Thamnophis cyrtopsisBlackneck Garter Snake (cyrtopsis

Geographic Range

Black-necked garter snakes occur in the United States in southern Colorado, southwestern Utah, central and eastern Arizona, New Mexico (except the far eastern portion of the state), and western and central Texas. They are also widespread in Mexico and are found as far south as Guatemala. (Hammerson, 2012; Hanson and Hanson, 1997; Purser, 2005)


Black-necked garter snakes are known to be semi-aquatic, and are most commonly found near steams and ponds. They prefer streams in valleys and canyons, as well as shallow rocky pools, where their preferred prey can be found. As water levels recede through the year, cattle tanks and other man-made containers may be used by these snakes as sources of water. At night, these snakes find cover in exposed roots along stream banks, and in crevices, rodent holes, and debris. Other typical habitats include riparian woodland, forests (including pine-oak forests) and scrub, as well as desert scrub communities and flats, dry grasslands, tropical lowlands, and cloud forest in mountainous areas. In Mexico, this snake can also be found in habitats ranging from desert through mixed conifer forests to tropical forests. Black-necked garter snakes share habitat with other Thamnophis species, particularly at higher elevations in the mountains and foothills in Mexico. These snakes are found at elevations ranging from sea level to 2700 m. (Degenhardt, et al., 1996; Hammerson, 2012; Hanson and Hanson, 1997; Ivanyi, et al., 2000; Lazaroff, et al., 2006; Leviton, 1971; Rossman, et al., 1996; Shaw and Campbell, 1974; Stebbins, 1954)

  • Range elevation
    0 to 2700 m
    0.00 to 8858.27 ft

Physical Description

Black-necked garter snakes range from 41 cm to 71 cm in length as adults (maximum reported size is 109 cm), and measure about 20 cm at birth. They are olive-gray or brown to darker brown dorsally, with a gray or blueish gray head, with white or slightly greenish or brownish coloration ventrally. They have a mid-dorsal stripe that may vary in color from yellow to white, shading to orange anteriorly. This stripe divides two black blotches, one on each side of the neck. There are two rows of alternating dark spots between the stripes, creating a checkerboard pattern. They also have lateral stripes on the second and third scale rows, which are cream, tan, or yellow in color. These snakes have 19 rows of keeled dorsal scales, 130-184 scales under the belly, 64-109 scales under the tail, and a single anal plate. They have 21-29 maxillary teeth. Males can be distinguished by their longer tails (1.1-1.3% longer than those of females). Younger snakes have brighter colors, with greater contrast than adults. There are three subspecies of black-necked garter snakes: western black-necked garter snakes (Thamnophis cyrtopsis cyrtopsis), eastern black-necked garter snakes (Thamnophis cyrtopsis ocellatus), and tropical black-necked garter snakes (Thamnophis cyrtopsis collaris). (Bartlett, et al., 2001; Conant and Collins, 1998; Degenhardt, et al., 1996; Ivanyi, et al., 2000; Rossman, et al., 1996; Stebbins, 1954)

  • Sexual Dimorphism
  • male larger
  • Range length
    41 to 71 cm
    16.14 to 27.95 in


Black-necked garter snakes are ovoviviparous. Fertilization occurs internally and young develop with the support of nutrients exchanged between embryonic and maternal membranes. These snakes undergo direct development. Young are born alive and are quite small, about 20 cm in length. They look similar to adults, except with brighter, more contrasting coloration. (Conant and Collins, 1998; Degenhardt, et al., 1996; Stebbins, 1954)


Black-necked garter snakes likely mate in the spring, after emerging from hibernation, as is the case for most Thamnophis species. The dominant mating system of Thamnophis species, and snakes in general, is polygynandry, but the system used by this particular species is not known. Generally, male Thamnophis tend to emerge from hibernation before females, returning to and clustering around a den in response to the release of female pheromones. Males follow the pheromone scent trail to a female in order to mate. Competition is quite high between males as females emerge from hibernation, not only due to the number of males waiting outside the den, but also to males obscuring the pheromone trails of females that they follow. In general, mating is comprised of three phases. The first is tactile chase, in which a male and female first make contact and he either lines up next to or loops over her body, using his pelvic spurs to scratch her vent. The second is tactile alignment, in which copulation attemps occur, with muscle contractions aligning the male's tail to the female's. The final stage, intromission coitus, is copulation, in which a female gapes her cloaca and allows the insertion of a male's hemipenis. Male-male combat is not uncommon, but does not typically result in injury to either party. (Degenhardt, et al., 1996; Rivas and Burghardt, 2005; Rossman, et al., 1996; Vitt and Caldwell, 2014)

It is most likely that mating occurs in the spring, after hibernation, and it has been documented that most births occur from June through August. Females have been found with enlarged ovarian follicles or developing embryos between March and June, suggesting an incubation time of approximately three months. Young black-necked garter snakes are usually born near water, with each female producing 3-24 offspring at a time, with an average clutch size of eight. Males mature at about 42-47 cm in length and females mature at about 50-76 cm. Age at maturity is not definitely known for this species; however other Thamnophis species mature in their second or third year. Sperm seems to be produced by males during the late summer and autumn; it is stored by males in their vasa deferentia until mating in the spring and summer. (Degenhardt, et al., 1996; Goldberg, 1998; Rivas and Burghardt, 2005; Stebbins, 1954; Vitt and Caldwell, 2014)

