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Differential energy budget and monopolization potential of harem holders and bachelors in hanuman langurs (Semnopithecus entellus) Preliminary results.

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American Journal of Primatology 55:57–63 (2001)
Differential Energy Budget and Monopolization Potential of
Harem Holders and Bachelors in Hanuman Langurs
(Semnopithecus entellus): Preliminary Results
OLIVER SCHÜLKE*
Abteilung Verhaltensforschung und Ökologie, Deutsches Primatenzentrum, Göttingen,
Germany
The demographic structure in the Hanuman langur (Semnopithecus
entellus) population of Jodhpur is extreme, in that some single males
monopolize harems with, on average, 25 adult females. It has been proposed that extratroop males, which live in all-male bands, inhabit lowquality habitats and suffer from reduced food provisioning and longer
daily travel distances. To compare the resulting energetic consequences
for harem holders and bachelors, I estimated their gross energy intake
and daily energetic expenditures. This analysis revealed no clear-cut
differences between the two classes of males in time spent feeding on
provisioned food, daily path length, gross energy intake, and energy expenditure. Due to the small sample size and other limitations of the study
design, the hypothesis under investigation can not be evaluated conclusively. The preliminary results suggest, however, that energy budgets of
harem holders and bachelors do not differ markedly. The importance
of direct ecological pressures to males for our understanding of variation in group composition is highlighted. Am. J. Primatol. 55:57–63,
2001. © 2001 Wiley-Liss, Inc.
Key words: number of males; energy budget; harem; all-male band; food
provisioning; Semnopithecus entellus
INTRODUCTION
Variation in the number of males in primate groups is thought to be determined mainly by two factors: the degree of estrus synchrony among females, and
female group size [reviewed in Nunn, 1999]. In general, this holds also for
Hanuman langurs (Semnopithecus entellus). Groups with more than 12 females
generally contain more than one male [Newton, 1988] and one-male groups are
common where female receptive periods are spread throughout the year
[Srivastava & Dunbar, 1996]. Thus, the langur population of Jodhpur (Rajasthan/
India) is exceptional with respect to their extremely large bisexual groups comprised of a single adult male and, on average, 25 females. Surplus males (bachelors) are living in all-male bands (AMBs). Based on the observations that food
Contract grant sponsor: Indian Council for Cultural Relationships.
*Correspondence to: Oliver Schülke, Abteilung Verhaltensforschung und Ökologie, Deutsches
Primatenzentrum, Kellnerweg 4, 37077 Göttingen, Germany. E-mail: oschuel@www.dpz.gwdg.de
Received 31 July 1998; revision accepted 7 June 2001
© 2001 Wiley-Liss, Inc.
58 / Schülke
provisioning is centered on harems and that AMBs have larger home ranges and
longer travel distances, Rajpurohit et al. [1995] and Sommer [1996, as cited in
Pereira, 1998] picked up an earlier suggestion by Sugiyama et al. [1965], who
proposed that AMBs are forced into low-quality habitats by bisexual groups. According to Sugiyama et al. [1965], poor habitat quality makes bachelors suffer
more from energy stress compared to males in harems. Extremely divergent energy budgets of harem holders and bachelors may explain why single males are
capable of monopolizing large groups of females. This paper addresses this hypothesis by comparing the energy budgets of harem holders and bachelors.
