American Toad

Anaxyrus americanus
Monday, June 24, 2013

            These toads are ubiquitous across most of the eastern half of the United States. However, they are quite uncommon at high elevations, and have not been found at all above 1200 m in the Allegheny Mountains. At 1160 m, the station is at the high end of their range.

            Anaxyrus americanus is a chubby toad with skin that ranges from reddish to brown to grey. The color of a toad’s skin can vary greatly based on temperature, humidity, and stress levels. Their backs are covered with differently-sized warts and marked with 3-4 pairs of dark spots, each accompanied by one large wart. The southern toad, an easily confused species found in the southeastern US, lacks this regular pattern and wart configuration; Fowler’s toad has three or more warts within each dark spot and an unblotched belly. Discrimination between the sexes is possible in American toads by noting the throat color; males have dark throats, while females have light throats and a lighter overall coloration. Females also typically reach larger sizes, up to 11 cm in length (males can grow to 8.5 cm).

            American toads are much less sensitive to habitat fragmentation caused by humans than many other species of amphibians. They thrive in forests, open areas, backyards, gardens, and agricultural land—virtually anywhere as long as they have access to a semi-permanent source of fresh water for breeding and the development of the tadpoles. In March-April, males migrate from their terrestrial homes to these breeding sites, where they establish territories and begin calling to attract the females. The quality of the call—a long trill produced by inflating the dewlap—and the quality of the territory defended have both been shown to influence females’ choice of mates when they arrive around a week later. During the breeding season, males develop dark, horny pads on the first and second toes of their forelegs. This allows them to cling to the females’ back in a position called amplexus while she searches for a suitable area to lay her eggs. Female toads can lay up to 8,000 eggs in long, spiral tubes of jelly reminiscent of the cords of rotary telephones—if those cords were 6 to 20 meters in length when fully extended! Males release sperm into the water as the female produces the eggs, fertilizing them externally.

            After 3-12 days, the eggs hatch into tiny dark-colored tadpoles which grow rapidly, grazing on aquatic vegetation over the next 40-70 days. Although the majority of tadpoles die before reaching the stage of metamorphosis, those that survive emerge from the water as miniature versions of their adult forms, often less than a centimeter in length. Around this time, they also abandon herbivory in favor of the generalist carnivory practiced by the adults. Adult toads are known to consume up to 1,000 insects in a single day, along with occasional snails, slugs, earthworms, and a variety of other invertebrates. The young achieve sexual maturity in 2-3 years, molting shedding their skin around four times per year. They collect the skin under their tongue before consuming it to reclaim nutrients and protein.

            The skin on toads’ backs has glands which produce secretions that irritate mucous membranes in many animals, making the toad very distasteful to predators. It has been shown that some of those toxins are deposited with the eggs when they are laid, affording the eggs and tadpoles a temporary protection against many types of predators which decreases with age.

            This toad was caught by Phoebe on the front steps of Lewis, where it is a regular visitor. It is a female, judging by its large size and lighter coloration.

Hazel Galloway