[8] During low temperatures in fall and winter, tens of thousands of male and female red-sided garter snakes hibernate together, in a shared underground communal den, for about eight months each year to protect themselves from cold weather and predators.

[6] Soon after feeding, close to the end of summer, the adult snakes return to their dens for winter hibernation.

[4] The young snakes, around one-year-old, spend more time in marshy areas to find food before the winter temperatures arrive and hibernate there as their small size makes them unnoticeable to predators.

[8] During hibernation, the energy stored from summer feeding is used for body growth and maintenance to survive in subzero temperatures for months.

[13] In red-sided garter snakes, the breeding season usually begins in early spring and lasts for about a month.

[15][4] After mating, males and females move to mossy summer grounds where they find food and water and birth the offspring.

[4][8] The reproductive system of male snakes includes testicles, epididymis, vas deferens, hemipenes, and renal sex segment.

After mating, it takes up to 6 weeks for sperm to travel through the oviduct and reach the developing follicles in the ovaries.

[18] When a male finds a female attractive, male shows increased frequency of tongue-flicking, chin pressing, tail searching, body contractions, caudocephalic waving, and attempts to align his body and cloaca to that of females to increase his mating possibility.

[15][11] However, as the sex ratio during emergence is highly male-biased, it is possible that multiple males court the same females together.

[15][18] After the male red-sided garter snake performs courtship behavior, if the female is sexually receptive, she remains steady, breathes rapidly, lifts her tail and gapes her cloacal opening.

[14][16][7] Around that time, most females either become unreceptive or start migrating to feeding grounds immediately after mating to avoid any injury or suffocation due to the mass gathering of males, who stay at dens till the end of breeding season.

[16] However, females prefer shorter copulation to prevent injury to their reproductive tract, suffocation, exhaustion, and risk of predation.

[19][20][7] Female genitalia often bleeds during and after copulation as penetration of the basal spine harms cloacal tissues while locking genitals.

[19] To shorten the copulatory period, female snakes perform axial rotation and roll their body to disturb cloacal alignment with males.