METHODS
In 1996, a total of 1,514 Hanuman langurs were living in 30 bisexual groups,
which were all organized as harems (Mohnot, Rajpurohit, and Chhangani, unpublished census data). The average group size of harems was 46 (8–128); average female group size was 25 (5–73). In addition, 136 bachelors (94 adults) lived
in at least 11 AMBs, with a mean band size of 10 (3–33). The study area around
Jodhpur has been described by Vogel [1988]. The four harem holders of the bisexual groups of Bhadreshwar (B28, 42), Kadamkandi-West (B27, 19), Kailana I
(B19, 9) and Sidhnath-Temple (B24, 25) were selected as focal animals (letter/
number codes refer to designations of earlier studies, and adult group size). Four
bachelors from the two AMBs, Chopasani (AMB 11, 2) and Soothla (AMB 9, 5),
were observed, including the only two adults of Chopasani and two mid-ranking
males (male #1 and male #5) from Soothla. All focal animals were approximately
the same age. From October 1996 through January 1997, I observed each focal
male continuously throughout his activity period from before dawn (6:15 AM) until after dusk (6:30 PM) on four consecutive days. I conducted instantaneous recording (30-sec intervals) of positional and feeding behavior [Martin & Bateson,
1993]. An attempt was made to sample bite rates for every male for every food
item by counting the number of bites, complete items (e.g., whole leaf or fruit) or
handfuls ingested during 10 independent 30-sec intervals. Daily path length was
measured with a pace counter during observations or on the following day. Plant
samples were collected for later identification at the Botanical Institute of the
JNV University of Jodhpur (Dr. Sundaramoorthy) and at the Central Arid Zone
Research Institute (CAZRI; Dr. B.K. Dutta) for chemical analysis (see below) and
in order to obtain the dry weight (SICO POPULAR® SB1 balance) of one “bite”
on an item-specific basis.
Phytochemical analyses were performed on 88 food items at the German Primate Center, Göttingen, using methods described elsewhere [Heiduck, 1997]. Calculation of energy content was based on the concentrations of nutrients per gram
dry weight by the following factors: 16.7 kJ/g for carbohydrates and proteins,
and 37.6 kJ/g for lipids [Janson et al., 1986]. Daily gross energy intake was calculated by multiplying the energy content of each particular food item with the
corresponding bite rate, bite weight, and daily feeding time [Rhine & Flaningon,
1978]. Since bite rate was not measured for every male for every item he ever fed
on, I calculated an average bite rate across all males for every item.
Daily energetic costs K were calculated using Equation 1 [Coelho, 1974]:
K = [B1*(24–P)] + L + ΣAi
[1]
where K = daily energy expenditure (kJ/day); B1 = basal metabolic rate per hour
(kJ/h) [Kleiber, 1961]; P = activity period of the individual (h); L = energy expen-
Differential Energy Budget of Langurs / 59
diture due to locomotion (kJ); and Ai = energy expenditure due to behavior i
performed during activity period (kJ).
Energetic costs of locomotion L were calculated as the costs of moving the
individual’s body mass over a measured distance [Coelho, 1974]. I assumed the
body mass of all focal males to be 18.1 kg (SD: 0.74 kg, range 17.0–19.0 kg), the
average body mass of five adult males weighed by Sommer [1985]. For daily
energetic costs Ai associated with a specific nonlocomotor behavior, i values from
the literature were used [Coelho et al., 1976]. For comparisons between bachelors and harem holders I used one value per male (mean value of 4 days of
observation) for a Mann-Whitney U-test. The statistical power of the test, however, is small due to small sample size, and in order to present the data in more
detail, means and standard deviations are given along with the medians.
RESULTS
Habitat quality in terms of abundance of provisioned food could not be compared directly, because patterns of provisioning varied markedly. Harems were
provisioned daily by one to 19 persons with high-quality food, such as wheat
preparations, fruit, vegetables, and sweets, near their sleeping site. In contrast,
bachelors had to move into human settlements and beg for food. Bachelors initiated the act of provisioning and were fed individually rather than as a group. As
a consequence, and contrary to the predicted pattern, the average time a male
spent feeding on provisioned food did not differ markedly between harem holders
and bachelors (NHH = NBA = 4, HH: median = 7.9% activity time, mean ± SD = 8.3
± 2,7, range = 5.7–11.5%, BA: median = 5.2% activity time, mean ± SD = 5.3 ±
1.3, range = 4.0–6.9%; one-tailed MWU: U = 3, Z = –1.44; 0.05 < P < 0.1).
