The Common Garter Snake is a medium-sized, diurnal snake. The head is large and distinct from the neck; eyes are large with round pupils. There are usually 7 yellow upper labials, often with red to black blotch markings. Male and female snakes are dark bodied with three distinct straight yellow to greenish-yellow dorsal/lateral stripes and generally have red hatching or bars along their sides (except on Vancouver Island where the red colouration is often absent). Dorsal body scales are strongly keeled and have up to 19 scale rows at mid-body. Snakes have a single anal plate scale and paired ventral scales posterior of the cloaca. Ventral surface ranges from light yellow to black, with the chin and neck lightest in colour. When a snake is nearing ecdysis (shedding their skin), the body color will be dull and eyes will appear blue-grey in colour as fluid lymph fills the area between the new and old layers of skin. Garter snakes range in size from under 200 mm when born to over a metre in total length when adult (Matsuda et al. 2006); however, significant size variation exists between populations. Males are smaller in head dimensions and body size than females, although they have longer tails relative to body length. Such sexual dimorphism has been attributed to reproductive investment in females, as fecundity increased with body size.