Autores: Diana Alejandra del Rosario Reyes Espinoza
ÍNDICE DE CUADROS, FIGURAS Y ANEXOS
2. CONCEPTOS IMPORTANTES
3.3. Ingreso de un Nuevo lote
The evolutionary history o f gibbons has been little understood, largely due to a poor fossil record (Tyler, 1993). Numerous fossil taxa have been nominated as possible gibbon ancestors on the basis of their small body size and simple molar cusp
morphology, including; Pliopithecus, Laccopithecus, Micropithecus, Dendropithecus,
Limnopithecus, Dionysopithecus and Platodontopithecus. M ost o f these taxa are now
generally regarded as early catarrhines (Tyler, 1993; Fleagle, 1999). For some, however one o f these fossils remains a strong contender for the position o f gibbon ancestor:
Laccopithecus robustus (e.g. Wu and Pan, 1984, 1985; Tyler, 1993; Jablonski, 1993a).
This late Miocene fossil (c. 8 Ma [million years ago]) is known from the Lufeng deposits in Yunnan, China. Evidence for the close phylogenetic relationship between
Laccopithecus and extant gibbons is largely based on cranial and dental anatomy (Wu
and Pan, 1984, 1985). Meldrum and Pan (1988) have also presented evidence that the only identified postcranial remains o f Laccopithecus, a proximal fifth phalanx, is similar to modem siamangs, and hence a brachiator. However, metric and
morphological examination o f the upper and lower dentition shows a sexual . dimorphism that far exceeds extant gibbons (Pan et al., 1989). According to Tyler (1993), if Laccopithecus is the ancestor o f modem gibbons, it should be an arboreal brachiator, and have minimal sexual dimorphism. Clarification o f the placement of
Laccopithecus within the Hylobatidae warrants further evidence from the skull, auditory
region and postcranium (Tyler, 1993; Jablonski, 2000).
Wu and Poirier (1995) list mammalian faunas from a site called Hudieliangzi (Butterfly Hill) in Yuanmou, Yunnan, south China which contains specimens identified
Undisputed gibbons do not appear in the fossil record until the Pleistocene (e.g. Hooijer, 1960; Gu, 1989; Tyler, 1993; Jablonski 2000). Hooijer (1960) presents
evidence o f fossil gibbons (mainly teeth) from Sumatra {H. syndactylus, H. agilis), Java
(H. syndactylus, H. moloch), Borneo {H. muelleri) and China {H. hoolock).
Subsequently, several authors have dated the sites which contain these fossil faunas. De Vos (1983) and Van den Berg et al. (1995) have dated the so-called Punung faunal assemblage on Java and deposits on Sumatra, which contain fossils identified as H.
syndactylus, to between 60,000 and 80,000 years old. Deposits containing the remains
o f H. moloch on Java have been dated as Recent (Van den Berg et a l, 1995). The
evidence for syndactylus on Java, between 60,000 and 80,000 years ago indicates this species was present on the island before moloch, Java’s only present day gibbon inhabitant.
Long et al. (1996) discuss additional evidence o f fossil gibbons {H. sp.) from the sites o f Lang Trang in Vietnam dated to 80,000 years and Niah in M alaysia dated to 50,000 years, although details o f taxonomic identification are not provided. Gu (1989) presents evidence o f fossil gibbons from Chinese Pleistocene deposits. The fossils, mainly teeth, are identified as representing two species: H. concolor and H. hoolock.
Palaeoenvironm ental history of SE Asia
The palaeoenvironmental history o f Southeast Asia is complex due to a combination o f orogenesis, plate tectonics and glacial activities. These factors have affected the palaeogeography o f SE Asia, plus its climate, temperature, fauna and flora. The Indo-Malaysian Islands are part o f a complex comprising several structural
divisions. At the boundaries o f these plates, tectonic activity has created series o f volcanoes and areas o f submergence, the most dramatic o f which can be seen at the margins o f the Sunda Shelf (Figure 1.1). The exposed part o f the Sunda Shelf is also known as Sundaland, and comprises the Malay Peninsula, Sumatra, Java, Borneo and other smaller island groups (Bellwood, 1997) (Figure 1.1).
Hall (1996, 1998) has reconstructed the palaeoenvironmental history of
Cenozoic SE Asia, including the distribution o f land and sea. Since the fossil evidence for gibbons and gibbon ancestors dates to no later than the middle Miocene
(approximately 15 Ma), this date has been used as a benchmark from which to briefly describe the palaeoenvironmental history o f SE Asia.
Between 20 and 10 Ma changes in the orientation o f several tectonic plate boundaries throughout SE Asia resulted in the tectonic pattern recognisable today (Hall,
1998). These changes, described in detail in Hall (1998) had dramatic effects on the palaeogeography o f the area. Marine deposits including fossils and sediments are used to reconstruct the extent o f land exposure, the position o f former coastlines, and the location o f former river systems. However, in many cases depending on the type o f depositional environment, the marine record is incomplete and patchy. Despite this, Hall (1998) has reconstructed the distribution o f land and sea in SE Asia at 5 million year intervals from the interpretation o f marine records from a variety o f sources.
u» M iddle M io cen e 15 M a VOLCANOES ^ HIGHLAND E jl a n d CARBONATE PLATFORMS □ SHALLOW SEA □ DEEP SEA
VXv///.- - L ate M io ce n e % 10 M a
. . .
