A female goldenrod crab spider (Misumena vatia) ambushing the female of a pair of mating flies
Ambush predators or sit-and-wait predators are carnivorous animals that capture or trap prey by stealth or by strategy (typically not conscious), rather than by speed or by strength. Ambush predators sit and wait for prey, often from a concealed position, and then launch a rapid surprise attack.
The ambush may be set by hiding in a burrow, by camouflage, by aggressive mimicry, or by the use of a trap. The predator then uses a combination of senses to assess the prey and to time the strike. Nocturnal ambush predators such as cats and snakes have vertical slit pupils,
helping them to judge the distance to prey in dim light. Different
ambush predators use a variety of means to capture their prey, from the
long sticky tongues of chameleons to the expanding mouths of frogfishes.
Ambush predators usually remain motionless (sometimes hidden) and
wait for prey to come within ambush distance before pouncing. Ambush
predators are often camouflaged, and may be solitary. Pursuit predation becomes a better strategy than ambush predation when the predator is faster than the prey.
Ambush predators use many intermediate strategies. For example, when a
pursuit predator is faster than its prey over a short distance, but not
in a long chase, then either stalking or ambush becomes necessary as
part of the strategy.
Bringing the prey within range
Concealment
Ambush often relies on concealment, whether by staying out of sight or by means of camouflage.
Ambush predators such as trapdoor spiders on land and mantis shrimps
in the sea rely on concealment, constructing and hiding in burrows.
These provide effective concealment at the price of a restricted field
of vision.
Trapdoor spiders excavate a burrow and seal the entrance with a
web trapdoor hinged on one side with silk. The best-known is the thick,
bevelled "cork" type, which neatly fits the burrow's opening. The other
is the "wafer" type; it is a basic sheet of silk and earth. The door's
upper side is often effectively camouflaged with local materials such as
pebbles and sticks. The spider spins silk fishing lines, or trip wires,
that radiate out of the burrow entrance. When the spider is using the
trap to capture prey, its chelicerae
(protruding mouthparts) hold the door shut on the end furthest from the
hinge. Prey make the silk vibrate, and alert the spider to open the
door and ambush the prey.
Many ambush predators make use of camouflage so that their prey can come within striking range without detecting their presence. Among insects, coloration in ambush bugs closely matches the flower heads where they wait for prey. Among fishes, the warteye stargazer buries itself nearly completely in the sand and waits for prey. The devil scorpionfish
typically lies partially buried on the sea floor or on a coral head
during the day, covering itself with sand and other debris to further
camouflage itself. The tasselled wobbegong is a shark whose adaptations as an ambush predator include a strongly flattened and camouflaged body with a fringe that breaks up its outline.
Many ambush predators actively attract their prey towards them before ambushing them. This strategy is called aggressive mimicry, using the false promise of nourishment to lure prey. The alligator snapping turtle is a well-camouflaged ambush predator. Its tongue bears a conspicuous pink extension that resembles a worm and can be wriggled around; fish that try to eat the "worm" are themselves eaten by the turtle. Similarly, some reptiles such as Elaphe rat snakes employ caudal luring (tail luring) to entice small vertebrates into striking range.
The zone-tailed hawk, which resembles the turkey vulture, flies among flocks of turkey vultures, then suddenly breaks from the formation and ambushes one of them as its prey. There is however some controversy about whether this is a true case of wolf in sheep's clothing mimicry.
Flower mantises are aggressive mimics, resembling flowers convincingly enough to attract prey that come to collect pollen and nectar. The orchid mantis actually attracts its prey, pollinator insects, more effectively than flowers do. Crab spiders, similarly, are coloured like the flowers they habitually rest on, but again, they can lure their prey even away from flowers.
Some ambush predators build traps to help capture their prey. Lacewings are a flying insect in the order Neuroptera. In some species, their larval form, known as the antlion,
is an ambush predator. Eggs are laid in the earth, often in caves or
under a rocky ledge. The juvenile creates a small, crater shaped trap.
The antlion hides under a light cover of sand or earth. When an ant,
beetle or other prey slides into the trap, the antlion grabs the prey
with its powerful jaws.
Some but not all web-spinningspiders are sit-and-wait ambush predators. The sheetweb spiders (Linyphiidae)
tend to stay with their webs for long periods and so resemble
sit-and-wait predators, whereas the orb-weaving spiders (such as the Araneidae) tend to move frequently from one patch to another (and thus resemble active foragers).
Detection and assessment
Many nocturnal ambush predators like this leopard cat have vertical pupils, enabling them to judge distance to prey accurately in dim light.
Ambush predators must time their strike carefully. They need to
detect the prey, assess it as worth attacking, and strike when it is in
exactly the right place. They have evolved a variety of adaptations that
facilitate this assessment. For example, pit vipers
prey on small birds, choosing targets of the right size for their mouth
gape: larger snakes choose larger prey. They prefer to strike prey that
is both warm and moving; their pit organs between the eye and the nostril contain infrared (heat) receptors, enabling them to find and perhaps judge the size of their small, warm-blooded prey.
The deep-sea tripodfish Bathypterois grallator uses tactile and mechanosensory cues to identify food in its low-light environment. The fish faces into the current, waiting for prey to drift by.
Several species of Felidae (cats) and snakes have vertically elongated (slit) pupils, advantageous for nocturnal
ambush predators as it helps them to estimate the distance to prey in
dim light; diurnal and pursuit predators in contrast have round pupils.
Capturing the prey
Mantis shrimp captures its prey rapidly with its mantis-like front legs.
Frogfish traps its prey by suddenly opening its jaws and sucking the prey in.
Ambush predators often have adaptations for seizing their prey rapidly
and securely. The capturing movement has to be rapid to trap the prey,
given that the attack is not modifiable once launched. Zebra mantis shrimp
capture agile prey such as fish primarily at night while hidden in
burrows, striking very hard and fast, with a mean peak speed 2.30 m/s
(5.1 mph) and mean duration of 24.98 ms.
Chameleons (family Chamaeleonidae) are highly adapted as ambush predators.
