Garter snakes are competent swimmers that regularly enter ponds, streams, marshes, and lakes to hunt prey or escape predators. Some species within the genus Thamnophis are classified as aquatic specialists that spend much of their foraging time underwater. As for danger to people, garter snakes are among the least threatening snakes you can encounter, though they are not entirely harmless. A few species produce mild toxic secretions that, in rare and prolonged bites, have caused localized swelling and bruising in humans.
How Garter Snakes Move Through Water
Garter snakes swim using the same basic lateral undulation they use on land, sending S-shaped waves down the length of their bodies. A kinematic study of common garter snakes (Thamnophis sirtalis) found that swimming speed is strongly tied to how fast the snake undulates. The faster the wave frequency, the faster the snake moves. Speed also increases when the snake starts its propulsive wave with a wider initial sweep near the head and amplifies that wave progressively toward the tail, generating more thrust with each cycle.1PubMed. Kinematics of swimming garter snakes (Thamnophis sirtalis) In practice, a swimming garter snake looks like a narrow ribbon rippling across the water’s surface, head slightly raised, body pushed along by those traveling waves.
One thing that affects all garter snakes in the water is temperature. Because they are ectotherms, their muscles work less efficiently when cold water pulls their body temperature down. Research comparing the common garter snake, the eastern ribbon snake, and the northern water snake found that all three species slowed down at lower water temperatures to a similar degree, regardless of how aquatic their lifestyle was.2Canadian Journal of Zoology. Locomotion and antipredator behaviour in three species of semi-aquatic snakes So the more aquatic species are not noticeably better at handling cold water. This matters in early spring and late fall, when a garter snake swimming through a chilly pond is operating well below peak performance.
Underwater Hunters
Swimming is not just an escape strategy for garter snakes. Many species actively forage in water, feeding on fish, tadpoles, frogs, leeches, and aquatic worms. Some are remarkably skilled at it. Aquatic specialists like the Sierra garter snake (T. couchii) and the narrow-headed garter snake (T. rufipunctatus) use rapid forward strikes to catch fish, lunging with head speeds that approach those of vipers and boas striking on land.3Functional Ecology. Forward attack modes of aquatic feeding garter snakes The two specialists have distinct styles: T. couchii hunts fish in open water, while T. rufipunctatus sits and waits for prey to come close, more like a classic ambush predator.
Water depth changes how garter snakes hunt. In shallow water, species tend to use forward strikes, keep their eyes more tightly focused on prey, and catch fish faster. Move the same snake into deeper water and behavior shifts. Foraging takes longer, gape angles widen, and some species that usually rely on sweeping lateral strikes switch to forward strikes instead.4Integrative Organismal Biology. Naïve Garter Snake Chemosensory Preferences Corroborate Diet of Wild Individuals and Reveal Behavioral Convergence for Foraging in Deep Water Western ribbon snakes, which normally feed right at the water’s edge, often refuse to forage in deep water altogether.5Ethology. Testing an Adaptive Hypothesis Through Context‐Dependence: Effects of Water Depth on Foraging Behaviour in Three Species of Garter Snakes So even among closely related garter snakes, comfort in water varies considerably.
Not All Garter Snakes Are Equally Aquatic
The genus Thamnophis includes roughly 35 recognized species, and they span a wide range from almost fully terrestrial to highly aquatic. Researchers have historically classified them along a spectrum: terrestrial-aquatic generalists at one end and aquatic specialists at the other. The generalists eat a mixed diet of earthworms, slugs, and whatever small prey they find on land or near water. The aquatic specialists spend much of their time crawling along stream bottoms and striking underwater.5Ethology. Testing an Adaptive Hypothesis Through Context‐Dependence: Effects of Water Depth on Foraging Behaviour in Three Species of Garter Snakes
The common garter snake, T. sirtalis, which is what most people in North America think of when they hear “garter snake,” falls somewhere in the middle. It swims perfectly well and catches aquatic prey, but it is also happy hunting earthworms in a garden. If you see a small snake swimming across a pond in the eastern United States or southern Canada, there is a good chance it is a common garter snake. Along the Pacific coast, you might encounter the Sierra garter snake or the aquatic garter snake (T. atratus), both of which are more firmly tied to streams and rivers.
