Frogs are a familiar sound in many natural environments, their distinctive calls often filling the air, especially during warmer months. These vocalizations are a crucial form of communication for these amphibians, serving several important biological purposes.
Primary Reasons for Calling
The most common reason male frogs call is to attract females for breeding, a behavior known as an advertisement call. These loud, species-specific calls signal a male’s presence and readiness to mate, allowing females to locate suitable partners. Females may assess the quality of a male by the characteristics of his call, influencing their mate choice.
Males also use calls to establish and defend their territory from rivals. These aggressive or territorial calls warn other males to stay away from a prime breeding spot, reducing physical confrontations. A “release call” is produced when a male mistakenly attempts to mate with another male or an unreceptive female, signaling for the other frog to let go. Frogs can also emit distress calls when threatened by a predator, which may startle the attacker and allow escape.
The Mechanics of Frog Calls
Frogs produce sounds using specialized anatomical structures. Air is pushed from the lungs, passing over vocal cords within the larynx. These vocal cords vibrate, generating the initial sound.
This sound then enters the vocal sac, an elastic pouch under the frog’s throat or on the sides of its head. The vocal sac inflates, acting as a resonating chamber that amplifies the sound, allowing it to travel further. Unlike many other vocalizing animals, frogs cycle air between their lungs and vocal sac without expelling it.
Beyond the Basics: Diversity in Frog Calls
Frog calls exhibit remarkable diversity, with each species possessing a unique vocal signature. This species-specific call enables frogs to identify and attract mates of their own kind, even in noisy environments where multiple species may be calling simultaneously. Females are often “tuned” to the specific call of their species, helping them locate a male amidst a chorus.
Subtle variations within a species’ call can convey additional information. For example, a male bullfrog’s call pitch can indicate its body size, with larger frogs producing deeper croaks. This allows females to assess a male’s size, an important factor in mate selection. Some frog species also use a repertoire of different calls for distinct situations, showcasing complex communication.
Environmental Triggers for Calling
Environmental conditions significantly influence when and where frogs call. Temperature and humidity are important factors, with many species increasing vocalizations after rainfall or during warm, humid evenings. A drop in barometric pressure, often preceding rain, can also prompt some tree frogs to call.
Most frog calling occurs at night, particularly from sunset to a few hours before sunrise. This nocturnal activity helps them conserve moisture due to their permeable skin and avoid daytime predators. Calling activity is also strongly tied to specific breeding seasons, which vary by species but generally occur during spring and summer, influenced by local weather conditions. The presence of suitable aquatic habitats, where eggs are typically laid, also dictates where calling is most common.