They can change colour to match their surroundings and often climb
through trees with a swaying motion, probably to mimic the movement of
the leaves and branches they are surrounded by. All chameleons are primarily insectivores and feed by ballistically projecting their tongues, often twice the length of their bodies, to capture prey. The tongue is projected in as little as 0.07 seconds, and is launched at an acceleration of over 41 g. The power with which the tongue is launched, over 3000 W kg−1, is more than muscle can produce, indicating that energy is stored in an elastic tissue for sudden release.
All fishes face a basic problem when trying to swallow prey:
opening their mouth may pull food in, but closing it will push the food
out again. Frogfishes
capture their prey by suddenly opening their jaws, with a mechanism
which enlarges the volume of the mouth cavity up to 12-fold and pulls
the prey (crustaceans, molluscs
and other whole fishes) into the mouth along with water; the jaws close
without reducing the volume of the mouth cavity. The attack can be as
fast as 6 milliseconds.
The gene-centered view of evolution, gene's eye view, gene selection theory, or selfish gene theory holds that adaptive evolution occurs through the differential survival of competing genes, increasing the allele frequency of those alleles whose phenotypic trait
effects successfully promote their own propagation, with gene defined
as "not just one single physical bit of DNA [but] all replicas of a
particular bit of DNA distributed throughout the world". The proponents of this viewpoint argue that, since heritable information is passed from generation to generation almost exclusively by DNA, natural selection and evolution are best considered from the perspective of genes.
Proponents of the gene-centered viewpoint argue that it permits understanding of diverse phenomena such as altruism and intragenomic conflict that are otherwise difficult to explain from an organism-centered viewpoint.
The gene-centered perspective can be traced to the philosopher Henri Bergson who wrote:
Life is like a current passing
from germ to germ through the medium of a developed organism. It is as
if the organism itself were only an excrescence, a bud caused to sprout
by the former endeavouring to continue itself in a new germ.
— Creative Evolution (1907)
Overview
John Maynard Smith
Richard Dawkins
The gene-centered view of evolution is a model for the evolution of social characteristics such as selfishness and altruism.
If the central dogma is true, and
if it is also true that nucleic acids are the only means whereby
information is transmitted between generations, this has crucial
implications for evolution. It would imply that all evolutionary novelty
requires changes in nucleic acids, and that these changes – mutations –
are essentially accidental and non-adaptive in nature. Changes
elsewhere – in the egg cytoplasm, in materials transmitted through the
placenta, in the mother's milk – might alter the development of the
child, but, unless the changes were in nucleic acids, they would have no
long-term evolutionary effects.
[t]he essence of the genetical
theory of natural selection is a statistical bias in the relative rates
of survival of alternatives (genes, individuals, etc.). The
effectiveness of such bias in producing adaptation is contingent on the
maintenance of certain quantitative relationships among the operative
factors. One necessary condition is that the selected entity must have a
high degree of permanence and a low rate of endogenous change, relative
to the degree of bias (differences in selection coefficients).
— Williams, 1966, pp. 22–23
Williams argued that "[t]he natural selection of phenotypes
cannot in itself produce cumulative change, because phenotypes are
extremely temporary manifestations." Each phenotype is the unique
product of the interaction between genome and environment. It does not
matter how fit and fertile a phenotype is, it will eventually be
destroyed and will never be duplicated.
Since 1954, it has been known that DNA is the main physical substrate to genetic information, and it is capable of high-fidelity replication through many generations. So, a particular gene coded in a nucleobase sequence of a lineage of replicated DNA molecules can have a high permanence and a low rate of endogenous change.
In normal sexual reproduction, an entire genome
is the unique combination of father's and mother's chromosomes produced
at the moment of fertilization. It is generally destroyed with its
organism, because "meiosis and recombination destroy genotypes as surely as death." Only half of it is transmitted to each descendant due to independent segregation.
And the high prevalence of horizontal gene transfer in bacteria and archaea
means that genomic combinations of these asexually reproducing groups
are also transient in evolutionary time: "The traditional view, that
prokaryotic evolution can be understood primarily in terms of clonal
divergence and periodic selection, must be augmented to embrace gene
exchange as a creative force."
The gene as an informational entity persists for an
evolutionarily significant span of time through a lineage of many
physical copies.
In his book River out of Eden, Dawkins coins the phrase God's utility function to explain his view on genes as units of selection. He uses this phrase as a synonym of the "meaning of life" or the "purpose of life". By rephrasing the word purpose in terms of what economists call a utility function, meaning "that which is maximized", Dawkins attempts to reverse-engineer the purpose in the mind of the Divine Engineer of Nature, or the utility function of god. Finally, Dawkins argues that it is a mistake to assume that an ecosystem or a species
as a whole exists for a purpose. He writes that it is incorrect to
suppose that individual organisms lead a meaningful life either; in
nature, only genes have a utility function – to perpetuate their own
existence with indifference to great sufferings inflicted upon the
organisms they build, exploit and discard.
Organisms as vehicles
Genes
are usually packed together inside a genome, which is itself contained
inside an organism. Genes group together into genomes because "genetic
replication makes use of energy and substrates that are supplied by the
metabolic economy in much greater quantities than would be possible
without a genetic division of labour."
They build vehicles to promote their mutual interests of jumping into
the next generation of vehicles. As Dawkins puts it, organisms are the "survival machines" of genes.
The phenotypic effect of a particular gene is contingent on its
environment, including the fellow genes constituting with it the total
genome. A gene never has a fixed effect, so how is it possible to speak
of a gene for long legs? It is because of the phenotypic differences
between alleles. One may say that one allele, all other things being
equal or varying within certain limits, causes greater legs than its
alternative. This difference enables the scrutiny of natural selection.
"A gene can have multiple phenotypic effects, each of which may
be of positive, negative or neutral value. It is the net selective value
of a gene's phenotypic effect that determines the fate of the gene."
For instance, a gene can cause its bearer to have greater reproductive
success at a young age, but also cause a greater likelihood of death at a
later age. If the benefit outweighs the harm, averaged out over the
individuals and environments in which the gene happens to occur, then
phenotypes containing the gene will generally be positively selected and
thus the abundance of that gene in the population will increase.
