Archives of Sexual Behavior, Vol. 20, No. 5, 1991
Homosexual Mounting in Free-Ranging Female
Hanuman Langurs (Presbytis Entellus)
Arun Srivastava, Ph.D., 1 Carola Borries, Ph.D., 2
and Volker Sommer, Ph.D. 2,3
Female-female mounting was studied for 3233 hr in a bisexual one-male troop
of free-ranging Hanuman langurs in northwestern India over a period of 6
years. The population breeds throughout the year and female-female mounts
(n = 524) occurred during all months. All 15 adult females mounted and
were mounted, although only 20% of the individuals exhibited more than half
of all mounter activity and 33% more than half of all mountee activity. Various
similarities with heterosexual mounting suggest that sexual arousal is a stimulus
of the activity. Cycling, pregnant, and lactating females acted as mounters as
well as mountees. However, ovulating individuals were greatly overrepresented
concerning their likelihood of mounting and being mounted. Females of any
given rank in the dominance hierarchy showed mounter as well as mountee
activity, although mounters were higher ranking than mountees in 84% of
cases. Young mounters were overrepresented and middle- to old-aged mounters
underrepresented, whereas no such differences existed for mountees. Mounting
relations among four young adult paternal half-sisters who occupied top
positions in the dominance hierarchy were significantly overrepresented.
Although the ultimate function of homosexual female mounting is not fully
understood, it is suspected to be connected with intrasexual competition: If
"pseudocopulations" induce mountees to reduce the number of solicitations
addressed to the male, the probability of insemination decreases and mounters
thus reduce the number of future competitors.
KEY WORDS: Presbytis entellus; mounting; homosexuality; dominance rank; reproductive
states.
1Department of Zoology, University of Jodhpur, Rajasthan, India.
2Institut f/ir Anthropologie, Universit~it G6ttingen, B/irgerstrasse 50, D-3400 G6ttingen,
Germany.
3To whom correspondence should be addressed.
487
0004-0002/91/1000-0487506.50/0© 1991Plenum Publishing Corporation
488
Srivastava, Borries and Sommer
INTRODUCTION
Mounting behavior among females has been described for a large
number of vertebrate animals (reviews in Young, 1941; Beach, 1968; Goy
and Goldfoot, 1975; Denniston, 1980). It has been recognized that such
behavior is generally neither associated with structural abnormalities or endocrinopathies nor represents "sex reversals," but "is instead to be
considered a normal element in the behavioral repertoire" (Beach, 1968,
p. 88). Unisexual mounts are of particular interest concerning the broader
question of how nonreproductive patterns of sexual behavior (e.g., heterosexual activities during infertile periods such as pregnancy or lactation as
well as homosexual activities) might have evolved.
Hormones and social factors are important in triggering the behavior
at the proximate level. In several species of nonhuman primates, female
mounting occurs mainly during the ovulatory period of the cycle, when
females are sexually receptive (Michael et al., 1974; Akers and Conaway,
1979). Age, sex, and dominance rank are likewise relevant to partner selection (e.g., Chevalier-Skolnikoff, 1976; Fedigan and Gouzoules, 1978).
However, the behavior shows considerable interspecies variability concerning its frequency, form, and correlation with certain hormonal and social
conditions. Therefore, the ultimate causes remain largely speculative. Functional explanations interpret female-female mounting as (i) a substitute for
heterosexual mounting (Talmage-Riggs and Anschel, 1973; Wolfe, 1984),
(ii) training for adult heterosexual roles (Akers and Conaway, 1979), (iii)
an expression of dominance (e.g., Kempf, 1917; Zuckerman, 1932), (iv) a
display of masculine behavior by receptive females which should stimulate
male sexual activity (Ford and Beach, 1952; Parker and Pearson, 1976), or
(v) an indication of Close affectional ties (Chevalier-Skolnikoff, 1976; Akers
and Conaway, 1979).
Most data on female-female mounting have been collected on domestic or laboratory animals and few quantitative studies exist for animals
under seminatural conditions confined to large enclosures (Hanby and
Brown, 1974; Gouzoules and Goy, 1983). Since there is general agreement
that captivity might change the frequency and expression of sexual behavior (Beach, 1968; Chevalier-Skolnikoff, 1976; Akers and Conaway, 1979;
Gouzoules and Goy, 1983) it seems necessary to investigate wild
populations.
For Hanuman langurs (Presbytis entellus), an Asian colobine monkey
inhabiting wide parts of the Indian subcontinent, female-female mounting
has been reported from several field sites (see Discussion). These observations are largely anecdotal and do not provide quantitative analyses of
the possible influences of reproductive status, dominance rank, age, and
Homosexual Mounting in Female Langurs
489
kinship. We examined these variables on the basis of data collected during
a 6-year study of a free-ranging troop of Hanuman langurs in northwestern
India and discuss them in the light of the potential adaptive significance
of female-female mounting.
MATERIAL AND METHODS
Study Site
In the vicinity of the city of Jodhpur (240 m above sea level) in
Rajasthan, India, lives an isolated population of 1200-1300 Hanuman langurs which has been studied by various Indian and German researchers
since 1967. The reproductive units are one-male-multi-female troops (harems). Females remain lifelong in their natal troop. Maturing males join
bands of extratroop males, which besiege harems from time to time and
try to oust the resident male. The climate is dry with maximum temperatures of up to 50 ° C, in May/June and minimum temperatures around 0 ° C
in December/January; 90% of the average 360 mm of precipitation is received during the monsoon in July-September (for details, see Mohnot,
1974; Winkler et al., 1984; Sommer and Rajpurohit, 1989).
Study Troop
Data presented here concern a one-male troop living about 6 km west
of Jodhpur city which is termed Kailana-I (KI; other publications sometimes use B19). The habitat is open shrub dominated by Prosopis juliflora,
Acacia senegal, and Euphorbia caducifolia. Water is available throughout
the year from artificial ponds. The langurs are not shy, due to provisioning
by local people for religious reasons, which accounts for one-third of the
total feeding time. Apart from feral dogs, natural predators are absent.
From 1977 through 1988, the troop was subject to a long-term study. All
members have been known individually since January 1977.
