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May 16, 2017 12:36 AMPublication: The East Hampton Press & The Southampton Press

Notes On The Bullfrog

Going…going…gone! Our largest frog, the bullfrog, is known for its voracious appetite. DANIELLE LEEF
May 16, 2017 9:14 AM

The bullfrog (Rana catesbeiana) is the largest frog in North America, averaging 4 to 6 inches in length (snout to vent) with maximum lengths of eight inches recorded. They are also quite large as tadpoles, growing over five inches long and dwarfing all our other native frog and toad tadpoles.An interesting aspect of this animal’s growth is that a four- or five-inch-long tadpole will metamorphose into a much smaller frog, measuring approximately two inches in length. A significant portion of the tadpole’s length is in the tail, which is lost as it transforms from its gilled, sub-aquatic larval stage to its adult stage where it can breathe on land via nostrils and lungs, and in the water where gas exchange takes place through its skin.

Bullfrogs only recently emerged from their overwintering sites on pond bottoms, the last of our frogs to become “active” in spring in terms of feeding and mating. However, this species can be observed in the winter through the pond ice moving on the bottom sediments, as is the case with some of our turtles, so active is a relative term. Even the winter sun can warm a pond’s dark bottom to some extent, with the clear ice acting as a greenhouse cover to lock in that heat. But I do not think any of these cold-blooded creatures’ digestive enzymes can function even in our warmest winter pond conditions, so why some turtles and frogs choose to move about in winter is a mystery to me.

Known for their voracious appetites, they must be particularly hungry after fasting from November through most of April. A “sit, wait and lunge” predator, the bullfrog’s main fare are invertebrates including a wide array of insects and crustaceans. But they are known to gulp down small ducklings, mice, hatchling turtles and other frogs. Last week, Danielle Leef happened upon a particularly gluttonous bullfrog in the process of swallowing another substantially sized frog. It looked to be quite an amazing feat!

The photos taken from the side of the animal show one of the best identification features to distinguish bullfrogs from other commonly encountered and similar-looking frogs here, such as the green frog. Notice the fold or ridge of skin extending from the aft end of the eye, over the top of the circular ear covering (tympanum), around the ear’s aft side and terminating near where the front legs join the main body.

The green frog’s skin fold or ridge continues past the ear along the side of the body toward the rear legs. Without seeing the fold on the “victim,” I’m not sure if it’s a green frog or another bullfrog.

The relative sizes of the round ear cover and eye are used to determine sex. Larger ears than eyes are males and smaller ears than eyes are females, making the bullfrog in the photo a male. The larger ear is not designed to aid in hearing, but in amplifying its breeding call, which is often described as “jug-o’rum.”

Males will start calling in earnest next month, when mating and egg-laying season commences. Being an inhabitant of permanent bodies of freshwater, they do not have to worry about their eggs and larva drying up in vernal ponds as some other amphibians do, for example the wood frog, which has to get a much earlier start in spring.

I’ve never seen their egg masses, thin sheets of black and white colored eggs averaging six to seven thousand per sheet. They are described by Tom Tyning as mats of frothy, tiny bubbles up to two feet in diameter floating near the surface of the water. Leeches are the main predators of the eggs; those that don’t get consumed hatch in as little as four days.

The tadpoles scrape algae, bacteria and other microorganisms from the pond bottom and the surface of aquatic vegetation. They will also scavenge dead animals. Their digestive system is so inefficient that they will ingest their own feces and derive nourishment from that second round.

Again, because they reside in permanent bodies of water, there’s no rush to metamorphose into the adult stage. Tadpoles will live up to three years before transforming into adults. The rate of development is constant throughout their natural range (southern Ontario and most of the United States east of the Rockies). But in the southeast the longer spell of warm weather allows them to transform in one summer season, while in the north it requires three summers. One of my references states that they take two years to metamorphose here on Long Island.

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