Alarm calls have been studied in many species, such as Belding's ground squirrels.
In animal communication, an alarm signal is an antipredator adaptation in the form of signals emitted by social animals in response to danger. Many primates and birds have elaborate alarm calls for warning conspecifics 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 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,
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 biologists
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)?".
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 and of the gene's "interest" in passing itself along to future generations." If alarm-calling is truly an example of altruism, then our understanding of natural selection becomes more complicated than simply "survival of the fittest gene".
Other researchers, generally those who support the selfish gene theory, question the authenticity of this "altruistic" behaviour. For instance, it has been observed that vervets
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 squirrels,
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.
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 animals alertness to the predator. One such case is the eastern swamphen (Porphyrio porphyrio), which gives conspicuous visual tail flicks.
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, we can apply these findings
to our understanding of altruism in human behaviour.
Monkeys with alarm calls
Vervet monkey in Dar es Salaam
Vervet monkeys
are the typical example of both animal alarm calls and of semantic
capacity in non-human animals. They have three distinct calls for leopards, snakes, and eagles,
and research shows that each call elicits different responses. When
vervets are on the ground they respond to the eagle alarm call by
looking up and running to cover, to leopard alarm calls primarily by
looking up and running into a tree, and to the snake alarm call
primarily by looking down. When in trees vervets responded to the eagle
alarm call by looking up and down and running out of trees, to the
leopard alarm call by running higher in the tree and looking both up and
down, and to the snake alarm call by looking primarily down.
Campbell's mona monkeys
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 to human morphology. Similarly, the cotton-top tamarin is able to use a limited vocal range of alarm calls to distinguish between aerial and land predators.
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.
That these three species use vocalizations to warn others of danger has been called by some proof of proto-language in primates.
However, there is some evidence that this behavior does not refer to
the predators themselves but to threat, distinguishing calls from words.
Another species that exhibits alarm calls is the Barbary macaque. Barbary macaque mothers are able to recognize their own offspring's calls and behave accordingly.
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 behavior of the alarm
signaler to help make a good decision about the best escape route for
themselves, without there having been any naming of predators.
False alarm calls
Deceptive vocalizations are given by male barn swallows
Deceptive alarm calls are used by male swallows (Hirundo rustica). Males give these false alarm calls when females leave the nest area during the mating season, and are thus able to disrupt extra-pair copulations. As this is likely to be costly to females, it can be seen as an example of sexual conflict.
Counterfeit alarm calls are also used by thrushes to avoid intraspecific competition.
By sounding a bogus alarm call normally used to warn of aerial
predators, they can frighten other birds away, allowing them to eat
undisturbed.
Vervets
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 alarm signals, communicated by chemicals known as pheromones. Minnows and catfish release alarm pheromones (Schreckstoff) when injured, which cause nearby fish to hide in dense schools near the bottom.
Animals are not the only organism to communicate threats to
conspecifics either; some plants are able to perform a similar trick. Lima beans release volatile chemical signals that are received by nearby plants of the same species when infested with spider mites.
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.
False chemical alarm signals are also employed. The aphid Myzus persicae is repelled by the wild potato Solanum berthaultii which releases a chemical from its leaves that acts as an allomone to disrupt aphid attacks.
