In
animal communication
Animal communication is the transfer of information from one or a group of animals (sender or senders) to one or more other animals (receiver or receivers) that affects the current or future behavior of the receivers. Information may be sent int ...
, an alarm signal is an
antipredator adaptation
Anti-predator adaptations are mechanisms developed through evolution that assist prey organisms in their constant struggle against predators. Throughout the animal kingdom, adaptations have evolved for every stage of this struggle, namely by avo ...
in the form of
signal
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
s emitted by social animals in response to danger. Many
primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
s and
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
s have elaborate alarm calls for warning
conspecifics
Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species.
Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organis ...
of approaching predators. For example, the alarm call of the
blackbird is a familiar sound in many gardens. Other animals, like fish and insects, may use non-auditory signals, such as
chemical messages. Visual signs such as the white tail flashes of many deer have been suggested as alarm signals; they are less likely to be received by conspecifics, so have tended to be treated as a signal to the
predator
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
instead.
Different calls may be used for predators on the ground or from the air. Often, the animals can tell which member of the group is making the call, so that they can disregard those of little reliability.
Evidently, alarm signals promote survival by allowing the receivers of the alarm to escape from the source of peril; this can evolve by
kin selection
Kin selection is the evolutionary strategy that favours the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin altruism can look like Altruism in animals, altruistic behavio ...
, assuming the receivers are related to the signaller. However, alarm calls can increase individual fitness, for example by informing the predator it has been detected.
Alarm calls are often high-frequency sounds because these sounds are harder to localize.
Selective advantage
This cost/benefit tradeoff of alarm calling behaviour has sparked many interest debates among
evolutionary biologist
Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life for ...
s seeking to explain the occurrence of such apparently "self-sacrificing" behaviour. The central question is this: "If the ultimate purpose of any animal behaviour is to maximize the chances that an organism's own genes are passed on, with maximum fruitfulness, to future generations, why would an individual deliberately risk destroying itself (their entire genome) for the sake of saving others (other genomes)?".
Altruism
Some scientists have used the evidence of alarm-calling behaviour to challenge the theory that "evolution works only/primarily at the level of the
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
and of the gene's 'interest' in passing itself along to future generations." If alarm-calling is truly an example of
altruism
Altruism is the principle and moral practice of concern for the welfare and/or happiness of other human beings or animals, resulting in a quality of life both material and spiritual. It is a traditional virtue in many cultures and a core as ...
, then human understanding of natural selection becomes more complicated than simply "survival of the fittest gene".
Other researchers, generally those who support the
selfish gene theory
With gene defined as "not just one single physical bit of DNA utall replicas of a particular bit of DNA distributed throughout the world", the gene-centered view of evolution, gene's eye view, gene selection theory, or selfish gene theory ...
, question the authenticity of this "altruistic" behaviour. For instance, it has been observed that
vervet
The vervet monkey (''Chlorocebus pygerythrus''), or simply vervet, is an Old World monkey of the family Cercopithecidae native to Africa. The term "vervet" is also used to refer to all the members of the genus ''Chlorocebus''. The five distinct ...
s sometimes emit calls in the presence of a predator, and sometimes do not. Studies show that these vervets may call more often when they are surrounded by their own offspring and by other relatives who share many of their genes. Other researchers have shown that some forms of alarm calling, for example, "aerial predator whistles" produced by
Belding's ground squirrel
Belding's ground squirrel (''Urocitellus beldingi''), also called pot gut, sage rat or picket-pin, is a squirrel that lives on mountains in the western United States. In California, it often is found at in meadows between Lake Tahoe and Kings C ...
s, do not increase the chances that a caller will get eaten by a predator; the alarm call is advantageous to both caller and recipient by frightening and warding off the predator.