Even so, it becomes necessary to model the genes in combination
with their vehicle as well as in combination with the vehicle's
environment.
Selfish-gene theory
The selfish-gene theory of natural selection can be restated as follows:
Genes do not present themselves
naked to the scrutiny of natural selection, instead they present their
phenotypic effects. [...] Differences in genes give rise to differences
in these phenotypic effects. Natural selection acts on the phenotypic
differences and thereby on genes. Thus genes come to be represented in
successive generations in proportion to the selective value of their
phenotypic effects.
— Cronin, 1991, p. 60
The result is that "the prevalent genes in a sexual population must
be those that, as a mean condition, through a large number of genotypes
in a large number of situations, have had the most favourable phenotypic
effects for their own replication." In other words, we expect selfish genes
("selfish" meaning that it promotes its own survival without
necessarily promoting the survival of the organism, group or even
species). This theory implies that adaptations
are the phenotypic effects of genes to maximize their representation in
future generations. An adaptation is maintained by selection if it
promotes genetic survival directly, or else some subordinate goal that
ultimately contributes to successful reproduction.
Individual altruism and genetic egoism
The
gene is a unit of hereditary information that exists in many physical
copies in the world, and which particular physical copy will be
replicated and originate new copies does not matter from the gene's
point of view.
A selfish gene could be favored by selection by producing altruism
among organisms containing it. The idea is summarized as follows:
If a gene copy confers a benefit B on another vehicle at cost C to its own vehicle, its costly action is strategically beneficial if pB > C, where p
is the probability that a copy of the gene is present in the vehicle
that benefits. Actions with substantial costs therefore require
significant values of p. Two kinds of factors ensure high values of p: relatedness (kinship) and recognition (green beards).
— Haig, 1997, p. 288
A gene in a somatic
cell of an individual may forego replication to promote the
transmission of its copies in the germ line cells. It ensures the high
value of p = 1 due to their constant contact and their common origin from the zygote.
The kin selection
theory predicts that a gene may promote the recognition of kinship by
historical continuity: a mammalian mother learns to identify her own
offspring in the act of giving birth; a male preferentially directs
resources to the offspring of mothers with whom he has copulated; the
other chicks in a nest are siblings; and so on. The expected altruism
between kin is calibrated by the value of p, also known as the coefficient of relatedness. For instance, an individual has a p = 1/2 in relation to his brother, and p = 1/8 to his cousin, so we would expect, ceteris paribus, greater altruism among brothers than among cousins. In this vein, geneticist J. B. S. Haldane famously joked, "Would I lay down my life to save my brother? No, but I would to save two brothers or eight cousins."
However, examining the human propensity for altruism, kin selection
theory seems incapable of explaining cross-familiar, cross-racial and
even cross-species acts of kindness.
Green-beard effect
Green-beard effects gained their name from a thought-experiment first presented by Bill Hamilton
and then popularized and given its current name by Richard Dawkins who
considered the possibility of a gene that caused its possessors to
develop a green beard and to be nice to other green-bearded individuals.
Since then, "green-beard effect" has come to refer to forms of genetic
self-recognition in which a gene in one individual might direct benefits
to other individuals that possess the gene. Such genes would be especially selfish,
benefiting themselves regardless of the fates of their vehicles. Since
then, green-beard genes have been discovered in nature, such as Gp-9 in fire ants (Solenopsis invicta), csA in social amoeba (Dictyostelium discoideum), and FLO1 in budding yeast (Saccharomyces cerevisiae).
Intragenomic conflict
As genes are capable of producing individual altruism, they are
capable of producing conflict among genes inside the genome of one
individual. This phenomenon is called intragenomic conflict
and arises when one gene promotes its own replication in detriment to
other genes in the genome. The classic example is segregation distorter
genes that cheat during meiosis or gametogenesis and end up in more than half of the functional gametes. These genes can persist in a population even when their transmission results in reduced fertility.
Egbert Leigh compared the genome to "a parliament of genes: each acts
in its own self-interest, but if its acts hurt the others, they will
combine together to suppress it" to explain the relative low occurrence
of intragenomic conflict.
Writing in the New York Review of Books, Gould has characterized the gene-centered perspective as confusing book-keeping with causality.
Gould views selection as working on many levels, and has called
attention to a hierarchical perspective of selection. Gould also called
the claims of Selfish Gene "strict adaptationism", "ultra-Darwinism", and "Darwinian fundamentalism", describing them as excessively "reductionist". He saw the theory as leading to a simplistic "algorithmic" theory of evolution, or even to the re-introduction of a teleological principle. Mayr went so far as to say "Dawkins' basic theory of the gene being the object of evolution is totally non-Darwinian."
Gould also addressed the issue of selfish genes in his essay "Caring groups and selfish genes".
Gould acknowledged that Dawkins was not imputing conscious action to
genes, but simply using a shorthand metaphor commonly found in
evolutionary writings. To Gould, the fatal flaw was that "no matter how
much power Dawkins wishes to assign to genes, there is one thing that he
cannot give them – direct visibility to natural selection."
Rather, the unit of selection is the phenotype, not the genotype,
because it is phenotypes that interact with the environment at the
natural-selection interface. So, in Kim Sterelny's summation of Gould's view, "gene differences do not cause evolutionary changes in populations, they register those changes." Richard Dawkins replied to this criticism in a later book, The Extended Phenotype,
that Gould confused particulate genetics with particulate embryology,
stating that genes do "blend", as far as their effects on developing
phenotypes are concerned, but that they do not blend as they replicate
and recombine down the generations.
Since Gould's death in 2002, Niles Eldredge has continued with counter-arguments to gene-centered natural selection. Eldredge notes that in Dawkins' book A Devil's Chaplain,
which was published just before Eldredge's book, "Richard Dawkins
comments on what he sees as the main difference between his position and
that of the late Stephen Jay Gould. He concludes that it is his own
vision that genes play a causal role in evolution," while Gould (and
Eldredge) "sees genes as passive recorders of what worked better than
what".
Predators may actively search for or pursue prey or wait for it,
often concealed. When prey is detected, the predator assesses whether to
attack it. This may involve ambush or pursuit predation,
sometimes after stalking the prey. If the attack is successful, the
predator kills the prey, removes any inedible parts like the shell or
spines, and eats it.
