Animal communication

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Metacommunications are signals that modify the meaning of subsequent signals. One example is the 'play face' and tail signals in dogs, which indicate that a subsequent aggressive signal is part of a play fight rather than a serious aggressive episode.
A squirrel communicating with its pup

Animal communication is any transfer of information on the part of one or more animals that has an effect on the current or future behaviour of another animal. The study of animal communication — sometimes called zoosemiotics (defined as the study of sign communication or semiosis in animals; distinguishable from anthroposemiotics, the study of human communication) — has played an important part in ethology, sociobiology, and the study of animal cognition.

Animal communication is a rapidly growing area of study. Even in the 21st century, many prior understandings related to diverse fields such as personal symbolic name use, animal emotions, animal culture, learning and even animal sexual behaviour, long thought to be well understood, have been revolutionized.

Modes

A lamb investigates a rabbit, an example of interspecific communication using body posture and olfaction.

Visual

Gestures: The best known form of communication involves the display of distinctive body parts, or distinctive bodily movements; often these occur in combination, so a distinctive movement acts to reveal or emphasize a distinctive body part. For example, the presentation of a parent Herring Gull’s bill to its chick signals feeding time. Like many gulls, the Herring Gull has a brightly coloured bill, yellow with a red spot on the lower mandible near the tip. When it returns to the nest with food, the parent stands over its chick and taps the bill on the ground in front of it; this elicits a begging response from a hungry chick (pecking at the red spot), which stimulates the parent to regurgitate food in front of it. The complete signal therefore involves a distinctive morphological feature (body part), the red-spotted bill, and a distinctive movement (tapping towards the ground) which makes the red spot highly visible to the chick. While all primates use some form of gesture,1 Frans de Waal came to the conclusion that apes and humans are unique in that only they are able use intentional gestures to communicate. He tested the hypothesis of gesture evolving into language by studying the gestures of bonobos and chimps.
Facial expression: Facial gestures play an important role in animal communication. Dogs for example express anger through snarling and showing their teeth. In alarm their ears will perk up. When fearful, dogs will pull back their ears, expose their teeth slightly and squint their eyes. Jeffrey Mogil studied the facial expressions of mice during increments of increasing pain; there were five recognizable facial expressions; orbital tightening, nose and cheek bulge, and changes in ear and whisker carriage.2
Gaze following: Coordination among social animals is facilitated by monitoring of each other's head and eye orientation. Long recognized in human developmental studies as an important component of communication, there has recently begun to be much more attention on the abilities of animals to follow the gaze of those they interact with, whether members of their own species or humans. Studies have been conducted on apes, monkeys, dogs, birds, and tortoises, and have focused on two different tasks: "follow[ing] another’s gaze into distant space" and "follow[ing] another’s gaze geometrically around a visual barrier e.g. by repositioning themselves to follow a gaze cue when faced with a barrier blocking their view". The first ability has been found among a broad range of animals, while the second has been demonstrated only for apes, dogs (and wolves), and corvids (ravens), and attempts to demonstrate this "geometric gaze following" in marmosets and ibis gave negative results. Researchers do not yet have a clear picture of the cognitive basis of gaze following abilities, but developmental evidence indicates that "simple" gaze following and "geometric" gaze following are likely to rely on distinct cognitive foundations.3
Active visual displays. Some cephalopods, such as the octopus and the cuttlefish, have specialized skin cells (chromatophores) that can change the apparent colour, opacity, and reflectiveness of their skin.4 In addition to being used for camouflage, rapid changes in skin colour are used while hunting and in courtship rituals.5 The colour changes in cuttlefish can be especially intricate as they are able to communicate two entirely different signals simultaneously from opposite sides of their body. When a male cuttlefish courts a female in the presence of other males, he displays two different sides: a male pattern facing the female, and a female pattern facing away, to deceive other males.6
Passive visual displays. Many animals communicate information about themselves without necessarily changing their behaviour. For example, sexual dimorphism in size or pelage communicates which sex the animal is. Other passive signals can be cyclical in nature. For example, in olive baboons, the beginning of the female's ovulation is a signal to the males that she is ready to mate. During ovulation, the skin of the female's anogenital area swells and turns a bright red/pink.7
Bioluminescent communication. Communication by the production of light occurs commonly in vertebrates and invertebrates in the oceans, particularly at depths (e.g. angler fish). Two well known forms of land bioluminescence are fireflies and glow worms. Other insects, insect larvae, annelids, arachnids and even species of fungi possess bioluminescent abilities. Some bioluminescent animals produce the light themselves whereas others have a symbiotic relationship with bioluminescent bacteria. (See also: List of bioluminescent organisms)

