Frogs rarely exhibit obvious clues that indicate gender, and most criteria that are suitable for clarifying the issue vary from one species to the next. That makes it difficult for pet owners and breeders to determine whether their captives are males or females. However, the females of most species tend to be larger than the males are, and most males have larger digits -- for grasping females during mating. The best time to determine the sex of a given frog is during the breeding season, when you can observe courting and reproductive behaviors.
Genitals and Mating
Most frogs have internal genitals, and the vast majority of species practice a method of external fertilization called amplexus. During amplexus, males mount females from behind and grasp their midbodies to immobilize them. The females then release clumps of eggs, over which the male releases his sperm. Obviously, egg laying is a behavior limited to females, but sometimes a male may try to breed with other another male during the chaos of breeding time.
One known exception to this rule exists: Male tailed frogs (Ascaphus spp.) of the Pacific Northwest have modified tails they use as intromittent organs. These are some of the only frogs known to engage in internal fertilization.
Courting and Calling
Most male frogs emit vocalizations -- colloquially called croaking -- that serve to announce claim to a territory and to attract females. By contrast, the majority of female frogs make no reproduction-related vocalizations, although they may emit alarm or release calls when approached or grabbed by a predator. Along with their penchant for singing to the ladies, males of many species have evolved enlarged tympanums, which likely play a role in call production. For example, bullfrog (Lithobates caetsbeianus) female tympanums are about the same size as their eyes, while male tympanums are much larger than their eyes. Males of some species also have dark skin on their throats.
Sexually Dimorphic Species
Males and females of some species exhibit slightly different colors. Males of some populations of strawberry poison dart frogs (Oophaga pumilio) are brighter than females are. North American wood frogs bear slightly different colors, as females are reddish and males are brown. Male Argentine toads (Rhinella icterica) are yellow, while females are dull brown.
A few frog lineages exhibit more distinct sexual dimorphism. For example, male fanged frogs (Limnonectes spp.) of Southeast Asia, have large teeth and larger heads than females, and they attain larger body sizes than their female counterparts do. Likely, this adaptation enables males to combat with each other for breeding rights and territory.
To make matters even more difficult for keepers, some frogs can spontaneously change sex. For example, African reed frogs (Hyperolius spp.) carry both testicular and ovarian tissues, although only one set of gonads is active at a time. Researchers from the University of California at Berkeley demonstrated that African clawed frogs (Xenopus laevis) may change from male to female when exposed to atrazine -- a common herbicide.