As the current school year draws to a close, I've been reflecting about what we've done in my high school biology classes since August. I, like my students, had to review for last week's semester exams--they in order to perform well and I so that I could make out a comprehensive final.

While pondering exams last Sunday, I was "distracted" at dusk by a series of rumbling sounds from several locations on the banks of Hilton Pond, where the spring mating ritual of the Bullfrog is peaking. I eavesdropped while three males tried to out-grunt each other in a vocal equivalent of sumo wrestling--an activity through which they attract gravid females.

The Bullfrog is the largest frog in North America, reaching a length of up to eight inches from tip of nose to rump; that females are a tad larger probably allows more room for egg production. These large amphibians are classified in genus Rana (Latin for "frog"), which in the Carolinas also contains the Green, Pig, River, Crawfish, Carpenter, Pickerel, and Southern Leopard Frogs. Of these, all are much smaller, and only the latter two occur in the Piedmont.

The Bullfrog is classified further into species catesbeiana in honor of the 18th century naturalist Mark Catesby, who deserves a note. An Englishman, Catesby first visited the American continent in 1712 and returned to delight his homeland with tales of exotic creatures he had seen. He found patrons to support further investigations, and using Charleston as his base he began The Natural History of Carolina, Florida, and the Bahama Islands. This publication established him as the first real ornithologist in America, and his artistic works rival those of John James Audubon and other naturalist/illustrators who followed.

Catesby wrote this of the giant new frog he observed in America: "The Noise they make has caused their Name; for at a few Yards Distance their Bellowing sounds very much like that of a Bull a quarter of a Mile off...." He was obviously impressed with this frog and its vocalizations and sent full scale paintings overseas to further enchant the Britons.

As the first accurate renderings of our Bullfrog (below left), the Catesby paintings show a leaping, full-color male with olive back and buffy belly. The large golden eyes protrude noticeably from the top of the skull, allowing the frog to float inconspicuously in the water with only eyeballs and nostrils breaking the surface. The eardrum, or tympanum, is a coppery-colored disc just behind the eye; in males it is larger than the eye itself. This external drum and the lack of an ear flap are two adaptations that provide streamlining and greater swimming efficiency.

One striking thing about Catebsy's Bullfrog painting is its depiction of the massive hind legs that are of special interest to epicures. There are probably many youngsters--and oldsters, for that matter--who spend summer evenings wandering around large ponds, lakes, or rivers with flashlights, looking for the white reflective eyeshine of a potential donor frog. Frog-gigging, the sport of skewering a live frog in the end of a miniature pitchfork, is one of the commonest ways to secure supper, and I also have seen skilled collectors fire .22 calibre rifle shells into the water just shy of the frog, thereby stunning it long enough to be picked up.

The Bullfrog was originally found throughout temperate North America east of the Rocky Mountains, but connoisseurs of frog legs have stocked them into ponds, lakes, and large rivers in the west. In California, Colorado, Arizona, and other western states, these introduced Bullfrogs compete heavily with native frogs, and in some cases whole populations of rare species have been wiped out by their bigger relatives from the east.

Bullfrogs in the northern United States breed when the water temperatures are up to about 70 degrees--as late as July in some cases--but in our Piedmont ponds females may lay egg masses as early as February. When the male calls from his riparian perch, a female (above right) moves toward him at the water's edge. The male then mounts the receptive female, digging his thumbs into her armpits; the pair may stay in this position (called "amplexus") for several days before the female releases her eggs for external fertilization.

Eggs mature in late summer, and in the north the hatchlings spend two winters as tadpoles on the unfrozen bottoms of large bodies of water. In South Carolina, earlier hatch dates and longer growing seasons allow the tadpoles to mature in one year, but they seldom get as big as their 6-1/2-inch long northern cousins.

The developing tadpole eats algae, and has a long, coiled intestine filled with micro-organisms that digest algal cell walls, providing nutrition for the microbes as well as the tadpole. When metamorphosis occurs, the tadpole goes through incredible changes as the gills are replaced by lungs, the tail is lost, and the gastrointestinal tract modifies to allow for a dietary shift from algae to animal matter.

Bullfrogs have big mouths, and although they consume mostly large insects, they are known to take mice, crayfish, other frogs, and small snakes. There are even several references to Bullfrogs leaping for, catching, and swallowing whole the unwary hummingbird or sparrow that flies too close.

By telling you all this, I've been avoiding the hardest part of this column, and that is how to describe adequately what got me started thinking about Bullfrogs in the first place--the voice of the amorous male. The commonest phoenetic description I've read is a deep "jug-o'-rum," but other writers have also likened it to "more rum," "blood 'n' ouns," "br-wum," "be drowned," "knee deep," and "bottle-o'-rum."

For me the best rendition was penned by naturalist Michael A. Godfrey, who described it as a multi-consonant "mmwonnng." If I close my eyes, I can hear this chorus in my mind, but none of us should try to reduce such a magical sound to words. Perhaps I'll be content instead to sit by Hilton Pond these summer nights as the basso profundo chorus echoes in the night air.

"Mmwonnng!"

Back to Piedmont Naturalist Volume 1 (1986)