Data Sampling
Between October 1981 and September 1987, data on female-female
mounting interactions totaling 3233 hr of ad-lib, sampling were collected
during three distinct blocks of observation, each covered by one of the
authors (Table I; for details, see Sommer, 1985; Borries, 1989; Srivastava,
1989). The numbers of adult females were 11, 13, and 12, respectively, but
490
Srivastava, Borries and Sommer
Table I. Observation Schedule and Composition of the Study Troop
Troop compositiona
Ad lib.
Observer
Sommer
Borries
Srivastava
E
Adult
Juvenile-young
adult
Infant
Period
sampling
(hr)
M
F
M
F
M F
Total
Oct. 81-Dec. 82
Jan. 85-Oct. 85
Oct. 86-Sep. 87
942.1
1018.5
1272.0
1
1
1
11
13
12
2
2
1
5
1
2
1
1
3 3
20
17
22
3232.6
a Censuses refer to 1 January of 1982, 1985, and 1987. Subsequently, 6 births occurred
in 1982, 5 in 1985, and 7 in 1987. The number of adult females remained constant
within a given block. The number of adult and immature males was higher during
periods of resident male changes (Oct.-Nov. 81, June 82, Sep.-Dec. 82, see Sommer,
1988).
remained constant within a given block. If not stated otherwise, all results
refer to mounting interactions between postmenarchal females only and
have been corrected for differences in observation time related to the number of females, their reproductive states, dominance ranks, and age-classes.
For each category of analysis (e.g., age class, reproductive state, dominance
rank) the number of female days were computed (expected value) and compared with the number of episodes during which female mounters or
mountees belonged to certain categories of age, reproductive state, or
dominance (observed value). The goodness-of-fit-test (Chi-square, twotailed, Siegel, 1985, p. 101 ft.) was applied, to reveal significant differences
between observed and expected scores.
Reproductive Parameters
Jodhpur langurs have no breeding season, although there is a birth
peak during March and a minimum in November (Sommer and Rajpurohit,
1989). The only nonbehavioral sign of the females' cycles are menstruations, which are mostly visible. Physical indications of estrus (such as genital
swellings) are absent. However, more than 90% of all sexual solicitations
addressed to males occur around the time of presumed ovulation. Copulations are almost invariably initiated by females (not males) through head
shaking, presentation of the hindquarters, and lowering of the tail. On the
basis of menstruations and solicitations/copulations, the duration of the follicular stage (menstruation to first copulation) and luteal stage (first
copulation to menstruation) could be assessed with fair accuracy, as
Homosexual Mounting in Female Langurs
491
revealed by comparison with hormonal studies (review in Sommer, 1985).
To calculate expected vs. observed scores for different stages of the menstrual cycle, the duration of menstruation has been subtracted from the
follicular stage and that of the ovulation phase from the luteal phase, resuiting in the following reproductive parameters: cycle length 24.1 _+ 3.6
days (n = 161); menstruation 1.9 _+ 1.0 days (n = 212); follicular stage
8.9 _+ 3.5 days (n = 80); ovulatory phase 4.0 - 2.3 days (n = 239); luteal
stage 14.9 -2_4.2 days (n = 69). Gestation lengths average 200.3 +- 3.4 days
(n = 31). On the basis of postconception estrous behavior, pregnancies
were divided into four periods (cf. Sommer, 1987): Days 1-52 (no estrous
signs); Days 53-88 (extended estrous behavior with frequent solicitations
and copulations); Days 89-136 (less frequent solicitations, no copulations);
Days 137-term (no estrous signs). The lactational amenorrhea is considered
to be over with the renewed onset of a regular menstrual cycle. Females
reach menarche at 2.4 _+0.3 years of age (n = 10; for details, see Sommer
et al., in press).
Dominance Hierarchy
The direction of dyadic displacement activities (n = 2940 episodes)
were used to generate a dominance hierarchy that produced the fewest
reversals. As a rule of thumb, females gain top positions in the hierarchy
as soon as they start reproducing and descend to bottom ranks as they
grow older (for details, see Borries et al., 1991).
Age Classes
Age classes include the following categories: young (menarche-9
years); middle-aged (10-19 years); old (20+ years) (Borries et al., 1991).
RESULTS
During the study, 524 episodes of mounting between adult females
were observed (207 in 1981/1982, 178 in 1985, 139 in 1986/1987), in which
mounter as well as mountee could be identified (Table II). Variables such
as dominance rank, reproductive status, age, etc. of the partners could be
determined for varying proportions of the total sample.
492
Srivastuva, Borries and Sommer
©
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[ ¢~
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Homosexual Mounting in Female Langurs
493
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Srivastava, Borries and S u m m e r
494
Table III. Age-Sex Class Distribution of Nonrcproductive Mounting in the
O n e Male Study Troop"
Mounter
Mountee
Episodes
Deviation from expectation
(for A F only )
(%)
AF
AF
JF
JM
AM
286
48
13
2
+ 33.1
-3.3
-8.4
-4.1
JF
AF
JF
JM
AM
24
+++
++
-9.0
(p < 0.01)
JM
AF
JF
JM
AM
14
++
+++
+
-8.2
(p < 0.01)
AM
JF
JM
+
+
Infant either sex
+
Infant either sex
(p < 0.001)
(ns)
(p < 0.01)
(p < 0.05)
a A F = adult female, JF = juvenile female, JM = juvenile male, A M = adult
male. Observed versus expected frequencies could be calculated for interactions
involving adult females only, since they were subjects to focal animal sampling.
F o r interactions b e t w e e n o t h e r age-sex classes, the following qualitative
categories were applied: - = not observed; + = rare, + + = moderate,
+ + + = frequent. Data for 1981-1982 and 1986-1987 only.
Age--Sex Class Distribution of
Nonreproductive Mounting
Nonreproductive mounting (i.e., mounts involving partners other
than sexually mature male mounters with adult female mountees) did not
occur only among adult females but involved all possible combinations
between different age--sex-classes. However, adult females clearly preferred each other whereas all other categories involving adult females
were significantly underrepresented, except for the combination adult female-juvenile female (Table III). The youngest female mounted by an
adult female was an infant of 7.6 months, the youngest male 7.9 months;
the youngest individuals, who mounted adult females (or at least tried
to do so) were a female of 3.1 months and a male of 5.1 months. If not
indicated otherwise, all results refer to adult female-adult female mounting only.
