| Six-lined racerunner | |
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| Male six-lined racerunner (Aspidoscelis sexlineata). | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Squamata |
| Family: | Teiidae |
| Genus: | Aspidoscelis |
| Species: | A. sexlineatus
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| Binomial name | |
| Aspidoscelis sexlineatus (Linnaeus, 1766)
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| Synonyms | |
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The six-lined racerunner (Aspidoscelis sexlineatus) is a species of lizard native to the United States and Mexico.
The six-lined racerunner is found throughout much of the southeastern and south-central portion of the United States, from Maryland to Florida in the east, across the Great Plains to southern Texas and northern Mexico.[3][4] In a study conducted on A. sexlineata in Mexico, the majority of the individuals found were inhabiting areas near the seashore that were formed and influenced from maritime climate and hurricanes.[5] The species' range also reaches north to Wisconsin and Minnesota. A small disjunct population is found in Tuscola County, Michigan.[6]
The six-lined racerunner is typically dark green, brown, or black in color, with six yellow or green-yellow stripes that extend down the body from head to tail. Between stripes are dark-colored fields ranging from dark brown to black and pale-colored fields near ventral scales on each side. There is a distinct lemon-yellow coloration on the sides of the head both above and below the lateral stripe. There are also white mental and gular stripes. The posterior gular fold is bordered by enlarged mesoptychial scales. The postantebrachial scales on the forearms are granular-like and only slightly enlarged.[7] The underside is usually white in color on females, but they may have some pale blue as adults. Males have a brightly blue-hued chest and abdominal stripes.[7] Males also sometimes have a pale green-colored throat. They are slender-bodied, with a tail nearly twice the body length. It is hypothesized that its long tail may serve as a counter-balance, allowing it to flee quickly from predators.[8]
Like other species of whiptail lizards, the six-lined racerunner is diurnal and insectivorous. A. sexlineata is most active between 9:00 am and 11:30 am on clear days between late spring and early summer when the temperature is closest to 90 °F.[9] In a different study, they found six-lined racerunners were most active between 11:00 am and 3:00 pm.[10] They are wary, energetic, and fast moving, with speeds of up to 18 mph (29 kmh), darting for cover if approached. Aggressive behavior is common, as the dominant will chase the subordinate and often follow up with a bite to signify dominance.[11]
Due to its extensive range, A. sexlineata is found in a wide variety of habitats including grasslands, woodlands, open floodplains, or rocky outcroppings. Populations have also been found abundantly in areas that experience frequent fires.[12] It prefers lower elevations, with dry loamy soils as well as open xeric environments.[13] In the lower elevations, the six-lined racerunner can be found in human-made disturbances like under voltage towers or along highways and railroads.[14] They lay their nests around activity burrows along secondary highways and dirt roads where it is well drained. [15]
Breeding takes place in the spring and early summer, with up to six eggs being laid in mid-summer and hatching six to eight weeks later. A second clutch of eggs may be laid several weeks after the first. Males have been seen to perform an act termed "cloacal rubbing" to show arousal during breeding. This act entails the male rubbing its cloaca and pelvic region rapidly on the ground, sometimes in a figure eight, while simultaneously moving forward.[16][9] When he finds a receptive female, the male will straddle her, curve his body over hers, and bite her on the opposite side to where his cloacal region is in order to start copulation.[9] Males will also perform an act termed "female tending" where the male will repeatedly charge at a female trying to leave its burrow until the female allows for the male to approach and begin the reproductive process.[17]
A. sexlineata are opportunistic insectivores; they target a prey that is currently abundant in their area.[18] The principal foods of A. sexlineata are spiders (Araneae 16.8%), grasshoppers (orthoptera 16.8%), and leaf hoppers (Homoptera 14.1%).[18] A. sexlineata also prey upon beetles and butterflies.[18]
Red-imported fire ants are a predator of the six-lined racerunner.[18]
There are three recognized subspecies of A. sexlineatus:
The six-lined racerunner is listed as a species of concern in the state of Michigan, due to its limited population but otherwise holds no official conservation status. In Michigan, the population is found in a single public game reserve, 322 km separated from the closest known other populations in Indiana.[19] This species has been observed to benefit from human development in some locations, with stable populations existing along road cuts and railroad tracks.[20] Playa Bagdad in Mexico has seen a large decrease in six-lined racerunner populations due to recreational vehicle damage to sand dunes.[21]