Auditory

Many animals communicate through vocalizations. Communication through vocalization is essential for many tasks including mating rituals, warning calls, conveying location of food sources, and social learning. Male mating calls are used to signal the female and to beat competitors in species such as hammer-headed bats, red deers, humpback whales and elephant seals.8 In whale species whale song has been found to have different dialects based on location.9 Other instances of communication include the warning cries of the Campbell monkey,10 the territorial calls of gibbons, the use of frequency in greater spear-nosed bats to distinguish between groups.11 Another example of an animal that uses auditory warning cries is the Vervet monkey. The Vervet monkey gives an acoustically distinct warning cry when each of its four different predators is sighted. Whichever warning cry is issued influences how the other monkeys react. For example, the Vervet monkey has separate warning cries for pythons and eagles because the response to each predator has to be different. If the warning cry for python is given then the monkeys climb trees to be out of reach of the snake, and if the warning cry for eagle is given then the monkeys get low to the ground and try to hide underneath of things. It is important for the different warning cries to be distinguished so that the monkeys can recognize the calls and have the appropriate response. Prairie dogs also have very complex communication when it comes to predator communication. According to Con Slobodchikoff and others, prairie dog calls contain specific information as to what the predator is, how big it is and how fast it is approaching. This is by no means a complete list, there are many different animals that use auditory communication.

Olfactory

Less obvious to most humans is olfactory communication. Many mammals, in particular, have glands that generate distinctive and long-lasting smells, and have corresponding behaviors that leave these smells in places where they have been. Often the scented substance is introduced into urine or feces. Sometimes it is distributed through sweat, though this does not leave a semi-permanent mark as scents deposited on the ground do. Some animals have glands on their bodies whose sole function appears to be to deposit scent marks: for example Mongolian gerbils have a scent gland on their stomachs, and a characteristic ventral rubbing action that deposits scent from it. Golden hamsters and cats have scent glands on their flanks, and deposit scent by rubbing their sides against objects; cats also have scent glands on their foreheads. Bees carry with them a pouch of material from the hive which they release as they re-enter, the smell of which indicates that they are a member of the hive and grants their safe entry. Ants use pheromones to create scent trails to food as well as for alarm calls, mate attraction and to distinguish between colonies. Additionally, they have pheromones that are used to confuse an enemy and manipulate them into fighting with each other.12 Some fish such as minnows communicate by chemical signals detected by olfactory nerves.

Electro

A rare form of animal communication is electrocommunication. It is seen primarily in aquatic animals, though some land mammals, notably the platypus and echidnas are capable of electroreception and thus theoretically of electrocommunication.13

Seismic

Sometimes called vibrational communication, this mode of communication describes the conveying of information through seismic vibrations of the substrate. The substrate may be the earth, a plant stem or leaf, the surface of a body of water, a spider’s web, a honeycomb, or any of the myriad types of soil substrates. Vibrational communication is an ancient sensory modality and it is widespread in the animal kingdom where it has evolved several times independently. It has been reported in mammals, birds, reptiles, amphibians, insects, arachnids, crustaceans and nematode worms.14 Vibrations and other communication channels are not necessarily mutually exclusive, but can be used in multi-modal communication. Vibrational gestures have also been observed in insects. An example of this is seen by bees in their waggle dance. A bee that has found food performs this waggle dance to the other bees in the hive to direct them to where the food is. The bee repeatedly walks in a figure 8 motion while shaking its abdomen. The direction and waggle duration corresponds to the direction and distance to the food from the hive. This tells the other bees exactly how to find the food once they leave the hive.

