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Frogs and toads produce a rich variety of sounds, calls, and songs during their courtship and mating rituals. The callers, usually males, make stereotyped sounds in order to advertise their location, their mating readiness and their willingness to defend their territory; listeners respond to the calls by return calling, by approach, and by going silent. These responses have been shown to be important for species recognition, mate assessment, and localization. Beginning with the pioneering experiments of Robert Capranica in the 1930s [1] using playback techniques with normal and synthetic calls, behavioral biologists and neurobiologists have teamed up to use frogs and toads as a model system for understanding the auditory function and evolution. It is now considered an important example of the neural basis of animal behavior, because of the simplicity of the sounds, the relative ease with which neurophysiological recordings can be made from the auditory nerve, and the reliability of localization behavior. Acoustic communication is essential for the frog's survival in both territorial defense and in localization and attraction of mates. Sounds from frogs travel through the air, through water, and through the substrate. Frogs and toads largely ignore sounds that are not conspecific calls or those of predators, [2] with only louder noises startling the animals. Even then, unless major vibration is included, they usually do not take any action unless the source has been visually identified. [3] The neural basis of communication and audition gives insights into the science of sound applied to human communication.
Frogs are more often heard than seen, and other frogs (and researchers) rely on their calls to identify them. Depending on the region that the frog lives in, certain times of the year are better for breeding than others, and frogs may live away from the best breeding grounds when it is not the species’ mating season. During the breeding season, they congregate to the best breeding site and compete for call time and recognition. Species that have a narrow mating season due to ponds that dry up have the most vigorous calls. [4]
In many frog species only males call. Each species has a distinct call, though even among the same species, different dialects are found in different regions. Although humans cannot detect the differences in dialects, frogs distinguish between regional dialects. For example, male bullfrogs can recognize the calls of their direct territorial neighbors. By ignoring the calls of these neighbors, they save energy, and only vocalize aggressively in response to an intruder's call. In this way, calls establish territories, but they also attract females. [4] Males may have a solitary call for times when there is no competition that uses less energy. During other times, when a frog must compete with hundreds or thousands of other frogs to be heard, together they perform a chorus call where each frog calls in turn, successively. The most important feature of the chorus is the shared pattern. Through this pattern, few individuals calls are drowned out. One frog's call may be dominant and trigger the calls of the responding frogs in symphony. Calling is linked to physical size and females may be attracted to more vigorous calls. [4] Frogs in the same region chorus within their species and between different species. Frogs of the same species will retune their frequency so it is distinct from other frogs of the same species. Different species of frogs living in the same region have more dramatically different call frequencies. [5] The frequency and durations of different species' calls vary similarly to the preference of that species' females. The neural circuitry of females of different species varies.
A frog which demonstrates vocalizations in male-male competition is the Lithobates clamitans aka the Green Frog. Typically, they have four types of calls each warning a different level of urgency and each being distinct. The first two calls are types of advertisement calls to establish dominance among the challengers. The other two calls are more directed towards agonistic encounters.
Like the males, females can distinguish the minute differences between individual frogs. However, males and females are attuned to different parts of the advertisement call. For example, males of the onomatopoeically named coqui species are more attuned to the low frequency co part of the call, whereas females are more attuned to the high frequency qui. [6] In fact, the order of the parts does not matter. Similarly, for females of the Tungara species, the female basilar papilla is biased towards a lower-than-average “chuck” portion of a male call. [7] Experiments that measure the vocal responses and approaches shows these attenuations.
Calls are often sent through the air, but other mediums have been discovered. Some species call while they are underwater and the sound travels through the water. This is adaptive in a region with many species competing for air time. Narins has found female frog species that use solid surfaces, such as blades of grass and logs, upon which they tap rhythmically to attract mates. Also, Feng, Narins and colleagues have found that some species of frogs use ultrasound.
