Slow lorises (Nycticebus coucang)


Taxonomy of slow lorises in Sumatra


Greater slow lorises can be found in Indonesia (Sumatra), Malaysia (continental Malaysia), Thailand, and Singapore. Sumatran slow lorises are endemic to the northern part of Sumatra. Lorises inhabit various areas ranging from a primary tropical rainforest, mixed or coniferous tropical forests, marshes, and savanna, to damaged, eroded, or mined forests. Their territory can range from approximately 0.004 km² to 0.25 km², depending on the habitat type.

Gender and mating

Determining the sex of slow lorises is difficult at first glance, however, males have short, thin penis and females have even shorter and much thinner clitoris. In their time of heat, females usually loudly and periodically “whistle” and mark their territory. A male, if interested, will reply with whistling and “overmarking” the territory marked by the female. Mating usually happens while both lorises hang from a branch. The oestrus lasts 29-45 days, while they can mate throughout the entire year. The gestation period lasts around 191 days resulting in a single (rarely two) grey young with silver-white legs. The young are already well developed when born, their eyes are open and their weight is around 15 g. An interesting fact is that females give birth during the day. Mothers start to regularly leave their offspring alone already from the first day of their lives. They feed them with milk for 2.5 to 6 months and the young can ingest firm food already in the 3rd or 4th week of age. They mature at the age of 6 months but stay with their mothers longer. Females can become sexually active after their 16th month and males in their 18th month. Compared to other mammals of similar size, slow lorises become sexually active relatively late, followed with a long gestation period with low reproductive capability characterized by few offspring and a long interval between gestations.


Slow lorises are active from sunset to sunrise. Most of their time is spent in solitarily looking for food and observing their surroundings. They communicate with each other by hissing, whistling, and the adolescents are able to even use ultrasound if under threat. The main communication method is based on using urine, as they leave urine marks everywhere they go. Slow lorises tend to use the same trails through trees or sometimes even on the ground. Greater slow lorises form a social structure called noyau, which means that smaller territories of females or submissive males are covered by the bigger territory of a dominant male. They regularly sleep in a different location but they sleep in a typical position, having their head hidden between their thighs, curled into a ball. If the surrounding temperature drops, they are able to minimize their movement and fall into a motionless state for up to several days. They main predators are reticulated pythons (Python reticulatus), changeable hawk-eagles (Spizaetus cirrhatus), and orangutans (Pongo pygmaeus). Due to their nocturnal and inconspicuous nature, there is very little known about their lifestyle.


The biggest threats to slow lorises in Sumatra are black animal markets, where they are sold as “house pets”. Other threats are loss and damage to their habitat to create oil palm (Elaeis guineensis) plantations, illegal wood mines, etc. Slow lorises are also often being shot as crop pests.

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