Rajpurohit [1994] found that one AMB had longer daily travel distances than
one harem, and concluded that bachelors have higher energy expenditures than
harem holders. My data did not support this prediction (NHH = NBA = 4, HH:
median = 2.7 km, mean ± SD = 2.7 ± 1.0, range = 1.5–4.0, BA: median = 3.0 km,
mean ± SD = 3.4 ± 1.5, range = 2.1–5.3; one-tailed MWU: U = 7.0, Z = –0.29,
n.s.). Rajpurohit [1994], however, measured the movement of the entire group,
whereas I measured individual movements. Because the energy consequences of
daily activities are the focus of this paper, the latter measure should be preferred here. It seems noteworthy, however, that bachelors often moved 5–6 km
per day, which was rare for harem holders. On all of these days bachelors left the
exclusive part of their home range to visit harems. On their way bachelors crossed
open shrub-savanna with low food abundance, and after arriving at the harems
they rarely got a share from the provisioned food because the harem holder kept
them away from the feeding females. In the evening bachelors usually returned
to the exclusive areas of their home range (median of the mean daily path length
of three bachelors when visiting = 5.4 km, mean ± SD = 5.5 ± 0.2, range = 5.7–
5.3). On the other hand, whenever bachelors stayed away from harems they never
moved farther than 3.5 km on a single day (median of the means of three bachelors = 1.8 km, mean ± SD = 1.7 ± 0.4) and approached humans to beg successfully for food several times a day. Visiting harems therefore may entail the doubled
costs of higher energy expenditure (visit: median of the means of three bachelors
= 3.7 kJ*103/day, mean ± SD = 3.7 ± 0.04 vs. stay: median = 3.5, mean ± SD = 3.5
± 0.07) and lower energy intake (visit: median of the three means = 5.5 kJ*103/
day, mean ± SD = 5.2 ± 0.6 vs. stay: median = 9.5, mean ± SD = 10.0 ± 3.6).
However, more data on the distribution of visits is needed to investigate this
point in detail.
60 / Schülke
Fig. 1. Individual gross energy intake plotted against energy expenditure for four harem holders (open
points: B28: Bhadreshwar; B27: Kadamkandi-West; B19: Kailana I, B24: Sidhnath-Temple) and four
bachelors (filled points: AMB11: Chopasani males #1 and #2; AMB 9: Soothla males #1 and #5) on four
days, each.
Daily gross energy intakes did not differ between harem holders and bachelors (Fig. 1; NHH = NBA = 4, HH: median = 10.2 kJ*103/d, mean ± SD = 10.3 ± 4.0,
BA median = 8.3, mean ± SD = 7.9 ± 1.9, one-tailed MWU: U = 4.0, Z = –1.16,
n.s.). Although medians of mean daily gross energy intake point in the predicted
direction, samples of harem holders and bachelors overlapped to such an extent
that no statistical difference was detected. In contrast to the prediction, the total
daily energetic costs of harem holders (median = 3.6 kJ*103/d, mean ± SD = 3.6 ±
1.3) were not significantly lower than those of bachelors (median = 3.6 kJ*103/d,
mean ± SD = 3.6 ± 1.1; one-tailed MWU: U = 8.0, Z = 0.0, n.s.), which may be due
to small sample size. Daily energy budgets were not balanced in the sense that
high energy expenditure was compensated for by high energy intake on the same
day (Fig. 1).
DISCUSSION
Variation in primate group composition is generally viewed from the females’
perspective. Ecological settings are thought to determine the distribution of fertile females in time and space, which both result in a certain level of monopolizability of females [Nunn, 1999]. The influence of ecology on males and the role of
males in shaping primate social systems are often neglected but should be emphasized [Pereira et al., 2000]. The present study investigated the influence of
two such ecological parameters, namely distribution of provisioned food and homerange size, on differential competitive abilities among males. In contrast to the
expected pattern, the presented data revealed no clear tendencies for differences
between males living in harems and males from AMBs. Differences in habitat
quality, namely food provisioning, did not translate into differences in feeding
times for provisioned food or daily energy intakes in the present sample. Bach-
Differential Energy Budget of Langurs / 61
elors could have compensated for low food abundance by longer search times and
travel distances. But neither daily path length nor daily energy expenditure differed markedly between the two classes of males. Sugiyama et al.’s [1965] suggestion that AMBs are forced into low-quality habitats by harem groups and
consequently suffer from energy disadvantages was not supported, but small
sample size definitely preludes a final evaluation of the hypothesis.
