Horsfield's tarsiers are strange looking primates. Their pelage coloration ranges from pale-olive or reddish brown to pale or dark grey-brown, possibly varying with age. These animals have an extremely long tail which can reach 181-224 mm and is hairless except for tufts of hair at the end. Horsfield's tarsiers have two grooming claws on each foot. The fingers are very long and have pads on the tips. The toes have flattened nailes except for the second and third toes on hind feet, which bear claw-like nails. Eyes of these small primates are large and do not reflect light.
Horsfield's tarsiers live in Brunei, Indonesia, and Malaysia. They are found in Southern Sumatra, Borneo and nearby islands. They occur in Sabah, Brunei, Sarawak, and Kalimantan. Horsfield's tarsiers can live in both primary and secondary forests, and they also live in forests along the coasts or on the edge of plantations.
Horsfield's tarsiers are nocturnal creatures. They prefer to sleep, rest, or remain stationary alone during the day on perches at a height of 3.5-5 meters. Before sunset, Horsfield's tarsiers wake up and spend 1.5-2 hours of the night foraging for food. They detect prey primarily by sound and catch it with its hands. Horsfield's tarsiers, like all tarsiers, are excellent leapers; they also move by climbing, quadrupedal walking, hopping and "cantilevering." Social grooming in this species only occurs between mothers and infants, removing dead skin and parasites by scratching with their toe claws and licking their fur, avoiding their faces. They clean their faces by rubbing on branches. Horsfield's tarsiers are territorial and mark their territory with urine and scent from glands, while scratching the surface with their hind-limb toe claws.
Horsfield's tarsiers are carnivorous. They mainly eat insects such as beetles, grasshoppers, cockroaches, butterflies, moths, praying mantis, ants, phasmids, and cicadas. They may also eat small bats, snakes (even poisonous) and birds, including: spiderhunters, warblers, kingfishers, and pittas.
Horsfield's tarsiers are monogamous which means that males and females mate only with one partner. During the breeding season, males perform courtship calls. They emit 2-3 chirrups while opening and closing the mouth. This call happens within 5 minutes of looking at the female. Females give birth to 1 baby after the gestation period that lasts around lasts 178 days. Infants are born with their eyes open and fully furred and are able to groom themselves. The mother will carry her infant in her mouth and when she forages for food she will park the infant on a branch. The infant has click-like sounds: "k", "tk", "ki", or a rapid "kooih" and can be heard when the baby is left alone or is cold. The mother stays in contact with her baby using high-pitched calls. Infants start first using the tails as support during resting at 7-10 days and are weaned at 80 days after birth. Young Horsfield's tarsiers leave their range at the onset of puberty, and find their own territory.
The main threat to Horsfield's tarsiers is the rapid loss of habitat due to forest conversion, oil palm plantations, fire, and logging. Additionally, these animals are also collected for the illegal pet trade and wrongly considered a pest to agricultural crops. They can suffer, directly and indirectly, from the use of agricultural pesticides.
The IUCN Red List and other sources don’t provide the number of the Horsfield's tarsier total population size. Currently, this species is classified as Vulnerable (VU) on the IUCN Red List and its numbers today are decreasing.
Feeding mainly on insects, Horsfield's tarsiers may help to control their numbers.