Lanthanotus borneensis Steindachner, 1878, Sarawak, Borneo.
OTHER COMMON NAMES None known.
One of the least known of all lizards, earless monitor lizards are medium-sized, with adults averaging 16.5-21.6 in (42-55 cm) total length with a relatively long cylindrical body, long neck, and long tail. They have short legs but long, curved, sharp claws. They can wrap their muscular bodies and prehensile tails around a branch in a manner that suggests that they might climb. Most of their scales are small, but six longitudinal rows of enlarged scales run from the head down the back, and two central rows run out on to the tail. Earless monitor lizard tails do not regenerate. They shed their skin in one piece as do some other anguimorphans and snakes. The brain case is similar to snakes in that it is more solidly encased than it is in varanids. The upper temporal arch has been lost and there is a hinge joint in the middle of the lower jaw, as in snakes. Also, like snakes, these lizards have sharp recurved teeth on their premaxillaries, maxillaries, palatines, pterygoids, and dentaries. Lanthanotus is the only species among anguimor-phans with translucent windows in its lower eyelids, which could be a precursor to the "spectacle" covering the eyes of snakes. Like snakes, Lanthanotus (Lanthan = hidden, otus = ear)
have no external ear openings and they have deeply forked tongues. Indeed, the earless monitor lizard could well be closely related to snakes. Viewed from below, males have blunt, rectangular jaws, whereas jaws of females are more pointed. Because of the hemipenes in males, the base of the tail is broader than in females.
These very secretive lizards are found only in lowland riverine regions of Sarawak on northern Borneo (there are unsubstantiated reports from nearby Kalimantan, Indonesia).
These lizards are found along banks of rivers and ponds.
Earless monitor lizards dig burrows in banks along watercourses and retreat into the water when threatened. By some accounts, the earless monitor is aquatic (individuals have been captured in fish seines and traps), by others, it is a burrower. The species does seem to prefer cool moist habitats. A number of these unusual lizards were collected after severe flooding in Sarawak in 1963. They could have been inactive in underground retreats and emerged when it flooded. They may also climb. Some reports of captive lizards suggest that the earless monitor could be nocturnal. In captivity, earless monitors appear to prefer relatively low ambient temperatures of about 75.2-82.4°F (24-28°C). In captivity, these are sluggish lizards that spend most of their time lying in water, seldom moving. Captives shed their skins very infrequently, less than once per year. Such observations could be mere artifacts of the unusual environmental conditions in captivity and could be largely irrelevant to behaviors of free-ranging wild lizards.
Captives have eaten squid, small bits of fish, earthworms, liver, and even beaten eggs. Earthworm setae have been found in stomachs of museum specimens, but their natural diet remains largely unknown.
Little is known about reproduction in the earless monitor lizard. One lizard was found with six large eggs in September 1976, but these eggs may not have been Lanthanotus eggs. Eggs have never been laid in captivity. Clutch sizes from dissected females is three to four. Eggs are large, about 1.2 in (30 mm) long.
The earless monitor lizard is not threatened, but should be considered gravely endangered as it occurs only in lowland riverine areas of Sarawak that have been dramatically altered by human activity.
SIGNIFICANCE TO HUMANS
As the sister species to varanids, this living fossil may offer hints as to what the ancestors of monitor lizards were like. ♦
I Varanus salvadorii I Varanus komodoensis I Lanthanotus borneensis
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