45%

34%

16%

3% 2%

Activity Budget in H. lar

Rest

Feed/Forage

Travel

Singing

Intragroup interaction

Figure 4. After [7].

4) Territory and Territoriality

H. lar families occupy mutually exclusive territories

up to 0.4km2, individuals may travel 1.5km per day.

Territory use is summarised in fig. 4.

Territories are primarily defended by early-morning

vocalisation by paired adults, and intergroup male-

male staring bouts. Physical violence is rare [8].

Territories incorporate many scattered fruit patches

The ecology and evolution of monogamy in the white-handed gibbon, Hylobates lar.

James R Lumbard

2) Reproductive Ecology

Female H. lar give birth to a single infant after a gestational period of 8 months. Weaning

occurs at around two years of age, similar to the theoretical inter-birth interval. A

reproductive lifespan of up to 20 years sets a maximum limit of 10 offspring raised to

weaning during a lifetime although this limit is rarely realised [2].

The female’s oestrus cycle lasts 22 days and is overlain by cyclical sexual swelling, the

maximal phase of which coincides with ovulation 80% of the time. The period of sexual

3) Social Structure and Interactions

H. lar lives in isolated family groups of up to six individuals consisting of two parents and up

to 4 offspring of various ages [4]. Neither parent exerts dominance over the other, although

pregnant and lactating females often assume feeding dominance [5].

The female is entirely responsible for care of the infant although very little time is dedicated

to other close proximity within-group interaction [6], [5]. It is the male’s job to maintain the

pair bond through asymmetric grooming sessions and presents to his mate (fig. 3).

6) Conclusions

Sexual swelling and the effort of maintaining the pair bond demonstrate the importance

of mate selection and monogamy.

The size and mutual exclusion of territories constrain socio-ecology as a function of food

availability and result in strict defensive regimes involving both sexes.

Single births, the distribution of resources and fast, efficient locomotion allow joint

defence of territory, which is necessitated by low sexual dimorphism.

0

5

10

15

20

25

30

35

40

45

50

Tarsius bancanus (monogamy)

Pan troglodytes (multi-M, multi -F)

Hylobates lar (monogamy)

Mandrillus sphinx (polygyny)

Species (Mating System)

Oestrus cycle length (days)

Duration of maximum sexual swelling (days)

Ratio swelling-to-oestrus (%)

swelling is unusually long

compared to the oestrus

cycle for a monogamous

primate (Fig. 2) [3].

Figure 2. Comparing oestrus cycle and sexual swelling period in a number of primate mating systems. Data after [3].

0

10

20

30

40

50

60

70

80

90

100

First groom carried out Proportion of interaction spent grooming partner

Activity weighting (%)

Male

Female

Figure 5. Comparison of Cercopithecus nictitans (left) and H. Lar (right) wrist skeletal anatomy. From [11].

5) Locomotion and Specialised Anatomy

H. lar is a true brachiator (right) allowing efficient horizontal

locomotion as well as gap-crossing, and spends 80% of its

time in the top two layers of the canopy [9].

High humeral head surface area allows large shoulder

excursion, while strong forelimbs compared to hind limbs are

adapted to resist high magnitude forces.

A highly notched radial articular surface helps prevent

dislocation of the elbow during brachiation [10].

Fig. 5 shows specialisations of the H. lar wrist for

brachiation. The reduced ulnar styloid has

decreased ulnar articulation with the carpals.

The semilunar meniscus (l) interacts with the

carpals, stabilising the wrist while allowing a

greater range of pronosupination [12].

Gibbons also possess highly developed forelimb

musculature [13].

1) Introduction

Gibbons diverged from the other hominoids

ca. 10.5 Mya and are regarded as

consisting of 4 genera within the clade

Hylobatidae: Hylobates, Hoolock,

Nomascus and Symphalangus, making

them the most diverse extant apes. Fig. 1

illustrates their geographic range.

Typical weight is 5 kg although

Symphalangus, which is sympatric with H.

lar over part of the latter’s range, is

typically twice as heavy.

Hylobates lar exhibits many ‘typical’ gibbon

traits including, monogamy, brachiation

and low sexual dimorphism [1].

Monogamy will be investigated in terms of

reproduction, social structure, territory

and locomotion.

Figure 1. H. lar are endemic to Thailand, Cambodia, Malaysia, Sumatra, Borneo and western Java. From [1].

Very little play-type

ricocheting brachiation

interaction is observed

between H. lar siblings

[5].

Figure 3. Male-female activity weighting of pair-bond maintenance. Data after [6].

References:

1. Geissmann, T. (2013).at http://www.gibbons.de/main/index.html [Accessed: 29/11/13] 2. Palombit, R. International Journal of Primatology 16, 739–760 (1995). 3. Barelli, C., Heistermann, M., Boesch, C. & Reichard, U.H. Hormones and Behavior 51, 221–30 (2007). 4. Fleagle, J.G.596 (Academic Press: San Diego, 1999). 5. Leighton, D.R. Primate Societies xi 578 (1987). 6. Palombit, R. Behaviour 133, 321–356 (1996).

7. Palombit, R. Fig. 3, p. 327. Folia Primatologica 68, 32–37 (1997).

8. Raemaekers, J.J. & Raemaekers, P.M. Animal Behaviour 33, 481–93 (1985). 9. Cannon, C.H. & Leighton, M. American journal of physical anthropology 93, 505–24 (1994). 10. Ruff, C.B. American Journal of Physical Anthropology 119, 305–42 (2002). 11. Lewis, O.J., Hamshere, R.J. & Bucknill, T.M. Fig. 1, p.540 Journal of Anatomy 106, 539–52 (1970). 12. Sarmiento, E.E. International Journal of Primatology 9, 281–345 (1988). 13. Michilsens, F., Vereecke, E.E., D’Août, K. & Aerts, P. Journal of Anatomy 215, 335–54 (2009).

to cater for a highly frugivorous diet, though optimal habitat is typically saturated with mated

pairs, resulting in delayed sub-adult dispersal from the family territory [2].

Above: A lar gibbon family; inset, close up of a central lar gibbon. Text panel 5): A brachiating lar gibbon [www.arkive.org – Accessed: 29/11/13].