The tail of a male snake is longer and thicker than that of a female, and tapers less quickly. This is due to the 2 hemipenises in the base of the male’s tail. In the picture below, the 3 upper pictures show the shapes of a typical female’s tail (from below and from the side) whereas the lower two pictures show the male’s tail, which is thicker, and tapers more gradually, due to the two hemipenises.
Sexing snakes visually is easier with adult or subadult snakes.
Another obvious feature in larger snakes is size. Females are far heavier-bodied and larger than males, and any garter snake approaching over 30 inches (75cm) in length is highly likely to be female.
A surgical probe can be inserted into the vent on either side of the mid-line and gently moved pushed towards the tail. In a male, the probe inserts several scale-lengths inside the inverted hemi-penis, whereas in females it will stop at around 2 or so scale-lengths. <BR>This technique should only be used by those experienced in the technique as it is easy to damage the snake.
This technique involves gently squeezing the tail base, with the result that in a male snake the inverted hemipenes are everted, and become visible as they ‘pop out’. This technique is only suitable for use on young snakes, and again should only be used by those experienced in the technique, as damage can easily be done.