As a herpetologist (one who studies reptiles and amphibians), I have spent the majority of my time researching the former group. However, there is a species of amphibian that has always intrigued me, not only due to its large size, but because of its uses to man as a food source, as well as a bait item. This species is the familiar Bullfrog.
Bullfrogs (Rana catesbeiana) belong to a family that has eight representatives here in Texas. This particular frog family is known as the “true” frogs based on their long legs, extraordinary jumping ability and habitat selection. As with other members of this family, Bullfrogs prefer to live in areas of permanent water such as lakes, ponds, rivers, and creeks. Immature specimens are often seen basking on or along the shorelines, while the adults, who have fewer predators, will frequent the deeper waters, choosing to bask in the sun while perched atop lily pads and other vegetation debris.
As a species, the Bullfrog naturally occurs throughout the eastern three quarters of this state. However, displaced specimens can occasionally be observed in the drier climates of the Trans-Pecos region. Outside of Texas it can be observed from western Oklahoma eastward to the mid-Atlantic states.
The dorsal coloration of this large species of frog is always some shade of green, ranging from lime to an almost black hunter green. The face area is typically where the brightest green appears. The front legs are short, and are generally flecked with dark spots. The back legs are disproportionately long, with feet that are extensively webbed. There is no webbing on the front feet, nor are there any toe-pads. The eardrums are exposed on the side of the head and are known as the tympani. The skin is relatively smooth, save for several small tubercles on the back and a prominent skin ridge that runs along the head from the back of the eye to just above the front leg.
As stated already, this species of frog is large. In fact, it is the largest species of amphibian that is indigenous to this state. Adult females are slightly the larger sex, with mature individuals approaching eight inches in body length. Males are smaller, seldom exceeding seven inches. Generally, most observations are of specimens that are between four and six inches. Females can be differentiated from males by the fact that their tympani is of similar size as the eye, while in males the eardrums are conspicuously larger.
Like other amphibians, Bullfrogs live a dual life. Like most frog species, breeding is initiated by the late spring thunderstorms that generally occur. The breeding call of the male is distinctive, and is a familiar sound to anyone who has spent time camping near bodies of permanent water. Their calling voice can be heard at distances of almost a mile and consists of two or three syllables of “brrr-ummmmm” or “jug-o-rum”. Once a receptive female approaches the male, he will clasp the female around the waist and externally fertilize up to twenty thousand eggs. This activity is known as amplexus. Once the eggs hatch, the fish-like tadpoles are primarily herbivorous, although they have been known to feed on small water-living invertebrates and even tadpoles of other species. Bullfrog tadpoles can grow quite large with individuals over five inches being reported. Metamorphosis can occur quickly (several weeks) or be prolonged, up to two years. The time parameters are chiefly dependant upon things such as water quality, pH levels, heat, and drought. Eventually, all the tadpoles metamorph into small froglets, developing air-breathing lungs rather than gills, absorbing the tail, and developing legs.
Bullfrogs are alert and shy, and can be extremely difficult to approach too closely, especially during the day. Once they hop into the water for safety, watch for them to emerge at the surface several feet from the shoreline. However, they are quite approachable at night, and many will sit completely still when the light from a flashlight is pointed in their eyes. This species is still hunted for food (their legs are the traditional frog legs that appear on many menus) and biological studies, but does not appear to be in decline. In fact, in many areas where this species has been introduced it is considered a menace to local frog life. Bullfrogs consume any smaller vertebrate that it can fit into its large mouth, including other frogs. These introductions have generally come about due to incidental releases by fisherman, as the tadpoles of this species are utilized for fish bait.