Cnemidophorus sexlineatus

The Six-Lined Racerunner: A Swift Reptile of North America

The six-lined racerunner, scientifically classified as Aspidoscelis sexlineatus, is a distinctive lizard native to the United States and Mexico. Its common name derives from the six prominent yellow or green-yellow stripes that run longitudinally along its dark-colored body, a striking feature that aids in its identification.

Habitat and Distribution

This agile reptile boasts an extensive geographic range, primarily inhabiting the southeastern and south-central United States, stretching from Maryland to Florida in the east, and westward across the Great Plains to South Texas and northern Mexico. Its distribution also extends northward to Wisconsin and Minnesota, with a notable disjunct population in Michigan. The six-lined racerunner is adaptable, found in a variety of habitats including grasslands, woodlands, open floodplains, and rocky outcroppings. It often favors lower elevations with dry, loamy soils and open, xeric environments. Interestingly, populations have also been observed thriving in human-modified landscapes such as areas under voltage towers and along highways and railroads, and in habitats that experience frequent fires.

Physical Characteristics and Behavior

The six-lined racerunner is characterized by its slender build and a tail that can be nearly twice the length of its body, which likely serves as a counterbalance for its rapid movements. Beyond its signature stripes, it exhibits coloration that includes distinct lemon-yellow markings on the sides of its head and white stripes on its chin and throat. Sexual dimorphism is present, with males often displaying a brightly blue-hued chest and abdomen, while females typically have a white underside. These lizards are diurnal, meaning they are active during the day, and are known for their wariness, energy, and remarkable speed, capable of reaching speeds up to 18 miles per hour to escape threats. Within their populations, a hierarchical social structure exists, marked by aggressive interactions where dominant individuals chase and may bite subordinates to assert dominance.

Diet and Reproduction

As opportunistic insectivores, the six-lined racerunner's diet consists mainly of spiders, grasshoppers, leafhoppers, beetles, and butterflies, with their prey selection often dictated by what is abundant in their immediate environment. Breeding typically occurs in the spring and early summer. Females lay clutches of up to six eggs in midsummer, with hatching occurring six to eight weeks later. A second clutch may be produced several weeks after the first. Males engage in specific courtship behaviors, including 'cloacal rubbing' and 'female tending,' to attract and initiate mating with receptive females.

Conservation Considerations

While the six-lined racerunner generally thrives in many parts of its range, it is listed as a species of concern in Michigan and Wisconsin due to limited populations. Conversely, some human developments have inadvertently benefited the species by providing stable habitat. However, threats such as recreational vehicle damage to sand dunes have led to significant decreases in populations in certain areas, such as Bagdad, Tamaulipas, Mexico.