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Animal language are those forms of animal communication that are considered to show similarities to human language. Animal communication may be considered complex enough to be called a form of language if the inventory of signs is large, the signs are relatively arbitrary, and if the animals produce them with a degree of volition (as opposed to conditioned instincts). Animal communication can also be evidenced through the use of lexigrams (See Yerkish), in addition to signs. A lexigram represents a word but is not necessarily indicative of the object referenced by the word. They are symbols that correspond to objects and ideas and have been used by chimpanzees and bonobos to express language. While the term "animal language" is widely used, researchers agree that animal languages are not as complex or expressive as human language.
Some researchers including the linguist Charles Hockett, who proposed a list of design features of Human Language, argue that there are significant differences separating human language from animal communication even at its most complex, and that the underlying principles are not related. Accordingly, Thomas A. Sebeok has proposed not to use the term 'language' in case of animal sign systems.
Others argue that an evolutionary continuum exists between the communication methods these animals use and human language. Examining this continuum could help explain how humanity evolved its incredibly sophisticated proficiency for language.
Aspects of human language 
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The following properties of human language have been argued to separate it from animal communication:
- Arbitrariness; There is not necessarily a rational relationship between a sound or sign and its meaning, i.e. symbolism. (There is nothing intrinsically "housy" about the word "house".)
- Cultural transmission: Language is passed from one language user to the next, consciously or unconsciously.
- Discreteness: Language is composed of discrete units that are used in combination to create meaning.
- Displacement: Languages can be used to communicate ideas about things that are not in the immediate vicinity either spatially or temporally, or both.
- Duality: Language works on two levels at once, a surface level and a semantic (meaningful) level.
- Metalinguistics: Ability to discuss language itself.
- Productivity: A finite number of units can be used to create an indefinitely large number of utterances.
Research with apes, like that of Francine Patterson with Koko or Herbert Terrace with Nim Chimpsky, suggested that apes are capable of using language that meets some of these requirements such as arbitrariness, cultural transmission, discreteness and productivity. However, there is an experiment in which they implemented human integration with a chimpanzee infant named Gua in order to demonstrate the effects of the environment on language acquisition.
The experiment entailed raising 7.5-month-old Gua with a 10-month-old child, Donald. Gua was introduced to a normal human life at a very early age in order to ensure that the chimpanzee was not already exposed to nonhuman response mannerisms. The parents made sure to treat Gua like a human, dressing her like a human, refusing to treat her as a pet, feeding her the same way that they fed Donald, and punishing her the same ways as well. Eventually, Gua was able to respond to around 95 words and phrases, but she was never able to acquire the words or phrases. It was also observed that Gua never randomly spoke out, babbled, or cooed like Donald did. We can attribute this to both the chimpanzees' vocal cord anatomy and possible deficiencies in chimpanzees' Broca's areas. 
Researchers then turned to sign language using a chimpanzee named Washoe. Washoe learned hundreds of words, but failed to combine them in a creative way to convey meaning or intent. This does not signify knowledge of the syntax of the language. Even if people could successfully teach animals a "language", it would never be equivalent to human language as human language is naturally occurring and naturally acquired. 
In the wild chimpanzees have been seen "talking" to each other, when warning about approaching danger. For example, if one chimpanzee sees a snake, he makes a low, rumbling noise, signalling for all the other chimps to climb into nearby trees. In this case, the chimpanzees' communication is entirely contained to an observable event, demonstrating a lack of displacement.
Arbitrariness has been noted in meerkat calls; bee dances show elements of spatial displacement; and cultural transmission has possibly occurred between the celebrated bonobos Kanzi and Panbanisha.
Human language may not be completely "arbitrary". Some research has shown that almost all humans naturally demonstrate limited crossmodal perception (e.g. synesthesia), as illustrated by the Kiki and Booba study. Other recent research has tried to explain how the structure of human language emerged, comparing two different aspects of hierarchical structure present in animal communication and proposing that human language arose out of these two separate systems 
Claims that animals have language skills akin to humans however, are extremely controversial. As Pinker illustrates in his book the "The Language Instinct", claims that chimpanzees can acquire language are exaggerated and rest on very limited or specious data.