The failure to detect significant differences between these two classes of males
may be due to a number of factors. First, this study used bachelors of intermediate rank, rather than only AMB alpha males, who alone are in direct competition
over leadership in bisexual troops. Second, only a small proportion of the entire
population was included in the analysis. Bachelors from other AMBs probably
experience harsher conditions than the males sampled in this study, which worked
against the hypotheses under investigation. Third, the data presented here are
restricted to the winter season, thereby missing the period of food shortage for
langurs at Jodhpur. However, if bachelors face higher costs in summer, the frequency of harem holder replacements by bachelors should drop during or after
the summer, which is not the case [Sommer & Rajpurohit, 1989]. Hence, seasonality in natural food and water abundance does not seem to affect harem holders
and bachelors differentially. Fourth, interindividual differences in body mass may
have influenced the males’ energy budgets differentially. And finally, the calculation of daily gross energy intake was based on average ingestion rates, rather
than individual rates. This is potentially problematic, because male classes may
differ in feeding speed.
With these potential limitations, a final conclusion concerning potential differences in energy budgets between harem holders and bachelors must await
additional data from year-round observation, individually measured body weights,
and a larger number of males, which should then be analyzed in a multivariate
approach. A detailed investigation of langur male energy budgets should include
the comparison of days when AMBs visit harems and days when bachelors stay
in the exclusive areas of their home range. It is possible that bachelors experience energy stress when visiting harems because they have to pass long distances and get less natural and provisioned food. On the other hand, bachelors
have the opportunity to restore energy by reducing costs for traveling, and to
gain energy from more frequent food provisioning, when they stay away from the
harems. Therefore, the frequency of harem visits will be decisive for a bachelor’s
overall energy budget.
Moreover, the frequency of visits is closely related to what Moore [1999] suggested to be responsible for variation in group structure among Hanuman langur
populations: intruder pressure. Moore [1999] proposed that intruder pressure is
most importantly influenced by population density. However, it is the density
and distribution of bisexual groups—not population density—that is decisive, if
the number of groups that a male regularly encounters is of interest. Population
density is moderate at Jodhpur [Newton, 1988], but the group density of 0.4/km2
[Vogel, 1988] is very low compared to other sites (1.3/km2 Ramnagar [Borries,
2000]; 1.8/km2 Kanha [Newton, 1988]; and 1.6/km2 Rajaji [Laws & Vonder Haar
Laws, 1984]). In addition, the distribution of bisexual groups at Jodhpur is
clumped around man-made water resources and temples. Thus, compared to other
sites, distances between patches of harems are extremely long, and the number
of bachelors visiting a harem should be low. This may favor the formation of onemale groups. The core of this argument again stresses the importance of direct
ecological pressures on extragroup males for progress with our understanding of
the variation in primate group composition.
62 / Schülke
ACKNOWLEDGMENTS
C. Borries and A. Koenig provided invaluable help and guided me through
all phases of my work. I thank S.M. Mohnot for his kind cooperation; S.M. Mohnot,
L.S. Rajpurohit, and A.K. Chhangani for their permission to use unpublished
census data; and B.K. Dutta and Sundaramoorthy for identification of plant
samples. I feel deeply indebted to L.S. Rajpurohit for guidance and encouragement. I thank J. Ganzhorn for his hospitality and for making it possible to analyze my plant samples at the DPZ; J. Ostner and V. Sommer for numerous
discussions; and P.M. Kappeler, W. Dittus, and four anonymous reviewers for
valuable comments on earlier drafts of this paper.
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