The smallest frogs expend much energy to produce calls. In order for vocalizations to be produced, the respiratory airflow goes from the lungs, passing through the larynx, and into the oral cavity. The vocal cords then oscillate as a result. [8] In addition, vocalizing muscles can make up 15% of a male spring peeper's body mass, while the same muscles are only 3% of females. Frogs produce sound from the air sac below their mouth that from the outside, is seen to inflate and deflate. Air from the lungs is channeled to the air sac, which resonates to make the sound louder. The larynx is larger and more developed in males, though not significantly different from females. [7]
Frogs produce two types of calls that most experiments tend to focus on, which are release calling and mating calling. Only the male frogs are able to produce mating calls to attract gravid female frogs. When male and non-gravid female frogs are clasped by sexually active male frogs, they produce a release call. In the leopard frog, there are three movements for their sound production. First, there are body wall contradictions to serve as a way for the intra-pulmonary pressure to increase. Second, in order for air flow to pass through the larynx, the glottis must be open. Third and last, in the larynx, the vocal cords must oppose each other at the midline so that the air flow can cause them to vibrate. [9] In addition, their release calls and movements of their throats and sides are correlated with laryngeal calling movements. [10] For the Concave-eared torrent frog (Amolops tormotus), they produce sounds in the ultrasonic range. [8] Three areas that are highly involved in frog calls are the preoptic area, the medulla-midbrain junction, and the medulla-spinal cord junction. The preoptic area is important in order the frog to initiate mate calling. The medulla-midbrain junction is responsible for producing the calling motor pattern. The medulla-spinal cord junction contains the hypoglossal and vagus nuclei, which are vital to organize the calling and breathing motor patterns. [9]
Biologists[ who? ] believed that frogs ears are placed too close together to localize sound accurately. Frogs cannot hear short, high frequency sounds. Sound is localized by the time difference when the sound reaches each ear. The “vibration spot” near the lungs vibrates in response to sound, and may be used as an additional measure to localize from. [4]
Dr. Feng's work applies the neuroethology of frog communication to medicine. A recent project on hearing aids is based on how female frogs find their mates. Females must recognize the male they choose by his call. By localizing where his call is coming from she can find him. An additional challenge is that she is localizing his call while listening to the many other frogs in the chorus, and to the noise of the stream and insects. The breeding pond is a very noisy place, and females must distinguish a male's calls from the other noise. How they recognize the sound pattern of the male they are pursuing from the surrounding noise is similar to how intelligent hearing aids help people hear certain sounds and cancel out others. The underlying neural mechanisms are fast neural oscillations, and synaptic inhibition to cancel out noise. The timing and frequency of the sound also play a part in frog communication and may be used in Feng's work. He also studies bat echolocation to create intelligent hearing aids. He is also working on cochlear implants. [11]
Toad is a common name for certain frogs, especially of the family Bufonidae, that are characterized by dry, leathery skin, short legs, and large bumps covering the parotoid glands.
The spring peeper is a small chorus frog widespread throughout the eastern United States and Canada. They prefer permanent ponds due to their advantage in avoiding predation; however, they are very adaptable with respect to the habitat they can live in. In northern regions, the frog is able to endure below freezing temperatures due to the capacity of their livers to exude and flush the bloodstream with a glucose cryoprotectant which acts both as an anti-freeze in their blood, and allows organs like the heart to enter into a state of protected dormancy. They are so called because of their chirping call that marks the beginning of spring. Crucifer is derived from the Latin root meaning "cross-bearing." This could be a reference to the cross-like pattern on the spring peeper's dorsal side.
Neuroethology is the evolutionary and comparative approach to the study of animal behavior and its underlying mechanistic control by the nervous system. It is an interdisciplinary science that combines both neuroscience and ethology. A central theme of neuroethology, which differentiates it from other branches of neuroscience, is its focus on behaviors that have been favored by natural selection rather than on behaviors that are specific to a particular disease state or laboratory experiment.
