Common Garter Snake
| Scientific Name | Thamnophis sirtalis |
| Survival Strategy | Brumation |
| Conservation Status | Least Concern |
Spring
The male common garter snake comes out of brumation as spring temperatures warm up. Mating begins as soon as the females emerge from the hibernaculum. Males release pheromones that attract the females, and if she finds a suitable male, they will mate, and the female will leave the area. A female garter snake can store sperm in its reproductive tract for several years, so she can still reproduce if she does not find a suitable mate.
Summer
The eggs of the garter snake remain inside the female until they hatch (ovoviviparous). After 2 to 3 months, 10 to 80 precocial young are born between late July and October.
Fall
Changes in day length and temperature trigger a search for a suitable underground winter shelter. As the temperature drops below 16°C (60°F), garter snakes enter a state of inactivity called brumation.
Winter
Garter snakes will spend the entire winter below the frost line in a state of brumation. Sluggish but not asleep, they will move farther underground if severe winter temperatures and a lack of insulating snow cover drive the frost line deeper. Garter snakes may come out to bask for a few hours on warm days before returning to the hibernaculum.
Behavior
Snake Ball
In spring, male garter snakes leave their den first, and even though they have not eaten in months, the urge to breed overcomes the need to feed. As each female garter snake emerges, they are besieged by every male who detects the female’s pheromones. At some den sites, there may be 100 males writhing around a single female forming a “snake ball,” with each male hoping to be the one that mates with her.
Heat Thief
Some males emerging late from their winter den will use chemical deception to trick other males into giving up some of their heat energy. After producing the female pheromone, the newly emerged (and colder) male is engulfed by other already-warmed males and “steals” some of their heat without tapping into his energy reserves, a behavior known as kleptothermy. The heat thief gains an energy advantage over the competition and is often more successful in mating.
Physiology
Brumation
As an ectotherm, the common garter snake’s body temperature is the same as the outside air. Garter snakes must maintain a body temperature above 32°C (90°F) to digest their food because digestive enzymes do not work efficiently at colder temperatures. The snake’s digestion slows down, and food in its digestive system may spoil, causing the snake to regurgitate the life-sustaining energy the meal provides.
Shorter day lengths and lower temperatures trigger brumation in the common garter snake. As fall weather cools, it will take refuge below the frost line by occupying an abandoned mammal tunnel as a hibernaculum (garter snakes cannot dig tunnels). Safely inside its underground shelter, the snake will enter a state of brumation, typically in September or early November. The hibernaculum protects the snake from the deadly sub-freezing temperatures above ground.
Snakes are awake during brumation, though very sluggish, and their metabolism slows down, conserving energy and reducing the amount of oxygen needed to survive until spring.
Tongue Flicking
Common garter snakes have poor hearing, are nearsighted, and cannot sense infrared heat. However, they have an excellent sense of smell, powered by two different olfactory organs. First, snakes use olfactory sensory cells in their nostrils to monitor their surroundings. If they pick up a scent, they will begin to flick their tongue.
The snake’s most sensitive olfactory tool is the vomeronasal organ located above the roof of the mouth. Its flicking tongue brings odor-containing compounds into the mouth. The compounds then pass through two tiny openings and into the vomeronasal organ. Each odor triggers a unique nerve impulse that carries the information to the brain. Using this powerful sense of smell, garter snakes can identify females and follow the trails used by prey.
Smelling in Stereo
Snakes can locate a scent source by smelling in stereo. By spreading their forked tongue and collecting odor-containing compounds from two locations simultaneously, the brain determines which side of the tongue contains a more intense smell and from which direction it originated.
Diet
Common garter snakes are carnivores that eat almost any animal small enough to swallow, including earthworms, baby mice and voles, frogs and toads, aquatic invertebrates, insects, and small fish.
Lifespan and Mortality
The average lifespan of the common garter snake is about two years in the wild.
Predators
Predators of the common garter snake include large fish, bullfrogs, snapping turtles, other snakes, American crows, hawks, great blue herons, sandhill cranes, raccoons, foxes, and squirrels. A benefit of being ovoviviparous is that it eliminates the predation of eggs.
Roadkill
Snakes will often bask on asphalt roads in spring and fall. The dark road surface provides much-needed heat. Basking on roads increases the likelihood of accidental or intentional death by automobiles.
Winter Mortality
A lack of insulating snow cover increases the risk of lethal temperature drops in dens of bromating garter snakes. One study estimated that denning sites in central Manitoba lost over 60,000 snakes in one winter due to winterkill.
Climate Vulnerability
The impact of a warming climate on the common garter snake is not well understood. One study found that when compared to data from 40 years ago, garter snakes are emerging from brumation two weeks earlier. Another study suggested prolonged drought conditions may reduce their immune function.
Never Stop Learning
Ecotourism: The Narcisse Snake Dens in Gimli, Manitoba, hosts the largest gathering of snakes anywhere in the world. Visitors will often see tens of thousands of garter snakes at this family-friendly snake refuge.
References
| Zimmerman, R. 2013. “Thamnophis sirtalis” (On-line), Animal Diversity Web. Accessed February 21, 2022 at https://animaldiversity.org/accounts/Thamnophis_sirtalis/ |
| Zug, George R.. “Jacobson’s organ”. Encyclopedia Britannica, 12 Apr. 2018, https://www.britannica.com/science/Jacobsons-organ |
| Shine, Richard & Mason, Robert. (2004). Patterns of mortality in a cold-climate population of garter snakes (Thamnophis sirtalis parietalis). Biological Conservation. 120. 201-210. 10.1016/j.biocon.2004.02.014. https://www.researchgate.net/publication/222527579_Patterns_of_mortality_in_a_cold-climate_population_of_garter_snakes_Thamnophis_sirtalis_parietalis |
| Garter snakes emerge for their grand coming-out party in March and April https://hitchcockcenter.org/earth-matters/garter-snakes-emerge-for-their-grand-coming-out-party-in-march-and-april/ |
| If You’re Scared of Snakes, Don’t Watch This | National Geographic https://youtu.be/-jTxiWmSpk8 |
| Narcisse Snake Dens – Gimli, Manitoba – Atlas Obscura https://www.atlasobscura.com/places/narcisse-snake-dens |
| Do Snakes Hibernate? How Brumation In Snakes Works https://www.snakesforpets.com/do-snakes-hibernate/ |