Wet Beast Wednesday: American bullfrog
It's a bit surprising that I've been doing these posts for over 2 years now and I haven't covered a single frog yet. Let's hop to it and talk about North America's largest frog: the American Bullfrog. These large lads are some of the largest frogs in the world and are so good at surviving that they have become an invasive species around the world. Also, I didn't intend for both posts this month to be about animals in the Mississippi River, but maybe I'll make this Mississippi River month.
(Image: an American bullfrog standing in shallow, algae-filled water. It is a large frog with olive-green skin and dark speckles on the back and limbs. I ridge of skin runs from the back of the eyes to the top of the front limbs around the tympanum, a disc-shaped structure right behind the eye. This one is identifiable as a male by the size of its tympanum and yellow throat. End ID)
The American bullfrog (Lithobates catesbeianus, sometimes alternatively called Rana catesbeiana) is a very large frog, with an average size of 9 to 15 cm (3.6 to 6 in) from snout to cloaca, but exceptionally large individuals can reach 20 cm (8 in). A mature bullfrog can reach an average of 500 g (1.1 lbs) with some specimens weighing up to 800 g (1.8 lbs). They are typically a dark green color, but individuals can be lighter green to almost black. The underbelly is white with mottled brown spots. American bullfrogs are visually similar to the related green frog (Lithobates clamitans) and share habitat with them. They can be distinguished by a flap of skin that starts behind the eye. In bullfrogs, this flap curved down toward the front leg, while in green frogs it extends down the side. Green frogs are also smaller than mature bullfrogs. Female bullfrogs grow larger than males. They can also be distinguished in a few other ways. Male bullfrogs have yellow throats while females have white throats. In addition, the tympanum (a disc-shaped organ behind the eye that acts as an eardrum) is different sizes in the two sexes. A male's tympanum is typically about twice the size as its eye, while a female's tympanum is about the same size as its eye.
(Image: a male bullfrog with its throat pouch inflated, indicating it is vocalizing. It looks like the bottom of its neck and mouth are inflated. End ID)
A bullfrog's hind legs are very long and highly muscular. A mature bullfrog can jump around 1.8 m (6 ft) in one go. The toes of the hind legs are webbed. The eyes and nostrils are set high on the head, allowing the frog to lurk just beneath the water's surface with only part of its head exposed. The bulbous eyeballs give them a very wide range of vision. When frogs blink, the eyes are pulled back into the head and protrude into the throat. Frogs use this to help swallow food. By blinking, the eyes help push food down the esophagus. Media near-universally depicts frog tongues as being slender, elastic, and extending a long way from the mouth like a chameleon's. This is not actually true. Frog tongues are shorter (about a third the length of the bod) and broad and attached to the back of the lower lip. They are still used to catch food. The frog can open its mouth and whip the tongue at prey. Frogs use a special kind of saliva that acts somewhat like a non-Newtonian fluid to catch prey. This saliva is secreted by the tongue just before it impacts prey. The saliva starts out very fluid, allowing it to enter crevices on the animal's body. Then, the force of the tongue causes the saliva to harden, making the prey stick to the tongue as it is pulled back into the mouth. Once inside the mouth, the tongue stops moving and the saliva becomes fluid again. American Bullfrogs have very thin skin. This comes with advantages and disadvantages. The thin skin allows them to oxygen from the water around them to supplement their breathing. This is known as cutaneous respiration. On the down side, this makes it easy for the frogs to absorb toxic substances through their skin and makes them highly vulnerable to drying out. Unlike the thick skin of toads, which holds water inside the body, the thin skin of frogs allows their internal water to evaporate out through the skin. As with most frogs, the bullfrog has a pouch on its throat called a gular pouch that inflates when it vocalizes.
(Image: a female bullfrog standing just out of the water. It is identifiable by its smaller tympanum ans white throat. End ID)
American Bullfrogs live in eastern North America from southeastern Canada to northeastern Mexico and throughout the Mississippi River drainage. They have also been introduced outside of their native range to the western part of North America and other places like Hawai'i, Japan, South America, southern Europe, the Caribbean, and southeast Asia. They rely on permanent slow-moving or still water bodies like lakes, streams, ponds, and rivers. American bullfrogs will die if they dry out, so they cannot venture far from the water unless it is raining. They are nocturnal ambush predators who are especially voracious and will eat anything they can fit in their mouths. Common prey animals of bullfrogs include invertebrates of all kinds, snakes, fish, smaller frogs, small mammals, and small reptiles. They have tiny teeth that are too small to cause damage but assist with holding onto prey. Prey will be held in the mouth until it suffocates and then swallowed whole. Bullfrogs will use their arms to help push large prey down the throat. They prefer hunting at the water's edge and can sit still for hours or days until prey ventures by. Once prey is in range, the bullfrog will leap at it. Bullfrogs always close their eyes when they leap. Predators of American bullfrogs include snakes, birds, large fish, raccoons, alligators, and larger frogs. A threatened bullfrog will often scream, which serves both to startle the attacker and alert any nearby bullfrogs to the threat. Bullfrogs will build burrows out of mud and leaves that they hibernate in during the winter.
