Why Do Frogs Croak at Night?
Frogs are frequently portrayed in cartoons and animated films as musicians in an orchestra or jazz band. In reality, they don’t need any actual instruments to make their signature sounds. If you’ve ever taken a nocturnal walk through a field of grass just after a generous drizzle, you may have found yourself serenaded by the bizarre, yet familiar, sound of hundreds of hidden frogs.
The masters of the croak, some of these amphibians can perfectly imitate the sounds of bells, chimes, crickets, horns, bouncing balls, and ducks. Considering there are over 5,000 species of frogs (Anura), each with their unique croak, the sounds they make range from subtle and charming to shocking and irritating.
As most frogs are nocturnal animals, their unmistakable concerts occur throughout the night. In tropical rainforests, where most species reside, the nocturnal hours are actually the loudest. The frogs can seem to compete with one another for a slot in the stream of sound waves. Every second is filled with calls and signals, predominantly for the purpose of finding a mate.
Most Croaks Are Mating Calls
The thousands of species of frogs across the globe have evolved to produce distinct croaks. Their sheer diversity of mating calls is necessary. If they all croaked in the same way, one species would hardly ever manage to find another. If a tree frog is led to a bullfrog, mating simply cannot take place.
Just as many animals produce unique chemical cues to attract female conspecifics, each species of frog has its signature croak. They are also able to identify the calls of their neighboring frogs. This audible-based form of neighbor recognition has been shown in the North American bullfrog (Rana catesbeiana).
Interestingly, only the male frogs are able to produce their mating or “advertisement” calls. Females remain largely silent. However, they are tasked with the incredibly important activity of deciphering between the croaks and moving toward the right one. They may make noise while mating, in the attempt to communicate with the males on their backs. Females of the Emei music frog (Nidirana daunchina) are incredibly fascinating; they make vocalizations to encourage their male partners to perform.
How Do Frogs Croak?
Frogs have to make loud noises to be heard, and they can do so without damaging their own eardrums. They croak by redirecting the movement of air from their lungs. Instead of coming out through their nostrils, air is pushed over the vocal cords (larynx) and windpipe and all the way through to the air sac. Underneath their chins, this chamber is seemingly surrounded by elastic skin as it can expand to multiple times its size in some males.
Depending on the species, the air sac can look much like a bubble as it expands. It functions as a resonance chamber that amplifies the sound. Frogs have one of the most anatomically complex laryngeal systems. A valve (i.e. glottis) that controls the movement of air into and out of the lungs determines the pitch, loudness, and frequency of the call.
Frogs don’t particularly have high-frequency hearing capabilities, so their croaks produce sound waves with frequencies that range from 250 – 10,000 Hz. An exception is the torrent frog, which can chirp at a frequency of 128,000 Hz and has a special head configuration for ultrasonic hearing! In comparison, some mammals can hear at frequencies of up to 60,000 Hz. As a result, frogs have developed a head anatomy that allows each of their ears to detect a single sound twice! When the same sound wave is perceived at two different phases, the variance in the peaks and troughs helps a frog detect its source location.
Frogs with low-frequency croaks may struggle to be heard above their neighbors. They must have adaptations that allow their mates to find them amidst the intense sonic competition. One South American frog, Leptodactylus ocellatus, has evolved to croak underwater instead. This is one croak that humans are unlikely to hear! By calling for mates underwater, where sound can travel well, L. ocellatus is a specialist within its own league.
At the other end of the spectrum are Pacific tree frogs (Pseudacris regilla), which are also known as chorus frogs. These have made local news in San Francisco for disrupting the night’s peace! Those who were sensitive to the sound of their croaks and couldn’t go to sleep amidst the persistent noise were advised to use earplugs.
A Puerto Rican frog has taken the cake for being the loudest frog in the world. Eleutherodactylus coqui, or simply coquí, grows to just 3 inches, but it can make a croak that may be as loud as 80 – 100 decibels when it is about a meter away. The deafening mating calls of this frog actually managed to bring down property values in Hawaii, where it has no natural predators!
Factors That Affect the Occurrence of Croaks
As most frogs are nocturnal or crepuscular, it follows that the majority of the sounds they make would be heard at night or just after dusk. Croaks may also be heard during the day, but they are more likely to be warning sounds, if not the calls, of diurnal frogs. When frogs are in distress, such as when a predator is closing in, they can actually wail or scream! This may have the purpose of startling their predators or inciting panic.
The rainiest months of the year are when nights come alive with croaking frogs. Rain means more fresh water, and more fresh water means more aquatic real estate for amphibian reproduction. Frogs must take the opportunity to call out to potential mates as they are more likely to locate a suitable place for their offspring. Wetland systems are usually rife with amphibians. In those that are located in the tropics, frogs sing all throughout the year.
If frogs occur in high densities, other audible types of croaks may include territorial calls or release calls. A higher ratio of males to females could also result in more croaks. Territorial males signal to other male frogs, usually in the form of a low groan, that they should stay away or back off. As the males move from place to place or attempt to mount other frogs, they can trigger audible responses. Females that have already mated, frogs of a different species, or other male frogs will respond to mating attempts by uttering a quick-release call. This chirp-like call is basically frog-speak for “let go!”.