Bullfrogs May Be Returning to Island After Decades of Decline Nature Notes By

Patricia Martin

The Bullfrogs are back! When I was young, in the 1960s and early 70s, there were all sorts of frogs and other amphibians on the Island, but less than a generation later, many amphibians, reptiles, and other animals seemed to have declined or even disappeared. At least in part, this was probably owing to the spraying of insecticides, including DDT, to control the fly and mosquito populations. This practice was discontinued in the 1960s, but some of the effects have lingered. As I mentioned earlier this year, a number of organisms have been reintroduced to the Island by private citizens in the last 20 years. Apparently the Bullfrog is one of them. Until this summer, I haven’t seen, nor heard of Bullfrogs on the Island. Recently, a neighbor told me about seeing and hearing one in the wetlands along the east shore of the Island.

Bullfrog

One reason that frogs and toads are particularly susceptible to toxins is because of the way they absorb oxygen. All frogs and toads in Michigan exist in two stages, first as a larval stage (tadpole) and then as an adult. In the early stage, the tadpole is rather fish-like, with no limbs, compressed tails, and gills. The gills that they use to absorb oxygen are internal. Because at this stage they “breathe” in the water, they can easily absorb any poison dissolved therein. Even in the adult stage, frogs and toads are more apt to take in toxins, because even though they have lungs and live, at least to some extent, on land, they also absorb oxygen through their skin. The skin of these animals is full of tiny blood vessels to absorb oxygen. To accomplish this type of “breathing,” the skin of the frogs and toads needs to be kept moist by thousands of tiny mucus glands. Most of them live in damp habitats. In addition to absorbing oxygen through the skin, they also absorb water and do not need to drink. For these reasons, frogs are susceptible to pollution and poisons and are called indicator species.

The Bullfrog ( Rana catesbeiana ) is the largest native frog in Michigan. As an adult, it can be between 3.5 and eight inches long. These frogs can be green, yellowish-green, olive, or brown and sometimes have dark brown spots on the back. They have no dorsolateral (ridges) folds and the skin is slightly bumpy. There are often dark bars on the upper surfaces of the legs. The belly is white and mottled with gray. In an adult male, the eardrum is much larger than the eye and the throat is yellow. The females have smaller eardrums, about the size of the eye, and the throat is white. The only frog that one might confuse with the Bullfrog in our area is the Green Frog, which is smaller on average and has dorsolateral (ridges) folds along both sides of the back.

Bullfrogs live in permanent ponds, lakes, and marshes. They’re most common in warm waters with abundant plant life. On the Island, a good place to look for them, beside the wetlands, would be Croghan Water, the ponds on Grand Hotel’s golf courses, and the marsh on the Miller/Berke property (this is private land).

The life cycle of the Bullfrog begins in the spring. They’re the last of the native Michigan frogs to become active. Breeding starts in June and may continue throughout the month. It is at this time that the male Bullfrog will often sit upright in the water with their yellow throats inflated, either to attract the females or discourage rivals. They will utter their low pitched, resonating call that sounds like “brrr-rr-rr-rum.” The males are territorial and will wrestle with an invading male. The females will choose their males by swimming over to the calling male. After mating, the female will produce between 5,000 to 20,000 eggs in a thin floating mass that may cover an area of up to five square feet. The eggs will hatch in three to six days. The tadpoles are greenish with black spots and a white belly, and are great algae eaters. They grow very quickly and may reach a length of six inches. Most over winter as tadpoles and do not change into adults until their second summer. Some even take a third year before they change.

Bullfrogs eat a variety of foods and almost any animal they can swallow, including insects, crayfish, smaller frogs, fish, small snakes, turtles, mice, and birds. They in turn are often food for other organisms, including humans, who enjoy tasty frog legs. If grabbed by a predator, they will often give a loud wailing scream, which may startle an attacker long enough for them to escape.

Over the past decades, Bullfrog populations have been in decline in Michigan. Because of the long time required to mature, and their vulnerability to predation, their populations can easily be disrupted. Over-harvesting has undoubtedly contributed to the decrease in population, but disruption of habitat and pollution have also played a role.

I’d be interested to hear if anyone else has seen these frogs recently. In addition, another frog that I haven’t seen here in years is the Northern Leopard Frog, and I would also like to know if anyone has spotted them.

P.S. One exciting sighting occurred Monday, August 22. An American White Pelican was seen flying over Haldimand’s Bay.

Trish Martin is a year-around resident of Mackinac Island, has earned a master’s degree in botany from Central Michigan University, and owns Bogan Lane Inn.