It is sometimes possible to tell if a frog is male or female by differences in the size and color of the frog. Most female frogs are larger than male frogs. The female red-eyed tree frog is not only larger than the male but is olive green and blue, while the male is pale green. Some scientists can tell male and female frogs apart by the size of their eardrum.
Differences between male and female frogs can be seen when the frogs reproduce. When frogs gather to breed around ponds or other bodies of water, the ones that are chirping or croaking are the males. They do this to attract females, guard their territories and intimidate other males. If two frogs are in the water and one is on top of the other and grasping the bottom frog around the middle, the top frog is male.
Sometimes the difference is seen by watching their parental care. If a gray toad is seen carrying eggs on its back legs, it is probably a male midwife toad. With pouched frogs, both males and females guard the eggs, but when they hatch, the male gathers the tadpoles in pouches on his body where they grow into tiny frogs.