Autocommunication

Autocommunication is a term used when the sender and receiver of the signal is the same animal. Examples include echolocation in bats and dolphins, and active electrolocation in weakly electric fish.

Functions

While there are as many kinds of communication as there are kinds of social behaviour, a number of functions have been studied in particular detail. They include:

  • Agonistic interactions: everything to do with contests and aggression between individuals. Many species have distinctive threat displays that are made during competition over food, mates or territory; much bird song functions in this way. Often there is a matched submission display, which the threatened individual will make if it is acknowledging the social dominance of the threatener; this has the effect of terminating the aggressive episode and allowing the dominant animal unrestricted access to the resource in dispute. Some species also have affiliative displays which are made to indicate that a dominant animal accepts the presence of another.
  • Mating rituals: signals made by members of one sex to attract or maintain the attention of potential mate, or to cement a pair bond. These frequently involve the display of body parts, body postures (gazelles assume characteristic poses as a signal to initiate mating), or the emission of scents or calls, that are unique to the species, thus allowing the individuals to avoid mating with members of another species which would be infertile. Animals that form lasting pair bonds often have symmetrical displays that they make to each other: famous examples are the mutual presentation of reeds by Great Crested Grebes, studied by Julian Huxley, the triumph displays shown by many species of geese and penguins on their nest sites and the spectacular courtship displays by birds of paradise and manakins.
  • Ownership/territorial: signals used to claim or defend a territory, food, or a mate.
  • Food-related signals: many animals make "food calls" that attract a mate, or offspring, or members of a social group generally to a food source. When parents are feeding offspring, the offspring often have begging responses (particularly when there are many offspring in a clutch or litter - this is well known in altricial songbirds, for example). Perhaps the most elaborate food-related signal is the Waggle dance of honeybees studied by Karl von Frisch. Young ravens signal to older, more experienced ravens when they come across new or untested food. Pheromones can also be released by many social insects to lead the other members of the society to the food source. Ants are one of the best examples of pheromone use. Ants leave a pheromone trail on the ground that can be followed by other ants to lead them to the food source.
  • Alarm calls: signals made in the presence of a threat from a predator, allowing all members of a social group (and often members of other species) to run for cover, become immobile, or gather into a group to reduce the risk of attack. Alarm signals are not always vocalized. An example of this is that crushed ants release an alarm pheromone that sends nearby ants into an attack frenzy and attracts more ants from further away.
  • Meta-communications: signals that modify the meaning of subsequent signals. One example is the 'play face' in dogs which signals that a subsequent aggressive signal is part of a play fight rather than a serious aggressive episode.

Interpretation

While many gestures and actions have common, stereotypical meanings, animal communication is often more complex and subtle than at first seems; the same gesture may have multiple meanings depending on context and other behaviours. Because of this, generalizations such as "X means Y" are often, but not always accurate. For example, even a domestic dog's simple tail wag may be used in subtly different ways to convey many meanings as illustrated in Charles Darwin's The Expression of the Emotions in Man and Animals published in 1872.

Combined with other body language, in a specific context, many gestures e.g. yawns, direction of vision, all convey meaning. Thus statements that a particular action "means" something, should always be interpreted as "often means". As with human beings, who may smile or hug or stand a particular way for multiple reasons, many animals also re-use gestures.15

A European starling (Sturnus vulgaris) singing.

Intraspecific

Much animal communication occurs between members of the species and this is the context in which it has been most intensively studied. Most of the forms and functions of communication described above are relevant to intraspecific communication.

Interspecific

Many examples of communication take place between members of different species. Animals communicate to other animals with various signs: visual, sound, echolocation, vibrations, body language, and smell.

Prey to predator

If a prey animal moves, makes a noise or vibrations, or emits a smell in such a way that a predator can detect it, this is consistent with the definition of "communication" given above. This type of communication is known as interceptive eavesdropping, where a predator intercepts the message being conveyed to conspecifics.

This Chihuahua is baring his teeth to signify an attack is imminent if the photographer comes closer to take his bone.