(Image: a bullfrog held in a hand to show its size. the hand is wrapped around the waist of the grog, leaving the upper body and legs free. The frog is larger than the hand. End ID)
American bullfrogs mate in spring, usually from early May to late July, though mating season will start later in colder latitudes. Water and air temperature seem to be the determining factors for when mating starts. During this time, males will congregate into groups called choruses wherein individuals are usually spaced 3 to 6 m (9.8 to 19.7 ft) apart. Males become highly territorial during this time and will fight other males that attempt to enter their spaces. A chorus usually lasts for a few days, after which it will break down and reassemble elsewhere. Males are very aggressive during mating season and will compete with each other for the best spots. Competition takes the form of vocalizing and posing at each other until one male backs down. If neither backs off, they will fight. Older and larger males are the most successful at claiming territory and mates. Small males may wait passively at the edge of the chorus and attempt to claim any territory a larger male abandons. At night, the chorus will vocalize to attract females. The calls of bullfrog males are similar to a bellowing bull and gave the species their common name. Females come to inspect the chorus and will mate with the males who they judge most fit. The chorus mating strategy is very similar to lekking, a mating strategy seen in birds. In lekking, male birds will display together and females will inspect them to choose the best mate. Male bullfrogs will attempt to mate with as many females as possible.
(video: a male bullfrog performing a mating call. The gular pouch inflates as it calls and its body vibrates enough to splash the water around it. End ID)
Once a female has selected a mate, they will move to the water's edge. The male climbs on top of the female and uses his front legs to hold onto her. This is called amplexus. The female then releases her eggs into the water and the male releases sperm to fertilize them. A female can release up to 20,000 eggs at a time, which form a sheet that floats on the water and can be up to a square meter in area. The water temperature must be between 15 and 32 degrees C (59 - 90 F). If the temperature is lower than this, development stops. At higher temperatures, the embryos develop abnormally. Development usually takes 3 to 5 days. The eggs hatch into tadpoles, which are larval frogs that look like a head with a tail. At this stage they have gills and cannot leave the water. Bullfrog tadpoles start out small, but will grow larger, up to 10.9 cm (4.3 in) from head to tail, which is quite large for a tadpole. Bullfrog tadpoles initially feed on algae and plankton, but will also rasp at plants and carrion as they grow older. The time it takes tadpoles to metamorphose into juvenile frogs is dependent on the weather temperature. It can take several months in tropical water and up to 3 years in colder water. During metamorphosis, the tadpole will develop limbs, take on its adult shape, lose the gills and grow lungs, and absorb the tail. After metamorphosis, it can take up to 2 more years for the frog to become sexually mature. Wild bullfrogs can live up to 10 years.
(image: a bullfrog tadpole held in hand to show its scale. It is longer than any of the fingers. Its body is an olive green mottled with lighter spots on top and white in the belly. It looks like a head with a tail attached. The tail has a fin running down both sides. A small leg is beginning to form at the base of the tail. End ID)
The American bullfrog is classified as least concern by the IUCN, meaning it is not in danger of extinction. They are extremely abundant in their home region and have become invasive in areas they have been introduced to. Bullfrogs can outcompete native frogs due to their larger size and voracious appetites and harm native species by eating too many of them. They are also suspected to be asymptomatic carrier of the fungal disease Chytridiomycosis, which is fatal to many frog species. Bullfrogs have thus introduced the disease to new areas, damaging populations of native frogs while the bullfrogs take over. Bullfrogs are edible to humans and are hunted for food across their range. Typically the hind legs are the only part eaten and they are comparable to chicken drumsticks in taste and texture (though they are smaller). Commercial rearing of bullfrogs is difficult as they typically do not eat non-live food and large populations of bullfrogs in small rearing ponds are vulnerable to disease.
(Image: a bullfrog in the process of eating a smaller frog. The smaller frog's head is entirely within the bullfrog's mouth, leaving the arms and lower body sticking out. End ID
