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Monday, June 4, 2018

Whiptails & Racerunners

Common Spotted Whiptail
Small to medium-sized slender lizards, whiptails and racerunners can be distinguished from other lizard species by their generally granular dorsal (topside) scales, larger rectangular ventral (underside) scales arranged in transverse rows, long tails, and forked, snake-like tongues. Additionally, these species belong to the genus Aspidoscelis, from the Greek aspido or ‘shield’ and skelos or ‘leg’, relating to their well-developed limbs that enable them to sprint at rapid speeds. Terrestrial and diurnal, these lizards are primarily carnivorous or insectivorous, actively foraging for food while temperatures are warm, and quickly skirting between objects for cover.  Of the 22 species occurring in the southwest, Central Texas is home to the Common Spotted Whiptail (Aspidoscelis gularis) and Six-lined Racerunner (Aspidoscelis sexlineata).  

Common Spotted Whiptail

The Common Spotted Whiptail has a brownish-green body with 7 to 8 longitudinal yellowish-white stripes and similarly colored spots in the margins.  Its tail is up to twice the length of its body, reaching a total length of just about 12 inches.  It is found in prairies, grasslands, rocky hillsides, dense thickets, and canyon bottoms, often near water.  Males are larger than females, having a red, orange, or pinkish throat and sometimes a blue or light blue belly.  Females have a white or cream-colored underside, and lay 1 to 8 eggs in July, typically in a separate chamber of their underground burrow, sometimes as deep as 11 inches.  These lizards scare easily and often retreat in a straight line, but can best be viewed from April through August when their insect prey are most abundant.

Six-lined Racerunner
Just over 10 inches long, the Six-lined Racerunner has seven light dorsal stripes with a greenish wash on the head and upper body that fades to brown posteriorly.  Males may also have a blue throat and belly, and juveniles often have more distinct stripes, lack the green wash, but have a bright blue tail. Females lay clutches of 1 to 6 eggs from May to August.  Associated more with sandy soils, this lizard can occupy a wide variety of habitats, including grasslands, riverbanks, floodplains, and juniper woodlands.  Spiders and grasshoppers are their chief prey, and adults actively forage during the day, but as summer temperatures rise by July, their activity becomes bimodal, peaking in the morning and late afternoon to avoid the hottest parts of the day.

Common Spotted Whiptail growing a new tail
Like many lizards, whiptails and racerunners have developed the ability to allow the tail to break free of the body when grasped by a predator. Called tail autonomy, this process involves wriggling the detached tail to distract the predator while the lizard itself is able to escape. Complex adaptations have evolved to enable the tail to be released along a series of fracture planes, which usually occur through weakened vertebrae, and not between them. Most amazingly, these species also have the ability to regenerate the tail, albeit slowly, so it can be lost again if necessary.