Why garter snakes give birth to live young, not eggs

When a female common garter snake is ready to give birth, nothing about the scene looks like a snake laying eggs. There is no nest, no clutch and no shell. She simply delivers living babies. The Animal Diversity Web, a University of Michigan database, gives a litter size of 4 to 80, with most litters between 10 and 40, and says each newborn is about 12.5 to 23 centimeters long (roughly 5 to 9 inches). The same account says the babies get no parental care, so the family reunion lasts about as long as the delivery.

That surprises people who learned that snakes lay eggs, and it is not a quirk of one species. The authors of a 2018 garter snake study in Ecology and Evolution write that live birth has evolved independently from egg-laying more than 100 times. Other snakes on this site, such as the Cottonmouth and the Yellow-bellied sea snake, also bear live young.

This article explains what live-bearing means for a snake (it is not quite a mammal’s pregnancy), why cold climates seem to favor it, and what a garter snake birth season looks like. It also sorts out a few mix-ups, including the argument over what to call it.

Meet the common garter snake

The common garter snake (Thamnophis sirtalis) is widespread. The Animal Diversity Web places it from Florida to coastal Quebec and west to British Columbia, and says it is largely absent from the dry southwestern United States. It lives in meadows, marshes, woodlands and on hillsides, prefers moist grassy ground, and does well in suburbs and cities that offer cover. Its menu is wide: earthworms, amphibians, leeches, slugs, snails, insects, crayfish and small fish, plus the odd lizard or baby bird.

The Reptile Database adds that it eats rough-skinned newts, which make a toxin called tetrodotoxin. It says resistance to the toxin has evolved in some garter snake populations, and that newt toxicity and snake resistance are strongly correlated from place to place. That arms race is the story in the Rough-skinned newt article. Here we stick to the babies.

Eggs, live young and the words in between

Biologists describe two broad styles. In oviparity, the mother lays eggs and the embryos finish developing outside her body. In viviparity, she keeps the young inside until they are ready to be born. The common garter snake does the second.

You will also see the word ovoviviparous, which is usually used for live-bearers whose embryos live mostly off yolk. The sources disagree on the label. The Animal Diversity Web says ovoviviparous, the Reptile Database lists both words, and the 2018 research paper says viviparous. All of them describe the same result: living babies, no eggshell. The labels differ because the mother’s part in feeding her embryos is a matter of degree, not a clean on-off switch.

What a mother garter snake provides

Here is where the 2018 study comes in. Amanda Sparkman and colleagues took a field-portable ultrasound scanner into mountain meadows near Eagle Lake in California’s Sierra Nevada, and from 2015 to 2017 they scanned pregnant common garter snakes and western terrestrial garter snakes (Thamnophis elegans). Yes, the snakes got ultrasounds.

The authors report that garter snake embryos are fed mostly by yolk (the technical word is lecithotrophic). But the placenta also moves respiratory gases, nutrients and water from mother to egg. In their data, egg volume rose by about 27% from early to late development in the common garter snake, against about 68% in the western terrestrial garter snake, a gap the authors attribute mainly to the mother passing water to the eggs. The average litter was about 11 for the common garter snake and about 7 for the other species. Those are numbers from one set of mountain meadows, so they describe those populations, not every garter snake.

Why keep the babies inside? The cold-climate idea

Live birth looks like a lot of work for a mother. So why does it keep evolving? The leading explanation is the cold-climate hypothesis. The logic is simple: in a cool place, an egg buried in soil develops slowly, while a mother can move to a sunny rock and carry her babies with her.

A global analysis by Anna Zimin, Richard Shine and dozens of co-authors tested that idea on 9,061 lizard and snake species. The Dryad record for the study reports that live birth was strongly associated with cold climates, both for single species and for whole local communities of species. It still occurs in some warm climates, which the authors attribute partly to evolutionary history. Live birth was not clearly linked to climate variability or elevation, and where elevation mattered, it seemed to act through temperature, not through thin air. Body size had only a weak link.

Garter snakes fit the pattern in one way: their range reaches cold country. But the sources say garter snakes in general bear live young, so the habit is probably older than this one species, and today’s climate may not be why it persists. We found no study that tests the reason for this species specifically.

A garter snake birth season, step by step

Spring at a den is crowded: the males leave first, and each female who comes out meets a welcoming committee she did not ask for. Females can store sperm until it is needed, and the account says a female may not mate if she does not find a proper partner. The account gives birth dates for northern females only, so we cannot say what the calendar looks like in the south. Larger mothers have larger litters.

Common mistakes and mix-ups about live-bearing snakes

Myth: all snakes lay eggs. Many do, but the garter snake does not, and the 2018 authors count more than 100 independent origins of live birth.

Mix-up: live birth means a mammal-style pregnancy. Mostly it does not. The embryos are fed mainly from yolk, with the placenta adding gases, water and some nutrients.

Mix-up: the right word. Ovoviviparous and viviparous both appear in good sources for this snake. Read them as two labels for live-bearing, not as two different animals.

Myth: the mother raises the young. The Animal Diversity Web says newborns get no parental care.

Mistake: live birth belongs to warm places, or only cold ones. The big picture points to cold climates, yet live birth also occurs in some warm ones. A pattern is not a rule.

Conservation status

The common garter snake is listed as Least Concern on the IUCN Red List of Threatened Species. That is the category as distributed by GBIF, the Global Biodiversity Information Facility, and we checked it on November 9, 2026. Least Concern means the species was evaluated and does not meet the criteria for a threatened or near-threatened category.

That does not mean no problems. The Animal Diversity Web lists pesticide use, habitat destruction, over-collection for the pet trade, water pollution and human persecution as threats. It says northern populations are more vulnerable because they hibernate in larger groups and produce fewer young each year. One subspecies, the San Francisco garter snake, is described there as endangered and was listed in 1967. We have no confirmed population trend for the species, because none of our sources states one.

Categories are reviewed and can change, and the copy GBIF distributes can trail the IUCN website by some months, which is why we print the date we checked. GBIF’s copy does not give the year of the IUCN assessment, so we do not state one.