The American Bullfrog (Lithobates catesbeianus) is the largest true frog found in North America. Identifying this imposing amphibian requires focusing on specific visual cues, including its structure, skin, and distinct anatomical markers.
Impressive Size and Structure
The bullfrog’s size is often the first noticeable feature, with adults reaching snout-to-vent lengths of up to 8 inches (20 cm) and weights exceeding one pound. This considerable size gives the frog a robust and bulky appearance, distinguishing it from generally smaller frog species. Its head is characteristically broad and flat, sitting atop a stout body.
The frog possesses powerful, long hind legs with fully webbed feet, making it an efficient and strong swimmer. This muscular build is an adaptation to its highly aquatic lifestyle, where it spends most of its time near large, permanent bodies of water like ponds and lakes. Its substantial dimensions contribute to its wide, powerful gape.
Defining Coloration and Skin Texture
The dorsal (upper) surface typically displays an olive-green to brownish-green coloration. This base color is often broken up by an irregular pattern of darker mottling, spots, or bands of grayish-brown pigment across the back. This natural patterning enhances its ability to blend into the shoreline mud and vegetation.
The skin texture is generally smooth, lacking the rough, warty appearance common to toads. While not entirely slick, the skin may sometimes appear slightly granular but remains relatively uniform across the body. The ventral (under) surface is usually off-white, cream, or pale yellow, sometimes showing dark mottling or a distinct yellow wash.
Key Identification Markers
Two anatomical features are useful for confirming the bullfrog’s identity: the tympanum and the absence of full dorsolateral folds. The tympanum, the external eardrum, is a large, conspicuous circular structure located just behind each eye. Its size is a significant trait for determining the frog’s sex.
The bullfrog lacks the distinct pair of raised ridges, known as dorsolateral folds, that run from behind the eye down the back in many other frog species. Instead, the bullfrog has a prominent supratympanic fold. This ridge of skin begins near the eye, arches over and around the tympanum, and then ends near the shoulder. This specific fold structure serves as a reliable differentiator from similar species like the Green Frog.
Visual Differences Between Sexes
Once an American Bullfrog is identified, a closer look at the tympanum and throat color can reveal its sex. In the adult male, the tympanum is noticeably larger than the diameter of the eye, often appearing almost twice its size. Conversely, in the adult female, the tympanum is roughly the same size as the eye or slightly smaller.
Throat coloration provides another distinct cue, especially during the breeding season. The male bullfrog often develops a bright yellow coloration on its throat, which is associated with its vocal sac. The female’s throat, in contrast, tends to remain a paler cream or white, without the prominent yellow pigmentation.