Are Garter Snakes Venomous?
For decades, garter snakes were considered completely non-venomous. That classification has become more nuanced. Garter snakes lack the hollow front fangs that vipers and cobras use to inject venom. What they do have is a pair of Duvernoy’s glands in the back of the jaw that produce a mild toxic secretion, delivered through small, rear-positioned teeth. In the wandering garter snake (T. elegans vagrans), this secretion turns out to be surprisingly potent when tested on mice, causing significant hemorrhaging in the lungs, diaphragm, and stomach lining.6Toxicon. The toxic Duvernoy’s secretion of the wandering garter snake, Thamnophis elegans vagrans
The important qualifier is that mouse-level toxicity does not translate directly to human-level danger. Garter snakes are small, their rear teeth are not efficient delivery devices, and meaningful envenomation requires a prolonged bite where the snake chews for an extended period. In everyday encounters, a garter snake will give a quick defensive nip and release, delivering little to no secretion. The Duvernoy’s gland system is primarily an aid for subduing small prey such as frogs, not a weapon designed for self-defense against large mammals.
What Happens When a Garter Snake Bites a Person
Documented cases of actual envenomation from garter snakes exist, but they are rare and consistently mild. A case report in an emergency medicine journal described a person bitten on the fingertip by a wandering garter snake who quickly developed local swelling, bruising, and fluid-filled blisters at the bite site. No systemic symptoms appeared, meaning the person felt fine overall aside from the sore finger.7Annals of Emergency Medicine. Human envenomation from a wandering garter snake The same paper noted that garter snake bites are “generally innocuous” to people.
A separate case involved a 13-year-old who received a prolonged bite from an eastern garter snake on the hand. The hand became cool, swollen, and bruised, and there was some lymph node involvement, but vital signs and lab work remained normal, and recovery was quick. Interestingly, the same person was bitten by garter snakes on later occasions without any symptoms, suggesting that the first reaction was not allergic and that envenomation depends on how long the snake holds on.8PubMed. Human envenomation from the bite of the eastern garter snake, Thamnophis s. sirtalis (Serpentes: Colubridae) A clinical toxicology paper on the wandering garter snake confirmed this pattern, stating that envenomation depends on the duration of the bite and the “inclination of the snake,” and that the resulting effects resemble a very mild version of a pit viper bite.9PubMed. Envenomation following the bite of a wandering garter snake (Thamnophis elegans vagrans)
So the honest answer on danger is: a quick defensive bite from a garter snake is about as medically significant as a cat scratch. Real envenomation is possible but requires the unusual circumstance of the snake latching on and chewing, which almost never happens outside of captive handling situations. No garter snake bite has been reported as life-threatening.
Bacteria in the Mouth
The bigger practical concern with any garter snake bite is not venom but infection. A microbiology study that cultured bacteria from garter snake mouths isolated 126 strains of aerobic and facultative bacteria, many of which are known human pathogens. The list included Pseudomonas aeruginosa, Salmonella species, Shigella species, and various other bacteria capable of causing wound infections.10PubMed Central. Aerobic bacterial oral flora of garter snakes: development of normal flora and pathogenic potential for snakes and humans The researchers concluded that garter snakes should be considered reservoirs of human pathogens and that pet owners and lab workers should take appropriate precautions.
If you are bitten by a garter snake, the standard advice applies for any animal bite: wash the wound thoroughly with soap and warm water, apply an antiseptic, and watch for signs of infection over the following days. Seeking medical attention is reasonable if you notice increasing redness, warmth, or swelling around the bite, but a routine garter snake nip treated with basic first aid almost never leads to complications.
How Garter Snakes Defend Themselves
Biting is actually one of the last things a garter snake wants to do when threatened. Their first line of defense is to flee. A warm garter snake can move quickly through grass or water, and escape is the preferred option. When fleeing is not possible, their defensive toolkit gets more creative. Many garter snakes will musk, releasing a foul-smelling secretion from glands near the base of the tail. If you have ever picked up a garter snake and had your hands smell terrible for the rest of the afternoon, you have experienced musking firsthand.