Predator attraction
Another theory suggests that alarm signals function to attract further predators, which fight over the prey organism, giving it a better chance of escape. Others still suggest they are a deterrent to predators, communicating the prey's alertness to the predator. One such case is the
western swamphen
The western swamphen (known as Purple Swamphen) (''Porphyrio porphyrio'') is a swamphen in the rail family Rallidae, one of the six species of purple swamphen. From the French name ''talève sultane'', it is also known as the sultana bird. This ...
(''Porphyrio porphyrio''), which gives conspicuous visual tail flicks (see also
aposematism
Aposematism is the advertising by an animal to potential predators that it is not worth attacking or eating. This unprofitability may consist of any defences which make the prey difficult to kill and eat, such as toxicity, venom, foul taste or ...
,
handicap principle
The handicap principle is a hypothesis proposed by the biologist Amotz Zahavi to explain how evolution may lead to "honest" or reliable signalling between animals which have an obvious motivation to bluff or deceive each other. It suggests that ...
and
stotting
Stotting (also called pronking or pronging) is a behavior of quadrupeds, particularly gazelles, in which they spring into the air, lifting all four feet off the ground simultaneously. Usually, the legs are held in a relatively stiff position. Ma ...
).
Further research
Considerable research effort continues to be directed toward the purpose and ramifications of alarm-calling behaviour, because, to the extent that this research has the ability to comment on the occurrence or non-occurrence of altruistic behaviour, these findings can be applied to the understanding of altruism in human behaviour.
Monkeys with alarm calls
Vervet monkeys
Vervet monkeys
The vervet monkey (''Chlorocebus pygerythrus''), or simply vervet, is an Old World monkey of the family Cercopithecidae native to Africa. The term "vervet" is also used to refer to all the members of the genus ''Chlorocebus''. The five distinct ...
(Chlorocebus Pygerythus) are some of the most studied monkeys when it comes to vocalization and alarm calls within the nonhuman primates. They are most known for making alarm calls in the presence of their most common predators (
leopards
The leopard (''Panthera pardus'') is one of the five extant species in the genus '' Panthera'', a member of the cat family, Felidae. It occurs in a wide range in sub-Saharan Africa, in some parts of Western and Central Asia, Southern Russia, ...
,
eagles
Eagle is the common name for many large birds of prey of the family Accipitridae. Eagles belong to several groups of genera, some of which are closely related. Most of the 68 species of eagle are from Eurasia and Africa. Outside this area, just ...
, and
snakes
Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more joi ...
). ''The alarm calls of the Vervet monkey are considered arbitrary in relation to the predator that it signifies. In the sense that the calls may be distinct to the threat that the monkeys are seeing but arbitrary in that it does not mimic the sounds of the predator. This is like yelling “Danger!” when seeing an angry dog instead of making barking sounds. This type of alarm calls is seen as the earliest example of symbolic communication'' (the relationship between signifier and signified is arbitrary and purely conventional) in nonhuman primates.
However, there is much debate on whether the vervet monkeys alarm calls are actual “words” in the sense of purposely manipulating sounds to communicate specific meaning or are unintentional sounds ''that are made when interacting with an outside stimulus. Like small children who cannot communicate words effectively make random noises when being played with or are stimulated by something in their immediate environment. As children grow and begin learning how to communicate the noises, they make are very broad in relation to their environment. They begin to recognize the things in their environment but there more things than known words or noises so a certain sound may reference multiple things. As children get older, they can become more specific about the noises and words made in relation to the things in their environment''. It is thought that as Vervet monkeys get older they are able to learn and break the broad categories into more specific sub categories to a specific context.
In an experiment conducted by Dr. Tabitha Price, they used custom software to gather the acoustic sounds of male and female Vervet monkeys from East Africa and male Vervet monkey from South Africa. The point of the experiment was to gather the acoustic sounds of these monkeys when stimulated by the presence of snakes (mainly
Python
Python may refer to:
Snakes
* Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia
** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia
* Python (mythology), a mythical serpent
Computing
* Python (pro ...
