I frequently receive questions about how to tell apart male and frogs. Unfortunately, there is no single external trait that distinguishes males from females across all 7,000 species of frogs. The reproductive organs of most frogs are located inside their bodies so you can't simply look for reproductive organs to tell apart males and females. However, there are some traits that can be used to tell males from females. Many of these traits are only present in mature adults during the breeding season. If you are able to find frogs during their breeding season, and you can identify the species of frog, you can use these traits to tell apart males and females.
More information on my research is available on the Benard Lab website.
Characteristics used to tell apart male and female frogs: Songs: Typically only male frogs sing.
Body size: In most species, females are on average larger than males.
Nuptial pads: Males in many species have rough patches of skin on their hands.
Loose skin on throat: Males in some species have loose skin on their throat.
Enlarged ears: In just a few species, male frogs have larger ears than small frogs.
Other traits: A variety of other traits like color and spines can distinguish males from females.
The banjo-plucking noise of male Green Frogs illustrates a hallmark of most frog species: males make vocalizations to attract mates or discourage rivals. Gray Treefrogs can be heard calling in the background.
So if you find a frog singing, it would almost certainly be a male frog, with very few exceptions. However, if you find a frog and it does not make a noise, it could be a male or a female. There are many reasons why a male frog may not sing. Immature male frogs don't call, and many frogs don't call outside of the breeding season. Startled or scared frogs usually don't sing. Singing is not a reliable way to determine if a frog is male or female.
Females are larger than males in many species of frog.
In most frogs and toads, males are on average smaller than females. However, there is variation around the average size of each sex, and for many species there is considerable overlap between male and female size. For example, in one population of Pacific Chorus Frogs, I found the body length of male frogs ranged from 28.6 mm to 40.1 mm (1.1 to 1.6 inches), and females ranged from 33.1 to 45.6 mm (1.3 to 1.8 inches). So body size on its own is usually not a great way to determine the sex of adult frogs.
Why are female frogs bigger than males?: Websites like WikiHow have spread the myth that male frogs are smaller than female frogs so that the male does not crush the female during mating. Of course, this is just not true. Because of the size overlap between males and females, larger males do end up mating with smaller females. Yet we don't find crushed female frogs when we watch frogs mate. Further, in a few frog species like the southeast asian Sylvirana nigrovittata , males are larger than females. Yet this species survives without the males crushing the females.
A pair of American Toads in amplexus.
Male and female frogs face different constraints that have affected their evolution. Male reproductive success is limited by how many females he can mate with, while female reproductive success is limited by how many eggs she can lay. Female frogs gain an advantage by delaying maturity so that they breed at an older age and a larger size, which allows them to produce more eggs. Unless there is a strong benefit to a larger size, males have higher success by maturing earlier and mating with as many females as possible.
Male wood frog's thumb with nuptial pads on left, female wood frog's thumb without nuptial pads on right.
During the mating season, the males of many species of frog have an enlarged thumb with a patch of textured skin. This patch of textured skin is called a nuptial pad. The nuptial pad helps the male frog cling securely to the female, a behavior known as amplexus. Most North American frogs use axillary amplexus, in which the male's arms wrap under the female's armpits. But a few North American frogs use inguinal amplexus, in which the male's arms wrap around the female's waist.
Among North American frogs, nuptial pads occur in males of the Spadefoot Toads (Scaphiopodidae), True Toads (Bufonidae), and True Frogs (Ranidae). They also occur in the invasive Cuban Treefrogs. However, if you catch one of these frogs and it doesn't have nuptial pads, it still might be a male. The nuptial pads only appear in sexually mature males. A frog lacking nuptial pads might be a female frog, or it might be an immature male.
Nuptial pads aren't a good indicator of sex for all species. For example, many native North American treefrogs in the family Hylidae, such as the Spring Peeper, lack nuptial pads.
View showing a female wood frog's belly, which is being grasped by a male wood frog.
Two pairs of wood frogs in amplexus, sitting on top of old and new wood frog eggs.
Nuptial pads are present on males of many frog species. Here are American toad nuptial pads.
Another way to identify the sex of some frogs is to look at their throats. For many species in which the males' expand their throat when inflating their vocal sacs, the males and females differ in the coloration of the skin on the throat. Males have loose skin on the throat, that is often a different color than the rest of the belly. In contrast, females have smooth skin with coloration that matches the belly.
Calling male Gray Treefrog. You can see the vocal sac expanding as he calls.
Male Spring Peeper: yellow-orange throat with loose skin.
Female Spring Peeper: cream throat with smooth skin.
Female Green Frog (Rana clamitans) illustrating that her tympanum is about the size of her eye. The tympanum is the circular organ located just behind the eye of most frogs.
The sex of a few species of frog can be identified by the size of their tympanum. The tympanum is a frog's ear, and it can be seen just behind the eye in most frogs. The Peterson Field Guide to Reptiles and Amphibians of Eastern and Central North America provides a nice list of frogs in which the males have a significantly larger tympanum than females: Mink Frog, Carpenter Frog, Bronze Frog, Green Frog, River Frog, Pig Frog, and the Bullfrog. The males of these species have tympana that are about twice the size of their eye, whereas the females have tympana that are about the size of their eye. In most other species of frog, both males and females have tympana about the size of their eye.
Interestingly, the North American frogs in which males have an enlarged tympanum are all closely related to one another (see Fig 2 in Pyron & Wiens 2011). This suggests that enlarged tympana in males evolved once, in the common ancestor of all of these frogs, and then was retained as they evolved into separate species. It is not clear why larger tympana in males evolved in the first place. Are larger tympana an adaptation, improving the ability of male frogs to survive or reproduce? Or is this a random evolutionary change that doesn't provide a benefit?
Male Green Frog with large ear
Male (right) and female (left) Pacific Chorus Frogs in amplexus showing they both have small tympanums ("ears")
Male (top) and female (bottom) Wood Frogs have tympanum ("ears") of the same size.
A few species of frogs exhibit sexual dichromatism, in which males and females have different colors. Amphibiaweb provides a nice, partially complete list of sexually dichromatic frog species. Wood Frogs are an example of a North American frog with sexual dichromatism. Males are typically light brown to dark brown. While some females are also brown, many females develop red coloration. The reddish color in female wood frogs varies seasonally, and develops before they are old enough to reproduce.
A few frog species also have specialized external morphological structures that distinguish males and females. The tailed frog, which inhabits the pacific northwest of North America is one such species. Mature male tailed frogs have a tail-like structure, which is used during mating to internally fertilize the eggs of female tailed frogs. Female tailed frogs lack the tail. Other examples of these specialized structures include male "moustaches" of spines, and males with hair-like papilla.
Female wood frogs vary in color from dark brown to bright red. Male wood frogs never develop the bright red color.