Frogs’ nocturnal croaking is a familiar part of many natural soundscapes. These nocturnal vocalizations are not random noises but serve specific purposes for the frogs. The timing of their calls is influenced by environmental factors that make nighttime particularly suitable for their communication.
The Primary Purposes of Nocturnal Croaking
The most prominent reason male frogs vocalize at night is to attract mates. These “advertisement calls” are crucial for reproduction, as females assess the quality of a potential partner based on his vocalizations. Each frog species possesses a distinct call, allowing females to locate males of their own kind even in a noisy environment with multiple species calling simultaneously.
Beyond attracting mates, male frogs also use their calls to defend their territory. They issue warning calls to signal other males to stay away from their territory. This defense secures a breeding spot. While males are the primary callers for mating and territorial purposes, both male and female frogs can produce “release calls” if an overzealous male attempts to mate with an already-bred female or another male. Frogs also produce “distress calls,” such as a high-pitched scream, when attacked by a predator, which can startle the attacker and allow the frog to escape.
The Mechanics of Frog Calls
Frogs produce sound using a specialized respiratory system. Air from their lungs is forced over their vocal cords, causing them to vibrate and generate sound. The sound then travels into a flexible pouch of skin called a vocal sac, which most male frogs possess.
The vocal sac inflates with air, acting as a resonance chamber that amplifies the sound produced by the vocal cords. This amplification allows the frog’s call to travel much further, with some species’ calls audible over a kilometer away, compared to a few meters for those without vocal sacs. Air cycles back and forth between the lungs and the vocal sac without being expelled, enabling prolonged and continuous calling.
Environmental Factors Influencing Nighttime Activity
Frogs often prefer to call at night due to several environmental advantages. As ectotherms, frogs cannot regulate their body temperature, making cooler nighttime temperatures more suitable for extended activity. Nighttime also offers higher humidity, which is beneficial for frogs given their permeable skin, reducing the risk of dehydration.
The darkness of night provides cover from many diurnal predators, such as birds, making it a safer time for frogs to engage in vocalizations and mating behaviors. The reduced background noise at night also allows their calls to travel farther and be heard more clearly by potential mates. While most frog species are nocturnal, some do call during the day.
Common Nighttime Croakers
Many frog species contribute to the nighttime chorus with their unique vocalizations. The American bullfrog is known for its deep, booming “jug-o-rum” call, which can be heard from a considerable distance. The Cope’s gray treefrog produces a short, bird-like trill, while the green treefrog has a distinct nasal honk or bell-like sound. Spring peepers are recognized for their high-pitched “peep” sounds, often heard in large choruses during the early spring. The specific frogs heard depend on geographic location and habitat.