), raptors, terrestrial animals (mostly Leopards), and aggression. Then to determine if the calls could be distinguished with a known context.
''The experiment determined that while the Vervet monkeys were able to categorize different predators and members of different social groups, however their ability to communicate specific threats is not proven. The chirps and barks that Vervet monkeys make as an eagle swoop’s in are the same chirps and barks that are made in moments of high arousal. Similarly, the barks made for leopards are the same that are made during aggressive interactions''. ''The environment that they exist in is too complex for their ability to communicate about everything in their environment specifically.''.
In an experiment conducted by Dr. Julia Fischer, a
drone
Drone most commonly refers to:
* Drone (bee), a male bee, from an unfertilized egg
* Unmanned aerial vehicle
* Unmanned surface vehicle, watercraft
* Unmanned underwater vehicle or underwater drone
Drone, drones or The Drones may also refer to:
...
was flown over Vervet monkeys and recorded the sounds produced. The Vervet monkeys made alarm calls that were almost identical to the eagle calls of East African Vervets. When a sound recording of the drone was played back a few days later to a monkey that was alone and away from the main group it looked up and scanned the sky. Dr. Fischer concluded that Vervet monkeys can be exposed to a new threat once and understand what it means.
It is still debated whether or not Vervet monkeys are actually aware of what the alarm calls mean. ''One side of the argument is that the monkeys give alarm calls because they are simply excited. The other side of the argument is that the alarm calls create mental representation of predators in the listeners minds.'' The common middle ground argument is that they give alarm calls because they want others to elicit a certain response, not necessarily because they want the group to think that there is a specific threat near.
Ultimately there is not enough evidence to support whether or not the calls are simply identifying a threat or calling for specific action due to the threat.
Campbell's mona monkeys
Campbell's mona monkey
Campbell's mona monkey (''Cercopithecus campbelli''), also known as Campbell's guenon and Campbell's monkey, is a species of primate in the family Cercopithecidae found in the Ivory Coast, Gambia, Ghana, Guinea, Guinea-Bissau, Liberia, Senegal, ...
s also generate alarm calls, but in a different way than vervet monkeys. Instead of having discrete calls for each predator, Campbell monkeys have two distinct types of calls which contain different calls which consist in an acoustic continuum of affixes which change meaning. It has been suggested that this is a
homology
Homology may refer to:
Sciences
Biology
*Homology (biology), any characteristic of biological organisms that is derived from a common ancestor
* Sequence homology, biological homology between DNA, RNA, or protein sequences
*Homologous chrom ...
to human
morphology
Morphology, from the Greek and meaning "study of shape", may refer to:
Disciplines
* Morphology (archaeology), study of the shapes or forms of artifacts
* Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...
. Similarly, the
cotton-top tamarin
The cotton-top tamarin (''Saguinus oedipus'') is a small New World monkey weighing less than . This New World monkey can live up to 24 years, but most of them die by 13 years. One of the smallest primates, the cotton-top tamarin is easily recogn ...
is able to use a limited vocal range of alarm calls to distinguish between aerial and land predators.
[Neyman, P. F. 1978. Aspects of the ecology and social organization of free-ranging cotton-top tamarins (Saguinus oedipus) and the conservation status of the species. In: The Biology and Conservation of the Callitrichidae (Ed. by D. G. Kleiman), pp. 39e71. Washington, D.C.: ]Smithsonian Institution Press
The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums and education and research centers, the largest such complex in the world, created by the U.S. government "for the increase and diffusion of knowledge". Founded ...
. Both the Campbell monkey and the cotton-top tamarin have demonstrated abilities similar to vervet monkeys' ability to distinguish likely direction of predation and appropriate responses.
[Sproul et al. Cottontop tamarin,'' Saguinus oedipus'', alarm calls contain sufficient information for recognition of individual identity. ]Animal Behaviour
Ethology is the scientific study of animal behaviour, usually with a focus on behaviour under natural conditions, and viewing behaviour as an evolutionarily adaptive trait. Behaviourism as a term also describes the scientific and objectiv ...