Predators are adapted and often highly specialized for hunting, with acute senses such as vision, hearing, or smell. Many predatory animals, both vertebrate and invertebrate, have sharp claws or jaws to grip, kill, and cut up their prey. Other adaptations include stealth and aggressive mimicry that improve hunting efficiency.
Spider wasps paralyse and eventually kill their hosts, but are considered parasitoids, not predators.
At the most basic level, predators kill and eat other organisms.
However, the concept of predation is broad, defined differently in
different contexts, and includes a wide variety of feeding methods; and
some relationships that result in the prey's death are not generally
called predation. A parasitoid, such as an ichneumon wasp,
lays its eggs in or on its host; the eggs hatch into larvae, which eat
the host, and it inevitably dies. Zoologists generally call this a form
of parasitism,
though conventionally parasites are thought not to kill their hosts. A
predator can be defined to differ from a parasitoid in that it has many
prey, captured over its lifetime, where a parasitoid's larva has just
one, or at least has its food supply provisioned for it on just one
occasion.
Relation of predation to other feeding strategies
There are other difficult and borderline cases. Micropredators are small animals that, like predators, feed entirely on other organisms; they include fleas and mosquitoes that consume blood from living animals, and aphids
that consume sap from living plants. However, since they typically do
not kill their hosts, they are now often thought of as parasites. Animals that graze on phytoplankton
or mats of microbes are predators, as they consume and kill their food
organisms; but herbivores that browse leaves are not, as their food
plants usually survive the assault. When animals eat seeds (seed predation or granivory) or eggs (egg predation), they are consuming entire living organisms, which by definition makes them predators.
Scavengers, organisms that only eat organisms found already dead, are not predators, but many predators such as the jackal and the hyena scavenge when the opportunity arises. Among invertebrates, social wasps (yellowjackets) are both hunters and scavengers of other insects.
Seed predation is restricted to mammals, birds, and insects and is found in almost all terrestrial ecosystems. Egg predation includes both specialist egg predators such as some colubridsnakes and generalists such as foxes and badgers that opportunistically take eggs when they find them.
A basic foraging cycle for a predator, with some variations indicated
To feed, a predator must search for, pursue and kill its prey. These actions form a foraging cycle.
The predator must decide where to look for prey based on its
geographical distribution; and once it has located prey, it must assess
whether to pursue it or to wait for a better choice. If it chooses
pursuit, its physical capabilities determine the mode of pursuit (e.g.,
ambush or chase). Having captured the prey, it may also need to expend energy handling it (e.g., killing it, removing any shell or spines, and ingesting it).
Search
Predators have a choice of search modes ranging from sit-and-wait to active or widely foraging. The sit-and-wait method is most suitable if the prey are dense and mobile, and the predator has low energy requirements. Wide foraging expends more energy, and is used when prey is sedentary or sparsely distributed. There is a continuum of search modes with intervals between periods of movement ranging from seconds to months. Sharks, sunfish, Insectivorous birds and shrews are almost always moving while web-building spiders, aquatic invertebrates, praying mantises and kestrels rarely move. In between, plovers and other shorebirds, freshwater fish including crappies, and the larvae of coccinellid beetles (ladybirds), alternate between actively searching and scanning the environment.
The black-browed albatross regularly flies hundreds of kilometres across the nearly empty ocean to find patches of food.
Prey distributions are often clumped, and predators respond by looking for patches where prey is dense and then searching within patches.
Where food is found in patches, such as rare shoals of fish in a nearly
empty ocean, the search stage requires the predator to travel for a
substantial time, and to expend a significant amount of energy, to
locate each food patch. For example, the black-browed albatross
regularly makes foraging flights to a range of around 700 kilometres
(430 miles), up to a maximum foraging range of 3,000 kilometres (1,860
miles) for breeding birds gathering food for their young. With static prey, some predators can learn suitable patch locations and return to them at intervals to feed. The optimal foraging strategy for search has been modelled using the marginal value theorem.
Search patterns often appear random. One such is the Lévy walk, that tends to involve clusters of short steps with occasional long steps. It is a good fit to the behaviour of a wide variety of organisms including bacteria, honeybees, sharks and human hunter-gatherers.
Having found prey, a predator must decide whether to pursue it or
keep searching. The decision depends on the costs and benefits involved.
A bird foraging for insects spends a lot of time searching but
capturing and eating them is quick and easy, so the efficient strategy
for the bird is to eat every palatable insect it finds. By contrast, a
predator such as a lion or falcon finds its prey easily but capturing it
requires a lot of effort. In that case, the predator is more selective.
One of the factors to consider is size. Prey that is too small
may not be worth the trouble for the amount of energy it provides. Too
large, and it may be too difficult to capture. For example, a mantid
captures prey with its forelegs and they are optimized for grabbing prey
of a certain size. Mantids are reluctant to attack prey that is far
from that size. There is a positive correlation between the size of a
predator and its prey.
A predator may also assess a patch and decide whether to spend time searching for prey in it. This may involve some knowledge of the preferences of the prey; for example, ladybirds can choose a patch of vegetation suitable for their aphid prey.
Capture
To capture prey, predators have a spectrum of pursuit modes that range from overt chase (pursuit predation) to a sudden strike on nearby prey (ambush predation). Another strategy in between ambush and pursuit is ballistic interception, where a predator observes and predicts a prey's motion and then launches its attack accordingly.
Ambush or sit-and-wait predators are carnivorous animals that capture
prey by stealth or surprise. In animals, ambush predation is
characterized by the predator's scanning the environment from a
concealed position until a prey is spotted, and then rapidly executing a
fixed surprise attack. Vertebrate ambush predators include frogs, fish such as the angel shark, the northern pike and the eastern frogfish. Among the many invertebrate ambush predators are trapdoor spiders on land and mantis shrimps in the sea.
Ambush predators often construct a burrow in which to hide, improving
concealment at the cost of reducing their field of vision. Some ambush
predators also use lures to attract prey within striking range. The capturing movement has to be rapid to trap the prey, given that the attack is not modifiable once launched.
