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Cnemidophorus sexlineatusSix-lined Racerunner

By cassie Bonavita

Geographic Range

The six-lined racerunner, Cnemidophorus sexlineatus, is native to the southeastern and central United States. The subspecies, Cnemidophorus sexlineatus viridis, or western prairie runner, dwells west of the Mississippi River as far as New Mexico, as far north as Colorado, and as far south as Texas. One six-lined racerunner subspecies (C. s. sexlineatus) resides east of the Mississippi River to the Coastal Plain of North Carolina, as far north as Maryland, to as far south as the Gulf coast of Florida. There is an isolated colony near the Great Lakes, in Tuscola County and in eastern-central Michigan at the northern end of the range. (Harding, 1997; Trauth, 1983; Warner, 2000)

Habitat

Six-lined racerunners are found in a variety of habitats such as savannas, prairies, fields, coastal dunes, and in fire-maintained open woodlands. Six-lined racerunners can thrive in fire-tolerant woodlands, especially those that burn annually during racerunner dormant period of mid- October to early May. They are unlikely to be found at elevations above 650 m and are commonly found at higher annual temperature ranges (26.1-43.1 °C) than most temperate-zone reptiles. Six-lined racerunners are commonly found in dry soils, sunny areas with eroded clay, and areas of sparse vegetation. In areas of human influence they can be found along roads, railroad tracks, and power lines. Six-lined racerunners dig burrows 3-12 cm into the soil for periods of inactivity and as a refuge when fleeing threats. During the nesting season, females dig burrows that face south to west to oviposit their eggs. (Carpenter, 1960; Harding, 1997; Martof, et al., 1980; Mount, 1996; Paulissen, 1988a; Steen, et al., 2013; Trauth, 1983)

  • Range elevation
    0 to 650 m
    0.00 to 2132.55 ft

Physical Description

Six-lined racerunners have slim bodies, pointed snouts, and tails that make up 70 percent of their total body length. Adult total length ranges from 15.2 cm - 26.7 cm and a SVL of greater than 6 cm. Males are larger than females.

Their coloration is often black, brown, and olive with 6 yellow or green longitudinal stripes that span from the back of the neck to the end of their tail. A subspecies of the six-lined racerunner, the western prairie racerunner, has a 7th stripe and bright green coloration on its ventral side.

Unlike other lizard species in the same range, six-lined racerunners are not shiny, but have velvety skin. They have pellet-like dorsal scales, and rectangular ventral scales. Their tails have rough rows of scales.

During mating season, adult males express bright blue coloration on their throats. Instead, females, which have light yellow stripes and white bellies year round.

Juveniles are, on average, 3 cm in total length when hatched. Their snout to vent length (SVL) is less than 50 mm. They have very distinct bright green stripes and bluish tails.

Six-lined racerunners are often mistaken for five-lined skinks, Plestiodon fasciatus, which are black, shiny lizards with just 5 yellow stripes. (Ballinger, et al., 1979; Carpenter, 1962; Harding, 1997; Martof, et al., 1980; Mount, 1996; Warner, 2000)

  • Sexual Dimorphism
  • male larger
  • sexes colored or patterned differently
  • male more colorful
  • Range length
    15.2 to 26.7 cm
    5.98 to 10.51 in

Development

After fertilization, six-lined racerunner eggs develop within the mother for approximately 38 days. During this time, the eggs increase to half their final size. The eggs are roughly 1 cm long when deposited and consist of soft, flexible shells. They continue to develop for 2 months.