There are however, some actions of prey species that are clearly communications to actual or potential predators. A good example is warning colouration: species such as wasps that are capable of harming potential predators are often brightly coloured, and this modifies the behaviour of the predator, who either instinctively or as the result of experience will avoid attacking such an animal. Some forms of mimicry fall in the same category: for example hoverflies are coloured in the same way as wasps, and although they are unable to sting, the strong avoidance of wasps by predators gives the hoverfly some protection. There are also behavioural changes that act in a similar way to warning colouration. For example, canines such as wolves and coyotes may adopt an aggressive posture, such as growling with their teeth bared, to indicate they will fight if necessary, and rattlesnakes use their well-known rattle to warn potential predators of their venomous bite. Sometimes, a behavioural change and warning colouration will be combined, as in certain species of amphibians which have most of their body coloured to blend with their surroundings, except for a brightly coloured belly. When confronted with a potential threat, they show their belly, indicating that they are poisonous in some way.

Another example of prey to predator communication is the pursuit-deterrent signal. Pursuit-deterrent signals occur when prey indicates to a predator that pursuit would be unprofitable because the signaler is prepared to escape. Pursuit-deterrent signals provide a benefit to both the signaler and receiver; they prevent the sender from wasting time and energy fleeing, and they prevent the receiver from investing in a costly pursuit that is unlikely to result in capture. Such signals can advertise prey’s ability to escape, and reflect phenotypic condition (quality advertisement), or can advertise that the prey has detected the predator (perception advertisement). Pursuit-deterrent signals have been reported for a wide variety of taxa, including fish (Godin and Davis, 1995), lizards (Cooper et al., 2004), ungulates (Caro, 1995), rabbits (Holley 1993), primates (Zuberbuhler et al. 1997), rodents (Shelley and Blumstein 2005, Clark, 2005), and birds (Alvarez, 1993, Murphy, 2006, 2007). A familiar example of quality advertisement pursuit-deterrent signal is stotting (sometimes called pronking), a pronounced combination of stiff-legged running while simultaneously jumping shown by some antelopes such as Thomson's gazelle in the presence of a predator. At least 11 hypotheses for stotting have been proposed. A leading theory today is that it alerts predators that the element of surprise has been lost. Predators like cheetahs rely on surprise attacks, proven by the fact that chases are rarely successful when antelope stot. Predators do not waste energy on a chase that will likely be unsuccessful (optimal foraging behaviour). Quality advertisement can be communicated by modes other than visual. The banner-tailed kangaroo rat produces several complex foot-drumming patterns in a number of different contexts, one of which is when it encounters a snake. The foot-drumming may alert nearby offspring but most likely conveys vibrations through the ground that the rat is too alert for a successful attack, thus preventing the snake's predatory pursuit.16

Predator to prey

Humpback anglerfish

Typically, predators attempt to reduce communication to prey as this will generally reduce the effectiveness of their hunting. However, some forms of predator to prey communication occur in ways that change the behaviour of the prey and make their capture easier, i.e. deception by the predator. A well-known example is the angler fish, which has a fleshy bioluminescent growth protruding from its forehead and dangling in front of its jaws; smaller fish attempt to take the lure and by doing so, place themselves in a better position for the angler fish to catch them. Another example of deceptive communication is observed in the genus of jumping spiders (Myrmarachne). These spiders are commonly referred to as “antmimicking spiders” because of the way they wave their front legs in the air to simulate antennae.

Human/animal

Various ways in which humans interpret the behaviour of domestic animals, or give commands to them, are consistent with the definition of interspecies communication. Depending on the context, they might be considered to be predator to prey communication, or to reflect forms of commensalism. The recent experiments on animal language are perhaps the most sophisticated attempt yet to establish human/animal communication, though their relation to natural animal communication is uncertain. Lacking in the study of human-animal communication is a focus on expressive communication from animal to human specifically. Horses are taught not to communicate (for safety).citation needed Dogs and horses are generally not encouraged to communicate expressively, but are encouraged to develop receptive language (understanding). Since the late 1990s, one scientist, Sean Senechal, has been developing, studying, and using the learned visible, expressive language in dogs and horses. By teaching these animals a gestural (human made) American Sign Language-like language, the animals have been found to use the new signs on their own to get what they need.17

