Grizzled Leaf Monkey

The Grizzled Leaf Monkey (Presbytis comata) is currently listed as endangered by the International Union for Conservation of Nature. It is estimated that there are less than 1000 of these animals living in the wild, and, due to clearing of rainforests in Indonesia, only 4% of their natural habitat remains. It is estimated that the population of the Grizzled Leaf Monkey has decreased at least by 50% in the last 10 years.

Grizzled Leaf Monkeys are found only on the island of Java in Indonesia. Java is part of the Sundaland Biodiversity hotspot that contains some 82 threatened species and 13 critically endangered species. They have been seen all throughout the tropical rainforest, in both high and low areas. They are active during the day and live in the trees, rarely touching the ground. They seem to travel in groups of 3-12 monkeys each, averaging 6-7. Sometimes the groups are centered around a mated pair, but not always. There have been reports of all male groups as well. These groups tend to be very territorial, defending their areas against other groups, but there does appear to be pretty extensive overlapping edges between territories. The small areas that they occupy are relatively crowded, with sometimes up to 91 individuals per square mile or 35 per square kilometer.

The average weight for a Grizzled Leaf Monkey is 13-18 pounds (6-8 kg). They are small and slender with small faces, tails, and long fur that ranges in color from dark gray to white on the underbelly. The fur on the top of their heads is darker, verging on black, and it is a longer fringe of hair. They also have large, rounded stomachs.

The Grizzled Leaf Monkey has an interesting adaptation to deal with the food that it eats. Since leaves are usually not very nutritious and are actually often toxic, most animals that live in the jungles of Indonesia supplement their diets with fruits, flowers, and the more nutritious parts of plants. Leaf Monkeys, however, have large stomachs with several chambers. The first chamber contains specialized bacteria that counteract the toxins in the leaves and break down the leaves so that the monkeys can get nutrition from them. About 70% of their diet consists of leaves, 62% young leaves and 6% mature leaves. They will also eat fungi and tips of branches.

Did You Know? The Grizzled Leaf Monkey’s stomach is so large that the contents alone account for 25% of their total weight.

There isn’t a lot of information available on mating and reproduction in the Grizzled Leaf Monkey. Other leaf monkeys, however, such as the Francois Monkey, have been studied in more detail, and the females reach sexual maturity at three to four years old and the males at four. They reach their full adult size at about six or seven. Usually only one baby is born at a time after a gestational period of about 200 days. Babies are usually nursed for about two years and don’t have any contact with their mothers after they are weaned.