Slow Lorises are incredibly unique mammals. One of the few nocturnal primates, they are the only primate that is venomous. Mixing secretions from a gland near their elbow with their saliva produces a potent toxin that can cause a painful, lingering wound if bitten. They also have two tongues!

The “slow” in their name is actually a misnomer – they usually move quite fast when not being blasted by the bright lights of a pet store or someone’s home. They’re used to living in the dark after all. 

Prosimians

Slow lorises (Nycticebus spp.) belong to an evolutionary old group of primates (Primates) – prosimians (Prosimiae). Prosimians differ from simians in many important characteristics, for example, the un-fused mandibular symphysis (two halves of the lower jaw), an orbit missing a lacrimal bone, underdeveloped stereoscopic vision, very well-developed olfaction on the other hand, and territorial scent marking using their scent glands. Prosimians also have relatively small brains compared to simians, with limited or no brain structuring into ridges and a big olfactory part of telencephalon. Prosimians also usually have conspicuous teeth specializations and the number of their teeth might be reduced.

Taxonomy of slow lorises in Sumatra

In 2020, there was a taxonomic division of the species of the greater slow loris (Nycticebus coucang) into two separate species – greater slow loris (Nycticebus coucang) and Sumatran slow loris (Nycticebus hilleri). The area in which we operate can be considered as a border area of the areas of distribution of these two species. As a result, our conservation activities include both species, particularly in the case of confiscated slow lorises accepted to our rescue centre. Slow lorises living in our field area should, according to the definition of the area on the IUCN Red List of Threatened Species, belong to the species of Sumatran slow loris (Nycticebus hilleri). These two species are supposed to differ mainly in the colour of the coat.

Distribution

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.

Behaviour

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.

Threats

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.