Key Words
Demography ⴢ Genetic diversity ⴢ Habitat fragmentation ⴢ Mitochondrial DNA ⴢ
Pan troglodytes verus ⴢ Emigration ⴢ Immigration ⴢ Small population

Abstract
Maternal kinship is important in primate societies because it affects individual behaviour as well as the sustainability of populations. All members of the Bossou chimpanzee community are descended from 8 individuals (herein referred to as original adults)
who were already adults or subadults when field observations were initiated in 1976 andwhose genetic relationships were unknown. Sequencing of the control region on thematernally inherited mtDNA revealed that 4 (1 male and 3 females) of the 8 original
adults shared an identical haplotype. We investigated the effects of the skewed distribution of mtDNA haplotypes on the following two outcomes. First, we demonstrated thatthe probability of mtDNA haplotype extinction would be increased under such a skewed
composition in a small community. Second, the ratio of potential mating candidates tocompetitors is likely to decrease if chimpanzees become aware of maternal kinship andavoid incest. We estimated that the magnitude of the decrease in the ratio is 10 times
greater in males than in females. Here we demonstrate a scenario in which this matrilineal skewness in a small community accelerates extinction of mtDNA haplotype, which
will make it more difficult to find a suitable mate within the community.
Copyright © 2008 S. Karger AG, Basel

Introduction

In recent years, the habitats of great apes have become degraded and fragmented due to human encroachment [Cowlishaw and Dunbar, 2000; Walsh et al., 2003].
Data from several long-term chimpanzee (Pan troglodytes) field studies indicate that© 2008 S. Karger AG, Basel
0015–5713/09/0801–0019$26.00/0
Fax +41 61 306 12 34
E-Mail karger@karger.ch
www.karger.com

Accessible online at:www.karger.com/fpr

Makoto K. Shimada, PhD
Institute for Comprehensive Medical Science
Fujita Health University
Toyoake, Aichi 470-1192 (Japan), Tel. +81 562 93 9378
Fax +81 562 93 8834, E-Mail mshimada@fujita-hu.ac.jp

community sizes typically range between 40 and 150 individuals [Goodall, 1986;
Boesch and Boesch-Achermann, 2000; Wrangham, 2000; Nishida et al., 2003; Mitani, 2006]. The wild chimpanzee (P. t. verus) community in Bossou, Republic ofGuinea, whose core area is restricted to approximately 6 km2 by human activity, hasbeen observed since 1976 [Sugiyama, 1984]. With a size of approximately 20 members since 1976, Bossou is one of the smallest chimpanzee communities known [Sugiyama, 1999, 2004]. Male chimpanzees usually remain in their natal group for lifeand form intermale bonds to defend the resources within their territory, while mostfemales leave their natal group and join neighbouring communities [Goodall, 1986;
Nishida, 1990]. According to the demographic record of the Bossou community,
however, female immigration from outside has never been observed, but the disappearance of both males and females from this community has regularly been observed [Sugiyama, 1999, 2004]. While none of these disappeared individuals has
been observed in the adjacent chimpanzee communities of the Nimba mountains todate, it is likely that they emigrated from Bossou since they were healthy at the timeof their disappearance [Sugiyama, 2004]. In this study, we assumed that all healthyadolescents who disappeared from the Bossou community had emigrated. In spiteof the frequent disappearance of individuals and no immigration from outside, thesize of the Bossou community has remained stable for nearly three decades due to its
relatively high reproductive rate [Sugiyama, 2004].
When our research team started field observations in 1976, we identified 7 females, consisting of 6 full adults and 1 subadult, and their offspring, in addition to 3
adult males, 2 of which did not remain in the community. We therefore refer to these
8 individuals (7 females and 1 male) as the ‘original adults’. Based on long-term mother-offspring records (fig. 1), we defined 7 ‘recorded matrilines’ (RMs), derived from the
7 original adult females. Generally, primates such as chimpanzees avoid mating withmaternally related individuals [for reviews, see Goodall, 1986; Smuts et al., 1986; Pusey,
2004]. Conversely, the effect of paternal kinship on behaviour is not as clear as maternal kinship; for example, the incidence of mating between females and their fathers
varies depending on individuals [Pusey, 2004]. In other behaviours such as male-malecooperation, a lower correlation has been observed for paternal kinship than maternal
kinship, prompting the hypothesis that chimpanzees are not capable of recognizing
paternal kinship [Langergraber et al., 2007]. If Bossou chimpanzees are capable of recognizing maternal kinship or genetic matrilineage (GM) beyond our mother-offspring
record, we might expect maternal genetic composition to affect mate choice.
The genetic diversity of the Bossou community is becoming increasingly important as female immigration has not been recorded and most of the adult females have
either ceased, or are close to ceasing, menstruation. In addition, the Bossou community lost a quarter of its members due to a flu-like epidemic at the end of 2003, resulting in 2 of the 7 RMs becoming extinct within 4 months [Matsuzawa, 2006]. This
marked decrease in numbers requires urgent attention, and comparisons need to bemade with previous demographic records to evaluate the threat of loss of diversity.
Although Sugiyama et al. [1993] analysed faecal and food wadge (remains ofeaten fruit) samples collected from the Bossou community to establish paternity using a microsatellite locus, these authors did not establish the relationships among the
RMs. In addition, despite Shimada et al. [2004] reporting the existence of 5 mitochondrial DNA haplotypes (mt types) in the Bossou community, the relationships
between these mt types and the RMs were not explored. Our primary goal in this20