Homosexual Mounting in Female Langurs
495
Fig. 1. Female-female mounts closely resembled male-female copulations. Female 4 (right)
solicited Female 3 through head shaking, whereupon Female 3 clings jusl below the knees of
Female 4. The pelvic thrusting of Female 3 is accompanied by grunt-and-grimacing
(22 Apr. 82).
Qualitative Features and Contexts
Female-female mountings were brief interactions, lasting about 5 sec
on average with a maximum of about 10 sec. In the majority of the cases,
mounters apply a double foot-clasp on the mountee's lower legs (Fig. 1).
Less frequently, they may stand on the ground.
Unlike mounting positions described for Macaca mulatta (Akers and
Conaway, 1979) or Macaca arctoides (Chevalier-Skolnikoff, 1976), the
mounter does not rub her genitals on the rump of the mountee but thrusts
against her buttocks. The clitoris of the mounter is thus not directly stimulated, although that of the mountee might be. However, indirect mounter
496
Srivastava, Borries and Sommer
Table IV. Behaviors Connected with Female-Female Mounting a
Behavior
Mountee solicits adult male or
copulates
Mountee solicits female mounter
Pelvic thrusts of mounter
Preceding
mounting
During Succeeding
mounting mounting
Proportion b
(%)
19
11
13.4
28
38
29.5
39.3c
29
10
18.8
4.9
88
Mounter grooms mountee
Mountee grooms mounter
13
1
Embracing, pulling of facial fur
Mounter mounts quadrupedicallyd
Jumping display of mounter
Jumping display of mountee
Mountee resists (slaps mounter)
Teeth grinding of mounter
14
Mounter displaces mountee
Mountee displaces mounter
Mountee displaces 3rd individual
3rd individual displaces mountee
3rd individual displaces mounter
"~
J
"~
42.9
23.7
J
7.6
2.7
I
1
1
0.9
I,.
2
J
1
1
1.3
1.3
0.9
4
0
1
4
1
3
0
3.1
0.0
0.4
1.8
0.9
}
1
2.2
6.3
a224 episodes, data for 1981-1982 only; in 66 cases, either mounter or mountee was a
nonadult individual, usually a perimenarchal female.
t'Totai > 100% since a given episode could be accompanied by several variables.
c During 1986-1987, pelvic thrusts occurred in 33.6% out of 128 closely monitored episodes.
aStanding on mountee's back.
stimulation might be achieved due to mechanical pressure on body portions
surrounding the clitoral region. There were no visible indications of orgasm
for either mounter or mountee, but these are likewise absent in mountees
participating in heterosexual copulations. Manual or oral stimulation of the
genitalia were not observed.
Moreover, a relationship existed between the mounting syndrome and
various agonistic behavioral patterns (Table IV). In the context of mounting, mounters sometimes exhibited head bobbing (a mild threat gesture),
grunt-and-grimacing (indicating excitement), teeth grinding (indicating tension or threat), or displaced mountees. Both mounter and mountee
sometimes participated in jumping displays or embraced each other (a common gesture of reassurance).
Behaviors accompanying female-female mounting were in some respect strikingly similar to the sequence of male-female copulations
(Table IV). Although the majority of interactions seemed to be initiated
by the mounter, in 19 cases (8.5%) out of 224 episodes, female mounters
responded to solicitations which were directed towards a male. In 28 cases
Homosexual
Mounting in Female Langurs
%
20
o AD. F E M A L E - AO. FEMALE
• ALL N O N R E P R O D U C T I V E
497
MOUNTS
MOUNTS
[n= 128
( n= 266
]
}
-
p,
/
10
-
"o__o
'
l
6
~
I
7
I
8
,',
I I i I I I I I I I DAy9 10 11 12 13 14 15 16 17 18 TIME
Fig. 2. Diurnal distribution of female-female mounting. Data for
1986-1987 only, when observation hours were equally distributed
over the day. Broken line indicates expectation.
(12.5%) the typical heterosexual solicitation was directly displayed towards
the subsequent female mounter. In 38 cases (16.7%) unsuccessful solicitations occurred after the mount. Pelvic thrusting occurred in more than
one-third of all cases. The mean number of thrusts was 6.0 (n = 41 episodes, range 2-11 thrusts). As during heterosexual contacts of estrous
females with males, mountings were frequently accompanied by allogrooming. Following copulations, females often groom the male, whereas males
rarely groom females. However, such a clear-cut role of "mountee = active
groomer" vs. "mounter = passive groomee" could not be found in femalefemale interactions. The reation was rather reversed, since out of 53
grooming interactions in the context of mounting, 79.2% were performed
by the mounter.
Heterosexual copulations were subject to sexual harassment by nonmating individuals, especially adult females, in more than 80% of all cases
(Sommer, 1989a). In few instances, unisexual female "couples" were likewise harassed.
Diurnal and Annual Distribution
On average, each female participated in a mounting episode every
59.6 hr, i.e., once every 5th day. The diurnal activity had a morning peak,
a depression around noon, and another less pronounced peak in the late
afternoon, )~z(12) = 46.60, p < 0.001 (Fig. 2). Thus it resembles the activity
pattern observed for monitoring, feeding, and locomotion tWinkler, 1988;
Srivastava, 1989). It is higher during the cool hours of the day and lower
during the hot hours. Mounts occurred during all months, although not
equally distributed, Z2 (11) = 78.56, p < 0.001 (Fig. 3). Seasonal fluctua-
Srivastava, Borries and S o m m e r
498
%
20
10
~
_
J FM A M J J A S 0 N D
Fig. 3. A n n u a l distribution of female-female mounting, n = 497
episodes recorded b e t w e e n 1981-1987. Broken line indicates
expectation.
tion did not correlate with the overall reproductive activity (e.g., conceptions, births).
Mounting and Reproductive Status
Cycling, pregnant, and lactating individuals all acted as mounter and
mountee, although several deviations from the given expectations were significant (Fig. 4). During menstruation, follicular, and luteal periods, participation was significantly underrepresented for mounters as well as mountees.
