The Cuban Tree Frog is the largest tree frog in North America. And with an appetite to match, it has become a serious threat to native Florida wildlife.
The Cuban Tree Frog, Osteopilus septentrionalis, is a member of the large Hylid tree frog family, which contains dozens of members, but it has a number of anatomic specialties that place it apart, and it is classed in a genus of its own. The skin is rough and warty, and one oddity is that the skin on the top of the head is firmly attached to the roof of the skull. Like other tree frogs, the Cuban is arboreal and spends all its time off the ground. It uses the large sticky pads on the ends of its toes to climb around on tree branches, rock faces, or the sides of buildings. Occasionally, the frogs will seek a daytime shelter inside an electrical circuit box, and end up frying themselves and shorting out the power.
To prevent itself from drying out, the skin is coated with a thin waxy layer that holds in moisture. When threatened, the Cuban Tree Frog can also produce a slimy mucus on its skin that is an irritant to the eyes and mouth of potential predators. Unlike the more familiar pond frogs, tree frogs are not very good swimmers and never enter water except to lay eggs.
During the tropical rainy season, the males develop a dark patch of rough skin on their thumbs (which helps them hold on to females for mating) and will congregate around any semi-permanent body of water that does not contain any fish to eat the eggs–drainage ditches, retention ponds, even swimming pools. The mating call, which is particularly enthusiastic after a rainstorm, is a grating trill that sounds sort of like a rusty gate. The females can lay as many as 1,000 eggs at a time, stuck together in a floating jellylike mass, which usually hatch in just a day or two. The tadpoles feed mostly on algae, but have been known to kill and eat tadpoles of other frog species. Depending on the water temperature, they can hatch and develop into froglets in as little as four weeks. The developed froglets are about half an inch long when they hop off into the trees. They can live as long as ten years in captivity, though wild frogs probably die long before that.
The Tree Frog is found naturally in Cuba, the Cayman Islands and the Bahamas, where it sleeps during the day (usually pressed up against a tree branch to be inconspicuous) and becomes active at night, hunting insects, smaller frogs, and other prey. The females are generally bigger than the males, and can reach a body length of 5.5 inches. Cuban Tree Frogs are usually a drab greenish grey color, but they can control the pigment cells in their skin and are able to change color, usually becoming dark brown or purplish in cooler conditions and light grey when warm. Most often there are darker bands on the rear legs, and the underside of the legs and belly has bright yellow patches. The eyes are very large, with lovely gold flecks in the iris.
Despite their large size, Cuban Tree Frogs can stay pretty inconspicuous. Because of this, they are often unintentionally carried around from place to place, stuck to the side of a packing crate or onto the stem of a potted plant. They have therefore hitchhiked their way around the world, becoming established throughout the Caribbean islands and other tropical areas like Hawaii. The frogs are also prominent in the pet trade, and escaped/released pets have added to their dispersal.
Reports of Cuban Tree Frogs in Florida go back to the 1920s in Miami, where they probably arrived as stowaways in cargo containers. The species was definitively established in South Florida by 1950, long before the pet trade began to import them by the thousands. Almost certainly they had been floating across the Caribbean to the mainland after storms and hurricanes, on floating vegetation mats and uprooted trees, for millenia, but were apparently never able to establish themselves. But once human cities appeared, the Cuban Tree Frogs found that habitat to their liking–there are fewer predators, lots of suitable prey, and urban areas are even a little warmer than their surroundings. Despite the fact that a variety of Florida predators, from racer snakes to raccoons, will eat them, the Cubans are apparently able to reproduce enough to keep expanding their population. The frogs are able to thrive in close contact with the humans, and have steadily spread north to the Jacksonville area. Since they cannot tolerate freezing conditions (the unusually cold Florida winters of 2010 and 2011 temporarily wiped many of the invaders out), that may be the limit of their expansion.
The frogs are harmless to humans (aside from the irritation sometimes caused by their skin secretions). But they are not so harmless to native Florida wildlife. Cuban Tree Frogs are big, perpetually hungry, and will eat anything that moves which they can fit in their mouths. The biggest impact seems to be on our native tree frog species, which are not only crowded out of habitat and breeding areas by the invaders, but are often swallowed whole as a Tree Frog lunch. While acknowledging that the Cuban Tree Frog is now probably a permanent resident, the State of Florida nevertheless encourages people to kill the frogs wherever they are found, by sealing them inside a plastic baggie and putting them in the freezer overnight.