Folia Primatol 2009;80:19–32

Shimada /Hayakawa /Fujita /Sugiyama /Saitou

study was therefore to investigate the maternal genetic relationship between the RMs(i.e. GMs) and skewness in matrilineal genetic composition, using information basedon these mt types. Since mtDNA is maternally inherited and the mothers of theoriginal adults were unknown, the 1 original adult male and 7 RMs were targeted for
mtDNA typing. Based on the resulting information regarding the relationships between RMs, we also investigated the impact of demographic changes on GM diversity within this small chimpanzee community.

Materials and Methods
Mother-Offspring Record
Long-term observations of the Bossou chimpanzee community in Guinea started in 1976.
All individuals have since been individually recognized and monitored. Seven RMs were recorded based on mother and offspring relationships gathered since 1976. Besides these RMs, 1
original adult male has remained in the community since the beginning of our observation(fig. 1).
Sample Collection
We collected non-invasive samples including hair, urine and faeces from the chimpanzeesof Bossou. During the sampling periods, there were 21 individuals in the community. We genotyped 13 of the 21 individuals encompassing all 7 RMs and the 1 original adult male of thecommunity. Names of the 13 chimpanzees from which we took samples are the following (withtheir abbreviations in parentheses): Fana (Fn), Jire (Jr), Juru (Ju), Kai (Ka), Nina (Nn), Nto (Nt),Pama (Pm), Pili (Pl), Tua (TA), Velu (Vl), Vuavua (Vv), Yo (Yo) and Yolo (YL) (table 1). The 8
original adults are indicated with an asterisk in table 1. The first character of the individual’s
name indicates its RM, indicated in italic capital letters in figure 1. The sampling period wasfrom January to March 1999, and samples were collected by M.K.S. and S.H. Additional sampling was also conducted from December 2001 to March 2002 by S.F. The 13 individuals genotyped are represented in bold circles and rectangles in figure 1.
To collect hair samples, we tracked a known individual until it slept in a bed it built in atree about 10 m above ground. The next morning, we climbed the tree and took hairs from the
bed using forceps after the chimpanzee had left. We did not collect samples from the bed when
we found dried leaves in it, as this is a sign of bed reuse. We stored hairs collected from a single
bed in a sterile plastic disposable tube after adding absolute ethanol. Some hairs had hair bulbs.
We collected urine if we observed a known individual dropping urine on the leaves of a tree orbush. Urine was collected and deposited in a sterile tube using sterile, disposable plastic syringes, and 2 vol of 70% ethanol was added. This method is the same as that of Hayakawa and
Takenaka [1999]. We collected faeces deposited by known individuals by wiping the surface ofthe faeces with a sterile cotton swab soaked in saline + 1 mM EDTA solution. The faecal samples
attached to the wet cotton swab were collected by shaking in the tubes containing the 2 ml ofsaline + 1 mM EDTA solution, and then 10 ml of absolute ethanol was added to the tube. Thismethod is the same as that used by Sugiyama et al. [1993].
DNA Extraction
The method of DNA extraction was as follows. We extracted DNA from single hairs using
Isohair (Nippon Gene Co. Ltd.) according to the manufacturer’s instructions. For urine samples, we followed the method described by Hayakawa and Takenaka [1999]. For faecal samples,
DNA was extracted using the QIAmp쏐 DNA Stool Kit (Qiagen) according to Morin et al. [2001].
After centrifugation (800 g, 10 min at room temperature), the precipitate was suspended in 1.6
ml ASL buffer and incubated at room temperature for 30–60 min. The final elution of DNA wasincubated in AE buffer for 20–30 min. To avoid cross-contamination during DNA extraction,
we wore disposable caps and gloves, and used pipettes with filtered tips.

