Why Do Bull Frogs Just Come Out at Night?

The American bullfrog is the largest frog in North America.

Bullfrogs are the largest and one of the most common frogs found in North America. It’s believed that bullfrogs are only active at night because they are often seen hopping around on warm summer evenings and their loud, distinctive call is a common late-night sound. Bullfrogs, however, are active at all hours of the day and their hours of activity vary with location. Most bullfrogs in warmer climates do spend the majority of their life cycle active at night as they search for mates, breed and hunt prey.

Taxonomy

The American bullfrog (Rana catesbeiana) is one of the largest members of the Ranidae family of frogs, which contains other species commonly referred to as the true frogs. Like many other true frogs, bullfrogs are primarily nocturnal in habit, smooth skinned and have long powerful legs with well-defined webbed feet that propel them swiftly through aquatic environments.

Range and Habitat

Bullfrogs are native to Eastern and Central North America but have been introduced to many areas in the Western United States. Bullfrogs are aquatic frogs and can be found along the edges of lakes, ponds, slow-moving rivers and other permanent bodies of water. Because of their large range and ability to live in multiple habitats, the behavior of bullfrogs may vary with location. Some frogs may be more active during the day in cooler parts of the country, while other bullfrogs may be more active at night in more temperate climates. Because of their ability to tolerate a range of environmental conditions, bullfrogs have been introduced to many countries around the world such as Europe, Asia, and Central and South America.

Description

Adult bullfrogs can grow up to 8 inches in length, with females being slightly larger than males on average. In addition to their large size, bullfrogs are easily identifiable by their large conspicuous tympanum (the hearing organ of frogs) located directly behind the eye; the tympanums in male bullfrogs can be larger in size than their eyes. Bullfrogs range in coloration based on location though they often have an olive base-color and gray or brown patterns on their back and legs. Their coloration helps them to blend in to their aquatic environments during the day and aid them in ambushing unsuspecting prey at nights.

Diet

American bullfrogs are opportunistic feeders that prefer to hunt at night. They will readily feed on anything that they can swallow. Bullfrogs will often construct small enclosures in the mud and wait for prey to pass by. When they spot prey, they wait for the right distance and then with a rapid motion grab the prey with their tongue and swallow it whole. Bullfrogs are renowned for eating prey that few amphibians can, such as rodents, snakes, other frogs (including other bullfrogs), and even bats.

Reproduction

The breeding season of bullfrogs varies with location, but breeding activities are done exclusively at night. In warmer climates such as Florida, breeding begins as early as February and can last through October. In more northern climates like Colorado or Montana, breeding may not begin until late June and can last just one month. Male bullfrogs mate with multiple females and are territorial over prime reproductive habitats. Female bullfrogs can lay large masses of eggs that contain as many as 20,000 eggs. Metamorphosis of bullfrogs can take up to two years depending on the location and temperature. Breeding activities occur late at night, which is why bullfrogs can be commonly seen and heard during these periods.

References

• University of Michigan Museum of Zoology: Animal Diversity Web: North American Bullfrog
• Amphibiaweb: Rana Catesbeiana
• University of Florida: Florida Wildlife Extension: Bullfrog
• University of Montana, Missoula: Northern Rockies Natural History Guide: Bullfrog
• Animal Planet: The Vanishing Frog: Anatomy of a Frog: Frog Ears

Resources

• California Herps: Sounds of Rana Catesbeiana: American Bullfrog