Ballistic interception
The chameleon attacks prey by shooting out its tongue.
Ballistic interception is the strategy where a predator observes the
movement of a prey, predicts its motion, works out an interception path,
and then attacks the prey on that path. This differs from ambush
predation in that the predator adjusts its attack according to how the
prey is moving.
Ballistic interception involves a brief period for planning, giving
the prey an opportunity to escape. Some frogs wait until snakes have
begun their strike before jumping, reducing the time available to the
snake to recalibrate its attack, and maximising the angular adjustment
that the snake would need to make to intercept the frog in real time. Ballistic predators include insects such as dragonflies, and vertebrates such as archerfish (attacking with a jet of water), chameleons (attacking with their tongues), and some colubrid snakes.
Pursuit
Humpback whales are lunge feeders, filtering thousands of krill from seawater and swallowing them alive.
In pursuit predation, predators chase fleeing prey. If the prey flees
in a straight line, capture depends only on the predator's being faster
than the prey.
If the prey manoeuvres by turning as it flees, the predator must react
in real time to calculate and follow a new intercept path, such as by parallel navigation, as it closes on the prey. Many pursuit predators use camouflage to approach the prey as close as possible unobserved (stalking) before starting the pursuit.
Pursuit predators include terrestrial mammals such as lions, cheetahs,
and wolves; marine predators such as dolphins and many predatory fishes,
such as tuna; predatory birds (raptors) such as falcons; and insects such as dragonflies.
An extreme form of pursuit is endurance or persistence hunting,
in which the predator tires out the prey by following it over a long
distance, sometimes for hours at a time. The method is used by human hunter-gatherers and in canids such as African wild dogs
and domestic hounds. The African wild dog is an extreme persistence
predator, tiring out individual prey by following them for many miles at
relatively low speed, compared for example to the cheetah's brief high-speed pursuit.
A specialised form of pursuit predation is the lunge feeding of baleen whales. These very large marine predators feed on plankton, especially krill,
diving and actively swimming into concentrations of plankton, and then
taking a huge gulp of water and filtering it through their feathery baleen plates.
Pursuit predators may be social, like the lion and wolf that hunt in groups, or solitary, like the cheetah.
Osprey tears its fish prey apart, avoiding dangers such as sharp spines.
Once the predator has captured the prey, it has to handle it: very
carefully if the prey is dangerous to eat, such as if it possesses sharp
or poisonous spines, as in many prey fish. Some catfish such as the Ictaluridae have spines on the back (dorsal) and belly (pectoral)
which lock in the erect position; as the catfish thrashes about when
captured, these could pierce the predator's mouth, possibly fatally.
Some fish-eating birds like the osprey avoid the danger of spines by tearing up their prey before eating it.
Solitary versus social predation
In social predation, a group of predators cooperates to kill prey.
This makes it possible to kill creatures larger than those they could
overpower singly; for example, hyenas, and wolves collaborate to catch and kill herbivores as large as buffalo, and lions even hunt elephants.
It can also make prey more readily available through strategies like
flushing of prey and herding it into a smaller area. For example, when
mixed flocks of birds forage, the birds in front flush out insects that
are caught by the birds behind. Spinner dolphins form a circle around a school of fish and move inwards, concentrating the fish by a factor of 200. By hunting socially chimpanzees can catch colobus monkeys that would readily escape an individual hunter, while cooperating Harris hawks can trap rabbits.
Social hunting allows predators to tackle a wider range of prey,
but at the risk of competition for the captured food. Solitary predators
have more chance of eating what they catch, at the price of increased
expenditure of energy to catch it, and increased risk that the prey will
escape. Ambush predators are often solitary to reduce the risk of becoming prey themselves. Of 245 terrestrial carnivores, 177 are solitary; and 35 of the 37 wild cats are solitary, including the cougar and cheetah. However, the solitary cougar does allow other cougars to share in a kill, and the coyote can be either solitary or social. Other solitary predators include the northern pike, wolf spiders and all the thousands of species of solitary wasps among arthropods, and many microorganisms and zooplankton.
Specialization
Physical adaptations
Under the pressure of natural selection, predators have evolved a variety of physical adaptations
for detecting, catching, killing, and digesting prey. These include
speed, agility, stealth, sharp senses, claws, teeth, filters, and
suitable digestive systems.
For detecting prey, predators have well-developed vision, smell, or hearing. Predators as diverse as owls and jumping spiders have forward-facing eyes, providing accurate binocular vision
over a relatively narrow field of view, whereas prey animals often have
less acute all-round vision. Animals such as foxes can smell their prey
even when it is concealed under 2 feet (60 cm) of snow or earth. Many
predators have acute hearing, and some such as echolocatingbats hunt exclusively by active or passive use of sound.
Predators including big cats, birds of prey, and ants share powerful jaws, sharp teeth, or claws which they use to seize and kill their prey. Some predators such as snakes and fish-eating birds like herons and cormorants
swallow their prey whole; some snakes can unhinge their jaws to allow
them to swallow large prey, while fish-eating birds have long spear-like
beaks that they use to stab and grip fast-moving and slippery prey. Fish and other predators have developed the ability to crush or open the armoured shells of molluscs.
Many predators are powerfully built and can catch and kill
animals larger than themselves; this applies as much to small predators
such as ants and shrews as to big and visibly muscular carnivores like the cougar and lion.
Skull of brown bear has large pointed canines for killing prey, and self-sharpening carnassial teeth at rear for cutting flesh with a scissor-like action
Size-selective predation: a lioness attacking a Cape buffalo, over twice her weight. Lions can attack much larger prey, including elephants, but do so much less often.
Predators are often highly specialized in their diet and hunting behaviour; for example, the Eurasian lynx only hunts small ungulates. Others such as leopards are more opportunistic generalists, preying on at least 100 species.
The specialists may be highly adapted to capturing their preferred
prey, whereas generalists may be better able to switch to other prey
when a preferred target is scarce. When prey have a clumped (uneven)
distribution, the optimal strategy for the predator is predicted to be
more specialized as the prey are more conspicuous and can be found more
quickly; this appears to be correct for predators of immobile prey, but is doubtful with mobile prey.
