Frogs: The Animal Answer Guide

4. Frog Behavior

35 Why do frogs make noise? Frogs make a lot of noise. The noise a large chorus of frogs produce can be deafening. Why do they make so much noise? Frogs are trying to secure a mate and calling, sometimes very loudly, is the main way they do this. In nearly all species, primarily the male calls to attract a mate. Such a call, which scientists refer to as an advertisement call, cannot only attract females but also, in many species, can warn other males that they are encroaching on the calling frog’s territory. The advertisement call is unique to each species—that is, a frog can nearly always be identified by its advertisement call. Male frogs gather near a suitable breeding site to call at a particular time of year known as the species’ breeding season. Female frogs can detect slight variations in males’ calls and are more attracted to males with calling characteristics that denote high reproductive fitness. Calling in most species occurs primarily at night but may occur during the day, especially for those that breed during colder times of the year. Although advertisement calls are best known and are the type of call most easily recognized by humans, most frogs and toads produce other types of calls for reasons other than advertising to attract mates. For example , many species produce a release call if grabbed by a male frog attempting to mate with an individual that is not receptive to mating (e.g., other males or females that have already laid their eggs). The release call informs the mating male that the union is unacceptable. Some frogs, such as bullfrogs, have specific territorial calls. If approached too closely by another male, bullfrogs will produce a quick “phoot” call to preserve their territory. Many frogs, especially frogs in the family Ranidae, will emit a Chapter 4 Frog Behavior 36 Frogs: The Animal Answer Guide loud “squeak” when startled. Other species, such as squirrel treefrogs (Hyla squirella), produce “rain calls” during the afternoon when summer rainstorms are approaching. In some species, such as the midwife toad (Alytes obstetricans) of Europe, the female will produce a call in response to the male, indicating she is willing to mate. The male then changes its advertisement call as a response to the female. A remarkable call emitted by many species of anurans is the distress call. The distress call of some species can be loud and is produced when the animal is threatened or grasped by a predator. Distress calls are made with an open mouth. The smoky jungle frog (Leptodactylus pentadactylus) is found in the New World Tropics and is well known for its large size and loud distress call that sounds like a traumatized cat. The bullfrog (Rana catesbeiana) also produces a loud distress call, most often when it is swallowed legs first by a watersnake. The loud scream is similar to the sound a rabbit emits when it is attacked. Many scientists think frogs’ distress calls attract predators that may attack the animal trying to eat them or at least disrupt the event and allow them time to escape. How do frogs make sounds? Frogs produce sounds similar to how humans vocalize; that is, air is passed over vocal chords that vibrate, producing sounds. Frogs have some anatomical modifications that help to increase the volume of their call, thus making them discernable to potential mates for long distances. The primary structure that increases the volume of frog calls is the vocal sac (or Most noises made by frogs are by males that call during the breeding season. However, some species will emit a scream when being attacked by a predator, such as this Cuban treefrog (Osteopilus septentrionalis) being eaten by a garter snake (Thamnophis sirtalis). Courtesy Chris Gillette 37 Frog Behavior sacs), which acts as a resonance chamber to amplify the sound they produce . When calling, the vocal sac is inflated with air and expands greatly. Many frogs have a single vocal sac below their chin, but some species, such as northern leopard frogs (Rana pipiens) and the edible frog (Rana esculenta) of Europe, have paired vocal sacs on either side of their head. Frogs can call from various locations, including from the ground, on vegetation, or while floating in the water. Calling, however, can attract not only mates but also potential predators. Thus, frogs frequently stop calling if they are...