The American linguist Charles Hockett theorized that there are 16 features of human language that distinguished human communication from that of animals. He called these the design features of language. The features mention below have so far been found in all human languages and each is missing from at least one animal communication system.
- Vocal-auditory channel: sounds emitted from the mouth and perceived by the auditory system. This applies to many animal communication systems, but there are many exceptions. Ex. An alternative to vocal-auditory communication is visual communication. An example is cobras extending the ribs behind their heads to send the message of intimidation or of feeling threatened. 
- Broadcast transmission and directional reception: this requires that the recipient can tell the direction that the signal comes from and thus the originator of the signal.
- Rapid fading (transitory nature): Signal lasts a short time. This is true of all systems involving sound. It doesn't take into account audio recording technology and is also not true for written language. It tends not to apply to animal signals involving chemicals and smells which often fade slowly. Ex. A skunks’ smell, produced in its glands, lingers to deter a predator from attacking. 
- Interchangeability: All utterances that are understood can be produced. This is different to some communication systems where, for example, males produce one set of behaviours and females another and they are unable to interchange these messages so that males use the female signal and vice versa. Ex. Heliothine moths have differentiated communication: females (only send) send a chemical to indicate preparedness to mate and males (only receive) cannot send the chemical
- Total feedback: The sender of a message is aware of the message being sent
- Specialization: The signal produced is intended for communication and is not due to another behavior. Ex. Dog panting is a natural reaction to being overheated and exercised, but is not produced to specifically relay a particular message
- Semanticity: There is a fixed relationship between a signal and a meaning.
Non-primates: studied examples 
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Recording of Humpback Whales singing and Clicking.
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Among the most studied examples of animal languages are:
- African Forest Elephants - Cornell University's Elephant Listening Project began in 1999 when Katy Payne began studying the calls of African Forest Elephants in Dzanga National Park, Central African Republic. Andrea Turkalo has continued Payne's work in Dzanga National Park, and she continues to observe the elephants' communication today. For nearly 20 years, Turkalo has spent the majority of her time using a spectrogram to record the noises that the elephants make. After extensive observation and research, she has become able to recognize elephants by their voices. Researchers hope to translate these voices into an elephant dictionary, but that will likely not occur for many years. Because elephant calls are often made at very low frequencies, this spectrogram is able to detect lower frequencies that human ears are unable to hear, allowing Turkalo to get a better idea of what she perceives the elephants to be saying. Cornell’s research on African Forest Elephants has challenged the idea that humans are considerably better at using language and animals only have a small repertoire of information that they can convey to others. As Turkalo said on 60 Minutes’ “The Secret Language of Elephants”, “Many of their calls are in some ways similar to human speech." 
- Bee dance - used to communicate direction and distance of food source in many species of bees.
- Bird songs - songbirds can be very articulate. African Grey Parrots are famous for their ability to mimic human language, and at least one specimen, Alex, appeared able to answer a number of simple questions about objects he was presented with. Parrots, hummingbirds and songbirds - display vocal learning patterns.
- Whale songs - Two groups of whales, the Humpback Whale and the subspecies of Blue Whale found in the Indian Ocean, are known to produce the repetitious sounds at varying frequencies known as whale song. Male Humpback Whales perform these vocalizations only during the mating season, and so it is surmised the purpose of songs is to aid sexual selection. Humpbacks also make a sound called the feeding call. This is a long sound (5 to 10 s duration) of near constant frequency. Humpbacks generally feed cooperatively by gathering in groups, swimming underneath shoals of fish and all lunging up vertically through the fish and out of the water together. Prior to these lunges, whales make their feeding call. The exact purpose of the call is not known, but research suggests that fish react to it. When the sound was played back to them, a group of herring responded to the sound by moving away from the call, even though no whale was present.
