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)
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)
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)
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)
The lifespan of these snakes, as measured in captivity, is 12-15 years. (Bartlett, et al., 2001)
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)
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)
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)
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)
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)
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)
There are currently no known negative economic impacts of this species on humans.
There are currently no known positive economic impacts of this species on humans.
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.
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