Around ovulation, mounter activity was 1.6 times and mountee activity even
4.7 times higher than expected. Mounters had additional significant peaks
during the main phase of postconception estrus (Days 53-88 of pregnancy)
and during lactation (Days 1-50, Days 151-200). Mountees had significant
minima during the last anestrous phase of pregnancy (Days 137-term) as
well as during late lactation (Days 151-200). Cases in which mounter as
well as mountee were anestrous accounted for only 14.5% of all episodes.
Mounting and Dominance
Mounting behavior clearly paralleled the female displacement hierarchy. Mounters were higher ranking than mountees in 84.2% of all cases
(N = 436), a significant difference found during each period of the study
(80.8% out of 130 cases during 1981/1982; 91.6% out of 178 cases during
1985; 77.3% out of 128 cases during 1986/1987).
Homosexual Mounting in Female Langurs
499
ICycling
I
4s2
MENSTRUATING
I
FOLLICULAR
34
] 11114 [ 231
OVULATORY
[ '°'11:~,
f ~ .,k ,,k
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I 4c'41
LUTEAL
Pregnant
ANESTROUS
DAy
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, :Ft'I~:
E S TR O U S / C O P U L A T I N G I
517 I DAY
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ESTROUS/SOLICITiNG
DAY
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TERM
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69
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MOUNTER
DAYS
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MOUNTEE
EPISODES -
!
EXPECTATION
i I
ii
,..
I
I
2
I
M
tttt~
I
6
[Mounts/Day
I
,//--7
I
22
10
3
X
10 - 2
Fig. 4. Reproductive status of female mounters and mountees. "Days" refer to the
total number of observation days of females in a given reproductive status, "episodes"
indicates the total number of mounting events. Significance values for observed versus
expected: *p < 0.05, **p < 0.01, ***p < 0.001.
Females of any rank position did act as mounter as well as mountee. Mounters were significantly overrepresented in the high-ranking
class and significantly underrepresented in the middle- and low-ranking
classes (Fig. 5). For mountees, a similar trend held true. When the actual rank difference between mounter and mountee was analyzed, dyads
with mounters ranking 1-3 positions higher in the hierarchy turned out
to be significantly overrepresented, whereas most dyads with mounters
ranking 3 or more positions lower were significantly underrepresented
(Fig. 6).
500
Srivastava, Borries and Sommer
DEVIATION
FROM
EXPECTATION
%
•
MOUNTER
[]
MOUNTEE
*'k'k
÷40
*20
-20
-k~Ik
-40
RANK
H I GH
EPISODES
305,/187
MIDDLE
78 / 126
LOW
53 / 123
Fig. 5. Female-female mounting: Distribution over rank classes.
High-ranking females (first third of displacement hierarchy; depending
on the total number of females, usually the first 4 positions);
middle-ranking (second third); low-ranking (last third, usually the last 4
positions). *p < 0.05, **p < 0.0t, ***p < 0.001.
Mounting and Age
Concerning age classes, there was no difference between the observed
and expected distribution for mountees, whereas young mounters were significantly overrepresented and middle-aged as well as old ones significantly
underrepresented (Fig. 7). These results closely resemble correlations between mounting and rank (see Fig. 5). This is not surprising, since there
is a significant negative correlation between age and dominance rank in
free-ranging female Hanuman langurs (Bottles et al., 1991).
The relative age difference of the partners was known for 97 dyads.
Out of these, 66.0% represented the combination "younger mounter/older
mountee" with a mean of 5.7 episodes per dyad (n = 367); 34.0% combined "older mounter/younger mountee" with a mean of 2.8 episodes per
dyad (n = 92).
Interindividual Variability
In the course of the study (1981-1987), all adult females acted as
m o u n t e r as well as m o u n t e e (see Table II; the number of individuals
Homosexual Mounting in Female Langurs
EPISODES
501
dr~rtk
*,A~r
]
EXPECTED
]
OBSERVED
84
'°1
78
i i~i !ililili~,!i~!~ii!!
72
30--
24--
12--
iiiii!
iii!,
i!~ii!ili!!iil~i!i!i!ii!!!i~iiii
'~|2 "11 410 ~'9 ~11 * 7 46 t S ~4 • 3 ¢'2 ÷1
MOUNTER
HIGHER
RANKING
ilill!
,,;',i!iiii
~iii!!ii!l
-1 - 2 - 3 - 4 - 5 - 6 - 7 - I - 9
MOUNTER LOWER RANKING
-10 - I ! -12
RANK
DIFFERENCE
Fig. 6. Observed vs. expected rank differences between female mounters and mountees.
*p < 0.05, **p < 0.01, ***p < 0.001.
acting as mounter/mountee was 10/11 for 11 females during 1981/1982,
10/12 for 13 females during 1985, and 8/12 for 12 females during
1986/1987). However, there was considerable interindividual variability:
20.0% of all mountees accounted for more than half (50.8%) of all episodes (n = 524) and 33.3% of all mountees accounted for 54.8%. Individual ratios of mounter:mountee activity varied from 12.0:1 (Female 3.2)
to 1:22.0 (Female 4).
Out of 194 possible dyads" between individuals (i.e., matrix cells in
Table II), 39.1% were zero dyads. Of the remaining nonzero dyads,
31.4% were reciprocal, i.e., both individuals involved acted sometimes as
mounter and sometimes as mountee. Of all dyads, 8.1% accounted for
more than half (50.4%) of the total events. Moreover the majority
(59.7%) of all mount-giving activity was restricted to 3 mountees only
and the majority (60.5%) of all mount-receiving activity to 2 mounters
only. Mounting relations were extraordinarily well developed between the
four paternal half-sisters, who occupied the top positions in the domi-
502
Srivastava, Borries and Summer
DEVIATION
FROM
EXPECTATION
l
MO U N T E R
[]
M O U N T E E
%
, k ~,,llr
÷
40
--
+
20
--
-
20
-
-
4 0
-
%
AGE
EPISODES
YOUNG
350/152
MIDDLE
50 /116
OLD
94
/ 196
Fig. 7. Female-female mounting: Distribution over age classes.
