Knob-scaled lizard  (Xenosaurus grandis)

The knob-scaled lizard is classified as Vulnerable (VU) on the IUCN Red List (1).

The knob-scaled lizard (Xenosaurus grandis) is a secretive reptile (3) with a flattened head and body, an adaptation to its habit of dwelling in narrow crevices (5). It is greyish to dark-brown, with tan to cream bands or blotches (6). Its common name refers to the tiny bumps or ‘knob-like scales’ on the upperparts, while the underparts are covered with smooth scales (7). It has a forked tongue, which it flicks in and out, and the mouth bears small, sharp, fang-like teeth (6).

Currently, five subspecies of the knob-scaled lizard are recognised, but this species’ taxonomy is currently under revision and several of these subspecies may be reclassified as distinct species (1).

The knob-scaled lizard’s range extends from central-southern Mexico to central Guatemala (1).

The knob-scaled lizard inhabits a variety of tropical forest types, including tropical rainforest, cloud forest, deciduous forest, and oak forest (1). It can also be found in more open, disturbed habitats (3), but is always found in association with rock crevices (1) (3).

The diurnal knob-scaled lizard feeds primarily upon insects, especially on Orthoptera (grasshoppers, crickets and locusts) and Lepidoptera larvae (caterpillars of moths and butterflies), but it also occasionally feeds on small vertebrates such as lizards (3). An ambush predator, the knob-scaled lizard waits hidden in a rocky crevice until launching a surprise attack when prey comes within range (6). This lizard is a solitary species which will aggressively defend its crevice from other lizards, including other individuals of the same species (3).

The knob-scaled lizard gives birth to live young in late spring or early summer (3) (8), after a nine-month gestation period (9). On average, each litter contains three young (10), but litters of two to seven young have been reported (3) (10). Male knob-scaled lizards reach maturity at 28 months, while females reach maturity a little later at 32 months (11). The growth rate for males and females is highly dependent on the seasonal availability of food, with growth rate being greatest during the wet season (11).

Many knob-scaled lizard populations are threatened by human activities and are suffering from deterioration to their habitat, which is declining in both extent and quality (1). One of the five subspecies, Xenosaurus grandis grandis, is believed to have been reduced to just two populations (1). One of these populations, located on Buena Vista Mountain, Mexico, is highly threatened by the activities of a cement factory, which includes demolition of the landscape to obtain raw materials, thereby degrading the lizard’s habitat. In addition, the knob-scaled lizard is captured for the international pet trade (1).

The knob-scaled lizard is known to occur within two protected areas in Mexico: the Los Tuxtlas Biosphere Reserve and Ocote Biosphere Reserve (1). If, following taxonomic revision, some of the subspecies are reclassified and elevated to distinct species, their conservation status will need to be re-evaluated and it is likely that each will be recognized as seriously threatened (1).

To find out about the conservation of lizards and other reptiles see:

• International Reptile Conservation Foundation:
  http://www.ircf.org/
• Amphibian and Reptile Conservation:
  http://www.arc-trust.org/