(2006) vol. 72 (6) pp. 1379-1385
That these three
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
use vocalizations to warn others of danger has been called by some proof of
proto-language
In the tree model of historical linguistics, a proto-language is a postulated ancestral language from which a number of attested languages are believed to have descended by evolution, forming a language family. Proto-languages are usually unattest ...
in
primates
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
. However, there is some evidence that this behavior does not refer to the predators themselves but to threat, distinguishing calls from words.
Barbary macaque
Another species that exhibits alarm calls is the
Barbary macaque
The Barbary macaque (''Macaca sylvanus''), also known as Barbary ape, is a macaque species native to the Atlas Mountains of Algeria, Libya, Tunisia and Morocco, along with a small introduced population in Gibraltar.
It is the type species of the ...
. Barbary macaque mothers are able to recognize their own offspring's calls and behave accordingly.
Diana monkeys
Diana monkey
The Diana monkey (''Cercopithecus diana'') is an Old World monkey found in the high canopy forests in Sierra Leone, Liberia, and western Côte d’Ivoire. Named for its white brow which is said to resemble the bow of the Roman goddess Diana, thi ...
s also produce alarm signals. Adult males respond to each other's calls, showing that calling can be contagious.
Their calls differ based on signaller sex, threat type, habitat, and caller
ontogenetic
Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the st ...
or lifetime predator experience.
Diana monkeys emit different alarm calls as a result of their sex. Male alarm calls are primarily used for resource defence, male–male competition, and communication between groups of conspecifics. Female alarm calls are mainly used for communication within groups of conspecifics to avoid predation.
Alarm calls are also predator-specific. In
Taï National Park
Taï National Park () is a national park in Côte d'Ivoire that contains one of the last areas of primary rainforest in West Africa. It was inscribed as a World Heritage Site in 1982 due to the diversity of its flora and fauna. Five mammal sp ...
,
Côte d'Ivoire
Ivory Coast, also known as Côte d'Ivoire, officially the Republic of Côte d'Ivoire, is a country on the southern coast of West Africa. Its capital is Yamoussoukro, in the centre of the country, while its largest city and economic centre is ...
, Diana monkeys are preyed on by leopards, eagles, and chimpanzees, but only emit alarm calls for leopards and eagles.
When threatened by chimpanzees, they use silent, cryptic behaviour and when threatened by leopards or eagles, they emit predator-specific alarm signals.
When researchers play recordings of alarm calls produced by chimpanzees in response to predation by leopards, about fifty per cent of nearby Diana monkeys switch from a chimpanzee antipredator response to a leopard antipredator response.
The tendency to switch responses is especially prominent among Diana monkey populations that live within the main range of the chimpanzee community.
This shift in antipredator response suggests that the monkeys interpret chimpanzee-produced, leopard-induced alarm calls as evidence for the presence of a leopard.
When the same monkeys are then played recordings of leopard growls, their reactions confirm that they had anticipated the presence of a leopard.
There are three possible cognitive mechanisms explaining how Diana monkeys recognize chimpanzee-produced, leopard-induced alarm calls as evidence for a nearby leopard:
associative learning
Learning is the process of acquiring new understanding, knowledge, behaviors, skills, values, attitudes, and preferences. The ability to learn is possessed by humans, animals, and some machines; there is also evidence for some kind of learnin ...
,
causal reasoning
Causal reasoning is the process of identifying causality: the relationship between a cause and its effect. The study of causality extends from ancient philosophy to contemporary neuropsychology; assumptions about the nature of causality may be ...
, or a specialized learning programme driven by adaptive antipredator behaviour necessary for survival.
In Taï National Park and
Tiwai Island
Tiwai Island ( Mende for 'Big Island') is a wildlife sanctuary and tourist site in Sierra Leone. Run by the non-governmental organization Environmental Foundation for Africa, Tiwai is 12 square kilometers in area and located on the Moa River in ...