Young (menarche-9 years), middle-aged (10-19 years), old
(20+ years). ***p < 0.001.
nance hierarchy for most of the time through at least 1987 (cf. Borries
et al., 1991). This age cohort performed 54.7% of all their combined
mount-giving activity among themselves and 72.0% of all mountreceiving, which is 10-13 times more than expected. Much of the pattern
of rank differences between mounters and mountees (see Fig. 6) is due
to the fact that Females 3.2 and 6.3 for most of the time maintained a
rank difference of 3 positions and Females 4.4 and 6.3 a difference of
1-2 positions.
The only other known kinship relations involved the four mothers
of the half-sisters. However, m o t h e r - d a u g h t e r dyads were clearly
underrepresented (n = 6 episodes). They account for only 1.3% of all
activity performed by daughters and 2.8% of all maternal activity.
DISCUSSION
F e m a l e - f e m a l e mounting among J o d h p u r langurs in general
matches the pattern reported for other study sites in India, Nepal, and
Sri Lanka (Table V). There is a single study site (Junbesi, Nepal) for
which the absence of female mounting is explicitly mentioned. Most of
Homosexual Mounting in Female Langurs
503
these qualitative reports likewise emphasize the participation of adult
females in nonreproductive mounting with other age-sex-classes, especially male and female immatures (cf. Table III). The extremely rare
occurrence of adult female-adult male mounts seems to be a speciesspecific pattern. Male-male mounts, which are especially frequent in
male bands, are the subject of a forthcoming paper (Sommer and
Moore, in preparation).
Hormonal Influences
Among mammals, the degree of pre- and postnatal endocrine influence on female-female mounting varies considerably (reviews in Beach,
1968; Goy and Goldfoot, 1975). As a rule, mounting activity is n o t n e c e s sarily dependent on certain hormonal conditions; nevertheless, it is m o r e
likely to coincide with certain reproductive states. This is true for langurs
as well; mounter as well as mountee activity occurred during all stages of
the menstrual cycle, pregnancy, and lactation, although significant peaks
and minima could be recognized (cf. Fig. 4).
In other mammals, mounting occurs frequently during estrus when
females are sexually receptive to males. Nevertheless, whereas the mounter might be estrous o r anestrous, the individual most likely to be
mounted is a second female in estrus (e.g., lions, domestic cats, dogs,
cattle, Beach, 1968; rhesus macaques, Carpenter, 1942; p. 150; Akers and
Conaway, 1979; Japanese macaques, Hanby and Brown, 1974; chimpanzees, Yerkes, 1939; gorillas, Harcourt, 1979; but see stumptail macaques,
Chevalier-Skolnikoff, 1976). Langurs behave similarly since both mounter
and mountee activity was significantly higher around the time of presumed ovulation (for hormonal studies, see Chowdhury and Chandra,
1984).
Mounting did not cease during pregnancy. However, the peak in
heterosexual postconception activity (Days 53--88) coincides with an increased activity of mounters but not mountees. This suggests that endocrine correlates of estrous activity during ovulation differ from those
postconception. Reports about postconception mountee activities are
lacking for other primates. In rhesus macaques, mounter activity ceases
with the onset of pregnancy (Akers and Conaway, 1979), whereas in
J a p a n e s e macaques, early p r e g n a n c y facilitated m o u n t e r activity
(Gouzoules and Goy, 1983).
During lactation, mounting activity showed an irregular distribution
for Jodhpur langurs. Mounter activity was overrepresented during Months
504
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Srivastava, Borries and Sommer
1-2 and from Month 6 onwards, whereas mountee activity was underrepresented from Month 6 onwards. One would expect that mounting activity
generally increases after infants develop motoric independence. However,
locomotor abilities of infants are probably not crucial in this regard since
langurs have a well-expressed system of infant transfer (review in Hrdy,
1976) which frees the mothers for foraging and social activities (Sommer,
1989b). In fact, almost all mounting occurred while troop members other
than the mother took care of the babies (Sommer, 1985). For stumptail
macaques, where mounting females were always lactating (Chevalier-Skolnikoff, 1976), caretaking behavior by nonmothers was likewise reported. In
Japanese macaques, however, having a dependent infant clearly inhibited
mounter activity (Gouzoules and Goy, 1983).
Sexuality, Dominance, and Gender Role
Wickler (1967) argued that in a heterosexual context, presentation of
the hindquarters (a "signal" commonly preceding mounting) can be liberated from a strictly sexual motivation and employed to turn aggressive
motivations of a dominant individual into sexual ones. Accordingly, for the
part of the signal sender, he believed that social and sexual presentation
have the same form but different motivations, whereas "the re.~ponse of the
partner has remained sexually unaltered" (Wickler, 1967; p. 108). However,
concerning same-sex mounting, Wickler used the term "homosexual" only
if an individual clearly prefers same-sex partners in a purely sexual context
(Wickler, 1969, p. 58). Moreover, same-sex mounts are generally interpreted as "social dominance gestures" (Wickler, 1967, p. 108). At the same
time, mount-giving behavior is linked with a male gender and mountreceiving with a female gender: "In the social context, male sexual actions
imply dominance and female sexual actions submissiveness--both sexes are
capable of exhibiting both types of interaction with an animal of either
sex" (Wickler, 1967, p. 109). Thus, the mountee is assumed to play the
"feminine" role in a male-male mounting whereas the mounter in a female-female mounting plays the "masculine" role. This concept of
"inversion," which originally referred to human homosexuality (e.g., Freud,
1905/1961), postulates a sex-reversal during all mounts that are not malefemale.
However, at least for Hanuman langurs it seems difficult to establish
such a dichotomy: Female mounters can hardly be labeled as "androgenized" or "masculinized" individuals because the same hormonal conditions
around midcycle stimulate to "play the female role" in a heterosexual
Homosexual Mounting in Female Langurs
507
mounting as well as to "play the male role" in a female-female mounting
(see Fig. 4).
Presbytis entellus clearly belongs to those species where the pattern
"dominant mounter-subordinate mountee" is followed rather rigidly (Figs.