- Prairie dog language: Dr. Con Slobodchikoff studied prairie dog communication and discovered:
- different alarm calls for different species of predators;
- different escape behaviors for different species of predators;
- transmission of semantic information, in that playbacks of alarm calls in the absence of predators lead to escape behaviors that are appropriate to the type of predator which elicited the alarm calls;
- alarm calls containing descriptive information about the general size, color, and speed of travel of the predator.
- Baboons - Humans are able to distinguish real words from fake words, based on the phonological order of the word itself, but it turns out that we are not alone with this ability. In a recent study, baboons have shown researchers that they also have this skill. The discovery has led us to believe that reading is not necessarily such an advanced technique (as we once thought), but instead based on the foundation of being able to recognize and distinguish letters from one another. The experimental setup consisted of six young adult baboons, and results were measured by allowing the animals to use a touch-screen and selecting whether or not the displayed word was indeed a real word, or a made-up word such as dran or telk. The study lasted for 1.5 months, with approximately 50,000 tests being completed in that time period. The experimenters explain the use of bigrams, which are just combinations of two (usually different) letters. They tell us that the bigrams used in nonwords are rare, while the bigrams used in real words are more common. Further studies will attempt to teach baboons how to use an artificial alphabet.
- Caribbean Reef Squid have been shown to communicate using a variety of color, shape, and texture changes. Squid are capable of rapid changes in skin color and pattern through nervous control of chromatophores. In addition to camouflage and appearing larger in the face of a threat, squids use color, patterns, and flashing to communicate with one another in various courtship rituals. Caribbean Reef Squid can send one message via color patterns to a squid on their right, while they send another message to a squid on their left.
- Bottlenose Dolphins - It was long believed that dolphins were never able to demonstrate the ability to communicate in their own language, but in a recent discovery, we may have been wrong all along. With the use of a CymaScope researchers have discovered that dolphins are now using their whistles to communicate more than just a simple hello to one another. They are discussing their demographics: names, ages, locations, genders, etc. It only makes sense that one of the world’s most intelligent animals has a language that they use to convey information to one another. Just like each person has his or her own unique voice, each dolphin has its own unique whistle sound, allowing each dolphin to maintain a separate identity, similar to humans. While this may be nothing spectacular, dolphins are able to create new sounds and whistles when trying to attract a mate. Another interesting point is that in most (if not all) groups, there is a designated leader. The leader is responsible for communicating with other groups’ leaders, discussing possible things such as demographics or locations of food and danger. Remarkably, dolphins can hear one another up to 6 miles apart underwater through a method called echolocation.
National Geographic has an article outlining the successes of a mother dolphin communicating with her baby using a telephone. Researchers noted that it appeared that both dolphins knew who they were speaking with and what they were speaking about. Nowadays, scientists are intrigued by the dolphins’ language and are attempting to “crack the code.” Not only do dolphins communicate via nonverbal cues, but they also seem to chatter and respond to other dolphin’s vocalizations.
- Sea Lions: Beginning in 1971 and continuing until present day, Dr. Ronald J. Schusterman and his research associates have studied sea lions’ cognitive ability. They have discovered that sea lions are able to recognize relationships between stimuli based on similar functions or connections made with their peers, rather than only the stimuli's common features. This is called “equivalence classification.” This ability to recognize equivalence may be a precursor to language.  Research is currently being conducted at the Pinniped Cognition & Sensory Systems Laboratory to determine how sea lions form these equivalence relationships. Sea lions have also been proven to be able to understand simple syntax and commands when taught an artificial sign language similar to the one used with primates.  The sea lions studied were able to learn and use a number of syntactic relations between the signs they were taught, such as how the signs should be arranged in relation to each other. However, the sea lions rarely used the signs semantically or logically. In the wild it's thought that sea lions use the reasoning skills associated with equivalence classification in order to make important decisions that can affect their rate of survival (e.g. recognizing friends and family or avoiding enemies and predators). Sea lions use the following to display their language:
- Sea lions use their bodies in various postural positions to display communication. 
