Knob-Scaled Lizards (Xenosauridae)
Medium-sized lizards with dorso-ventrally flattened body and relatively flat, triangular head
Maximum snout-to-vent length between 4.7 and 5.1 in (120 and 130 mm)
Number of genera, species
1 genus; 6 species
Tropical scrub, tropical rainforest, cloud forest
Mexico and Guatemala
Evolution and systematics
Originally the family Xenosauridae included two genera, Xenosaurus from Mexico and Central America and Shinisaurus from China. However, in 1999, J. Robert Macey and colleagues removed Shinisaurus from Xenosauridae and placed it into its own family (Shinisauridae) on the basis of DNA evidence. This same study places Xenosauridae in close relationships with the families Shinisauridae, Anniellidae, and Anguidae.
As of 2002, there were six described species of xenosaurs, including one described in 2000 (X. penai) and one in 2001 (X. phalaroantheron). Given the isolation of many Xenosaurus populations and their low mobility, it will not be surprising if further research finds several more undescribed species of Xenosaurus. No subfamilies are recognized.
There is no fossil record for the extant genus Xenosaurus. Fossils of three extinct xenosaurid genera have been found in North America and Europe.
The bodies of xenosaurids are relatively unique, at least compared to the more common lizards, such as Sceloporus and Anolis, of Central America and Mexico. In fact, the name Xenosaurus means "alien lizard." Compared to more typical lizards, xenosaurids have a flattened body and a flat, somewhat triangular head. Presumably the flattened body shape is related to the crevice- or hole-dwelling habit of these lizards.
The flattened head of these crevice-dwellers may influence how hard they can bite. Anthony Herrel and his colleagues found that xenosaurids with taller heads were able to bite more strongly than those with shorter heads. It is not clear whether the reduced bite strength of these flat-headed lizards is important ecologically because the jaws are still strong enough to easily crush their arthropod prey.
Male and female Xenosaurus often differ in body and head size, although the extent of sexual dimorphism varies between species. In most cases, females are bigger than males (e.g., X. newmanorum, X. platyceps), but no difference has been seen in some (e.g., X. grandis grandis, X. rectocollaris). While
females are often bigger in body size, males typically have larger heads, perhaps related to aggression between males.
Xenosaurids are found in Middle America, from southwestern Tamaulipas, Mexico, at the northern end of its range to central Guatemala at the southern end of its range. Within this broad region, the distribution is far from continuous, with most populations isolated from each other on particular mountain ranges.
The habitats in which xenosaurids are found range from cool, tropical cloud forests to fairly dry and hot scrub habitats. All species apparently share a crevice- or hole-dwelling habit. In fact, very few Xenosaurus have ever been found out-side of a crevice or hole.
Habitat appears to influence the ability of xenosaurids to thermoregulate. Species and populations from rain or cloud forests are unable to raise their body temperatures independently of the environment, whereas those from more open, scrub habitats are able to elevate their body temperatures above the environment's temperature.
There appears to be a range in the extent of social inter-actions in the Xenosauridae. Some species, such as X. grandis, are solitary animals that will aggressively interact or even fight when placed in proximity. Such fights may escalate to biting, especially in fights between males, sometimes leaving scars on the lizards' heads. Other species, such as X. platyceps and X. newmanorum, seem to be much more sociable and often occur in pairs.
Perhaps the most interesting social behavior observed in this family is apparent parental care. In two species (X. newmanorum and X. platyceps), adult females have been found in the same crevice as neonates. In each case, the female had recently been pregnant but was no longer. What is particularly interesting about these female/neonate associations is that the adult female is always seen closer to the crevice opening than the neonate, almost as if she were trying to protect the neonate from a predator trying to enter the crevice.
Feeding ecology and diet
Living almost exclusively in crevices or holes, xenosaurids are likely to be sit-and-wait foragers who eat what comes into or near their crevices. Diet analyses of three species by Julio Lemos-Espinal and colleagues found a generalist diet made up primarily of arthropods. However, the diet of X. newmanorum included a small amount of mammal and plant material. This diet analysis suggests that xenosaurids are generalists and opportunistic foragers.
Many lizards use their tongues to help detect prey odors or chemicals, and when presented with potential prey items, lizards will typically increase the rate at which they flick their tongues. William E. Cooper Jr. and colleagues tested X. platyceps for this response to prey chemicals. Tests of juveniles and adults outside of their crevices found that juveniles responded to prey chemicals but that adults did not. However, when tested in their crevices, both juveniles and adults responded to prey chemicals with elevated tongue-flicking rates, suggesting that the behavior of the adults depends on their location relative to their crevices.
All species studied are viviparous with many species having small litter sizes ranging from one to three offspring; however, X. grandis from a population in Veracruz, Mexico, has larger litters ranging up to six. Females likely give birth between June and August.
Because ecological and population studies of xenosaurids are few, their conservation status is unknown. However, given their relatively long lives (X. newmanorum can live to be at least seven years), relatively small litters, low mobility, and their specialized way of life (crevice-dwelling), it would seem that these species might be susceptible to any alteration of their habitats by humans. At least one population of X. newmanorum does coexist with low levels of human activity: they live in rock walls in a lime and coffee plantation.