5, 6). But even then, the equation of dominance and mounter role can
nevertheless be criticized, because Wickler assumed that same-sex mounts
almost always demonstrate dominance. However, cause and effect can
hardly be distinguished, i.e., whether mounting actually influences the hierarchy or simply reflects its existence. Moreover, female-female mounts
include behavioral patterns which--in a heterosexual context--would be
viewed as clear indicators of sexual interest: solicitation, pelvic thrusting,
grooming, touching of the genital regions (see Table IV) might be expressions of true "homosexual" motivations (as already stated by Weber and
Vogel, 1970, Vogel, 1976, p. 132). Hence, it seems virtually impossible to
separate "sexual mounting" from "dominance mounting" as tried by Ripley
(1965, p. 213).
Furthermore, it seems questionable to almost equate the mounter
position with dominance and that of the mountee with submissiveness,
notwithstanding the fact that in many species female mounters are usually
dominant individuals (nonprimates, e.g., white leghorn hens, Guhl, 1948;
mountain sheep, Geist, 1968; primates, e.g., Papio cynocephalus, Anthoney,
1968; Saimiri sciureus, T a l m a g e - R i g g s and Anschel, 1973, Macaca
arctoides, Chevalier-Skolnikoff, 1976; Macaca mulatta, A k e r s and
Conaway, 1979; cf. review in Wickler, 1967, p. 109). However, this pattern
does not always hold even within the same species: for example, males of
Macaca mulatta do tolerate mounting by females although males clearly
dominate females (Michael et al., 1974). In other species (e.g., Miopithecus
talapoin, Wolfheim and Rowell, 1972; Macaca fuscata, Gouzoules and
Goy, 1983) there is no relation at all between the dominance hierarchy
and same-sex mounts.
Sexual arousal and dominance are obviously not mutually exclusive
in langur females, since female-female mounting is related to both dominance and sexuality.
The Question of Function
Ford and Beach (1952) proposed that the temporary female display
of "masculine behavior," such as mounting, functions to prompt sexually
sluggish males into action. Beach (1976) considered female-male mounting
one form of "proceptivity" that should arouse males specifically at the time
508
Srivastava, Borries and S o m m e r
females are fertile. Parker and Pearson (1976) assumed that, by mimicking
a potential rival through display of male copulatory patterns, a female
mounter could attract a male's attention, resulting in copulation. In captive
rhesus monkeys, if males failed to ejaculate despite repeated female sexual
solicitations, females were more likely to mount the males who could be
aroused by such mountings (Michael et al., 1974).
Although female-male mountings are very rare in langurs (cf. Tables
III, V), female-female mounts might theoretically stimulate a male as well,
by either "arousing" him or making him "jealous." In fact, harem residents
do often not respond to female solicitations (cf. Mohnot, 1974, p. 165;
Vogel, 1976, p. 119; Hrdy, 1977, p. 141; Sommer, 1985, p. 68). The hypothesis draws some support because in 8.5% of all cases, unsuccessful
solicitations of males preceded a mounting episode between females and
in another 4.9% solicitations of males followed female-female mounts (of.
Table IV). Since males sometimes inspect the genital region of presenting
females, it is likely that males try to assess the reproductive condition of
females. If female-female mounting would in fact stimulate males, one
must not necessarily assume that mounters act cooperatively (by increasing
the chances to copulate for the mountee) if a mounter is herself motivated
to copulate.
However, several indications suggest that langur female mounting did
not evolve to stimulate males. First, female mounting often occurred when
males were definitely out of sight. Second, mounters were potentially fertile
in only 14.3% of all episodes and mountees in 40.7%, whereas the majority
of mounting took place during infertile stages (see Fig. 4). Similarly,
Gouzoules and Goy (1983) rejected the hypothesis for Japanese macaques,
since a significantly higher proportion of female-male and female-female
mounts occurred after the females had conceived.
Nevertheless, we believe that the functional explanations of Beach
(1976) and Parker and Pearson (1976) have to be rejected for a different
reason. In langur one-male troops, copulations during infertile periods
might be a strategy of females to deplete sperm otherwise available to competing fertile females (Sommer, 1989a, Sommer and Rajpurohit, 1989; cf.
also Small, 1988): (i) 81% of all copulations were harassed. Higher ranking
females harassed copulations of lower rankings twice as often as vice versa,
copulations of top-ranking females were only rarely disturbed and 75% of
all copulations of pregnant females were likewise harassed. (Female-female
mounts, on the other hand, were rarely harassed; see Table V.) Sperm competition is assumed to be more intense in larger harems. In fact, interbirth
intervals in larger harems were significantly longer than those in smaller
harems, although this difference cannot be ascribed to food availability
(Srivastava, 1989, p. 219). (iii) Females copulated significantly more fre-
Homosexual Mounting in Female Langurs
509
quently during conception estrus. During days of conception, the average
number of females copulating at the same day was significantly smaller
than at given nonconception days (Sommer et aL, in press).
Therefore, female-female mounting could be related to femalefemale competition in order to minimize the probability that a competitor
conceives. Weber and Vogel (1970) ascribed a "drive-reducing function"
to langur male-male mounting. Similarly, pseudocopulations among females-which often include pelvic thrusting (see Tables IV, V)--could
provide a surrogate sexual satisfaction to the mountee especially if a male
ignores heterosexual solicitations (Sommer, 1985, 1988). An estrous female
might consequently reduce her frequency of solicitation. With that, the
probability of insemination (and corresponding sperm depletion of the
male) decreases. Mounters thus reduce the number of future resource competitors (and increase their own chances to conceive). The likelihood of
being rejected is low for mounters (see Table IV) probably because the
mountee is usually lower ranking and estrous (see Figs. 4, 6). Hrdy (1977)
reported that estrous females sometimes tried to avoid mounting by other
females by placing her rump closely to the resident male while soliciting
him. However, during the present study, many solicitations were addressed
to females, which is somewhat contradictory to the proposed hypothesis.
On the other hand, estrous females gave the impression of being under a
considerable "hormonal pressure" prompting them to accept mounters of
any sex.