Other aspects

Evolution

A male peacock displaying the eye-spots on his tail

The importance of communication is evident from the highly elaborate morphology, behaviour and physiology that some animals have evolved to facilitate this. These include some of the most striking structures in the animal kingdom, such as the peacock's tail, the antlers of a stag and the frill of the frill-necked lizard, but also include even the modest red spot on a European Herring Gull's bill. Highly elaborate behaviours have evolved for communication such as the dancing of cranes, the pattern changes of cuttlefish, and the gathering and arranging of materials by bowerbirds. Other evidence for the importance of communication in animals is the prioritisation of physiological features to this function, for example, birdsong appears to have brain structures entirely devoted to its production. All these adaptations require evolutionary explanation.

There are two aspects to the required explanation:

  • identifying a route by which an animal that lacked the relevant feature or behaviour could acquire it;
  • identifying the selective pressure that makes it adaptive for animals to develop structures that facilitate communication, emit communications, and respond to them.

Significant contributions to the first of these problems were made by Konrad Lorenz and other early ethologists. By comparing related species within groups, they showed that movements and body parts that in the primitive forms had no communicative function could be "captured" in a context where communication would be functional for one or both partners, and could evolve into a more elaborate, specialised form. For example, Desmond Morris showed in a study of grass finchescitation needed that a beak-wiping response occurred in a range of species, serving a preening function, but that in some species this had been elaborated into a courtship signal.

The second problem has been more controversial. The early ethologists assumed that communication occurred for the good of the species as a whole, but this would require a process of group selection which is believed to be mathematically impossible in the evolution of sexually reproducing animals. Altruism towards an unrelated group is not widely accepted in the scientific community, but rather can be seen as reciprocal altruism, expecting the same behaviour from others, a benefit of living in a group. Sociobiologists argued that behaviours that benefited a whole group of animals might emerge as a result of selection pressures acting solely on the individual. A gene-centered view of evolution proposes that behaviours that enabled a gene to become wider established within a population would become positively selected for, even if their effect on individuals or the species as a whole was detrimental;18

In the case of communication, an important discussion by John Krebs and Richard Dawkins established hypotheses for the evolution of such apparently altruistic or mutualistic communications as alarm calls and courtship signals to emerge under individual selection. This led to the realisation that communication might not always be "honest" (indeed, there are some obvious examples where it is not, as in mimicry). The possibility of evolutionarily stable dishonest communication has been the subject of much controversy, with Amotz Zahavi in particular arguing that it cannot exist in the long term. Sociobiologists have also been concerned with the evolution of apparently excessive signalling structures such as the peacock's tail; it is widely thought that these can only emerge as a result of sexual selection, which can create a positive feedback process that leads to the rapid exaggeration of a characteristic that confers an advantage in a competitive mate-selection situation.

One theory to explain the evolution of traits like a peacock's tail is 'runaway selection'. This requires two traits - a trait that exists, like the bright tail, and a prexisting bias in the female to select for that trait. Females prefer the more elaborate tails, and thus those males are able to mate successfully. Exploiting the psychology of the female, a positive feedback loop is enacted and the tail becomes bigger and brighter. Eventually, the evolution will level off because the survival costs to the male do not allow for the trait to be elaborated any further. Two theories exist to explain runaway selection. The first is the good genes hypothesis. This theory states that an elaborate display is an honest signal of fitness and truly is a better mate. The second is the handicap hypothesis. This explains that the peacock's tail is a handicap, requiring energy to keep and makes it more visible to predators. Regardless, the individual is able to survive, even though its genes are not as good per se.

Cognitive aspects

Ethologists and sociobiologists have characteristically analysed animal communication in terms of more or less automatic responses to stimuli, without raising the question of whether the animals concerned understand the meaning of the signals they emit and receive. That is a key question in animal cognition. There are some signalling systems that seem to demand a more advanced understanding. A much discussed example is the use of alarm calls by vervet monkeys. Robert Seyfarth and Dorothy Cheney showed that these animals emit different alarm calls in the presence of different predators (leopards, eagles, and snakes), and the monkeys that hear the calls respond appropriately - but that this ability develops over time, and also takes into account the experience of the individual emitting the call. Metacommunication, discussed above, also seems to require a more sophisticated cognitive process.