Hatchlings are roughly 3 cm in length and they forage in order to develop needed fat stores to survive the 9-month hibernation period. Males require less fat reserves than females, because female reproductive development requires more energy. Age-class is determined by SVL; those below 60 mm are considered juveniles. This age class will end hibernation about 2 months after adults >60 cm due to their late start to hibernation. Sexual maturity occurs at a SVL above 70 mm and is reached within 1-2 years. These lizards exhibit indeterminate growth. (Clark, 1976; Etheridge, et al., 1983; Harding, 1997; Martof, et al., 1980; McKenna and Packard, 1975; Paulissen, 1987a; Paulissen, 1987b; Paulissen, 1988b; Trauth, 1983; Warner, 2000; Yanfu, et al., 2011)

Reproduction

During early May, fat stores are used up more quickly in order to have enough energy to begin seasonal enlarging of the gonads in both sexes. Fifteen days after exiting hibernation, females’ ovarian follicles and yolk deposits are full sized for reproduction. Generally the right ovary is larger and contains more eggs. In males, seasonal sexual enlargement occurs when fat stores lessen during hibernation. Reduced fat correlates directly with increased adrenal function.

Males use pheromones and visual adaptations to find females in the active season. Arousal of the female occurs by male cloacal rubbing, which consist of pelvic thrusting and forward movements of about 8 cm, before pausing, changing direction, and thrusting the pelvis. The motion resembles a figure-8 shape and occurs until females allow the male close enough for internal fertilization. Another mating ritual is known as female tending. This occurs when a male comes across a female’s burrow. He then stands near the entrance, deterring other males by chasing them away and keeping the female trapped. If she attempts to leave, the male will rush her, causing her to retreat back into her burrow. The male may continue this for a few hours a day, for several days. The female retreating pattern is a form of sexual recognition between members of the species. Once the female allows the male close enough to her, he straddles her, wrapping his tail around her body on one side and biting her for grip on the opposite side of the posterior body. The male six-lined racerunner inserts one of his hemipenes’ into the submissive female’s cloaca and begins copulation, which can last 2-15 minutes. Afterward, the male will dismount and leave. The female may remain in a submissive position for several minutes afterward. Six-lined racerunners are polygynandrous; both males and females will have multiple partners each mating season. (Brackin, 1978; Brackin, 1979; Carpenter, 1960; Carpenter, 1962; Etheridge, et al., 1986; Johnson and Jacob, 1984; Paulissen, 1988b)

As adults, the six-lined racerunners begin hibernation in mid August and exit it by mid May. They begin the seasonal reproductive cycle in the last few weeks of hibernation. Within 15 days of exiting hibernation, are fully developed for mating. Six-lined racerunners produce on average, 4 eggs in each clutch. Fully developed females may produce 1 or 2 clutches a year. In warmer climates, like populations native to Texas, females may produce up to 3 clutches annually. The gestational period lasts 36-40 days (average = 38); hatchlings appear in late July and early August. Sexual maturity is determined by (SVL) that is more than 65 mm and this occurs within 1-2 years after hatching. Depending on the environmental conditions, older females that have a SVL of above 70 mm can have multiple clutches at about 45-day breeding intervals. The number of eggs and number of clutches produced annually are dependent on size. Males become extremely aggressive during the mating season. Hierarchies are formed with the largest, most active males at the top and the smallest, more inactive males at the bottom. Large, aggressive males have more sexual encounters and do not compete with lower ranking males for food. However the lower ranking males are subject to aggressive attacks during mating season, which are correlated to lower fertility rates. When males come into contact with one another during late spring and early summer, the higher ranking male will chase the lesser ranking males. If the higher ranking male catches the lower ranking male, he will often bite or mount and thrust sexually on the lower ranking male due to additional hyperactive adrenal hormones during mating season. During these attacks, the lower ranking male will get into the submission position, which entails laying flat on the ground with his limbs outstretched, head down, and eyes closed. They may remain in this position for several minutes after completion of the attack. (Carpenter, 1960; Carpenter, 1962; Clark, 1976; Harding, 1997; Hoddenbach, 1966; Johnson and Jacob, 1984; Paulissen, 1988b; Trauth, 1983; Warner, 2000)

  • Breeding interval
    2 times per season, in 45 days intervals
  • Breeding season
    May-July
  • Range number of offspring
    1 to 8
  • Average number of offspring
    4
  • Range gestation period
    36 to 40 days
  • Average time to independence
    0 minutes
  • Range age at sexual or reproductive maturity (female)
    1 to 2 years
  • Average age at sexual or reproductive maturity (female)
    1 years
  • Range age at sexual or reproductive maturity (male)
    1 to 2 years
  • Average age at sexual or reproductive maturity (male)
    1 years