,
Sierra Leone
Sierra Leone,)]. officially the Republic of Sierra Leone, is a country on the southwest coast of West Africa. It is bordered by Liberia to the southeast and Guinea surrounds the northern half of the nation. Covering a total area of , Sierra ...
, specific acoustic markers in the alarm calls of Diana monkeys convey both threat type and caller familiarity information to a receiver. In Taï National Park, males respond to eagle alarm signals based on predator type and caller familiarity. When the caller is unfamiliar to the receiver, the response call is a 'standard' eagle alarm call, characterized by a lack of frequency transition at the onset of the call.
When the caller is familiar, the response call is an atypical eagle alarm call, characterized by a frequency transition at onset, and the response is faster than to that of an unfamiliar caller.
On Tiwai Island, males respond in the opposite way to eagle alarm signals.
When the caller is familiar, the response call is a 'standard' eagle alarm call, without a frequency transition at onset. When the caller is unfamiliar, the response call is an atypical eagle alarm call, with a frequency transition at onset.
The differences in alarm call responses are due to differences in habitat. In Taï National Park, there is a low predation risk from eagles, high primate abundance, strong intergroup competition, and a tendency for group encounters to result in high levels of aggression.
Therefore, even familiar males are a threat to whom males respond with aggression and an atypical eagle alarm call.
Only unfamiliar males, who are likely to be solitary and non-threatening, do not receive an aggressive response and receive only a typical alarm call.
On Tiwai Island, there is a high predation risk from eagles, low primate abundance, a tendency for group encounters to result in peaceful retreats, low resource competition, and frequent sharing of foraging areas.
Therefore, there is a lack of aggression towards familiar conspecifics to whom receivers respond with a 'standard' eagle call.
There is only aggression towards unfamiliar conspecifics, to whom receivers respond with an atypical call.
Simply put, a response with a typical eagle alarm call prioritizes the risk of predation, while a response with an atypical alarm call prioritizes social aggression.
Diana monkeys also display a predisposition for flexibility in acoustic variation of alarm call assembly related to caller
ontogenetic
Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the st ...
or lifetime predator experience. In Taï National Park and on Tiwai Island, monkeys have a predisposition to threat-specific alarm signals.
In Taï National Park, males produce three threat-specific calls in response to three threats: eagles, leopards, and general disturbances.
On Tiwai Island, males produce two threat-specific calls in response to two groups of threats: eagles, and leopards or general disturbances.
The latter are likely grouped together because leopards have not been present on the island for at least 30 years.
Other primates, such as Guereza monkeys and
putty-nosed monkey
The greater spot-nosed monkey or putty-nosed monkey (''Cercopithecus nictitans'') is one of the smallest Old World monkeys. It is a guenon of the '' C. mitis'' group, native to West Africa and living to some extent in rain forests, but more often ...
s, also have two main predator-specific assemblies of alarm calls.
Predator-specific alarm signals differ based on call sequence assembly. General disturbances in Taï National Park and both general disturbances and leopards on Tiwai Island result in alarm calls assembled into long sequences.
Conversely, leopards in Taï National Park result in alarm calls that typically begin with voiced inhalations followed by a small number of calls.
These differences in alarm call arrangement between habitats are due to ontogenetic experience; specifically, a lack of experience with leopards on Tiwai Island causes them to be classified in the same predator category as general disturbances, and accordingly, leopards receive the same type of alarm call arrangement.
Sexual selection for predator-specific alarm signals
In
guenon
The guenons (, ) are Old World monkeys of the genus ''Cercopithecus'' (). Not all members of this genus have the word "guenon" in their common names; also, because of changes in scientific classification, some monkeys in other genera may have co ...
s, selection is responsible for the evolution of predator-specific alarm calls from loud calls. Loud calls travel long distances, greater than that of the home range, and can be used as beneficial alarm calls to warn conspecifics or showcase their awareness of and deter a predator.