Resource competition is reduced if the competitor is closely related. In fact, mounters obviously discriminated according to the degree
of relatedness (r). Mother-daughter pairs rarely mounted; with both future grandchildren and half-siblings, r is on average 0.25. Half-sisters
(different mothers, same fathers) mounted each other significantly more
than expected; with a half-sister's offspring, r is on average 0.125 (see
Table II).
The hypothesis calls for another prediction: Females, who are seldom
mounted should produce more offspring than females who are often
mounted. In fact, during a 12-year period, high-ranking females of the study
troop produced significantly more offspring than low-ranking females
(Borries et al., 1991). However, it would be premature to draw a causal
conclusion to female-female mounting, since the likelihood of conception
might well be connected with other rank-related factors such as food resources, physical condition, or frequency of sexual harassment. Moreover,
it remains unclear why mountees so rarely resist.
Further clarification of the proximate and ultimate causes underlying
female-female mounting in langurs requires an interspecific comparison
Srivastava, Borries and Sommer
510
that relates the degree of same-sex m o u n t i n g to different social and mating
structures.
ACKNOWLEDGMENTS
T h e fieldwork was sponsored by grants from M a n - a n d - t h e - B i o s p h e r e
P r o g r a m m e , D e p a r t m e n t o f Environment, G o v e r n m e n t of India, New Delhi
(A.S.), the I n d o - G e r m a n Cultural Exchange P r o g r a m m e o f the G e r m a n
A c a d e m i c E x c h a n g e Service ( D A A D ) , Bonn and the Ministry o f Education,
Culture, and Social Welfare, G o v e r n m e n t of India, New Delhi (C.B., V.S.).
S u b s e q u e n t f i n a n c i a l s u p p o r t was g r a n t e d by D A A D (A.S.), Graf~Sg
Niedersachsen, the Richard-Winter-Foundation
(C.B.), t h e G e r m a n
R e s e a r c h Council (V.S.), a n d the A c a d e m y o f Sciences, University o f
G6ttingen.
W e are gratefully indebted to S. M. M o h n o t , J o d h p u r , and Christian
Vogel, G 6 t t i n g e n , for supervision. F o r critical c o m m e n t s on an earlier
draft, we t h a n k S u z a n n e Chevalier-Skolnikoff, Jutta K~ister, Jim M o o r e ,
A n d r e a s Paul, E c k a r t Voland, Paul Winkler, and 3 a n o n y m o u s j o u r n a l
referees.
REFERENCES
Akers, J. S. and Conaway, C. H. (1979). Female homosexual behavior in Macaca mulatta.
Arch. Sex. Behav. 8: 63-80.
Anthoney, T. (1968). The ontogeny of greeting, grooming, and sexual motor patterns in captive
baboons (superspecies Papio cynocephalus). Behavior 31: 399-412.
Beach, F. A. (1968). Factors involved in the control of mounting behavior by female mammals.
In Diamond, M. (ed.), Perspectives in Reproduction and Sexual Behavior, Indiana
University Press, Bloomington, pp. 83-131.
Beach, F. A. (1976). Sexual attractivity, proceptivity, and receptivity in female mammals. Horm.
Behav. 7: 105-138.
Bishop, N. (1975). Social behavior of langur monkeys (Presbytis entellus) in a high altitude
environment. Ph.D. thesis, University of California, Berkeley.
Boggess, J. E. (1976). Social behavior of the Himalayan langur (Presbytis entellus) in eastern
Nepal. Ph.D. thesis, University of California, Berkeley.
Borries, C. (1989). Konkurrenz unter freilebenden Langurenweibchen (Presbytis entellus).
Ph.D. thesis, University of G6ttingen, Germany.
Borries, C., Sommer, V., and Srivastava, A. (1991). Dominance, age, and reproductive success
in free-ranging female Hanuman langurs (Presbytis entellus). Int. J. Primatol. 12: 231-258.
Carpenter, C. R. (1942). Sexual behavior of the free-ranging rhesus monkeys (Macaca
mulatta). II. Periodicity of estrus, homosexual, autoerotic, and non-conformist behavior.
J. Com. Psychol. 33: 143-162.
Chevalier-Skolnikoff, S. (1976). Homosexual behavior in a laboratory group of stumptail
monkeys (Macaca arctoides): Forms, context, and possible social functions. Arch. Sex.
Behav. 5: 511-527.
Homosexual Mounting in Female Langurs
511
Chowdhury, S. R., and Chandra, H. (1984). Induction of menstrual cyclicity and circulating
levels of plasma progesterone and estradiol in the iangurs (Presbytis entellus). In Roonwal,
M. L., Mohnot, S. M., and Rathore, N.S. (eds.), Current Primate Researches, University
of Jodhpur, India, pp. 437-440.
Denniston, R. M. (1980). Ambisexuality in animals. In Marmor, J. (ed.), Homosexual
Behaviour- A Modern Reappraisal, Basic Books, New York, pp. 25-40.
Fedigan, L. M., and Gouzoules, H. (1978). The consort relationship in a troop of Japanese
monkeys. In Chivers, D. J., and Herbert, J. (eds.), Recent Advances in Primatology, VoL
1: Behaviour, Academic Press, London, pp. 493-495.
Ford, C. S., and Beach, F. A. (1952). Patterns of Sexual Behavior, Eyre & Spottiswoode,
London.
Freud, S. (1961). Drei Abhandlungen zur Sexualtheorie, Fischer, Frankfurt am Main.
(Originally published 1905).
Geist, V. (1968). On the interrelation of external appearance, social behavior and social
structure of mountain sheep. Z. TierpsychoL 25" 199-215.
Gouzoules, H., and Goy, R. W. (1983). Physiological and social influences on mounting
behavior of troop-living female monkeys (Macaca fuscata). Am. J. PrimatoL 5: 39-49.
Goy, R., and Goldfoot, D. (1975). Neuroendocrinology: Animal models and problems of
human sexuality. Arch. Sex. Behav. 4: 405-420.
Guhl, A. M. (1948). Unisexual mating in a flock of white leghorn hens. Transact. Kansas
Acad. Sci. 51: 107-111.
Hanby, J. P., and Brown, C. E. (1974). The development of sociosexual behaviors in Japanese
macaques, Macaca fuscata. Behavior 49: 152-196.