Skewed Matrilineal Genetic Composition in a
Small Wild Chimpanzee Community

Folia Primatol 2009;80:19–32

21

TA

T

Jr

*

*

1980

1977

*

1988

J

*

1993

*

JJ

Ju

Ka

F

Jy

2003

2001
Fn

JM

1992

K
2003

*
1980

*

FF

Ft

1983

*

Fl

1980

2004
FK

* 1991
*

*

1977

FE

1991

2004

Vl

V

Nn

N
2003

*
1980

*

1983

*
1990

VI

*

Vv
2004

1999

1977

*

1983

*

1988

*

1996

*

Nt

1991
2001

Ve
2003
Pm

*

1991

*

1986

Pl
2000

Yo

P

PO

PE

Y

*

*

*

*

*

1980

1983

1983

1993

1990

YL

2003

PK
2000

Fig. 1. The mother-offspring relationships for the RMs and the original adult male of the Bossou

chimpanzee population. Genealogies were constructed based on observations from 1976 to
2007. Circles and rectangles indicate females and males, respectively. The three kinds of lines –dotted, bold and solid – represent individuals that disappeared before sampling began, individuals that were used for mtDNA genotyping and those that were not used, respectively. Fourdigit numbers below figures show the year of disappearance, and figures without a year indicate
present members. Individuals with the year underlined disappeared during the period of the
flu-like epidemic. Although the year for the individual Vv is underlined, she is presumed tohave emigrated and not died due to illness. Italicized characters show the names for the RMs.
Individual names are abbreviated here and are described in the text.

22

Folia Primatol 2009;80:19–32

Shimada /Hayakawa /Fujita /Sugiyama /Saitou

Table 1. Summary of observed haplotypes in each sample

RM1

Individualname2

Sampletype

Sample3

Majority4

InconRM
sistency5 haplotype6

Consistent/
total7

F

Fana (Fn) +*hairfaeces

1
1

1
1

0
0

Bs-TJFK

2/2

J

Jire (Jr) +*Juru (Ju)hairfaeces

2
1

2
1

0
0

Bs-TJFK

3/3

K

Kai (Ka)*

hair
urine

1
11

0
10

1
1

Bs-TJFK

10/12

N

Nina (Nn)*Nto (Nt)hairfaeces

2
1

2
1

0
0

Bs-N

3/3

P

Pama (Pm)*Pili (Pl) +faeces
hair

1
1

1
1

0
0

Bs-P

2/2

T

TUA (TA)*

urine

3

3

0

Bs-TJFK

3/3

V

Velu (Vl)*
Vuavua (Vv)

urine
urine

5
11

5
11

0
0

Bs-V

16/16

Y

Yo (Yo)*hair
faeces
urine

3
1
4

3
1
4

0
0
0

Bs-Y

8/8

YOLO (YL)

1 RMs and 1 original male by field observation are depicted in figure 1.
2
Names of males are represented by capital letters. Plus signs represent females with infants;
asterisks indicate original adults (see text).
3
Number of samples whose mtDNA sequences were obtained.
4
Number of samples showing RM haplotype, which is determined by majority-rule whenmore than 2 haplotypes were detected for each RM, assuming that the majority was an authentic haplotype.
5 Number of samples not showing RM haplotype.
6 The most prevailing haplotype (i.e. major mt type, Bs-TJFK) was underlined.
7 Number of samples showing RM haplotype per number of samples yielding mtDNA sequence.

mtDNA Sequence Determination and Analyses
We determined about 605 bp of the hypervariable region I of the mtDNA control region.
We designed primer sets, L15926 (5ⴕ-TAC ACT GGT CTT GTA AAC C-3ⴕ, corresponding topositions 15326–15344 of the complete chimpanzee mtDNA sequence of Horai et al. [1995],
DDBJ/EMBL/GenBank accession No. D38113) and H16555 (5ⴕ-TGA TCC ATC GTG ATG TCT
TA-3ⴕ, corresponding to positions 15971–15990 of D38113). We performed PCR under the following conditions: each 50 ␮l reaction contained 1–5 ng template DNA, 1 ! EX Taq amplification buffer, 1.25 units EX Taq enzyme (Takara), 0.5 ␮ M each primer and 200 ␮ M each dNTP.
Amplification conditions were: denaturation at 94 ° C for 5 min, followed by 30 cycles of 94 ° C
for 30 s, 50 ° C for 30 s and 72 ° C for 60 s, and 20 cycles of 94 ° C for 30 s, 52 ° C for 30 s and 72 ° C
for 60 s, and a final extension at 72 ° C for 3 min. To avoid cross-contamination, we used pipettes
dedicated for the preparation of PCR with filtered tips. The PCR products were purified by using MicrospinTM S-400 HR columns (Amersham Pharmacia), or gel excision was followed by