  • Breeding interval
    Black-necked garter snakes breed once yearly, after emerging from hibernation.
  • Breeding season
    Black-necked garter snakes likely breed in the spring and summer months.
  • Range number of offspring
    3 to 24
  • Average number of offspring
  • Average gestation period
    3 months
  • Range age at sexual or reproductive maturity (female)
    2 to 3 years
  • Range age at sexual or reproductive maturity (male)
    2 to 3 years

After mating, males have no further involvement with their offspring. Young black-necked garter snakes are born live and disperse after birth, having no further association with their mothers. (Conant and Collins, 1998; Degenhardt, et al., 1996)

  • Parental Investment
  • no parental involvement
  • precocial


The lifespan of these snakes, as measured in captivity, is 12-15 years. (Bartlett, et al., 2001)

  • Typical lifespan
    Status: captivity
    12 to 15 years


These snakes are most often found near water. They are most active during the day, but night time activity is not unusual. If they are threatened, they attempt to escape into surrounding cover or use nearby water as an escape route. If pursued by predators, they tend to flatten and broaden their heads in order to appear larger. As with other Thamnophis species, if black-necked garter snake are closely threatened or captured, they release a foul-smelling pungent musk from their anal glands, sometimes accompanied by feces. These snakes are non-venomous and rarely use their teeth to bite. Body temperature ranges most often from 22.5-35.0ºC and, as with other snakes, body temperature is largely dependent on environmental temperature; they hibernate in dens during winter months. (Degenhardt, et al., 1996; Hanson and Hanson, 1997; Stebbins, 1954)

Home Range

Although these snakes have been observed returning daily to the same basking locations, most typically grassy areas along streams, specific home ranges or territories have not been identified. (Degenhardt, et al., 1996; Hanson and Hanson, 1997)

Communication and Perception

Thamnophis species communicate with each other mainly through the release and recognition of chemical pheromones. Visual channels are also used, although they are not the primary means of communication; in fact, males may mistakenly attempt to mate with other males, due to their inability to visually distinguish between sexes. During mating, these snakes interact through touch as well, with males rubbing against females before copulation. (Vitt and Caldwell, 2014)

In addition to their visual and chemical senses, these snakes have an inner ear with which they can detect sounds, and are also able to detect vibrations of the ground through their jawbone. (Vitt and Caldwell, 2014)

Food Habits

Frogs, toads, and tadpoles are the principal food items for these snakes. Individual diet depends on availability of prey, but may also include small fishes, skinks, crustaceans, and earthworms. Hunting techniques change during the spring and summer, depending on the availability of prey items (due to changes in water levels). For example, water and prey items are abundant in the spring, so these snakes actively forage to locate sedentary prey. During summer, when food items are lower in abundance, these snakes tend to use the "sit-and-wait" method to capture more active prey such as tadpoles and frogs. (Degenhardt, et al., 1996; Greene, 1997; Jones, 1990; Williams, 2013)

  • Primary Diet
  • carnivore
    • eats terrestrial vertebrates
  • Animal Foods
  • amphibians
  • reptiles
  • fish
  • insects
  • terrestrial worms
  • aquatic crustaceans


Black-necked garter snakes serve as prey for a variety of other vertebrates, including bullfrogs, sunfish, and other snake species. The expulsion of foul-smelling excrement and anal secretions serves to deter predators when these snakes are threatened. (Bartlett, et al., 2001; Degenhardt, et al., 1996; Hanson and Hanson, 1997; Ivanyi, et al., 2000)

Ecosystem Roles

In addition to their roles as predator and prey, black-necked garter snakes may serve as hosts to a number of endoparasites. (Bartlett, et al., 2001; Degenhardt, et al., 1996; Goldberg and Bursey, 2002; Hanson and Hanson, 1997; Rossman, et al., 1996; Williams, 2013)

Commensal/Parasitic Species

Economic Importance for Humans: Positive

There are currently no known positive economic impacts of this species on humans.

Economic Importance for Humans: Negative

There are currently no known negative economic impacts of this species on humans.

Conservation Status

The habitat of black-necked garter snakes is disappearing as a result of destruction and fragmentation of habitat throughout the Sonoran Desert. Populations of this species have also been impacted by introduced species such as bullfrogs (Lithobates catesbeianus) and sunfish (Lepomis sp.) that not only feed upon this species, but compete for their preferred prey items. However, there are currently no major recognized threats to the species as a whole, and its broad distribution and the existence of multiple populations has led to the IUCN categorizing this species as one of "Least Concern." (Hammerson, 2012; Ivanyi, et al., 2000)


Abdul Ahmad (author), Sierra College, Jennifer Skillen (editor), Sierra College, Jeremy Wright (editor), University of Michigan-Ann Arbor.



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


uses sound to communicate

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.


an animal that mainly eats meat


uses smells or other chemicals to communicate


active at dawn and dusk

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.

  1. active during the day, 2. lasting for one day.

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


union of egg and spermatozoan


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.


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.


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).


having the capacity to move from one place to another.


This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

native range

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


active during the night


reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.


chemicals released into air or water that are detected by and responded to by other animals of the same species


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


Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

scrub forest

scrub forests develop in areas that experience dry seasons.

seasonal breeding

breeding is confined to a particular season


remains in the same area


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


lives alone


mature spermatozoa are stored by females following copulation. Male sperm storage also occurs, as sperm are retained in the male epididymes (in mammals) for a period that can, in some cases, extend over several weeks or more, but here we use the term to refer only to sperm storage by females.


uses touch to communicate


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).


Living on the ground.


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

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.


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.


movements of a hard surface that are produced by animals as signals to others


uses sight to communicate

young precocial

young are relatively well-developed when born


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