The leaf monkeys, or langur, group of monkeys are three related genuses of Asian colobine monkeys. The Asian colobines' ancestors were the African colobine monkeys, which entered Asia from north of the Himalayas around 15 million years ago. The Asian Colobines split around 12 million years ago, into ancestors of Semnopithecus (the grey langurs) and the odd-nosed monkeys, and ancestors of Trachypithecus (lutungs) and Presbytis (langurs). Trachypithecus and Presbytis split around 11 million years ago. The ancestors of Semnopithecus split from the odd nosed  monkeys around 10 million years ago.

Semnopithecus monkeys inhabit South Asia, Presbytis monkeys live on Java, Borneo, Sumatra and the Malaysian peninsula, and Trachypithecus monkeys live in South Asia and South East Asia. DNA evidence suggests that there was continued hybridisation between Semnopithecus and Trachypithecus monkeys, which occurred when individual Semnopithecus males invaded populations of Trachypithecus monkeys.This occurred up to around 2.5 million years ago.

Colobine monkeys are generally arboreal (living in trees), and all have multi-chambered stomachs which are adapted to digest leaves (in a similar manner to ruminants) or other plant matter, although most colobine species also eat other things, and some are primarily frugivores (fruit eaters). They are medium-sized monkeys, which in most cases have babies of different colour to adults. Asian langurs specifically have tri-partite stomachs, with a single fementation chamber. This distinguishes them from the odd-nosed monkeys, which have four-part stomachs, including a pre-fermantation chamber. Langurs live in all-male groups, plus all-female groups with a single male leader.

Changing sea levels (resulting in islands being separated or joined from each other, as well as the formation of rivers), and climate change (due to ice ages and loss of rain forest habitat) are largely responsible for the speciation of the Asian colobines.

There are two leaf monkeys found in most areas of Sumatra, the silvery lutung, plus one Presbytis species. In Bukit Lawang the Presbytis species is Presbytis thomasi, Thomas' leaf monkey. Sightings of Thomas' leaf monkey are more common than those of the silvery lutung in Bukit Lawang. To the south of Bukit Lawang, beyond the Wampu river, the Presbytis species is the Sumatran surili.

The Presbytis genus of langur, known as surilis, are a genus of fruit-eating leaf monkeys, found mostly in Sumatra and Borneo. 

In general, each area of Sumatra is home to only one surili species. This occurs because Sumatran surilis are generalist feeders, occupying the same ecological niche, and therefore each area can sustain only one species. In Borneo, however, several surili species occupy a single area, as poor soils in Borneo mean that primates occupy more specialised niches.

The boundaries between species are generally major rivers, which the monkeys cannot cross. There may be small areas of overlap between species.

The full list of Presbytis species is:

• Thomas' leaf monkey (Presbytis thomasi) - DNA evidence suggests that it is the oldest Presbytis species, having diverged from other Presbytis species around 8 million years ago. Found in Aceh, and North Sumatra. Its barriers are the Wampu river in  North Sumatra, and the Alas river, as well as Lake Toba.
• The Sumatran surili (Presbytis melalophos), also known as the mitred leaf monkey (four subspecies in Sumatra, largely differentiated by their fur colour differences)
• The Javan surili (Presbytis comata), which is closely related to the South Sumatran subspecies (P. melalophos mitrata) of Sumatran surili
• The also related maroon langur (Presbytis rubicunda), which is found in Borneo, but is also closely related to the Sumatran surili
• The white-thighed surili (Presbytis siamensis). Four subspecies, found (P. siamensis siamensis) in peninsular Malaysia, (P. siamensis rhionis) in Pulau Bintang, (P. siamensis cana) in southern Riau province of Sumatra,  and (P. siamensis paenulata) in northern Riau province of Sumatra. Ancestry unknown.
• The banded leaf monkey (Presbytis femoralis) - with three subspecies, two found in the Malayan peninsula, and one,  P. femoralis percura, found between the Rokan and Siak rivers in the Pekanbaru area of Sumatra. 
• The Mentawai langur (Presbytis poteziani) with two subspecies, both on the Mentawai islands. It shares an ancestor with the Sumatran/Javan surili.
• The white-fronted surili (P. frontata), Hose's langur (P. hosei) and the Sarawak surili (Presbytis chrysomelas) - three closely related species, all found on Borneo.
• The Natuna Island surili (Presbytis natunae). Ancestry unknown. Found on Natuna Besar island.