Skewed Matrilineal Genetic Composition in a
Small Wild Chimpanzee Community

Folia Primatol 2009;80:19–32

23ethanol precipitation if more than one PCR band appeared on a 0.9% agarose gel. The purified
PCR products were labelled directly using the ABI PrismTM Big Dye Terminator Cycle Sequencing kit (Perkin Elmer). Sequencing with automated sequencers (ABI377 or ABI310, Perkin Elmer) was done from both ends using the same primers as used in the PCR. We multipliedaligned nucleotide sequences with the use of Clustal W [Thompson et al., 1994]. We sequenced
at least 2 samples per RM regardless of the individual (table 1). If several multitypes were foundin a RM, the most common type was regarded as the authentic one. We excluded the data if thequality was low in the chromatogram and/or the sequences diverged from corresponding regions of published chimpanzee mtDNA sequences.
Probability of mt Type Extinction
The probability that any mt type in the community would become extinct if 1 or 2 females
disappeared was calculated on an annual basis. We restricted the examination to females because males do not contribute to the inheritance of mtDNA. We then compared these annual
probabilities using the observed composition of mt types (Pobs) with those calculated under theassumption that mt types (Pexp) were uniformly distributed. These distributions were calculated every year as follows:
P = n1C nd/nCnd, if nd = 1
P = 0, if n1 = 0
P = [(n – n1) n1+ n2 + n1C nd]/nCnd, if nd = 2 and n1 6 nd,
P = [(n – n1) n1+ n2]/nP nd, if nd = 2 and n1 ! nd,
where n, nd, n1 and n2 are the total number of females, the number of disappeared females, number of mt types observed in 1 female and number of mt types observed in 2 females, respectively.
For comparison, we generated a hypothetical population consisting of the same numberof members as Bossou with an even mt type distribution as follows:In case (x – 1)n s ^ n ! xn s
n x = MOD(n/n s),n(x – 1) = n s – n x,n(x – 2) = 0,
where n s, n x and MOD() are the number of mt types, the number of mt types observed inx females and the remainder after division in parentheses. Note that (x兩 x ^ 3), (n兩n !
10), (n s兩n s ! 5) for the Bossou community (see online suppl. appendix for an example,
www.karger.com/doi/10.1159/000181187).
Analysis of Demography
Previous studies [Sugiyama, 1999, 2004] classified individuals by age into 4 categories –infant (0–3 years old), juvenile (4–7 years), adolescent (8–11 years) and adult (12 years andmore) – and indicated that the disappearance of individuals was most apparent in the adolescent and adult categories. A field record suggests that sexual maturation of females in this community is earlier than that of other communities: age at first swelling and age at first parturitionare 7.6 and 9.9 years, respectively, on average [Fujita, 2003]. We therefore referred to individuals
over 8 years old as ‘independent’ and analysed their demographic records. At the time of thedisappearance of independent individuals, we counted the number of available mating candidates (i.e. individuals of the opposite sex) and mating competitors (i.e. individuals of the samesex), and calculated the ratio between them (i.e. sex ratio for mate, or SRM), based on either
condition: (1) the ‘regardless of mt type’ (MIX) or (2) the ‘mt type distinction’ (DTN). Under
the MIX condition, we defined [SRM]MIX for each emigration event by an individual e as:[SRM]MIX(e) = [no]MIX(e)/[n s]MIX(e),<SRM >MIX e 

24<no >MIX e<ns >MIX e

Folia Primatol 2009;80:19–32

Shimada /Hayakawa /Fujita /Sugiyama /Saitou

where [no]MIX(e) is the number of independent individuals of the opposite sex to e regardless of
mt type, and [n s]MIX(e) is the number of independent individuals of the same sex as e regardlessof mt type, including e. While we also defined [SRM]DTN for each emigration event by an individual e:<SRM >DTN e 

1<no >DTN e

1,
<no >DTN e o e <ns >DTN o< >DTN

where [o]DTN(e) is an independent individual of the opposite sex to e with an mt type different
from e, [no]DTN(e) is the number of [o]DTN(e), and [n s]DTN(e) is the number of independent individuals of the same sex as e, including e, with an mt type different from [o]DTN(e) (see online
suppl. appendix for example, www.karger.com/doi/10.1159/000181187).