A spectrogram of a subadult male call shows that the call is a composition of elements from a female alarm call and male loud call, suggesting the transition from the latter to the former during puberty and suggesting that alarm calls gave rise to loud calls through
sexual selection
Sexual selection is a mode of natural selection in which members of one biological sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of t ...
.
Evidence of sexual selection in loud calls includes structural adaptations for long-range communication, co-incidence of loud calls and sexual maturity, and sexual dimorphism in loud calls.
Controversy over the semantic properties of alarm calls
Not all scholars of animal communication accept the interpretation of alarm signals in monkeys as having semantic properties or transmitting "information". Prominent spokespersons for this opposing view are
Michael Owren and Drew Rendall, whose work on this topic has been widely cited and debated. The alternative to the semantic interpretation of monkey alarm signals as suggested in the cited works is that animal communication is primarily a matter of influence rather than information, and that vocal alarm signals are essentially emotional expressions influencing the animals that hear them. In this view monkeys do not designate predators by naming them, but may react with different degrees of vocal alarm depending on the nature of the predator and its nearness on detection, as well as by producing different types of vocalization under the influence of the monkey's state and movement during the different types of escape required by different predators. Other monkeys may learn to use these emotional cues along with the escape behaviour of the alarm signaller to help make a good decision about the best escape route for themselves, without there having been any naming of predators.
Chimpanzees with alarm calls
Chimpanzee
The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative th ...
s emit alarm calls in response to predators, such as leopards and snakes.
They produce three types of alarm calls: acoustically-variable 'hoos', 'barks', and 'SOS screams'.
Alarm signalling is impacted by receiver knowledge and caller age, can be coupled with receiver monitoring, and is important to the understanding of the evolution of
hominoid
Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister g ...
communication.
Receiver knowledge
Alarm signalling varies depending on the receiver's knowledge of a certain threat. Chimpanzees are significantly more likely to produce an alarm call when conspecifics are unaware of a potential threat or were not nearby when a previous alarm call was emitted.
When judging if conspecifics are unaware of potential dangers, chimpanzees do not solely look for behavioural cues, but also assess receiver mental states and use this information to target signalling and monitoring.
In a recent experiment, caller chimpanzees were shown a fake snake as a predator and were played pre-recorded calls from receivers. Some receivers emitted calls that were snake-related, and therefore represented receivers with knowledge of the predator, while other receivers emitted calls that were not snake-related, and therefore represented receivers without knowledge of the predator.
In response to the non-snake-related calls from receivers, the signallers increased their vocal and nonvocal signalling and coupled it with increased receiver monitoring.
Caller age
Chimpanzee age impacts the frequency of alarm signalling. Chimpanzees over 80 months of age are more likely to produce an alarm call than those less than 80 months of age.
There are several hypotheses for this lack of alarm calling in infants zero to four years of age.
The first hypothesis is a lack of motivation to produce alarm calls because of mothers in close proximity that minimize the infant's perception of a threat or that respond to a threat before the infant can.
Infants may also be more likely to use distress calls to catch their mother's attention in order for her to produce an alarm call.
Infants might also lack the physical ability to produce alarm calls or lack the necessary experience to classify unfamiliar objects as dangerous and worthy of an alarm signal.
Therefore, alarm calling may require advanced levels of development, perception, categorization, and social cognition.
Other factors
Other factors, such as signaller arousal, receiver identity, or increased risk of predation from calling, do not have a significant effect on the frequency of alarm call production.
Receiver monitoring
However, while alarm signals can be coupled with receiver monitoring, there is a lack of consensus on the definition, starting age, and purpose of monitoring. It is either defined as the use of three subsequent gaze alternations, from a threat to a nearby conspecific and back to the threat, or as the use of two gaze alternations.
Moreover, while some studies only report gaze alternation as starting in late juveniles, other studies report gaze alternation in infants as early as five months of age.
