During the breeding season, one way to tell the difference between male and female wood frogs (Rana sylvatica) is to look at their thumbs. Male wood frogs have an enlarged thumb with a patch of textured skin. This patch of textured skin is called a nuptial pad.

Wood frog breeding aggregations are chaotic congregations full of activity, in which males battle for opportunities to mate with females. The nuptial pads help the males to hang on to the female during mating. The photos below show how the male frog clings to the back of the female. This mating will lead to eggs, which hatch into tadpoles, which metamorphose into juvenile frogs.