Thamnophis ordinoidesNorthwestern Garter Snake

Geographic Range

Northwestern garter snakes (Thamnophis ordinoides) are found in the Nearctic region. They inhabit the northwestern United States from the northern half of Humboldt County, California, into Oregon and Washington. They range eastward to the Cascade Range and westward to the Pacific Ocean. They are also found in southwestern Canada, with the northernmost extent of their range at Bella Coola, British Colombia. Their range in Canada extends eastward, into Vancouver, and westward to the Pacific, including Vancouver Island. (Ditmars, 1946; Hammerson, 2007; Rossman, et al., 1996)

Habitat

Northwestern garter snakes can be found in habitats ranging from grasslands to forests, as well as shrub-lands and wetlands. This species thrives within 0 to 1800 meters in elevation. They are most commonly found next to the wood line of a forest or thicket. They are also found in open areas, like grasslands and meadows, adjacent to forests. Unlike other species of garter snakes, northwestern garter snakes are not commonly associated with areas of open water, but are occasionally found near them. (Hammerson, 2007; Hammerson, 2007; Rossman, et al., 1996)

  • Range elevation
    0 to 1800 m
    0.00 to 5905.51 ft

Physical Description

Northwestern garter snakes exhibit numerous color patterns, including brown, gray or black dorsal coloring, and sometimes exhibit a green/bluish tint. They have a vertebral stripe that runs the length of the body along the center of the dorsum. This can fade or be non-existent in some individuals. When it is present, the stripe is most often a shade of yellow or orange, but can also be red, white or blue. Unlike some garter snakes, northwestern garter snakes have an indistinct or absent lateral stripe. Small dark makings on scales surrounding the vertebral stripe are common and can be found above the lateral stripe when one is present. As with most garter snakes, their tongue is red. Adult northwestern garter snakes have body length ranges from 22.5 to 95 cm, with most averaging 40 cm. Their mass ranges from 85 to 141 g. Male and female northwestern garter snakes exhibit the same color patterns. Females are larger than males, though the males generally have longer – about 2.6% longer than the females. Other species of the genus Thamnophis that have been measured upon birth average about 15 cm in length. It is assumed neonatal northwestern garter snakes are similar in length. The juveniles resemble the adults in color and patterning. There are no subspecies that are formally recognized for this snake. (Fox, 1948; Rossman, et al., 1996; Simon, 1979; Tuttle and Patrick, 2014)

  • Sexual Dimorphism
  • female larger
  • sexes shaped differently
  • Range mass
    85 to 141 g
    3.00 to 4.97 oz
  • Average mass
    112 g
    3.95 oz
  • Range length
    22.5 to 95 cm
    8.86 to 37.40 in
  • Average length
    40 cm
    15.75 in

Development

Northwestern garter snakes begin as a singled celled zygote following fertilization of an egg. From here, the single cell divides into a cell clump. Embryonic stages of garter snakes have been documented, though exact dates of stages throughout development are unknown. A major step in a developing embryo of a northwestern garter snake is the mandibular segment becoming visible. From this stage embryo continue to develop until margins of the eyes are clearly distinguishable from surrounding tissue. From there, the hemipenes of males becomes visible, and a determination of male or female is possible. In the final stages of development before birth, scales completely cover the head and scales begin to pigment, causing the brain to be no longer visible.

Garter snakes are ovoviviparous, meaning females give birth to live young, after they hatch out of eggs inside their mother. Garter snakes grow until they reach sexual maturity – around two or three years of age. Male northwestern garter snakes reach maturity at a smaller size than females. Like all snakes, northwestern garter snakes exhibit indeterminate growth, but growth is slower past sexual maturity. (Zehr, 1962)

Reproduction

Northwestern garter snakes breed from late March to early April, and again from September to early October. After males come out of hibernation, they search for female mates. Female snakes release pheromones that attract males to them. These pheromones have been reported to draw hundreds of male snakes at a time. Northwestern garter snakes are polygynandrous. Females have been found to store sperm from several males, which allows them to have multiple mates. Male garter snakes will mate with multiple females during the mating season. Northwestern garter snakes have not been observed to fight for mates or defend a possible mate. Mating does not affect social structure. (Rossman, et al., 1996)

Reproduction in northwestern garter snakes begins as males come out of hibernation. Once awake, males will bask in the sun for up to a week until females awake from hibernation. Females awake in smaller numbers and at a later time than males. Once females awaken, they release chemical cues, or pheromones, to attract the males. Because of the sex-staggered exit from hibernation, hundreds of male snakes often are attracted to the scent of a few females. The mating season of garter snakes is regulated by rising temperatures near the end of hibernation, not by increasing sexual hormone levels.