_ A - .
VOLCANOES HIGHLAND L 'l LAND PZI3 CARBONATE 'p l a t f o r m s □ SHALLOW SEA n DEEP SEA^ E arly Pliocene 5 M a VOLCANOES HIGHLAND E ] LAND CARBONATE PLATFORMS □ SHALLOW SEA □ DEEP SEA
Between 15 and 5 Ma large parts o f Sundaland were exposed. According to H all’s distribution map (Figures 1.5-1.7), during the middle Miocene at 15 Ma emergent land persisted from China, Vietnam, Laos, Cambodia, Thailand, and Malay Peninsula, connecting these areas with large parts o f Borneo. During this time only the southern parts o f Borneo were covered by shallow seas. A ridge o f volcanoes running east to west across central Borneo (on the Sarawak-Kalimantan border) created areas o f highland. This situation persisted through the late Miocene, approximately 10 Ma (Figure 1.6)
Subduction o f the Indian plate under the Sunda Shelf also created a ridge of volcanoes across Sumatra and Java, with patches of emergent land appearing at 15 Ma. By 10 Ma (Figure 1.6) these isolated patches o f land joined to form a long, thin strip o f land linking Sumatra, Java and Sundaland.
Between 15 and 10 Ma Hainan Island was also joined to mainland China, however by 5 Ma this corridor was covered by shallow seas (Figure 1.7). In H all’s most recent reconstruction o f emergent land, at 5 Ma, the area o f emergent land is reduced. However, even at this time Malay Peninsula, Borneo, Sumatra and Java are linked via emergent land on the Sunda Shelf. Also at this time, patches o f emergent land become evident at the position o f the present day Mentawai Islands, although these patches are not connected to Sumatra. Throughout this time, east o f Borneo deep basins had formed which may have represented barriers to dispersal to islands such as the Philippines, Sulawesi and New Guinea.
Thus, there is evidence indicating that emergent land probably extended from Indochina to Borneo in the Miocene, and according to Morley and Flenley (1987) both seasonal and everwet rain forests were present. Other evidence indicates that tropical rain forest extended as far north as southern China, southern Japan, and westward to
therein). According to Heaney (1991) by approximately 5 Ma the insular and tropical nature o f SE Asia was well established.
The age o f the Plio-Pleistocene boundary is controversial (Lowe and Walker, 1997). Some authors favour a relatively young date for the age o f this boundary, 1.64 Ma, based on palaeomagnetic evidence. Other suggest that the boundary is much older, around 2.5 Ma, based on ocean core and faunal change evidence (Lowe and Walker,
1997). The view taken here is that the Plio-Pleistocene boundary dates to no later than about 2.5 Ma.
Batchelor (1979) suggests that up until the end o f the Pliocene the extent o f exposed Sundaland covered some 2,000 kilometres east to west, and incorporated much o f Malay Peninsula, Sumatra and Borneo. However, at this time major changes in sea- level began as a result o f glacial activity on a global scale. The major world-wide effects o f glaciation were to alter sea levels, temperature and the extent o f vegetation zones. Evidence from deep sea cores and deeply stratified terrestrial gastropod- and pollen-bearing cores indicates that since about 2.5 Ma there have been a number (approximately 20) o f glacial and interglacial cycles. During glaciations the vast
quantities o f water trapped in ice sheets across the globe, immobilised large amounts o f '^O and the cold seas were as a result relatively rich in '*0. During interglacials the ratio was reversed. Fluctuations in these ratios have been plotted from deep-sea cores, and indicate the timing and extent o f glacial waxing and waning (Shackleton and Opdyke,
1973; Lowe and Walker, 1997).