Little parental investment occurs in six-lined racerunners. After fertilization, the eggs spend roughly 38 days in the mother before she lays the eggs into hibernation burrows 10 cm deep in the soil. After this, the young are independent. (Clark, 1976; Etheridge, et al., 1983; Hoddenbach, 1966; Trauth, 1983; Warner, 2000)

  • Parental Investment
  • precocial
  • pre-fertilization
    • provisioning

Lifespan/Longevity

The six-lined racerunner has a maximum longevity of 6 years in the wild. With an average life expectancy of 1.9 years, Clark Jr. (1976) suggested that females have longer lifespans than males, likely due to aggression. Larger racerunners with a SVL of greater than 60mm and those with short active seasons have longer life expectancies, because there is less chance of predation. (Clark, 1976; Tacutu, et al., 2012; Tinkle, 1969)

  • Range lifespan
    Status: wild
    6 (high) years
  • Average lifespan
    Status: wild
    1.9 years
  • Average lifespan
    Status: captivity
    6 years
    AnAge

Behavior

Six-lined racerunners are diurnal, solitary animals that live in close proximity to others in the population. Population size depends on the amount of vegetation and available food, but a great deal of territory overlap occurs between members of the population. For this reason they have complex social interactions; the largest, most active, and experienced individuals in the population are dominant over the rest. However, there is no correlation between aggression and territory. Because individuals choose prey positively correlated to their SVL, competition among individuals of different sizes is limited.

Racerunners move with quick, short, bursts throughout their territory. They are most active in the morning, spending their afternoon basking on rocks and their nights inside burrows to maintain a similar temperature range. Adults forage from late May until midsummer and then enter their hibernation burrows, maximizing their chances to survive the winter. Juveniles must forage into early October to grow and produce adequate fat stores before winter. (Barden, 1942; Brackin, 1978; Brackin, 1979; Carpenter, 1960; Carpenter, 1962; Clark, 1976; Etheridge, et al., 1983; Harding, 1997; McKenna and Packard, 1975; Paulissen, 1988b; Paulissen, 1988a; Warner, 2000; Witz, 2000)

Home Range

Six-lined racerunners have a home range of roughly 800 square meters. They do not defend a territory. (Harding, 1997)

Communication and Perception

Six-lined racerunners are solitary animals; they do not share reproductive burrows or care for their offspring after oviposit. This species has a complex hierarchy system based on size; larger, more active racerunners are higher ranking.

Six-lined racerunners are very aggressive when encountering each other. The dominant lizard will chase the lower ranking racerunner and upon catching it, will bite it repeatedly. They communicate through tactile means, like biting and straddling. Visual cues, body movements, and biting are especially important during courting.

Six-lined racerunners perceive their environment with visual and chemical cues. When foraging, they will flick their tongues repeatedly to locate, identify, and assess prey. Chemical recognition appears to only work with prey chemicals, not chemicals of plants. They use biting to confirm perceived signals as well as communicate with others. (Carpenter, 1960; Carpenter, 1962; Cooper, et al., 2000; Harding, 1997)

Food Habits

Six-lined racerunners are an actively foraging lizard species, with an adaptive tongue that senses prey chemicals. They are opportunistic insectivores that eat most arthropods and some mollusks. Adults are preferential to grasshoppers, while juveniles focus on cicadas. Prey selection varies with season and body size, but both adults and juveniles commonly consume beetle larva, spiders, and ants. Adults forage from late spring until to midsummer where as juveniles are not hatched until late summer and must forage into the middle of fall to survive the hibernating months. Six-lined racerunners utilize different microhabitats to digest their meals, areas of direct sun light and higher heat are preferred. (Cooper, et al., 2000; Harding, 1997; Paulissen, 1987b; Paulissen, 1988b; Yanfu, et al., 2011)

  • Animal Foods
  • insects
  • terrestrial non-insect arthropods
  • mollusks

Predation

Not much is known about the predators of six-lined racerunners. However, during the active season, May-August, adult racerunners are prey for northern black racer snakes Coluber constrictor. In addition, Solenopsis invicta, fire ants, attack egg clutches in burrows while embryos are developing.

