Frogs are a diverse group of amphibians whose presence in a local environment is often first announced by their vocalizations. These sounds, commonly known as croaking or calling, are a form of acoustic communication produced by forcing air over the vocal cords, often amplified by a vocal sac. While the act of calling is a fundamental behavior for these animals, the timing of when a frog chooses to vocalize is precisely regulated by a combination of long-term seasonal and short-term environmental cues. The familiar sound of a frog chorus primarily revolves around reproduction and survival.
Seasonal Triggers for Croaking
The annual onset of frog croaking is fundamentally tied to the species’ breeding season, which is determined by its internal biological clock and the sustained increase in temperature following winter dormancy. For many temperate-zone species, the beginning of the spring calling season is directly related to the temperature of the environment, particularly the water where breeding occurs. Warmer winters, for instance, have been observed to advance the start of breeding and calling activity in some populations, sometimes by several weeks.
The required temperature threshold for initiating the breeding chorus varies significantly among species, but it generally involves a period of sustained warmth rather than a single warm day. Early spring breeders, such as wood frogs or spring peepers, may begin calling when water temperatures are just above freezing, sometimes as low as 4 to 7 degrees Celsius. Species that breed later in the season often wait for warmer conditions, sometimes requiring temperatures that consistently exceed 15 degrees Celsius. This temperature dependency also creates a distinct geographical variation, as the breeding season is far shorter in colder, high-latitude regions and can be continuous or year-round in the stable, warmer climates of the tropics.
Environmental Factors That Influence Calling
Once the breeding season has been initiated by seasonal temperature shifts, the daily and momentary calling activity is modulated by short-term environmental factors, with moisture being a highly influential trigger. Because frogs are susceptible to desiccation, high relative humidity and the presence of rainfall often lead to bursts of vocal activity. Rainfall can act as a powerful momentary cue, causing a significant increase in calling intensity and duration, sometimes even triggering sporadic calling during the day among species that are typically nocturnal.
The time of day is another factor that finely controls when a frog chorus will be heard, as most species are crepuscular or nocturnal. Vocalization often peaks around dusk, when temperatures are still favorable but the risk of desiccation is lower than during the heat of the day. Sudden drops in temperature or the presence of strong winds can cause calling activity to cease temporarily, as these conditions are less conducive to maintaining the necessary body temperature and moisture levels.
The Different Meanings of Frog Calls
The primary function of the frog chorus is the advertisement call, sometimes called the mating call. This loud, repetitive vocalization is produced exclusively by males to attract a receptive female of the same species to a breeding site. Each species possesses a unique “love song,” which allows females to identify and select a mate even when dozens of different species are calling simultaneously in a mixed-species chorus.
Beyond the reproductive purpose, frogs also employ other specialized calls for different social and defensive interactions. A territorial or encounter call is a shorter, often aggressive sound used by a male to warn off a competing male that has encroached on his calling site. In cases of mistaken identity during the excitement of breeding, a male or an already-mated female that is clasped by another male will emit a release call, signaling that they are not a suitable mate. Finally, a distress call, which can sound like a loud, startling scream, may be used defensively when a frog is captured by a predator, potentially to startle the attacker into releasing its grip.