Body temperature and the location of the threat both influence what a garter snake does. Research on wild common garter snakes found that warmer snakes were more likely to flee, while cooler snakes, being sluggish, tended to rely on other tactics. The study also found that where the snake was touched mattered: a simulated attack to the head caused snakes to coil up and hide their heads, often wriggling their tails as a distraction. An attack to the midbody prompted either escape or an open-mouthed threat display. Larger snakes were more likely to stand their ground and display than smaller ones.11Ethology. Predatory Attacks to the Head vs. Body Modify Behavioral Responses of Garter Snakes The tail-wriggling behavior is thought to redirect a predator’s bite away from the vulnerable head toward the expendable tail tip.
Garter Snakes That Eat Poison
One of the most striking things about garter snakes has nothing to do with swimming or biting. In parts of the Pacific Northwest, common garter snakes prey on rough-skinned newts, which are loaded with tetrodotoxin, the same paralytic neurotoxin found in pufferfish. This toxin is lethal to most vertebrates, but certain garter snake populations have evolved resistance to it and can eat newts without dying. After consuming a newt, these snakes actually harbor active tetrodotoxin in their own tissues, making them temporarily toxic to anything that might eat them.12PubMed. A resistant predator and its toxic prey: persistence of newt toxin leads to poisonous (not venomous) snakes
This relationship between garter snakes and newts is a textbook example of a coevolutionary arms race. In areas where newts are highly toxic, the local garter snakes tend to be highly resistant. Where newt toxicity is lower, snake resistance is also lower. This geographic pattern has been documented across the shared ranges of multiple garter snake and newt species, with the strongest toxin levels and highest snake resistance found at southern latitudes.13PubMed. The geographic mosaic in parallel: Matching patterns of newt tetrodotoxin levels and snake resistance in multiple predator-prey pairs The resistance comes at a cost, though. Highly resistant snakes tend to be slower, because the genetic changes that block tetrodotoxin also affect the sodium channels in their muscles. So a garter snake that can eat the most toxic newts pays for that ability with reduced locomotor speed.
Overwintering Underwater
Garter snakes in northern climates face a serious challenge every winter. They need to find shelter that stays above freezing but also prevents them from drying out over months of dormancy. In some hibernation dens, garter snakes actually submerge themselves in water for the winter. A study that compared submerged and air-kept garter snakes at cold temperatures found that snakes kept in air gradually dehydrated and died, while those kept in water stayed hydrated and survived.14Canadian Journal of Zoology. Effects of humidity, temperature, and submergence behavior on survivorship and energy use in hibernating garter snakes, Thamnophis sirtalis The submerged snakes also conserved more stored energy, which could give them a reproductive advantage when mating season kicks off in early spring. This is not exactly “swimming,” but it does illustrate how comfortable garter snakes are with water and how central aquatic environments are to their survival in harsh climates.
Parasites Garter Snakes Carry
Because garter snakes eat aquatic prey like fish, frogs, and tadpoles, they inevitably pick up parasites that cycle through aquatic food webs. Studies have documented trematodes, nematodes, and acanthocephalans inside garter snakes, acquired either through ingesting infected prey or through direct penetration by parasite larvae in the water.15Diversity. Parasite Fauna of the Blackbelly Garter Snake (Thamnophis melanogaster) in Central Mexico One particularly interesting parasite is Alaria, a trematode whose larval stages (mesocercariae) tend to aggregate in garter snake tails. Research on red-sided garter snakes in Manitoba identified multiple Alaria species in their tails using both physical examination and genetic analysis.16PubMed Central. Alaria mesocercariae in the tails of red-sided garter snakes: evidence for parasite-mediated caudectomy
The concentration of parasites in the tail is thought to be connected to the tail-wriggling defensive behavior described earlier. If a predator bites off the tail, it gets a mouthful of parasites, and the snake loses only an expendable body part while the parasites get transmitted to their next host. Some researchers have suggested that Alaria parasites may even manipulate the snake’s behavior to make tail loss more likely, since the parasite benefits from being eaten by a bird or mammal. This is speculative but fits a broader pattern seen in other host-parasite systems where parasites influence host behavior to complete their life cycles. For people, the practical relevance is minimal: you are not going to acquire a garter snake’s internal parasites from handling one. The parasites that garter snakes carry are adapted to complex life cycles involving intermediate hosts like snails and fish, not direct transmission to humans through casual contact.