Harcourt, A. H. (1979). Social relationships among adult female mountain gorillas. Anita.
Behav. 27: 251-264.
Hrdy, S. B. (1976). Care and exploitation of nonhuman primate infants by conspecifics other
than the mother. In Rosenblatt, J. S., Hinde, R. A., Shaw, E. A., and Beer, C. (eds.),
Advances in the Study ofBehavios, VoL 6, Academic Press, New York, pp. 101-158.
Hrdy, S. B. (1977). The Langurs of Abu, Harvard University Press, Cambridge, MA.
Jay, P. (1963). The social behavior of the langur monkey, Ph.D. thesis, University of Chicago,
Chicago.
Kempf, E. J. (1917). The social and sexual behavior of infrahuman primates with some
comparable facts in human behavior. PsychoanaL Rev. 4: 127-154.
McKenna, J. J. (1975). An analysis of the social roles and behavior of seventeen captive
Hanuman langurs (Presbytis entellus). Ph.D. Thesis, University of Oregon, Eugene.
Michael, R. P., Wilson, M. I., and Zumpe, D. (1974). The bisexual behavior of female rhesus
monkeys. In Friedman, R. C., Richart, R. M., and Vande Wiele, R. L. (eds.), Sex
Differences in Behavior, Wiley, New York, pp. 399-412.
Mohnot, S. M. (1974). Ecology and behavior of the common Indian langur, Presbytis entellus
Dufresne. Ph.D. thesis, University of Jodhpur, India.
Parker, G. A., and Pearson, R. G. (1976). A possible origin and adaptive significance of the
mounting behavior shown by some female mammals in oestrus. Z Natur. Hist. 10: 241-245.
Ripley, S. (1965). The ecology and social behavior of the Ceylon gray langur, Presbytis entellus
thersites. Ph.D. Thesis, University of California, Berkeley.
Siegel, S. (1985). Nichtparametrische statistische Methoden, 2nd ed., Fachbuchhandlung fiir
Psychologie, Eschborn.
Small, M. F. (1988). Female primate sexual behavior and conceptionRAre there really sperm
to spare? CULT.Anthropol. 29: 81-100.
Sommer, V. (1985). Weibliche und miinnliche Reproduktionsstrategien der Hanuman
Languren (Presbytis entellus) von Jodhpur, Rajasthan/Indien. Ph.D. Thesis, University of
Grttingen, Germany.
Sommer, V. (1987). Infanticide among free-ranging langurs (Presbytis entellus) at Jodhpur
(Rajasthan/India): Recent observations and a reconsideration of hypotheses. Primates 28:
163-197.
Sommer, V. (1988). Female-female mounting in langurs (Presbytis entelhts), hTt. Z PrimatoL
8: 478.
512
Srivastava, Borries and Sommer
Sommer, V. (1989a). Sexual harassment in langur monkeys (Presbytis entellus): Competition
for nurture, eggs, and sperm? Ethology 80: 205-217.
Sommer, V. (1989b). Infant mistreatment in langur monkeys-Sociobiology tackled from the
wrong end? In Rasa, A. E., Vogel, C., and Voland, E. (eds.), The Sociobiology of Sexual
and Reproductive Strategies, Chapman and Hall, London, pp. 110-127.
Sommer, V., and Rajpurohit, L. S. (1989). Male reproductive success in harem troops of
Hanuman langurs (Prbsbytis entellus). Int. Z PrimatoL 10: 293-317.
Sommer, V., Srivastava, A., and Borries, C. (in press). Cycles, sexuality, and conception in
free-ranging langurs (Presbytis entellus). Am. J. Primatol.
Sommer, V., and Moore, J. (in preparation). Male-male mounting in free-ranging langurs
(Presbytis entellus).
Srivastava, A. (1989). Feeding ecology and behavior of Hanuman langur (Presbytis entellus).
Ph.D. thesis, University of Jodhpur, India.
Talmage-Riggs, G., and Anschel, S. (1973). Homosexual behavior and dominance in a group
of captive squirrel monkeys (Saimh~ sciureus). Folia PrimatoL 19: 61-72.
Vogel, C. (1976). Okologie, Lebensweise und Sozialverhalten der grauen Languren in
versehiedenen Biotopen Indiens. Fortsehr. Verhaltensforschung, Beihefte zur Z. Tierpsychol.
17, parey, Berlin.
Weber, I. (1973). Tactile communication among free-ranging langurs. Am. Z Phys. Anthropol.
38: 481-486.
Weber, I., and Vogel, C. (1970). Sozialverhalten in ein- und zweigeschlechtlichen
Langurengruppen. Homo 21: 73-80.
Wickler, W. (1967). Socio-sexual signals and their intra-specific imitation among primates. In
Morris, D. (ed.), Primate Ethology, Aldine, Chicago, pp. 69-147.
Wickler, W. (1969). Sind wir Siinder? Natulgesetze der Ehe, Droemer/Knaur, Miinchen.
Winkler, P. (1988). Feeding behavior of a food-enhanced troop of Hanuman langur (Presbytis
entellus) in Jodhpur, India. In Fa, J. E., and Southwick, C. H. (eds.), The Ecology and
Behavior of Food-Enhanced Primate Groups, Alan R. Liss, New York, pp. 3-24.
Winkler, P., Loch, H., and Vogel, C. (1984). Life history of Hanuman langurs (Presbytis
entellus): Reproductive parameters, infant mortality, and troop development. Folia
Primatol. 43: 1-23.
Wolfe, L. (1984). Mounting patterns of female Japanese macaques. Am. J. Phys. AnthropoL
63: 235.
Wolfheim, J. and Rowell, T. (1972). Communication among captive talapoin monkeys
(Miopithecus talapoin). Folia Primatol. 1: 224-255.
Yerkes, R. M. (1939). Social dominance and sexual status in the chimpanzee. Quart. Rev.
Biol. 14: 115-136.
Young, W. C. (1941). Observations and experiments on mating behavior in female mammals.
Quart. Rev. Biol. 16: 135-156.
Zuckerman, S. (1932). The Social Life of Monkeys and Apes, Kegan Paul, London.