It has been reported 19 that bottlenose dolphins can recognize identity information from whistles even when otherwise stripped of the characteristics of the whistle; making dolphins the only animals other than humans that have been shown to transmit identity information independent of the caller’s voice or location. The paper concludes that:

The fact that signature whistle shape carries identity information independent from voice features presents the possibility to use these whistles as referential signals, either addressing individuals or referring to them, similar to the use of names in humans. Given the cognitive abilities of bottlenose dolphins, their vocal learning and copying skills, and their fission–fusion social structure, this possibility is an intriguing one that demands further investigation.

—V. M. Janik, et al. 19

Animal communication and human behaviour

Another controversial issue is the extent to which humans have behaviours that resemble animal communication, or whether all such communication has disappeared as a result of our linguistic capacity. Some of our bodily features - eyebrows, beards and moustaches, deep adult male voices, perhaps female breasts - strongly resemble adaptations to producing signals. Ethologists such as Irenäus Eibl-Eibesfeldt have argued that facial gestures such as smiling, grimacing, and the eyebrow flash on greeting are universal human communicative signals that can be related to corresponding signals in other primates. Given the recency with which spoken language has emerged, it is very likely that human body language does include some more or less involuntary responses that have a similar origin to the communication we see in other animals.

Humans also often seek to mimic animals' communicative signals in order to interact with the animals. For example, cats have a mild affiliative response involving closing their eyes; humans often close their eyes towards a pet cat to establish a tolerant relationship. Stroking, petting and rubbing pet animals are all actions that probably work through their natural patterns of interspecific communication.

Dogs have shown an ability to understand communication from a species other than their own, specifically human communication. They were able to use human communicative gestures such as pointing and looking to find hidden food and toys.20

A new approach in the 21st century of studying animal communication uses applied behavioral analysis (ABA), specifically Functional Communication Training (FCT). This FCT previously has been used in schools and clinics with humans with special needs, such as children with autism, to help them develop language. Sean Senechal, at the AnimalSign Center has been using an approach similar to this FCT with domesticated animals, such as dogs (since 2004) and horses (since 2000) with encouraging results and benefits to the animals and people. Functional communication training for animals, Senechal calls "AnimalSign Language". This includes teaching communication through gestures (like simplified American sign language), Picture Exchange Communication System, tapping, and vocalisation. The process for animals includes simplified and modified techniques.

Animal communication and linguistics

For linguistics, the interest of animal communication systems lies in their similarities to and differences from human language:

  1. Human languages are characterized for having a double articulation (in the characterization of French linguist André Martinet). It means that complex linguistic expressions can be broken down in meaningful elements (such as morphemes and words), which in turn are composed of smallest phonetic elements that affect meaning, called phonemes. Animal signals, however, do not exhibit this dual structure.
  2. In general, animal utterances are responses to external stimuli, and do not refer to matters removed in time and space. Matters of relevance at a distance, such as distant food sources, tend to be indicated to other individuals by body language instead, for example wolf activity before a hunt, or the information conveyed in honeybee dance language. It is therefore unclear to what extent utterances are automatic responses and to what extent deliberate intent plays a part.
  3. In contrast to human language, animal communication systems are usually not able to express conceptual generalizations. (Cetaceans and some primates may be notable exceptions).
  4. Human languages combine elements to produce new messages (a property known as creativity). One factor in this is that much human language growth is based upon conceptual ideas and hypothetical structures, both being far greater capabilities in humans than animals. This appears far less common in animal communication systems, although current research into animal culture is still an ongoing process with many new discoveries.