Results

We successfully obtained mtDNA sequences from 10 of 17 (59%) hair samples
collected from night beds, 34 of 45 (76%) urine samples, and 5 of 6 (83%) faecal samples (table 1). We assigned the 5 mt types, Bs-TJFK, Bs-N, Bs-P, Bs-V and Bs-Y, to the
8 original adults, i.e. all of the RMs in Bossou, T, J, F, K, N, P, V and Y (table 1). Fourof these individuals (T, J, F and K ) shared an identical haplotype (mt type Bs-TJFK),
while the other 4 RMs (N, P, V and Y ) had haplotypes that differed from each other.
The observed sequences diverged sufficiently from one another to facilitate clear
identification of haplotype, and sequence data were deposited in the DDBJ/EMBL/
GenBank International Nucleotide Sequence Database under Accession No.
AB189231–AB189235. The details of sequence alignment and generation of phylogenetic trees for these sequences are reported in a supplementary figure together withother wild chimpanzee sequences in Shimada et al. [2004]. Since we genotyped themt type in all of the RMs, it is impossible that other mt types will be found in thiscommunity given the absence of female immigration from the outside and maternal
inheritance of mtDNA.
Figure 2a shows the frequency of mt types within individuals in the Bossou
community. Since the 4 RMs (T, J, F and K ) share the identical mt type, Bs-TJFK, the
Bs-TJFK has accounted for nearly 50% in this community since 1976 (fig. 2a). We
define the most prevailing mt type, Bs-TJFK, as ‘major type’ and the others as ‘minor
types.’ In the same way, we define the GM bearing the major type as the ‘major GM’and the others as ‘minor GMs’. A similar finding was made for the major mt typewhen only independent females were considered (fig. 2c). Given the small number of
independent males in the community, only one mt type, Bs-TJFK, was observed in
independent males for approximately half of the field observation period (fig. 2b).
Figure 3 shows the probability of mt type extinction in females of the community if 1 or more females disappeared over time. In the event that 1 female disappeared, the average probability of extinction was 0.48 (max.: 0.6, min.: 0.2, solid triangle in fig. 3). In the event that 2 females disappeared, the average extinction probability was 0.77 (max.: 1.0, min.: 0.42, solid square in fig. 3). In both cases, theseprobabilities are higher than those obtained assuming even mt type distribution
within the community, increased in average by 61 and 22% if 1 or 2 females disappeared, respectively. In all cases, extinction probabilities were highest in 2005 andlowest in 2000, which correspond to the maximum and minimum total number offemales, respectively (fig. 2c).

Skewed Matrilineal Genetic Composition in a
Small Wild Chimpanzee Community

Folia Primatol 2009;80:19–32

25

GM
Y

75

V

V

50

P

P

25

N

N

19 19 18 19 19 16 17 19 15 16 18 19 20 18 19 20 18 17 17 17 17 18 19 21 20 18 18 19 13 12 12

K
06
20

04

02

20

20

19

19

19

19

19

19

19

19

V

19

V

19

GM
Y

19

1 1 1 1 3 1 1 1 1 1 1 2 2 3 3 3 2 2 3 4 4 3 3 3 3 3 4 4 3 3 4

19

20

00

98

96

94

TJFK

RM
Y

06
20

04
20

02
20

00
20

98
19

96
19

94

T

19

92
19

88

86

90
19

19

19

84

80

82

19

19

19

19

76
19

78

Year

100
75
50
25
0

F
J

a

Frequency (%)

92

90

88

86

84

82

80

78

0
76

Frequency (%)

RM
Y

100

P

P

N

N

K
F

TJFK

Year

J

7 7 7 8 8 7 7 7 7 8 8 8 8 8 8 8 8 9 7 7 8 8 8 8 10 9 9 9 6 5 6

RM
Y

GM
Y

V

V

b

100
75
50
25
0

P

P

N

N

Year

06
20

04
20

02
20

00
20

98
19

96
19

94
19

92
19

88

86

84

90
19

19

19

80

82

19

19

19

19

19

c

78

K
76

Frequency (%)

T

F

TJFK

J

Fig. 2. Proportion of RM and GM in the Bossou community (1976–2006). The proportion of
RM and GM in the total Bossou population (a), the independent (68 years old) males (b) andthe independent females (c) are shown. The total numbers are indicated above each bar. The

four RMs that shared the same mt type (Bs-TJFK) are crosshatched. The youngest individual(FE) born in 2007 was not included.