Map showing approximate distribution of Presbytis species in Sumatra (Malaysia, Singapore and Java are not shown).

Thomas Leaf Monkey

Thomas leaf monkey in Bukit Lawang

Thomas leaf monkey/Thomas langur is the northenmost Sumatran surili species. Its range is considered to be the land to the west of the Wampu river, and west of the Simpang Kiri river. The Wampu river rises on the eastern half of the Bukit Barisan range, near Berastagi, flowing north into the sea past Stabat, on the east coast of Sumatra, while the Simpang Kiri river rises on the western half of the Bukit Barisan range, flowing south into the sea at Singkil, on the west coast of Sumatra.

Thomas leaf monkeys live at altitudes of up to 2500m, but population densities decrease at higher altitudes. They range from west to east across the Bukit Barisan range, living in primary and secondary rainforest, as well as rubber plantations on their borders. The different populations of Thomas leaf monkeys from the west coast to east coast of Sumatra are somewhat isolated by the mountain range, and the Alas river, which runs down the centre of the Bukit Barisan range, blocking passage for most of its length. This means that individuals between west and east coast populations have some differences from each other.

Their preferred food is fruit, when available, and overall their diet consists of around 45% fruit, and 40% young leaves. Thomas's langurs spend most of their time feeding, or resting. Both female and male Thomas leaf monkeys weigh around 5-8kg. They reach sexual maturity at age five, and give birth to single young. They live to around 20 years.

Thomas leaf monkeys are born to females living in groups of between one and six females with a single male. The females in the group give birth to infants, and after around five years following the group's formation, when the infants have grown sufficiently large, the females leave the group, leaving the male and his sons, who will try to form groups of their own.

Infanticide has been recorded in several langur species, including Thomas' langurs. Infanticide occurs when a male attempts to kill the infants in a different group, in order to take over that group for himself. It is suggested that the relatively small group size of Thomas' langur (larger groups would otherwise provide more protection against predator species), is a defence against infanticide.

Around 1.25 million years ago some of the population of Javan lutungs, moved from Java to the Malaysian peninsula, as a result of falling sea levels, which subsequently rose, separating the populations and meaning that the separate populations evolved into different species. At 0.5 million years ago sea levels fell further, allowing the Malaysian population to reach Sumatra, from where they spread across to Borneo, which was connected to Sumatra. With subsequent rising sea levels, the land bridge to peninsular Malaysia was the first to be sunk, while the link to Borneo remained for longer. This means that the Malaysian population of silvery lutung is now a separate subspecies from that living in Sumatra and Borneo.

The silvery lutung is a very specialised leaf eater, which particularly feeds on older leaves. They are the most folivorous of all colobines, eating around 90% leaves. In comparison, Presbytis monkeys eat fruit and young leaves. Silvery lutung live in groups of around 20 females and infants, with a single male. The group sleeps together in a single tree by night. They tend to stay near to rivers.

As with other lutungs, baby silvery lutungs are orange when born. Their skin changes from orange to adult black within a few days after birth, but the fur takes longer, around 4 months. Silvery lutungs practice communal nursing, meaning that after the infant is weaned, there is no more contact with the baby's mother than with other females. Silvery lutungs give birth to a single baby after a 6 month gestation, and can give birth from the age of two, although they do not reach full adulthood until aged five. Babies are born approximately every 2 years, and the adults can live up to 30 years.

Silvery lutung with baby. This baby is starting to change from infant (orange) to adult (black) colouring.