In infants and juveniles, it is potentially a means of social referencing or
social learning through which younger chimpanzees check the reactions of more experienced conspecifics in order to learn about new situations, such as potential threats.
It has also been proposed to be a communicative behaviour or simply the result of shifts in attention between different environmental elements.
Evolution of hominoid communication
The evolution of hominoid communication is evident through chimpanzee 'hoo' vocalizations and alarm calls. Researchers propose that communication evolved as
natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
diversified 'hoo' vocalizations into context-dependent 'hoos' for travel, rest, and threats.
Context-dependent communication is beneficial and likely maintained by selection as it facilities cooperative activities and social cohesion between signallers and receivers that can increase the likelihood of survival.
Alarm calls in chimpanzees also point to the evolution of hominoid language. Callers assess conspecifics' knowledge of threats, fill their need for information, and, in doing so, use social cues and intentionality to inform communication.
Filling a gap in information and incorporating social cues and intentionality into communication are all components of human language.
These shared elements between chimpanzee and human communication suggest an evolutionary basis, most likely that the last common human ancestor with chimpanzees also possessed these linguistic abilities.
False alarm calls
Deceptive
Deception or falsehood is an act or statement that misleads, hides the truth, or promotes a belief, concept, or idea that is not true. It is often done for personal gain or advantage. Deception can involve dissimulation, propaganda and sleight o ...
alarm calls are used by male swallows (''
Hirundo rustica
The barn swallow (''Hirundo rustica'') is the most widespread species of swallow in the world. In fact, it appears to have the largest natural distribution of any of the world's passerines, ranging over 251 million square kilometres globally. ...
''). Males give these false alarm calls when females leave the nest area during the
mating
In biology, mating is the pairing of either opposite-sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. ''Copulation'' is the union of the sex organs of two sexually reproduc ...
season, and are thus able to disrupt
extra-pair copulation
Extra-pair copulation (EPC) is a mating behaviour in monogamous species. Monogamy is the practice of having only one sexual partner at any one time, forming a long-term bond and combining efforts to raise offspring together; mating outside this pai ...
s. As this is likely to be costly to females, it can be seen as an example of
sexual conflict
Sexual conflict or sexual antagonism occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly over the mode and frequency of mating, potentially leading to an evolutionary arms race between male ...
.
Counterfeit alarm calls are also used by
thrushes
The thrushes are a passerine bird family, Turdidae, with a worldwide distribution. The family was once much larger before biologists reclassified the former subfamily Saxicolinae, which includes the chats and European robins, as Old World flycat ...
to avoid
intraspecific competition
Intraspecific competition is an interaction in population ecology, whereby members of the same species compete for limited resources. This leads to a reduction in fitness for both individuals, but the more fit individual survives and is able to r ...
. By sounding a bogus alarm call normally used to warn of aerial predators, they can frighten other birds away, allowing them to eat undisturbed.
Vervet
The vervet monkey (''Chlorocebus pygerythrus''), or simply vervet, is an Old World monkey of the family Cercopithecidae native to Africa. The term "vervet" is also used to refer to all the members of the genus ''Chlorocebus''. The five distinct ...
s seem to be able to understand the referent of alarm calls instead of merely the acoustic properties, and if another species' specific alarm call (terrestrial or aerial predator, for instance) is used incorrectly with too high of a regularity, the vervet will learn to ignore the analogous vervet call as well.
Alarm pheromones
Alarm signals need not be communicated only by
auditory means. For example, many animals may use
chemosensory
A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance ( endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorecep ...
alarm signals, communicated by chemicals known as
pheromone
A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
s. Minnows and catfish release alarm pheromones (''
Schreckstoff
In 1938, the Austrian ethologist Karl von Frisch made his first report on the existence of the chemical alarm signal known as ''Schreckstoff'' (fright substance) in minnows. An alarm signal is a response produced by an individual, the “sender” ...