We have not observed any differences in the emigration rate among GMs within
the Bossou community to date (number of major haplotypes per total are 6/10, 2/7, 3/4,
2/4, for emigrated females, males, remaining females, and males, respectively; fig. 1).
Table 2 shows the averaged SRM in the Bossou community at the time of thedisappearance events of independent individuals (see Methods section and online
suppl. appendix, with its suppl. table 1, www.karger.com/doi/10.1159/000181187). In
the DTN condition, since we did not consider members of the opposite sex in thesame GM as mating candidates, SRM diminished by 52–88% for all disappearance
events compared to the MIX condition. Within these disappearance events, the disappearance of males was associated with a greater decrease (i.e. MIX – DTN) in SRM
than when females disappeared. In both sexes, SRM of the major mt type decreased
more than those of minor mt types did.

26

Folia Primatol 2009;80:19–32

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Table 2. Averaged sex ratio between opposite to same sex in the Bossou community when young
adults disappeared

mt type1

Sex

Females

Males

n

Major

6

Minors4

Subtotal

10

Major

2

Minors5

Subtotal

7

SRM

Difference

DTN2

MIX3

MIX – DTN

%4

0.136
(0.001)0.170(0.002)0.150(0.001)

0.373(0.004)0.356(0.003)0.366(0.003)0.237(0.001)
0.187(0.001)0.217(0.002)

63.75(19.7)
52.45(54.9)
59.26(63.0)

0.438(0.008)0.601(0.042)0.554(0.036)3.500(0.500)2.833(0.556)3.024(0.560)3.063(0.383)2.232(0.355)2.470(0.464)

87.57
(0.0)
78.97
(22.2)
81.46(32.3)

Figures in parentheses indicate variances. SRM = Sex ratio for mate, opposite/same-sex ratio.
1 The major mt type is Bs-TJFK, and others, i.e. Bs-N, Bs-P, Bs-V and Bs-Y, are minors.
2 The number was counted under mt type distinction, i.e. opposite-sex individuals in thesame mt type were not mating candidates; see text for calculation method.
3 The number was counted regardless of mt type, i.e. opposite-sex individuals in the same
mt type were counted as mating candidates; see text for calculation method.
4 100 ! (MIX – DTN)/DTN. 5 t = 2.7, d.f. = 4, p = 0.05.
6
t = 6.7, d.f. = 15, p < 0.00001. 7 t = 3.2, d.f. = 5, p = 0.02.

Figure 4 shows the observed decrease in SRM from MIX to DTN per disappearance event and the expected decrease in SRM assuming an even distribution of GM.
This indicates that a skewed mt type composition has a greater effect on minor GMs
compared to the major GM.
Discussion

Relationship among Maternal Lineages
Since the infinite site model in molecular evolution assumes that each mutation
occurs at a new site, two haplotypes showing identical sequence always come fromthe same origin [Kimura, 1969]. Accordingly, the observation that identical mt types
in chimpanzees have arisen in geographically distinct areas can be attributed to the
low level of population structure within chimpanzee subspecies [Morin et al., 1994;
Goldberg and Ruvolo, 1997]. This suggests that ancestral populations of chimpanzee
subspecies were panmictic and that establishment of the current geographic distribution pattern was so recent that mutations in mtDNA do not accumulate to differentiate among current local populations [Gagneux et al., 2001; Shimada et al., 2004].
Although mt types with identical sequence are regarded as maternal kin, there are

Skewed Matrilineal Genetic Composition in a
Small Wild Chimpanzee Community

Folia Primatol 2009;80:19–32

27

Pobs (1)Probability of mt type loss

1.0

Pobs (2)Pexp (1)Pexp (2)

0.8
0.6
0.4

0.2

04
20

00
20

96
19

92
19

88
19

84
19

80
19

19

76

0

Year

Fig. 3. Chronological change of the probability of mt type loss in females of the Bossou community from 1976 to 2007. The probability that mt types are extinct from females in Bossou
community if 1 (triangles) or 2 (squares) females disappear per year is calculated. Solid andopen symbols indicate the annual probabilities calculated using the observed mt type composition (Pobs) and those expected for an even distribution of mt type (Pexp), respectively.