The vocal sac is the flexible membrane of skin possessed by most male frogs and toads. The purpose of the vocal sac is usually as an amplification of their mating or advertisement call. The presence or development of the vocal sac is one way of externally determining the sex of a frog or toad in many species; taking frogs as an example;
The common coquí, widely known as the coquí, is a species of frog native to Puerto Rico belonging to the family Eleutherodactylidae. The species is named for the loud call the males make at night, which serves two purposes; "CO" serves to repel other males and establish territory while the "KEE" serves to attract females. The auditory systems of males and females respond preferentially to different notes of the male call, displaying sex difference in a sensory system. The common coquí is a very important aspect of Puerto Rican culture, and it has become an unofficial territorial symbol of Puerto Rico. The frog is also found elsewhere, and is usually considered an invasive species outside Puerto Rico.
The Mexican burrowing toad is the single living representative of the family Rhinophrynidae. It is a unique species in its taxonomy and morphology, with special adaptations to assist them in digging burrows where they spend most of their time. These adaptations include a small pointed snout and face, keratinized structures and a lack of webbing on front limbs, and specialized tongue morphology to assist in feeding on ants and termites underground. The body is nearly equal in width and length. It is a dark brown to black color with a red-orange stripe on its back along with splotches of color on its body. The generic name Rhinophrynus means 'nose-toad', from rhino- (ῥῑνο-), the combining form of the Ancient Greek rhis and phrunē.
The American green tree frog is a common arboreal species of New World tree frog belonging to the family Hylidae. This nocturnal insectivore is moderately sized and has a bright green to reddish-brown coloration. Commonly found in the central and southeastern United States, the frog lives in open canopy forests with permanent water sources and abundant vegetation. When defending its territory, the frog either emits aggressive call signals or resolves to grapple with intruders, seldom leading to injury or death. To avoid predation, the frog will leap into the water or jump into the treetops.
Animal song is not a well-defined term in scientific literature, and the use of the more broadly defined term vocalizations is in more common use. Song generally consists of several successive vocal sounds incorporating multiple syllables. Some sources distinguish between simpler vocalizations, termed “calls”, reserving the term “song” for more complex productions. Song-like productions have been identified in several groups of animals, including cetaceans, avians (birds), anurans (frogs), and humans. Social transmission of song has been found in groups including birds and cetaceans.
The Bibron's toadlet or brown toadlet is a species of Australian ground-dwelling frog that, although having declined over much of its range, is widespread through most of New South Wales, Victoria, south-eastern Queensland, and eastern South Australia, including Kangaroo Island. Bibron's toadlet settles in a wide variety of habitats within these region but they mainly reside in dry forests, woodland, shrubland, grassland, coastal swamps, heathland, and sub-alpine areas. They deposit their eggs in leaf litters during the flooding season, which is essential for the proper development of the egg. This species has high sexual dimorphism within the species and utilizes chemosignals to attract potential mates.
The northern cricket frog is a species of small hylid frog native to the United States and northeastern Mexico. These frogs are majorly in grey, green, and brown color with blotching patterns. Many have a brown or orange stripe down the center of their back and a triangular marking on the top of their head. Despite being members of the tree frog family, they are not arboreal. These frogs prefer habitats near the edges of slow-moving bodies of water, and in close proximity to shelter items, like rocks. It has two recognized subspecies, A. c. crepitans and A. c. paludicola.
The gray treefrog is a species of small arboreal holarctic tree frog native to much of the eastern United States and southeastern Canada.
Anomaloglossus beebei is a species of frog in the family Aromobatidae. This frog is endemic to Guyana, specifically in the Kaieteur National Park. It mainly survives on the giant bromeliad called Brocchinia micrantha. The phytotelmata of this bromeliad is the site of oviposition and tadpole rearing and are defended over time by the males. The females of this species are more brightly golden coloured whereas males are more of a dull tan with brown pigmentation. Males take care of offspring and are preferred due to the elongation of their calls.