In size-selective predation, predators select prey of a certain size.
Large prey may prove troublesome for a predator, while small prey might
prove hard to find and in any case provide less of a reward. This has
led to a correlation between the size of predators and their prey. Size
may also act as a refuge for large prey. For example, adult elephants are relatively safe from predation by lions, but juveniles are vulnerable.
In aggressive mimicry, certain predators, including insects and fishes, make use of coloration and behaviour to attract prey. Female Photurisfireflies, for example, copy the light signals of other species, thereby attracting male fireflies, which they capture and eat. Flower mantises are ambush predators; camouflaged as flowers, such as orchids, they attract prey and seize it when it is close enough. Frogfishes are extremely well camouflaged, and actively lure their prey to approach using an esca,
a bait on the end of a rod-like appendage on the head, which they wave
gently to mimic a small animal, gulping the prey in an extremely rapid
movement when it is within range.
Venom
Many smaller predators such as the box jellyfish use venom to subdue their prey, and venom can also aid in digestion (as is the case for rattlesnakes and some spiders). The marbled sea snake that has adapted to egg predation has atrophied venom glands, and the gene for its three finger toxin contains a mutation (the deletion of two nucleotides) that inactives it. These changes are explained by the fact that its prey does not need to be subdued.
Several groups of predatory fish have the ability to detect, track, and sometimes, as in the electric ray, to incapacitate their prey by generating electric fields using electric organs. The electric organ is derived from modified nerve or muscle tissue.[94]
Physiology
Physiological adaptations to predation include the ability of predatory bacteria to digest the complex peptidoglycan polymer from the cell walls of the bacteria that they prey upon.
Carnivorous vertebrates of all five major classes (fishes, amphibians,
reptiles, birds, and mammals) have lower relative rates of sugar to amino acid transport than either herbivores or omnivores, presumably because they acquire plenty of amino acids from the animal proteins in their diet.
Antipredator adaptations
To counter predation, prey have a great variety of defences. They can
try to avoid detection. They can detect predators and warn others of
their presence. If detected, they can try to avoid being the target of
an attack, for example, by signalling that a chase would be unprofitable
or by forming groups. If they become a target, they can try to fend off
the attack with defences such as armour, quills, unpalatability or
mobbing; and they can escape an attack in progress by startling the predator, shedding body parts such as tails, or simply fleeing.
Avoiding detection
Prey
can avoid detection by predators with morphological traits and
coloration that make them hard to detect. They can also adopt behaviour
that avoids predators by, for example, avoiding the times and places
where predators forage.
Prey animals make use of a variety of mechanisms including camouflage and mimicry
to misdirect the visual sensory mechanisms of predators, enabling the
prey to remain unrecognized for long enough to give it an opportunity to
escape. Camouflage delays recognition through coloration, shape, and
pattern. Among the many mechanisms of camouflage are countershading and disruptive coloration. The resemblance can be to the biotic or non-living environment, such as a mantis resembling dead leaves, or to other organisms. In mimicry, an organism has a similar appearance to another species, as in the drone fly, which resembles a bee yet has no sting.
Behavioural mechanisms
Black woodpecker attending its chicks, relatively safe inside an excavated hole in a tree
Animals avoid predators with behavioural mechanisms such as changing
their habitats (particularly when raising young), reducing their
activity, foraging less and forgoing reproduction when they sense that
predators are about.
Eggs and nestlings are particularly vulnerable to predation, so birds take measures to protect their nests. Where birds locate their nests can have a large effect on the frequency of predation. It is lowest for those such as woodpeckers that excavate their own nests and progressively higher for those on the ground, in canopies and in shrubs.
To compensate, shrub nesters must have more broods and shorter nesting
times. Birds also choose appropriate habitat (e.g., thick foliage or
islands) and avoid forest edges and small habitats. Similarly, some
mammals raise their young in dens.
By forming groups, prey can often reduce the frequency of
encounters with predators because the visibility of a group does not
rise in proportion to its size. However, there are exceptions: for
example, human fishermen can only detect large shoals of fish with sonar.
Detecting predators
Recognition
Prey species use sight, sound and odor to detect predators, and they can be quite discriminating. For example, Belding's ground squirrel
can distinguish several aerial and ground predators from each other and
from harmless species. Prey also distinguish between the calls of
predators and non-predators. Some species can even distinguish between
dangerous and harmless predators of the same species. In the
northeastern Pacific Ocean, transient killer whales prey on seals, but
the local killer whales only eat fish. Seals rapidly exit the water if
they hear calls between transients. Prey are also more vigilant if they
smell predators.
Eurasian jay is constantly alert for predators, warning of their presence with loud alarm calls.
The abilities of prey to detect predators do have limits. Belding's ground squirrel cannot distinguish between harriers flying at different heights, although only the low-flying birds are a threat.
Wading birds sometimes take flight when there does not appear to be any
predator present. Although such false alarms waste energy and lose
feeding time, it can be fatal to make the opposite mistake of taking a
predator for a harmless animal.
Vigilance
Prey must remain vigilant, scanning their surroundings for
predators. This makes it more difficult to feed and sleep. Groups can
provide more eyes, making detection of a predator more likely and
reducing the level of vigilance needed by individuals. Many species, such as Eurasian jays, give alarm calls
warning of the presence of a predator; these give other prey of the
same or different species an opportunity to escape, and signal to the
predator that it has been detected.
If predator and prey have spotted each other, the prey can signal to
the predator to decrease the likelihood of an attack. These honest signals may benefit both the prey and predator, because they save the effort of a fruitless chase. Signals that appear to deter attacks include stotting, for example by Thomson's gazelle; push-up displays by lizards; and good singing by skylarks after a pursuit begins.
Simply indicating that the predator has been spotted, as a hare does by
standing on its hind legs and facing the predator, may sometimes be
sufficient.
Many prey animals are aposematically coloured or patterned as a warning to predators that they are distasteful or able to defend themselves. Such distastefulness or toxicity is brought about by chemical defences, found in a wide range of prey, especially insects, but the skunk is a dramatic mammalian example.