two kinds of maternal kin: maternal relationship sufficiently close for chimpanzees
to recognize kinship, and maternal relationship not sufficiently close for kin recognition. Although an additional genetic marker is necessary to estimate the time
elapsed from the common maternal ancestor of the 3 RMs sharing the identical mt
type (Bs-TJFK), we hypothesized that the chimpanzees belonging to the 3 RMs recognize maternal kinship. This provides a new clue to explain field observation data.
Interestingly, based on our field observations, individuals that are related to the original adult male and the 3 RMs (T, J, F and K ) tend to stay together significantly more
often than the other RMs [Sugiyama, 1988; Sakura, 1994]. This may corroborate previous studies on apes in which it was proposed that individuals sharing identical mt
types within a community are likely to be closely related maternal kin [Hashimoto
et al., 1996; Goldberg and Wrangham, 1997; Mitani et al., 2000].
Skewed mt Type Distribution
Based on our genotyping results, we observed a skewed distribution of mt types
within the Bossou community. Figure 3 shows that this skewness in mt type enhances the probability of mt type extinction. Consequently, because of this skewness,
Bossou is more susceptible to mt type extinction compared to a population of equal
size with even distribution of mt type. This skewed distribution of GM may affectchimpanzee behaviours such as migration patterns and mate choice to avoid incest.
To estimate the effect of the skewed GM distribution on the migration behaviour of
this chimpanzee community, we evaluated the difficulty associated with finding amating candidate among individuals within the community by introducing SRM,

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Observed decrease
Expected decrease
SRM – DTN

100

1.2
1.0
0.8
0.6

50

SRM

Decrease (%)

75

0.4
25
0.2
0

0.0
Ja

Ju

Fi

Ft

Ki

Kr

Nt

Major

Pl

Vv Yu JM JZ NA NO PR

Minors

Major

Females

VI

VU

Minors

Males

Disappeared individuals

Fig. 4. Decrease in SRM per disappeared individual when GMs are considered. Hatched bars
indicate the observed decrease in SRM from the MIX condition (counted independently of mttype) to the DTN condition (estimated using distinct mt type). Open bars indicate the expected
decrease in SRM assuming an even distribution of GMs.

which represents the ratio of potential mating candidates (i.e. the opposite sex) topotential competitors (i.e. the same sex) for the disappeared individuals (table 2; online suppl. appendix, suppl. table 1, www.karger.com/doi/10.1159/000181187).
Migration Behaviour and Kinship within the Bossou Community
If we assume that individuals with the same mt type are sufficiently closely related to recognize their kin, the decrease in the SRM between the sexes becomes significant (59.2% decrease in females vs. 81.4% decrease in males, p ! 10 –6, t = 6.73)
when we change the condition from MIX to DTN (table 2). Since SRM calculations
are not currently available for other chimpanzee communities, primarily due to thelack of information on GMs for all individuals, we were not able to compare the
SRMs between Bossou and other communities and thus to test whether the skewed
distribution in GMs at Bossou indeed explains the unique migration patterns exhibited by this small community of chimpanzees. However, we can compare the averagesex ratios (males/females) across chimpanzee communities for which long-term data
are available. The average sex ratio for independent individuals in the Bossou community is 0.33 (table 3), which is among the lowest of any chimpanzee community
for which equivalent data are available (table 3). This finding suggests that it is generally easier for males in the Bossou community to find a potential mate than it isfor males in other communities, if we consider sex ratio independently of kinship.
However, males in the Bossou community show a 10 times greater SRM decrease(MIX – DTN) on average than females (2.47 in males vs. 0.22 in females, table 2).
This means that the consideration of mt type to calculate sex ratio affects males more

Skewed Matrilineal Genetic Composition in a
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Folia Primatol 2009;80:19–32

29

Table 3. Comparison of averaged sex ratio
Communities

PeriodSex ratio (M/F) Average number of individuals
in adults and
total
males
femalesadolescents

Reference

BossouTai

1976–2007
1982–1996

0.33
0.38

10.91
35.34

2.69
9.67

8.22
25.67

MahaleGombe

1980–1999
1965–1971
1972–1983
1965–1983

0.44
0.95
0.53
0.67

52.40
38.14
32.67
34.68

16.10
18.57
11.25
13.95

36.30
19.57
21.42
20.74

this study
Boesch and BoeschAchermann [2000]
Nishida et al. [2003]
Goodall [1986]
Goodall [1986]
Goodall [1986]