A recent and interesting area of development is the discovery that the use of syntax in language, and the ability to produce "sentences", is not limited to humans either. The first good evidence of syntax in non-humans, reported21 in 2006, is from the greater spot-nosed monkey (Cercopithecus nictitans) of Nigeria. This is the first evidence that some animals can take discrete units of communication, and build them up into a sequence which then carries a different meaning from the individual "words":

The greater spot-nosed monkeys have two main alarm sounds. A sound known onomatopoeiacally as the "pyow" warns against a lurking leopard, and a coughing sound that scientists call a "hack" is used when an eagle is flying nearby.
"Observationally and experimentally we have demonstrated that this sequence [of up to three 'pyows' followed by up to four 'hacks'] serves to elicit group movement... the 'pyow-hack' sequence means something like 'let's go!' [a command telling others to move]... The implications are that primates at least may be able to ignore the usual relationship between an individual alarm call, and the meaning it might convey under certain circumstances... To our knowledge this is the first good evidence of a syntax-like natural communication system in a non-human species."

Similar results have also recently been reported in the Campbell's Mona Monkey.22

See also

References

  1. ^ de Waal
  2. ^ Langford, D.J. et al., (2010). Coding of facial expressions of pain in the laboratory mouse. Nature Methods, May 9th. pp. 1-3. doi:10.1038/nmeth.1455
  3. ^ Range F. and Virányi, Z. (2011). Development of gaze following abilities in wolves (Canis Lupus). PLoS ONE 6(2): e16888. doi:10.1371/journal.pone.0016888
  4. ^ Cloney, R.A. & E. Florey 1968. Ultrastructure of cephalopod chromatophore organs. Z. Zellforsch Mikrosk. Anat. 89: 250-280. PMID 5700268
  5. ^ Hanlon, R.T.; Messenger, J.B. (1996). Cephalopod Behaviour. Cambridge University Press. p. 121. ISBN 0-521-64583-2. 
  6. ^ Williams, Sarah (2012). "Two-faced fish tricks competitors". Science Now. Retrieved March 16, 2013. 
  7. ^ Motluk, Alison (2001). "Big Bottom". New Scientist 19 (7). 
  8. ^ Slabbekoorn, Hans, Smith, Thomas B. "Bird song, ecology and speciation." Philosophical Transactions: Biology Sciences 357.1420 (2002). 493-503.
  9. ^ Carey, Bjorn. Whales Found to Speak in Dialects. Live Science. 3 Jan. 2006.
  10. ^ Zuberbühler, Klause. "Predator-specific alarm calls in Campbell's monkeys, Cercopithecus campbelli." Behavioral Ecology and Sociobiology 50.5 (2001). 414-442
  11. ^ Boughman, Janette W. "Vocal learning by greater spear-nosed bats." Proceedings: Biological Sciences 265.1392 (1998). 227-233
  12. ^ Bickerton, Derek. Adam’s Tongue: How Humans Made Language, How Language Made Humans. New York, NY: Hill and Wang, 2009. Print
  13. ^ "Electrocommunication". Davidson College. Retrieved 2011-03-03. 
  14. ^ Hill, P.S.M., (2008). Vibrational Communication in Animals. Harvard, Cambridge, London
  15. ^ Burnie, David (1991). Animal Behavior Communication. New York, NY: Aladdin Books. ISBN 1-57335-167-9. 
  16. ^ "Web of Life:Vibrational communication in animals". Retrieved 8 December 2012. 
  17. ^ Sean Senechal: Dogs can sign, too. A breakthrough method of teaching your dog to communicate to you, 2009, Random House/Crown/TenSpeed Press
  18. ^ discussed at length by Richard Dawkins under the subject of his book The Selfish Gene
  19. ^ a b V. M. Janik, L. S. Sayigh, and R. S. Wells: "Signature whistle shape conveys identity information to bottlenose dolphins", Proceedings of the National Academy of Sciences, vol. 103 no 21, May 23, 2006
  20. ^ Hare, B., Call, J. & Tomasello, M.: "Communication of food location between human and dog (Canis familiaris).", Evolution of Communication, 2, 137–159, 1998.
  21. ^ The Times May 18, 2006, p.3
  22. ^ Brandon Kiem. "Rudiments of Language Discovered in Monkeys". Wiredscience. Retrieved 2013-03-15. 

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