Forming groups
By forming groups, prey can reduce attacks by predators. There are several mechanisms that produce this effect. One is dilution,
where, in the simplest scenario, if a given predator attacks a group of
prey, the chances of a given individual being the target is reduced in
proportion to the size of the group. However, it is difficult to
separate this effect from other group-related benefits such as increased
vigilance and reduced encounter rate. Other advantages include confusing predators such as with motion dazzle, making it more difficult to single out a target.
Chemical defences include toxins, such as bitter compounds in leaves
absorbed by leaf-eating insects, are used to dissuade potential
predators. Mechanical defences include sharp spines, hard shells and tough leathery skin or exoskeletons, all making prey harder to kill.
Some species mob predators cooperatively, reducing the likelihood of attack.
Escaping an attack
When a predator is approaching an individual and attack seems
imminent, the prey still has several options. One is to flee, whether by
running, jumping, climbing, burrowing or swimming. The prey can gain some time by startling the predator. Many butterflies and moths have eyespots, wing markings that resemble eyes. When a predator disturbs the insect, it reveals its hind wings in a deimatic or bluffing display, startling the predator and giving the insect time to escape. Some other strategies include playing dead and uttering a distress call.
Predators and prey are natural enemies, and many of their adaptations
seem designed to counter each other. For example, bats have
sophisticated echolocation
systems to detect insects and other prey, and insects have developed a
variety of defences including the ability to hear the echolocation
calls.
Many pursuit predators that run on land, such as wolves, have evolved
long limbs in response to the increased speed of their prey. Their adaptations have been characterized as an evolutionary arms race, an example of the coevolution of two species. In a gene centered view of evolution, the genes of predator and prey can be thought of as competing for the prey's body.
However, the "life-dinner" principle of Dawkins and Krebs predicts that
this arms race is asymmetric: if a predator fails to catch its prey, it
loses its dinner, while if it succeeds, the prey loses its life.
Eastern coral snake,
itself a predator, is venomous enough to kill predators that attack it,
so when they avoid it, this behaviour must be inherited, not learnt.
The metaphor of an arms race implies ever-escalating advances in
attack and defence. However, these adaptations come with a cost; for
instance, longer legs have an increased risk of breaking,
while the specialized tongue of the chameleon, with its ability to act
like a projectile, is useless for lapping water, so the chameleon must
drink dew off vegetation.
The "life-dinner" principle has been criticized on multiple
grounds. The extent of the asymmetry in natural selection depends in
part on the heritability of the adaptive traits. Also, if a predator loses enough dinners, it too will lose its life.
On the other hand, the fitness cost of a given lost dinner is
unpredictable, as the predator may quickly find better prey. In
addition, most predators are generalists, which reduces the impact of a
given prey adaption on a predator. Since specialization is caused by
predator-prey coevolution, the rarity of specialists may imply that
predator-prey arms races are rare.
It is difficult to determine whether given adaptations are truly
the result of coevolution, where a prey adaptation gives rise to a
predator adaptation that is countered by further adaptation in the prey.
An alternative explanation is escalation, where predators are adapting to competitors, their own predators or dangerous prey.
Apparent adaptations to predation may also have arisen for other
reasons and then been co-opted for attack or defence. In some of the
insects preyed on by bats, hearing evolved before bats appeared and was
used to hear signals used for territorial defence and mating.
Their hearing evolved in response to bat predation, but the only clear
example of reciprocal adaptation in bats is stealth echolocation.
A more symmetric arms race may occur when the prey are dangerous,
having spines, quills, toxins or venom that can harm the predator. The
predator can respond with avoidance, which in turn drives the evolution
of mimicry. Avoidance is not necessarily an evolutionary response as it
is generally learned from bad experiences with prey. However, when the
prey is capable of killing the predator (as can a coral snake
with its venom), there is no opportunity for learning and avoidance
must be inherited. Predators can also respond to dangerous prey with
counter-adaptations. In western North America, the common garter snake has developed a resistance to the toxin in the skin of the rough-skinned newt.
One way of classifying predators is by trophic level. Carnivores that feed on herbivores are secondary consumers; their predators are tertiary consumers, and so forth. At the top of this food chain are apex predators such as lions. Many predators however eat from multiple levels of the food chain; a carnivore may eat both secondary and tertiary consumers. This means that many predators must contend with intraguild predation, where other predators kill and eat them. For example, coyotes compete with and sometimes kill gray foxes and bobcats.
Biodiversity maintained by apex predation
Predators may increase the biodiversity of communities by preventing a single species from becoming dominant. Such predators are known as keystone species and may have a profound influence on the balance of organisms in a particular ecosystem.
Introduction or removal of this predator, or changes in its population
density, can have drastic cascading effects on the equilibrium of many
other populations in the ecosystem. For example, grazers of a grassland
may prevent a single dominant species from taking over.
The elimination of wolves from Yellowstone National Park had profound impacts on the trophic pyramid.
In that area, wolves are both keystone species and apex predators.
Without predation, herbivores began to over-graze many woody browse
species, affecting the area's plant populations. In addition, wolves
often kept animals from grazing near streams, protecting the beavers'
food sources. The removal of wolves had a direct effect on the beaver
population, as their habitat became territory for grazing. Increased
browsing on willows and conifers
along Blacktail Creek due to a lack of predation caused channel
incision because the reduced beaver population was no longer able to
slow the water down and keep the soil in place. The predators were thus
demonstrated to be of vital importance in the ecosystem.
In the absence of predators, the population of a species can grow exponentially until it approaches the carrying capacity of the environment. Predators limit the growth of prey both by consuming them and by changing their behavior.
Increases or decreases in the prey population can also lead to
increases or decreases in the number of predators, for example, through
an increase in the number of young they bear.
Cyclical fluctuations have been seen in populations of predator
and prey, often with offsets between the predator and prey cycles. A
well-known example is that of the snowshoe hare and lynx. Over a broad span of boreal forests
in Alaska and Canada, the hare populations fluctuate in near synchrony
with a 10-year period, and the lynx populations fluctuate in response.
This was first seen in historical records of animals caught by fur hunters for the Hudson Bay Company over more than a century.