Six-lined racerunners have evolved long tails, which make them extremely fast to escape predators. This species can lose its tail as a last-ditch effort to survive. Without their tail, the racerunner is 36 percent slower and extremely clumsy. Further, female six-lined racerunners lack bright coloration of some males, which helps avoid predators. (Ballinger, et al., 1979; Mitchell, 1994; Mount, et al., 1981; Steen, et al., 2013)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

The six-lined racerunner forages for insects for roughly 5 months in the active season and could act as a pest controller for the surrounding ecosystem. Further, digging burrows helps aerate the soil.

Racerunners act as a host species for the tapeworm Oochoristica bivitellobata. This parasitic species lives within their intestinal tract, averaging 6 worms per lizard, without rupturing the coelom lining. The literature suggests that transmission of these worms comes from ingestion of the larva. (Cooper, et al., 2000; Loewen, 1985; Paulissen, 1987a; Paulissen, 1987b; Trauth, 1983)

Commensal/Parasitic Species
  • Tapeworm Oochoristica bivitellobata

Economic Importance for Humans: Positive

There are no known positive economic benefits of six-lined racerunners for humans.

Economic Importance for Humans: Negative

There are no known adverse economic effects of Cnemidophorus sexlineatus on humans.

Conservation Status

Six-lined racerunners are considered least concern on IUCN and considered stable in the US, except for Michigan where they are listed as threatened. There is an isolated population in Tuscola County, which is considered threatened and protected by state legislation. All other populations of racerunners are not considered threatened in Michigan and reside in the south and western portions of the state.

Six-lined racerunners have adapted to fire-tolerant environments in the state of Florida and act as bioindicators in forest restoration. For many decades wildfires were suppressed in the Sand Hills, which changed the vegetative structure and had a severe negative effect on reptile reproduction. Pinus palustris, the longleaf pine, used to span the southeastern Coastal Plain. These forests had open canopies and light vegetation that was ideal for racerunners. In recent years a great deal of research on controlled burning and selective cutting has occurred. The results thus far show six-lined racerunners returning to these areas, which suggests that the treatments are working and the forest is being restored to original conditions ("Michigan Natural Features Inventory", 2007; Greenberg, et al., 1994; Steen, et al., 2013)

Other Comments

Six-lined racerunners and all female Cnemidophorus tesselatus commonly called the checkered whiptail have been discovered mating in Otero County, Colorado. The hybrids have either triploid or tetraploid chromosomes. Each type of hybrid has a unique dorsal coloration pattern and SVL size. Current research addresses diploid, triploid, and tetraploid annectant subspecies of Teiidae lizard. (Axtell, 1961; Walker, 1986)

Contributors

cassie Bonavita (author), Radford University, Karen Powers (editor), Radford University, April Tingle (editor), Radford University, Emily Clark (editor), Radford University, Cari Mcgregor (editor), Radford University, Jacob Vaught (editor), Radford University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

agricultural

living in landscapes dominated by human agriculture.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

diurnal
  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fossorial

Referring to a burrowing life-style or behavior, specialized for digging or burrowing.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

indeterminate growth

Animals with indeterminate growth continue to grow throughout their lives.

insectivore

An animal that eats mainly insects or spiders.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

molluscivore

eats mollusks, members of Phylum Mollusca

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

polymorphic

"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.

scrub forest

scrub forests develop in areas that experience dry seasons.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

soil aeration

digs and breaks up soil so air and water can get in

solitary

lives alone

suburban

living in residential areas on the outskirts of large cities or towns.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

visual

uses sight to communicate

young precocial

young are relatively well-developed when born

References

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