Significance to humans
List of SpeciesKnob-scaled lizard
Newman's knob-scaled lizard
Xenosaurus grandis Gray, 1856, Mexico, near Cordova [= Veracruz]. Five subspecies are recognized: X. g. agrenon, X. g. arboreus, X. g. grandis, X. g. rackhami, and X. g. sanmartinensis.
other common names
In general form X. grandis is similar to other xenosaurids, although it differs in some aspects of scalation and dorsal coloration. Xenosaurus grandis also has strikingly red eyes.
Xenosaurus grandis has the largest geographic range of any xenosaurid, ranging from west-central Veracruz, Mexico, to Guatemala. However, it is possible that X. grandis is actually composed of several, as yet undescribed, species.
Xenosaurus grandis is one of the more solitary species of xenosaurids, with the vast majority of individuals living alone in their crevices.
feeding ecology and diet
Xenosaurus grandis eats a wide range of arthropods.
The reproduction of X. grandis has been studied in two populations, one in Veracruz, Mexico (X. grandis grandis), and one in Oaxaca, Mexico (X. grandis agrenon). The Veracruz population has the largest litters in the family (mean = 5.1), whereas the Oaxaca population has a litter size similar to the rest of the family (mean = 3.2).
Unknown, but likely susceptible to habitat changes.
significance to humans
Newman's knob-scaled lizard
Xenosaurus newmanorum Taylor, 1949, Xilitla region, San Luis Potosi, Mexico. No subspecies are recognized.
other common names
This species is similar to other xenosaurids except for some elements of scalation. The eyes of X. newmanorum are greenish yellow.
Veracruz and southeastern San Luis Potosi, Mexico.
Xenosaurus newmanorum was the first species of xenosaurid found with apparent parental care in the form of females protecting their young. Xenosaurus newmanorum is also one of the more gregarious species of xenosaurid, with male-female pairs often seen in the same crevice.
feeding ecology and diet
Xenosaurus newmanorum's diet consists primarily of arthropods, but some individuals occasionally eat mammals and vegetation.
Xenosaurus newmanorum has a mean litter size of 2.6.
Unknown, but populations may be susceptible to habitat alteration. However, X. newmanorum is known to coexist with limited human activity.
significance to humans
Ballinger, Royce E., Julio A. Lemos Espinal, and Geoffrey R. Smith. "Reproduction in Females of Three Species of Crevice-dwelling Lizards (Genus Xenosaurus) from Mexico." Studies on Neotropical Fauna and Environment 35 (2000): 179–183.
Cooper, William E., Jr., Julio A. Lemos-Espinal, and Geoffrey R. Smith. "Presence and Effect of Defensiveness or Context on Detectability of Prey Chemical Discrimination in the Lizard Xenosaurus platyceps." Herpetologica 54 (1998): 409–413.
Herrel, Anthony, Ed De Grauw, and Julio A. Lemos-Espinal. "Head Shape and Bite Performance in Xenosaurid Lizards." Journal of Experimental Zoology 290 (2001): 101–107.
Lemos-Espinal, Julio A., Geoffrey R. Smith, and Royce E. Ballinger. "Diets of Three Species of Knob-Scaled Lizards (genus Xenosaurus) from México." Southwestern Naturalist 48 (2003).
——. "Ecology of Xenosaurus grandis agrenon, A Knob-Scaled Lizard from Oaxaca, Mexico." Journal of Herpetology 37 (2003): In press.
——. "Natural History of the Mexican Knob-scaled Lizard, Xenosaurus rectocollaris." Herpetological Natural History 4 (1996): 151–154.
——. "Natural History of Xenosaurus platyceps, A Crevice-dwelling Lizard from Tamaulipas, Mexico." Herpetological Natural History 5 (1997): 181–186.
——. "Neonate-Female Associations in Xenosaurus newmanorum: A Case of Parental Care in a Lizard?" Herpetological Review 28 (1997): 22–23.
——. "Thermal Ecology of the Crevice-dwelling Lizard, Xenosaurus newmanorum." Journal of Herpetology 32 (1998): 141–144.
Macey, J. Robert, James A. Schulte II, Allan Larson, Boris S. Tuniyev, Nikolai Orlov, and Theodore J. Papenfuss. "Molecular Phylogenetics, tRNA Evolution, and Historical Biogeography in Anguid Lizards and Related Taxonomic Families." Molecular Phylogenetics and Evolution 12 (1999): 250–272.
Nieto-Montes de Oca, A., Jonathan A. Campbell, and O. Flores-Villela. "A New Species of Xenosaurus (Squamata: Xenosauridae) from the Sierra Madre del Sur of Oaxaca, Mexico." Herpetologica 57 (2001): 32–47.
Pérez Ramos, E., L. Saldaña de la Riva, and Jonathan A. Campbell. "A New Allopatric Species of Xenosaurus (Squamata: Xenosauridae) from Guerrero, Mexico." Herpetologica 56 (2000): 500–506.
Smith, Geoffrey R., Julio A. Lemos-Espinal, and Royce E. Ballinger. "Sexual Dimorphism in Two Species of Knob-Scaled Lizards (Genus Xenosaurus) from Mexico." Herpetologica 53 (1997): 200–205.
Geoffrey R. Smith, PhD