- Sea lion's vocal cords limit their ability to convey sounds to a range of barks, chirps, clicks, moans, growls and squeaks.
- There has yet to be an experiment which proves for certain that sea lions use echolocation as a means of communication. 
The effects of learning on auditory signaling in these animals is of special interest. Several investigators have pointed out that some marine mammals appear to have an extraordinary capacity to alter both the contextual and structural features of their vocalizations as a result of experience. Janik and Slater (2000) have stated that learning can modify the emission of vocalizations in one of two ways: (1) by influencing the context in which a particular signal is used and/or (2) by altering the acoustic structure of the call itself. Male California sea lions can learn to inhibit their barking in the presence of any male dominant to them, but vocalize normally when dominant males are absent (Schusterman and Dawson, 1968). Recent work on Gray seals show different call types can be selectively conditioned and placed under biased control of different cues ( Shapiro et al.,2004; Schusterman, in press) and the use of food reinforcement can also modify vocal emissions. “Hoover”, a captive male harbor seal demonstrated a convincing case of vocal mimicry. However similar observations have not been reported since. Still shows under the right circumstances pinnipeds may use auditory experience, in addition to environmental consequences such as food reinforcement and social feedback to modify their vocal emissions.
In a 1992 study, Robert Gisiner and Ronald J. Schusterman conducted experiments in which they attempted to teach Rocky, a female California sea lion, syntax. Rocky was taught signed words, then she was asked to perform various tasks dependent on word order after viewing a signed instruction. It was found that Rocky was able to determine relations between signs and words, and form a basic form of syntax.  A 1993 study by Ronald J Schusterman and David Kastak found that the California sea lion was capable of understanding abstract concepts such as symmetry, sameness and transitivity. This provides a strong backing to the theory that Equivalence relations can form without language.
The distinctive sound of sea lions is produced both above and below water. To mark territory, sea lions “bark,” with non-alpha males making more noise than alphas. While females also bark, they do so less frequently and most often in connection with birthing pups or caring for their young. Females produce a highly directional bawling vocalization, the pup attraction call which helps mother and pup locate one another. As noted in Animal Behavior, their amphibious life style has made them need acoustic communication for social organization while on land.
Sea Lions can hear frequencies as low as 100 Hz and as high as 40,000 Hz and vocalize between the ranges of 100 to 10,000 Hz.
- Mustached Bats: Since these animals spend most of their lives in the dark, they rely heavily on their auditory system to communicate. This acoustic communication includes echolocation or using calls to locate each other in the darkness. Studies have shown that Mustached Bats use a wide variety of calls to communicate with one another. These calls include 33 different sounds, or "syllables," that the bats then either use alone or combine in various ways to form "composite" syllables.
Comparison of the term with "animal communication" 
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It is worth distinguishing "animal language" from "animal communication", no matter how complex the latter may be. In general the term "animal language" is reserved for the modeling of human language in animal systems; though there is some comparative interchange in certain cases (e.g. Cheney & Seyfarth's vervet monkey call studies). Thus "animal language" typically does not include bee dancing, bird song, whale song, dolphin signature whistles, prairie dogs, nor the communicative systems found in most social mammals. The features of language as listed above are a dated formulation by Hockett in 1960. Through this formulation Hockett made one of the earliest attempts to break down features of human language for the purpose of applying Darwinian gradualism. Although an influence on early animal language efforts (see below), is today not considered the key architecture at the core of "animal language" research.