'') when injured, which cause nearby fish to hide in dense schools near the bottom. At least two species of freshwater fish produce chemicals known as disturbance cues, which initiates a coordinated antipredator defence by increasing group cohesion in response to fish predators.
Chemical communication
A semiochemical, from the Greek σημεῖον (''semeion''), meaning "signal", is a chemical substance or mixture released by an organism that affects the behaviors of other individuals. Semiochemical communication can be divided into two broa ...
about threats is also known among plants, though it is debated to what extent this function has been reinforced by actual selection.
Lima bean
A lima bean (''Phaseolus lunatus''), also commonly known as the butter bean, sieva bean, double bean, Madagascar bean, or wax bean is a legume grown for its edible seeds or beans.
Origin and uses
''Phaseolus lunatus'' is found in Meso- and Sou ...
s release
volatile chemical signals that are received by nearby plants of the same species when infested with
spider mite
Spider mites are members of the Tetranychidae family, which includes about 1,200 species. They are part of the subclass Acari (mites). Spider mites generally live on the undersides of leaves of plants, where they may spin protective silk webs, a ...
s. This 'message' allows the recipients to prepare themselves by activating defense genes, making them less vulnerable to attack, and also attracting another mite species that is a predator of spider mites (''
indirect defence''). Although it is conceivable that other plants are only intercepting a message primarily functioning to attract "bodyguards", some plants spread this signal on to others themselves, suggesting an indirect benefit from increased
inclusive fitness
In evolutionary biology, inclusive fitness is one of two metrics of evolutionary success as defined by W. D. Hamilton in 1964:
* Personal fitness is the number of offspring that an individual begets (regardless of who rescues/rears/supports them ...
.
Deceptive chemical alarm signals are also employed. For example, the wild potato, ''
Solanum berthaultii
''Solanum berthaultii'' (syn. ''Solanum tarijense'') is a species of wild potato in the family Solanaceae, native to Bolivia and northwestern Argentina. It is being extensively studied for its resistance to ''Phytophthora infestans'', the late ...
'', emits the aphid alarm-pheromone, (E)-β-
farnesene
The term farnesene refers to a set of six closely related chemical compounds which all are sesquiterpenes. α-Farnesene and β-farnesene are isomers, differing by the location of one double bond. α-Farnesene is 3,7,11-trimethyl-1,3,6,10-dodec ...
, from its leaves, which functions as a repellent against the green peach aphid, ''
Myzus persicae
''Myzus persicae'', known as the green peach aphid, greenfly, or the peach-potato aphid, is a small green aphid belonging to the order Hemiptera. It is the most significant aphid pest of peach trees, causing decreased growth, shrivelling of the ...
''.
See also
*
Group selection
Group selection is a proposed mechanism of evolution in which natural selection acts at the level of the group, instead of at the level of the individual or gene.
Early authors such as V. C. Wynne-Edwards and Konrad Lorenz argued that the behav ...
*
Kin selection
Kin selection is the evolutionary strategy that favours the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin altruism can look like Altruism in animals, altruistic behavio ...
*
Mobbing call
References
External links
Chickadees' alarm-call carry information about size, threat of predatorThe Trek of the Pika"A story complete with sounds of pika and marmot calls" 2002-10-30
Characteristics of arctic ground squirrel alarm callsOecologia
''Oecologia'' is an international peer-reviewed English-language journal published by Springer since 1968 (some articles were published in German or French until 1976). The journal publishes original research in a range of topics related to plant ...
Volume 7, Number 2 / June, 1971
Why do Yellow-bellied Marmots Call? Daniel T. Blumstein & Kenneth B. Armitage
Department of Systematics and Ecology, University of Kansas
Alarm calls of Belding's ground squirrels to aerial predators: nepotism or self-preservation?
{{evo ecol
Animal communication
Antipredator adaptations
Emergency communication
Survival skills
Articles containing video clips
Chemical ecology