The Earless monitor lizard (Lanthanotus borneensis) is a semiaquatic, brown lizard native to the Southeast Asian island of Borneo. It is the only living species in the family Lanthanotidae and it is related to the true monitor lizards.
The Earless monitor lizard has a cylindrical body, long neck, short limbs, long sharp claws, small eyes, semitransparent lower eyelids, and six longitudinal rows of strongly keeled scales. Despite the name, these lizards are capable of hearing, although lack a tympanum, an ear-opening, and other externally visible signs of ears. The upper parts of their body are orangish-brown, and the underside is mottled dark brown and whitish, pale yellowish, ochre, or rusty. The tail is prehensile and if it is lost, it is not regenerated. Overall the sexes are alike, but males have a distinctly broader head and broader tail base than females.
Earless monitor lizards are found on the Southeast Asian island of Borneo. Here it is known from Sarawak in East Malaysia, as well as West and North Kalimantan in Indonesia. They also occur in Brunei. These lizards prefer to live in lowlands near rocky streams and marshes. These are typically in rainforests, but they are also found in streams flowing through degraded habitats such as agricultural land, mature fruit tree gardens, and palm oil plantations, and reportedly may occur in rice paddies.
Earless monitor lizards are generally strictly nocturnal animals, although exceptionally daytime observations in the open have been reported. Their day is usually spent near water in burrows that can be up to 30 cm (1 ft) long or under logs, rocks, or vegetation. They are generally quite inactive and not agile but can make surprisingly fast spurts when startled. They sometimes oscillate the throat (similar to frogs) and may flick their forked tongue, similar to snakes. They can also make a gentle, squeaky vocalization. Monitor lizards are generally solitary creatures, but in areas with limited water resources, they will gather in groups. During one study where 19 individuals were located during the night, about half were in the water and the other half near the water on land. When underwater, the semitransparent lower eyelids are generally closed, covering the eyes. It has been speculated that the prehensile tail is wrapped around stones, roots, and other things underwater to avoid being swept along during floods. Unusually for a lizard, Earless monitor lizards can swallow prey while submerged underwater. They appear to be able to do this by draining water from their nostrils, similar to turtles.
Earless monitor lizards have a carnivorous diet eating mainly earthworms, crustaceans, and fish.
Like their closest relatives, Earless monitor lizards are oviparous (egg-laying), although little is known about their reproduction. Adult males are likely territorially aggressive, as a survey of a locality found twice as many females as males, and most of the males (but no females) had various injuries, such as loss of toes or tail, and scarring on the head or neck. In the wild mating has been recorded in February, and a female caught in April was likely gravid. The 2-12 (average 8) oval eggs measure about 3 cm (1.2 in) long and have a leathery white shell. They are deposited on land. In captivity, the eggs hatch after about 3 months at a temperature of 27 °C (81 °F). When hatching the total length of the young is about 7-14 cm (3-6 in). When one year old, their total length is about 23-25 cm (9-10 in).
The main threat to Earless monitor lizards comes from humans who trap them for the pet trade and hunt them for their skin in the leather trade. Habitat loss represents another serious threat to this species, as forests in Borneo rapidly are being replaced by oil palm plantations.
Presently, the Earless monitor lizard is not included in the IUCN Red List and its conservation status has not been evaluated.