A simple model of a system with one species each of predator and prey, the Lotka–Volterra equations, predicts population cycles.
However, attempts to reproduce the predictions of this model in the
laboratory have often failed; for example, when the protozoan Didinium nasutum is added to a culture containing its prey, Paramecium caudatum, the latter is often driven to extinction.
The Lotka-Volterra equations rely on several simplifying assumptions, and they are structurally unstable, meaning that any change in the equations can stabilize or destabilize the dynamics. For example, one assumption is that predators have a linear functional response
to prey: the rate of kills increases in proportion to the rate of
encounters. If this rate is limited by time spent handling each catch,
then prey populations can reach densities above which predators cannot
control them.
Another assumption is that all prey individuals are identical. In
reality, predators tend to select young, weak, and ill individuals,
leaving prey populations able to regrow.
Many factors can stabilize predator and prey populations.
One example is the presence of multiple predators, particularly
generalists that are attracted to a given prey species if it is abundant
and look elsewhere if it is not. As a result, population cycles tend to be found in northern temperate and subarctic ecosystems because the food webs are simpler. The snowshoe hare-lynx system is subarctic, but even this involves other predators, including coyotes, goshawks and great horned owls, and the cycle is reinforced by variations in the food available to the hares.
A range of mathematical models have been developed by relaxing
the assumptions made in the Lotka-Volterra model; these variously allow
animals to have geographic distributions, or to migrate; to have differences between individuals, such as sexes and an age structure, so that only some individuals reproduce; to live in a varying environment, such as with changing seasons; and analysing the interactions of more than just two species at once. Such models predict widely differing and often chaotic predator-prey population dynamics. The presence of refuge areas, where prey are safe from predators, may enable prey to maintain larger populations but may also destabilize the dynamics.
Evolutionary history
Predation dates from before the rise of commonly recognized
carnivores by hundreds of millions (perhaps billions) of years.
Predation has evolved repeatedly in different groups of organisms. The rise of eukaryotic
cells at around 2.7 Gya, the rise of multicellular organisms at about 2
Gya, and the rise of mobile predators (around 600 Mya - 2 Gya, probably
around 1 Gya) have all been attributed to early predatory behavior, and
many very early remains show evidence of boreholes or other markings
attributed to small predator species. It likely triggered major evolutionary transitions including the arrival of cells, eukaryotes, sexual reproduction, multicellularity, increased size, mobility (including insect flight) and armoured shells and exoskeletons.
The earliest predators were microbial organisms, which engulfed
or grazed on others. Because the fossil record is poor, these first
predators could date back anywhere between 1 and over 2.7 Gya (billion
years ago). Predation visibly became important shortly before the Cambrian period—around 550 million years ago—as evidenced by the almost simultaneous development of calcification in animals and algae, and predation-avoiding burrowing. However, predators had been grazing on micro-organisms since at least 1,000 million years ago, with evidence of selective (rather than random) predation from a similar time.
The fossil record
demonstrates a long history of interactions between predators and their
prey from the Cambrian period onwards, showing for example that some
predators drilled through the shells of bivalve and gastropod molluscs, while others ate these organisms by breaking their shells.
Among the Cambrian predators were invertebrates like the anomalocaridids with appendages suitable for grabbing prey, large compound eyes and jaws made of a hard material like that in the exoskeleton of an insect.
Some of the first fish to have jaws were the armoured and mainly predatory placoderms of the Silurian to Devonian periods, one of which, the 6 m (20 ft) Dunkleosteus, is considered the world's first vertebrate "superpredator", preying upon other predators.
Insects developed the ability to fly in the Early Carboniferous or Late Devonian, enabling them among other things to escape from predators.
Among the largest predators that have ever lived were the theropod dinosaurs such as Tyrannosaurus from the Cretaceous period. They preyed upon herbivorous dinosaurs such as hadrosaurs, ceratopsians and ankylosaurs.
Meganeura monyi, a predatory Carboniferousinsect related to dragonflies,
could fly to escape terrestrial predators. Its large size, with a
wingspan of 65 cm (30 in), may reflect the lack of vertebrate aerial
predators at that time.
Humans, as omnivores, are to some extent predatory, using weapons and tools to fish, hunt and trap animals. They also use other predatory species such as dogs, cormorants, and falcons to catch prey for food or for sport.
Two mid-sized predators, dogs and cats, are the animals most often kept as pets in western societies.
Human hunters, including the San of southern Africa, use persistence hunting, a form of pursuit predation where the pursuer may be slower than prey such as a kudu
antelope over short distances, but follows it in the midday heat until
it is exhausted, a pursuit that can take up to five hours.
In biological pest control,
predators (and parasitoids) from a pest's natural range are introduced
to control populations, at the risk of causing unforeseen problems.
Natural predators, provided they do no harm to non-pest species, are an
environmentally friendly and sustainable way of reducing damage to crops
and an alternative to the use of chemical agents such as pesticides.
Among poetry on the theme of predation, a predator's consciousness might be explored, such as in Ted Hughes's Pike. The phrase "Nature, red in tooth and claw" from Alfred, Lord Tennyson's 1849 poem "In Memoriam A.H.H." has been interpreted as referring to the struggle between predators and prey.
In mythology and folk fable, predators such as the fox and wolf have mixed reputations.
The fox was a symbol of fertility in ancient Greece, but a weather
demon in northern Europe, and a creature of the devil in early
Christianity; the fox is presented as sly, greedy, and cunning in fables
from Aesop onwards. The big bad wolf is known to children in tales such as Little Red Riding Hood, but is a demonic figure in the Icelandic Edda sagas, where the wolf Fenrir appears in the apocalyptic ending of the world. In the Middle Ages, belief spread in werewolves, men transformed into wolves. In ancient Rome, and in ancient Egypt, the wolf was worshipped, the she-wolf appearing in the founding myth of Rome, suckling Romulus and Remus. More recently, in Rudyard Kipling's 1894 The Jungle Book, Mowgli is raised by the wolf pack. Attitudes to large predators in North America, such as wolf, grizzly bear
and cougar, have shifted from hostility or ambivalence, accompanied by
active persecution, towards positive and protective in the second half
of the 20th century.
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