severely than females in the Bossou community. If averaged SRM decreases (MIX –DTN) become available in other chimpanzee communities, SRM will indicate howdifficult it is for males in the Bossou community to mate. Moreover, competition
among females is not expected to be significant because the oestrous cycles in females are not synchronous. The relationship between difficulty for males in thiscommunity in mating with maternally non-related females and male emigration at
Bossou merits further research.
Chronological Changes in Genetic Diversity within the Bossou Population
Our field observations have shown that all of the females born in Bossou haveleft the community, and that no immigration of females into the community has occurred since 1976. As mtDNA is inherited maternally, the lack of female immigration significantly affects mtDNA diversity.
A flu-like epidemic that struck the Bossou community in December 2003 resulted in deaths of 4 chimpanzees (Ka, JM, Ve, PO) and the disappearance (presumed
death) of an old female chimpanzee (Nn). Once the epidemic had abated, another female chimpanzee (Vv), whose baby (Ve) died shortly after the epidemic, disappearedin May 2004 and is suspected to have emigrated (underlined in fig. 1). During theepidemic, the RMs K and N were lost. Because RM K carried mt type Bs-TJFK, whichis common to RMs T, J and F, the number of mt types decreased from 5 to 4, i.e. only
mt type Bs-N was lost; the first extinction of an mt type in the Bossou community
since the initiation of our observations. Furthermore, each of the RMs harbouring
haplotypes Bs-V, Bs-Y and Bs-P consists of only 1 adult female, as the males’ mtDNAs
are not inherited by the offspring (fig. 1). These 3 females, Vl, Yo and Pm, are original
adults and have not born offspring since 1991, 1991 and 1998, respectively. In theevent that these 3 adult females are lost without bearing any new female offspring thatremain in Bossou, only 1 mt type, Bs-TJFK, will be passed on to the next generationof the Bossou community. Currently, all fully adult females are estimated to be over
40 years old (fig. 1), which, based on hormonal assays conducted in captive chimpanzees, is the approximate age for the onset of menopause [Videan et al., 2006]. Females
younger than 40 years are Jy and Fl born in 2004 and 1997, respectively, both of whichcarry the mt type Bs-TJFK. Thus, if no female immigration occurs and the biased
proportion of GMs is maintained, only one mt type, Bs-TJFK, will remain in the
Bossou community in the near future regardless of community size.

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A previous study indicated that the mtDNA diversity of the chimpanzees at
Bossou is similar to that recorded in the neighbouring Nimba communities and suggested that female immigration into Bossou had occurred prior to our observations[Shimada et al., 2004]. Although assays of nuclear diversity should be undertaken toaccurately estimate the risk of inbreeding, the possibility of rapid GM extinction issuggestive of a relatively higher risk of inbreeding in the Bossou community than inother communities that have been studied for an extended period.
This study provides an actual example for predicting the potential risk associated with the migration of females between distinct populations of animals such as
chimpanzees. Considering the skewed proportion of GMs revealed by this study, theconservation status of the Bossou community is more sensitive than can be inferredfrom demographic data alone. Our research team as well as local people are currently conducting an afforestation project called the ‘Green Passage Plan’ to promote
the migration of chimpanzees between the Bossou and Nimba forests [Hirata et al.,
1998; Matsuzawa, 2006]. The maintenance of mt type diversity in the Bossou community is thus highly dependent upon whether new females will immigrate throughthe passage.
Acknowledgments
We would like to thank l’Institut de la Recherche Environnementale de Bossou and the
Direction Nationale de la Recherche Scientifique et Technologique of Guinea, as well as Mr. J.
Koman and Mr. G. Goumi for their assistance with field work. We would like to thank Drs. T.
Matsuzawa, H. Takemoto, G. Yamakoshi, M. Myowa-Yamakoshi and S. Hirata for assistingwith the collection of samples, and extend our gratitude to the local assistants P. Cherif and P.
Goumi. We would like to thank Drs. P. Gagneux and Y. Kureta for teaching M.K.S. how to climb
trees, and Dr. L. Vigilant and other members of the Primatology Department at the Max Planck
Institute for Evolutionary Anthropology for teaching M.K.S. DNA extraction techniques. We
also thank Dr. K. Chambers and Dr. T. Humle for helpful discussions and revisions of earlier
versions of the manuscript, Ms. Y. Noaki for her skillful technical assistance and members of
the Saitou laboratory for productive discussions. This study was supported by the Nissan Science Foundation (1999) to M.K.S., the Inamori Foundation (2000) to M.K.S., Grants-in-Aid for
Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan: Research on Priority Areas (No. 14011248) to N.S., International Scientific Program (No. 10041168)
to Y.S. and COE Research (No. 10CE2005) to the Primate Research Institute, Kyoto University.
M.K.S. received the COE fellowships to Primate Research Institute Kyoto University and National Institute of Genetics during this study.

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