KNOB-SCALED LIZARDS: Xenosauridae

PHYSICAL CHARACTERISTICS

With their flat heads and bodies and lumpy scales, the knob-scaled lizards have an unusual look. The head is usually triangular in shape, coming to a point at the tip of the snout. Some have a very noticeable ridge above the eye and extending forward to the snout and backward to the rear of the head. Often, the females have larger bodies than the males, but the males typically have bigger heads. Their bodies are usually dark brown to black, often with lighter-colored bands or blotches. The largest specimens grow to 4.7 to 5.1 inches (12 to 13 centimeters) long from the tip of the snout to the vent, a slitlike opening on the belly side of the animal at the beginning of the tail. The tail stretches nearly as long as the body.

Until 1999, this family only had four species. Discoveries of two new species—one in 2000 and one in 2002—increased the number to six. The two new species are known only by their scientific names: Xenosaurus penai and Xenosaurus phalaroantheron. Scientists believe additional species are yet to be identified. In particular, they suspect that a closer look at some of the already known knob-scaled lizards may reveal that they should actually be separated into two or more similar-looking species. This type of splitting is especially common in animals that live in small groups that are separated from one another, so the individuals from one group, or population, never see individuals from another population.

GEOGRAPHIC RANGE

Knob-scaled lizards live in typically small populations widely scattered from the Tamaulipas in northeastern Mexico on the Gulf of Mexico south to the middle of Guatemala in Central America.

HABITAT

Most knob-scaled lizards live in the mountains. Some species make their homes in cool cloud forests, while others prefer drier climates and live in hot, shrubby areas. In both cases, the knob-scaled lizards take advantage of their flattened shape and seek out cracks and holes in rocks and bark and other hiding places, where they spend much of their lives.

At one time, scientists included the Chinese crocodile lizard in this family. This lizard is now in its own family. Unlike the knob-scaled lizards, the Chinese crocodile lizard lives most of its life in or near shallow forest ponds, where it eats tadpoles and fishes.

DIET

These lizards are ambush hunters, which means that they sit very still and wait for their meal to come to them. Their meals are usually made up of insects that happen to come too close to their hiding places, which are usually in rock crevices. The lizards quickly grab the insects and gulp them down. Like other lizards, these species flick their tongues to pick up chemical odors from their insect prey. They cannot smell with their tongues, but they can smell with a special organ, called a Jacobson's organ, that sits above a small opening on the roof of the mouth. The lizard picks up the chemicals with its tongue and places them on the opening. A study of tongue-flicking behavior in Xenosaurus platyceps found that the young ones flicked their tongues to smell prey whether the lizards were in their hiding places or not, while the adult lizards did most of their tongue-flicking only when they were in holes or cracks. In other words, the adults were much more interested in finding prey when they were out of sight than when they were in the open.

At least one species of knob-scaled lizards, the Newman's knob-scaled lizard, will also eat bits of plants and some mammal meat. This suggests that the lizards may prefer insects but will eat just about anything they can find. Scientists call such animals opportunistic (ah-por-toon-ISS-tik), because they include almost any kind of plant or animal in their diet—if they are hungry and the opportunity presents itself.

BEHAVIOR AND REPRODUCTION

These lizards stay hidden away most of the time. Individuals in some species, including the one known simply as the knob-scaled lizard, live alone and defend their homes. Males will even bite one another on the head, which can leave behind noticeable scars. Other species, like Newman's knob-scaled lizard, are much more welcoming. In this species, pairs of male and female lizards often live together in peace in the same crevices. Members of this family usually stay in the same area throughout their lives, which can be quite long. Newman's knob-scaled lizards, for example, can live to be at least seven years old.

Females in all species give birth to baby lizards, rather than laying eggs as many other lizards do. A typical clutch for a lizard from this family is one to three babies, but some of those in the knob-scaled lizard species can have six young at a time. Once the females have their babies, usually from June to August, some stay with their young. Scientists have found that mothers in Newman's knob-scaled lizard species and the species known as Xenosaurus platyceps remain with their babies in their hidden-away homes, often keeping the young farther inside the hole or crevice, while the mothers stay nearer the entrance as if guarding the babies from possible land predators.

Some reports indicate that the lizards are most active at dawn and dusk and during the night. Because populations are scattered, their numbers are low, and they usually stay out of sight, much about their behavior and reproduction is still unknown.

REALLY RELATED?

Many scientists once included the Chinese crocodile lizard as one of the species in the family of knob-scaled lizards. The Chinese crocodile lizard is very similar in appearance to the other knob-scaled lizards, which all live in Mexico and Central America. A 1999 study, however, compared their DNA and found that the Chinese crocodile lizard is different enough to have its own family, which is now called Shinisauridae. DNA is genetic material, essentially, an instruction booklet for making a living thing, that is passed down from parents to babies. By looking at differences in these "instructions," scientists can tell how closely two species are related.

KNOB-SCALED LIZARDS AND PEOPLE

At least one population of Newman's knob-scaled lizard lives in the cracks of a rock wall on a plantation, but for the most part the lizards in this family and people hardly ever see one another. Since the lizards seem to make their homes in very small areas and travel very little from those areas, however, farming or other human activity that might destroy their habitat could mean disaster for the lizards, which would likely be unable to find a new home nearby.

CONSERVATION STATUS

Although scientists still know little about these species or their overall population sizes, they are not considered endangered or threatened.

KNOB-SCALED LIZARD (Xenosaurus grandis): SPECIES ACCOUNT

Physical characteristics: With a flat head and body and tall, bumpy scales, the knob-scaled lizard looks much like the other lizards in this family. This species, however, has bright red eyes and usually a dark-brown body, often with tan to cream bands or blotches. It grows to about 10 inches (25 centimeters) long from the tip of its snout to the end of its tail. The tail is a bit shorter than the rest of the body. Males and females are about the same size.

Geographic range: The knob-scaled lizard lives in both Central America and Mexico, stretching from Guatemala in the south to Veracruz, Mexico, on the Gulf of Mexico in the north.

Habitat: This species lives in wooded areas containing numerous cracks and crevices in rocks and bark where they can remain out of sight.

Diet: Like other species in this family, the knob-scaled lizard mainly eats insects, which it captures by ambush.

Behavior and reproduction: These lizards typically live alone in their crevices, which they defend against other members of their species. Male-to-male fights sometimes break out, with the males biting at one another's head. Females give birth to one to six baby lizards at a time.

Knob-scaled lizards and people: Knob-scaled lizards and people rarely see or bother one another.

Conservation status: Although much about this species is unknown, it is not considered endangered or threatened. ∎

FOR MORE INFORMATION

Books:

Halliday, Tim, and Kraig Adler, eds. The Encyclopedia of Reptiles and Amphibians. New York, NY: Facts on File, 1986.

Mattison, Chris. Lizards of the World. New York, NY: Facts on File, 1989.

Web sites:

"Chinese Crocodile Lizard." Center for Research on Concepts and Cognition, Indiana University. http://www.cogsci.indiana.edu/farg/harry/bio/zoo/shinisau.htm (accessed on December 8, 2004).

"Xenosauridae." Animal Diversity Web. http://animaldiversity.ummz.umich.edu/site/accounts/information/Xenosauridae.html (accessed on November 16, 2004).

"Xenosauridae." Virtual Museum of Natural History. http://www.curator.org/LegacyVMNH/WebOfLife/Kingdom/P_Chordata/ClassReptilia/O_Squamata/InfraAnguimorphan/SupFDiploglossa/FXenosauridae/xenosauridae.htm (accessed on December 8, 2004).