Northwestern garter snakes breed in late March and early April, and again in late September through early October, with each breeding season lasting approximately 3 weeks.

Male garter snakes have postnuptial reproductive cycles. In summer, following the mating season, testosterone levels in males rise and they begin to produce large amounts of sperm, which they store during hibernation. Upon waking from hibernation, the abundant sperm produced earlier are readily available in the vas deferens. Activity level during the fall mating season has yet to be determined, but it is thought to be lower than in spring.

Female northwestern garter snakes have prenuptial reproductive cycles. Upon entering hibernation, female garter snakes have previtellogenic follicles, the beginnings of egg yolk.Following mating, female garter snakes ovulate. Female garter snakes have been found to store the sperm of males and continue to mate with others. This practice helps to ensure that all eggs are fertilized. Ovulation for female northwestern garter snakes occurs from late May to early June. After a nine-week gestation period, female garter snakes give birth, most often in late August.

Clutch sizes in northwestern garter snakes range from 5 to 9, with an average snout-vent length of newborns at 12.7 cm and an average weight of 2 g. While male northwestern garter snakes reach sexual maturity by 12 months, females reach maturity at around 24 months. As with most garter snakes, northwestern garter snakes are independent of their mother immediately following birth. (Bronikowski, 2000; Rossman, et al., 1996)

  • Breeding interval
    Northwestern garter snakes breed for 2-3 weeks in the spring and 2-3 weeks in the fall.
  • Breeding season
    Northwestern garter snakes breed in late March and early April and again in late September through early October.
  • Range number of offspring
    5.0 to 9.0
  • Average gestation period
    9 weeks
  • Average time to independence
    0 minutes
  • Average age at sexual or reproductive maturity (female)
    24 months
  • Average age at sexual or reproductive maturity (male)
    12 months

Female northwestern garter snakes are ovoviviparous and must consume a large amount of calories to support the neonatal snakes growing within them. Female garter snakes nourish their young that are developing inside them until birth. Following birth, newborn garter snakes are left to fend for themselves. Beyond the act of mating, males have no parental investment. (Rossman, et al., 1996)

  • Parental Investment
  • no parental involvement
  • pre-fertilization
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

The longest known lifespan of northwestern garter snakes is 15.8 years in captivity, compared to 14 years for eastern garter snakes (Thamnophis sirtalis). In the wild, checkered garter snakes (Thamnophis marcianus) have a maximum longevity of 7 years. It is assumed that northwestern garter snakes have a similar lifespan in the wild. (Robert, et al., 2007; Snider and Bowler, 1992)

  • Range lifespan
    Status: captivity
    15.8 (high) years

Behavior

Northwestern garter snakes are a solitary species. They constantly move around in search for food. Northwestern garter snakes are neither social creatures nor do they have a social hierarchy. These snakes are diurnal, foraging for food during the day. Garter snakes, especially those in colder climates, hibernate throughout the winter months and are awoken by rising temperatures in spring. Upon awakening from hibernation, northwestern garter snakes begin mating. Females store the sperm of multiple males until ovulation. Once mating season is over, testosterone levels in males rise significantly, allowing them to produce large numbers of sperm. This sperm will be used in the fall mating season, of which the significance is not highly understood. Females store sperm throughout the hibernation period, using it to fertilize eggs the following spring mating season.

Northwestern garter snakes rely heavilytheir tongue and the extremely sensitive receptors on it. Tongue-flicking allows males to locate females via pheromones. Tongue-flicking also allows garter snakes to locate food by picking up and perceiving chemical cues left by prey. (Arnold, 1981; Bronikowski, 2000; Houck, 2009; Rossman, et al., 1996; Seigel and Collins, 1993; Shrine, et al., 2003; Tuttle and Patrick, 2014)

Home Range

The home range for northwestern garter snakes has not yet been published, but other members of the genus have reported home ranges around 0.8 ha. Dixon-MacCallum (2013) reported that larger northwestern garter snakes, as well as gravid (pregnant) females, have home ranges that include more open, non-vegetated areas as a way to more efficiently thermoregulate.