Animal Language results are controversial for several reasons. (For a related controversy, see also Clever Hans.) In the 1970s John Lilly was attempting to "break the code": to fully communicate ideas and concepts with wild populations of dolphins so that we could "speak" to them, and share our cultures, histories, and more. This effort failed. The very early [chimpanzee] work was with chimpanzee infants raised as if they were human; a test of the nature vs. nurture hypothesis. Chimpanzees have a laryngeal structure very different from that of humans, as well as no voluntary control of their breathing. This combination made it very difficult for the chimpanzees to reproduce the vocal intonations required for human language. Researchers eventually moved towards a gestural (sign language) modality, as well as "keyboard" devices laden with buttons adorned with symbols (known as "lexigrams") that the animals could press to produce artificial language. Other chimpanzees learned by observing human subjects performing the task. This latter group of researchers studying chimpanzee communication through symbol recognition (keyboard) as well as through the use of sign language (gestural), are on the forefront of communicative breakthroughs in the study of animal language, and they are familiar with their subjects on a first name basis: Sarah, Lana, Kanzi, Koko, Sherman, Austin and Chantek.
Perhaps the best known critic of "Animal Language" is Herbert Terrace. Terrace's 1979 criticism using his own research with the chimpanzee Nim Chimpsky was scathing and basically spelled the end of animal language research in that era, most of which emphasized the production of language by animals. In short, he accused researchers of over-interpreting their results, especially as it is rarely parsimonious to ascribe true intentional "language production" when other simpler explanations for the behaviors (gestural hand signs) could be put forth. Also, his animals failed to show generalization of the concept of reference between the modalities of comprehension and production; this generalization is one of many fundamental ones that are trivial for human language use. The simpler explanation according to Terrace was that the animals had learned a sophisticated series of context-based behavioral strategies to obtain either primary (food) or social reinforcement, behaviors that could be over-interpreted as language use.
In 1985 during this anti-Animal Language backlash, Louis Herman published an account of artificial language in the bottlenosed dolphin in the journal Cognition. A major difference between Herman's work and previous research was his emphasis on a method of studying language comprehension only (rather than language comprehension and production by the animal(s)), which enabled rigorous controls and statistical tests, largely because he was limiting his researchers to evaluating the animals' physical behaviors (in response to sentences) with blinded observers, rather than attempting to interpret possible language utterances or productions. The dolphins' names here were Akeakamai and Phoenix. Irene Pepperberg used the vocal modality for language production and comprehension in an African Grey Parrot named Alex in the verbal mode, and Sue Savage-Rumbaugh continues to study Bonobos such as Kanzi and Panbanisha. R. Schusterman duplicated many of the dolphin results in his California Sea Lions ("Rocky"), and came from a more behaviorist tradition than Herman's cognitive approach. Schusterman's emphasis is on the importance on a learning structure known as "equivalence classes."
However, overall, there has not been any meaningful dialog between the linguistics and animal language spheres, despite capturing the public's imagination in the popular press. Also, the growing field of language evolution is another source of future interchange between these disciplines. Most primate researchers tend to show a bias toward a shared pre-linguistic ability between humans and chimpanzees, dating back to a common ancestor, while dolphin and parrot researchers stress the general cognitive principles underlying these abilities. More recent related controversies regarding animal abilities include the closely linked areas of Theory of mind, Imitation (e.g. Nehaniv & Dautenhahn, 2002), Animal Culture (e.g. Rendell & Whitehead, 2001), and Language Evolution (e.g. Christiansen & Kirby, 2003).
There has been a recent emergence in animal language research which has contested the idea that animal communication is less sophisticated than human communication. Denise Herzing has done research on dolphins in the Bahamas whereby she created a two-way conversation via a submerged keyboard. The keyboard allows divers to communicate with wild dolphins. By using sounds and symbols on each key the dolphins could either press the key with their nose or mimic the whistling sound emitted in order to ask humans for a specific prop. This ongoing experiment has shown that in non-linguistic creatures brilliant and rapid thinking does occur despite our previous conceptions of animal communication. Further research done with Kanzi using lexigrams has strengthened the idea that animal communication is much more complex then we once thought.
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- Discussion: Starling Study: Recursion (Linguist List)
- International Bioacoustics Council research on animal language.
- The Animal Communication Project. More information on animal communication.
- Excellent compendium of links to the websites of all of the major animal language studies
- Listen to Nature includes article "The Language of Birds"
- Jarvis Lab homepage Evolution of Brain Structure for Vocal Learning