In SE Asia, as in many parts o f the world, the periodicity o f glaciations had the effect o f altering sea level and hence the extent o f exposed land, as well as affecting climate and vegetation. During periods o f low sea level large parts o f Sundaland were exposed by between 120 and 160m below present sea level (Morley and Flenley, 1987;
Heaney, 1991; Bellwood, 1997). This had the effect o f linking the islands o f Sumatra, Borneo and Java to the mainland, creating a geography similar to that seen during the Miocene (Figures 1.5-1.7). Several authors present evidence o f the environmental effects o f Quaternary glaciations in SE Asia (e.g. De Vos, 1983; Morley and Flenley,
1987; Heaney, 1991; van den Berg et a l, 1995; Jablonski, 1993a; Brandon-Jones, 1996, and references therein). These studies provide evidence o f the palaeo-ecological
implications o f environmental change, plus fossil locality and dating information for numerous sites across SE Asia. However, the exact nature and timing o f these variations is still actively debated (Jablonski, 1993b, 1997, and references therein)
In summary, it is apparent that the pre-history o f gibbons remains unclear due to a lack o f crucial fossil evidence from the late Miocene and Pliocene. However,
according to reconstructions by Hall (1996, 1998) and others (e.g. Heaney, 1991; Jablonski, 1993a) much o f the area uniting Sumatra, Borneo and Java to mainland Malaysia was exposed for long periods during the late Miocene, Pliocene and
periodically throughout the Pleistocene. Furthermore, several studies have shown that tropical rain forests were present in these areas, and that these habitats supported a diverse primate fauna (Morley and Flenley, 1987; Heaney, 1991; Jablonski, 1993a; and references therein). Despite evidence that climatic deterioration from the late Miocene onwards affected some primate fauna, this does not appear to be the case with respect to the gibbons (Jablonski, 1993a). Jablonski (1993a, 1998, 1999, 2000) presents evidence from the palaeontological and palaeoenvironmental record o f China which indicates that in spite o f increased seasonality and habitat fragmentation, gibbons were among the most successful primates. Finally, Jablonski (1998, 2000) suggests that this success was facilitated by a small body size and efficient life history parameters, such as an
1.4 B ackground to previous studies
Previous studies have used a variety o f morphological, behavioural and molecular characteristics to address taxonomic issues and infer phylogenetic
interrelationships. These have provided much information regarding general differences among the various gibbon taxa and sub-genera, and it is relevant at this stage to briefly outline these characteristics.
Gibbons are well known for their elaborate forms o f communication involving vocal and visual specialisations. All o f the gibbon species produce species-specific and sex-specific vocalisations, or songs (Marshall and Sugardjito, 1986; Geissmann, 1995). These songs have been shown to vary considerably and have been used by several authors to investigate taxonomic affinities and phylogenetic interrelationships (e.g. Groves, 1972; Chivers, 1977; Marshall and Sugardjito, 1986; Geissmann, 1995).
Regarding overall body size, the siamang is the largest o f all the gibbons. On average, the siamang weighs approximately 11 kg. Species in the subgenus Nomascus
are the second largest after the siamang, weighing over 7 kg. The hoolock gibbon averages 6.8 kg. Species in the subgenus Hylobates are the smallest, weighing.between about 5.5 - 6.5 kg. These values are mean, combined male and female body weights obtained from documentation relating to wild-shot museum specimens collected by Geissmann (1993). It is also well known that gibbons exhibit minimal sexual dimorphism (e.g. Groves, 1972; Creel and Preuschoft, 1976, 1984).
Pelage in gibbons is highly variable interspecifically. Furthermore, coat colour in gibbons is complex due to phase changes in colour which vary intraspecifically, during maturation (via colour phases = polymorphism) and differences between the sexes (sexually dichromatic). Several previous studies have concentrated on pelage differences among gibbons (e.g. Kloss, 1929; Groves, 1972; Marshall and Sugardjito,
1986; Geissmann, 1993, 1995). As a preliminary to the phylogenetic study observations were made on the pelts o f museum specimens, during skeletal data collection. These observations were assessed in association with published literature to produce a guide to the inter- and intra-specific differences in pelage among all eleven currently recognised species o f gibbon (Appendix 1). Due to a lack o f time, however, this was not pursued further in the present study. In the following section only broad differences and general coat colour will be described (Table 1.6).
H. syndactylus, klossii, moloch and muelleri are monochromatic, showing no
sharp colour phases; agilis and lar are polymorphic showing dark and light phases; and
pileatus, hoolock, concolor, leucogenys and gabriellae are sexually dichromatic, the
adult males being black and the adult females being light. There are several vernacular names in use which give an idea o f the general colour and/or distinguishing pelage features among the different taxa (Table 1.6).
While pelage and vocalisation differences among gibbons are well understood, detailed morphometric and molecular variability are less so. Thus, these two areas were chosen as the focus for this study. Since one o f the key aims o f this research is.to use molecular and morphological data to interpret phylogenetic relationships among gibbons, the next two sections o f this chapter (1.4.1, 1.4.2) outline previous studies in these areas. Furthermore, since a second key aim o f this study is to use the results of phylogenetic investigation to reconstruct the biogeographic history o f gibbons, section
T able 1.6 General pelage descriptions and commonly used vernacular terms for the different gibbon species (Geissmann, 1993, 1995).
Species C olour phase /
chrom atism
com m on v e rn a cu la r term s
Hylobates syndactylus monochromatic siamang
Hylobates hoolock sexually dichromatic hoolock, white-browed gibbon
Hylobates concolor sexually dichromatic concolor, black (crested) gibbon
Hylobates leucogenys sexually dichromatic white-cheeked (crested) gibbon
Hylobates gabriellae sexually dichromatic yellow-cheeked (crested) gibbon.
red-cheeked (crested) gibbon
Hylobates lar polymorphic lar, white-handed gibbon
Hylobates agilis polymorphic agile, black-handed gibbon
Hylobates muelleri monochromatic M üller’s gibbon, Bornean gibbon.
grey gibbon
Hylobates moloch monochromatic Javan gibbon, silvery gibbon
Hylobates pileatus sexually dichromatic pileated gibbon, capped gibbon
Hylobates klossii monochromatic Kloss gibbon, dw arf siamang.