These snakes, like others, are not known to actively defend a territory. (Dixon-MacCallum, 2013; Macartney, et al., 1988)

Communication and Perception

Northwestern garter snakes communicate with one another and perceive their environments through pheromones, sight, and tactile responses. Female garter snakes release a pheromone out of their skin that attracts males. Male garter snakes sense this by tongue-flicking the body of a female, or at her odor trail. Shrine et al. (2003) showed that the sensory capacity of male garter snakes is so great that it can identify the body condition and length of a prospective female mate. Northwestern garter snakes find their prey by sensing chemical cues, such as that produced by slugs. Northwestern garter snakes have been found to use the sun for orientation, allowing them to better locate den sites. Male and female northwestern garter snakes use tactile responses to communicate during the mating process. (Arnold, 1981; Houck, 2009; Rossman, et al., 1996; Seigel and Collins, 1993; Shrine, et al., 2003; Tuttle and Patrick, 2014)

Food Habits

Northwestern garter snakes consume mainly slugs and terrestrial earthworms. This snake is referred to by many as a slug specialist. Britt and Bennett (2008) investigated the diet of northwestern garter snakes and found that they primarily eat banana slugs (Ariolimax columbianus), which make up the main portion of their diet. Gignac and Gregory (2005) reported that they consume dew worms (Lumbricus terrestris) in the field. They have also been known to eat amphibious creatures such as small frogs and salamanders. (Britt and Bennett, 2008; Drummond, 1985; Fox, 1948; Gignac and Gregory, 2005; Rossman, et al., 1996)

  • Animal Foods
  • amphibians
  • mollusks
  • terrestrial worms

Predation

Northwestern garter snakes rely heavily on their striped pattern to evade predators. When a predator is in pursuit, northwestern garter snakes will perform reversals, sudden changes in direction, which allows them to evade predators. These reversals create an optical illusion from the movement of their striped dorsal patterns, confusing potential predators. When touched, other species of garter snakes in the genus Thamnophis will defecate and excrete a foul-smelling fluid from their anal glands. It is believed northwestern garter snakes do as well. This is done as a last line of defense against predators that have successfully captured them.

Suggested predators of northwestern garter snakes are American crows (Corvus brachyrhynchos), northwestern crows (Corvus caurinus) and various hawk species. (Brodie, 1992; Fleharty, 1967)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Northwestern garter snakes prey heavily on slugs, and are commonly referred to as slug specialists. Northwestern garter snakes are preyed upon by crows and hawks.

A species of trematode parasite Zeugorchis syntomentera has been found within the mouth and esophagus of northwestern garter snakes. These parasites hatch and develop on gastropods and continue to develop once eaten by garter snakes. An unidentified haematozoan (in the group Apicomplexa) was also identified in northwestern garter snakes. Fajfer (2012) reported the mite Ixodorhynchus liponyssoides on these snakes. (Clark and Bradford, 1969; Fajfer, 2012; Ingles, 1933)

Commensal/Parasitic Species
  • Trematode Zeugorchis syntomentera
  • Unidentified haematozoan (in the group Apicomplexa)
  • Mite Ixodorhynchus liponyssoides

Economic Importance for Humans: Positive

There are no positive economic effects of northwestern garter snakes on humans.

Economic Importance for Humans: Negative

There are no negative economic effects of northwestern garter snakes on humans.

Conservation Status

Northwestern garter snakes are listed as a species of "Least Concern" on the IUCN Red List webpage and have no special status on the US Federal list, CITES, or the State of Michigan List.

Threats to northwestern garter snakes include natural predation by birds of prey as well as human encroachment into their environment. A commonly overlooked threat towards this species is the lack of information and research conducted on habitats of northwestern garter snakes.

The main conservation measure currently practiced is continued research into this species and its required habitat. With this knowledge, proper conservation measures can be taken, if they are needed. (Dixon-MacCallum, 2013; Hammerson, 2007)

Contributors

Isaac Smith (author), Radford University, Lauren Burroughs (editor), Radford University, Logan Platt (editor), Radford University, Karen Powers (editor), Radford University, Galen Burrell (editor).

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

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.

delayed fertilization

a substantial delay (longer than the minimum time required for sperm to travel to the egg) takes place between copulation and fertilization, used to describe female sperm storage.

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

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

forest

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

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.

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).

marsh

marshes are wetland areas often dominated by grasses and reeds.

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.

pheromones

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

polygynandrous

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

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

solitary

lives alone

sperm-storing

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.

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).

terrestrial

Living on the ground.

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

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

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Bronikowski, A. 2000. Experimental evidence for the adaptive evolution of growth rate in the garter snake Thamnophis elegans. Evolution, 54/5: 1760-1767.

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