The Panama cross-banded tree frog or pug-nosed tree frog is a species of frog in the family Hylidae found in the humid Pacific lowlands of southwestern Costa Rica to eastern Panama and in the Caribbean lowlands of Panama and northern Colombia. Males of the species utilize synchronous calling to hide their position from predators. Females create basins during amplexus and deposit fertilized eggs onto the surface of the water.
The Túngara frog is a species of frog in the family Leptodactylidae. It is a small nocturnal terrestrial frog found in Mexico, Central America, and the northeastern regions of South America.
Odorrana graminea, the large odorous frog, inhabits fast-flowing streams in elevated mountainous regions of Southern China and Northern Indochina. It is one of 56 species in the genus Odorrana. Male O. graminea are noted for their ultrasonic call characteristics and are one of three frog species able to detect ultrasonic frequencies, likely evolved to facilitate communication amidst noisy streams and waterfalls. Studies on O. graminea courtship vocalizations suggest female preference for increased proportion of nonlinear vocal phenomena (NLP).
Vocal learning is the ability to modify acoustic and syntactic sounds, acquire new sounds via imitation, and produce vocalizations. "Vocalizations" in this case refers only to sounds generated by the vocal organ as opposed to by the lips, teeth, and tongue, which require substantially less motor control. A rare trait, vocal learning is a critical substrate for spoken language and has only been detected in eight animal groups despite the wide array of vocalizing species; these include humans, bats, cetaceans, pinnipeds, elephants, and three distantly related bird groups including songbirds, parrots, and hummingbirds. Vocal learning is distinct from auditory learning, or the ability to form memories of sounds heard, a relatively common trait which is present in all vertebrates tested. For example, dogs can be trained to understand the word "sit" even though the human word is not in its innate auditory repertoire. However, the dog cannot imitate and produce the word "sit" itself as vocal learners can.
A mating call is the auditory signal used by animals to attract mates. It can occur in males or females, but literature is abundantly favored toward researching mating calls in females. In addition, mating calls are often the subject of mate choice, in which the preferences of one gender for a certain type of mating call can drive sexual selection in a species. This can result in sympatric speciation of some animals, where two species diverge from each other while living in the same environment.
An organism is said to be sexually dimorphic when male and female conspecifics have anatomical differences in features such as body size, coloration, or ornamentation, but disregarding differences of reproductive organs. Sexual dimorphism is usually a product of sexual selection, with female choice leading to elaborate male ornamentation and male-male competition leading to the development of competitive weaponry. However, evolutionary selection also acts on the sensory systems that receivers use to perceive external stimuli. If the benefits of perception to one sex or the other are different, sex differences in sensory systems can arise. For example, female production of signals used to attract mates can put selective pressure on males to improve their ability to detect those signals. As a result, only males of this species will evolve specialized mechanisms to aid in detection of the female signal. This article uses examples of sex differences in the olfactory, visual, and auditory systems of various organisms to show how sex differences in sensory systems arise when it benefits one sex and not the other to have enhanced perception of certain external stimuli. In each case, the form of the sex difference reflects the function it serves in terms of enhanced reproductive success.
Sexual selection in amphibians involves sexual selection processes in amphibians, including frogs, salamanders and newts. Prolonged breeders, the majority of frog species, have breeding seasons at regular intervals where male-male competition occurs with males arriving at the waters edge first in large number and producing a wide range of vocalizations, with variations in depth of calls the speed of calls and other complex behaviours to attract mates. The fittest males will have the deepest croaks and the best territories, with females making their mate choices at least partly based on the males depth of croaking. This has led to sexual dimorphism, with females being larger than males in 90% of species, males in 10% and males fighting for groups of females.
Darcy Brisbane Kelley, is an American neurobiologist and currently a Weintraub and HHMI Professor in the Department of Biological Sciences at Columbia University. She is also Co-Director of Columbia’s Graduate Program in Neurobiology and Behavior and Editor of Developmental Neurobiology, and well known for her contributions to neuroethology, particularly the neural control of vocalization in Xenopus and the cellular and molecular mechanisms of sexually differentiated acoustic communication.