Alaska stands alone among United States regions for its remarkably low reptile diversity. The state’s cold climate and geology are unsuitable for reptiles, which are ectotherms relying on external heat sources. The list of native snakes is short, and the presence of even a single species is confined to the warmest, most geographically advantageous corner of the state.
Alaska’s Single Native Snake Species
The only snake species consistently reported in Alaska is the Common Garter Snake (Thamnophis sirtalis). While the status of a truly established, breeding population remains unconfirmed, credible sightings occur in the extreme southeastern Panhandle region. These reports concentrate near major river drainages like the Taku, Stikine, and Unuk Rivers, which provide potential corridors from British Columbia.
The Common Garter Snake is a slender, non-venomous reptile, typically ranging from eighteen inches to about three feet long. Its coloration is highly variable, generally featuring a dark body (black, brown, or olive-green) with three distinct longitudinal stripes. These stripes are most frequently yellow, but they can also appear white or greenish-yellow.
To endure the northern environment, this species uses specific behavioral adaptations. The most significant is its use of communal dens, known as hibernacula, where dozens or even hundreds of snakes gather to overwinter beneath the frost line. This mass hibernation helps the snakes conserve heat and survive the cold season.
Garter snakes are ovoviviparous, meaning they give birth to live young instead of laying eggs. This reproductive strategy is advantageous in cold climates because the mother can internally regulate the temperature and protect her developing offspring. Females typically bear between ten and forty young in a single litter, usually emerging in the summer months.
Factors Limiting Snake Population
The primary restriction on snake distribution across Alaska is the limiting effect of low temperatures on ectotherms. Snakes must bask in the sun to regulate their body temperature, and the short duration of warm, sunny weather curtails the time they have for essential activities like foraging, growth, and reproduction. Average annual temperatures across most of the state are too low to support a sustainable reptile life cycle.
A major geological barrier is the widespread presence of permafrost, which is permanently frozen ground beneath the surface. Snakes require deep, stable, frost-free underground shelters, or hibernacula, to survive the months-long winters. Permafrost across much of Alaska prevents the formation of these necessary deep, insulated retreats.
Historically, Quaternary glaciation events covered much of North America, including Alaska, in thick ice sheets. This ice coverage wiped out existing fauna and created long-term barriers to the northward migration of species as the ice retreated. The current distribution of snakes results from species slowly re-expanding their ranges from southern refugia. Alaska’s location at the northernmost extent means that only the most cold-tolerant species, like the Common Garter Snake, have a chance of survival.