Appunti dettagliati di Etologia

PREDATOR-PREY INTERACTION

Every organism will encounter and interact with individuals of other species in their lifetime and these

interactions have consequences that can affect the individual’s fitness over long term, they can lead to

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evolutionary changes in one or both interacting species and, in particular, some types of interactions result

in COEVOLUTION: an adaptation in one species lead to the evolution of an adaptation in a species that

interacts with it (reciprocal adaptation).

The continuous interaction between species bring to the so called EVOLUTIONARY ARMS RACE: continuous

feedback between two species to develop new way to increase fitness against the other species acting on it

predators can over evolutionary time become more efficient at capturing their prey and in response prey

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can evolve changes to decrease the likelihood of being consumed: the evolution of the traits that increase

the fitness of a predator species exerts a selective pressure on its prey to counter the consumer’s adaptation,

the prey exerts a selective pressure on the consumer to improve its fitness. The RED QUEEN HYPOTHESIS is

related to the coevolution of species: it states that species must constantly adapt and evolve to pass on genes

to the next generation and also to keep from going extinct when other species within a symbiotic relationship

are evolving (the queen run to stay in the same place) this back-and-forth coevolution of species is a

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constant change with smaller adaptations accumulating over long periods of time.

There can be

• symmetrical arms race: the coevolving species are changing in the same way, so they basically adapt

the same traits. Generally it is a result of competition over a limited resource, for example two plants

that live in an area with low levels of water will both keep evolving longer and longer roots.

• asymmetrical arms race: the species adapt in different ways, but the result is always the coevolution

of the two species. Most cases of asymmetrical arms race come from a predator-prey interaction:

for example, in the relationship lion-zebra, the zebra will become faster and stronger to escape the

lion and the lions will become furtive and a better hunter (even if they both become faster, the arms

race is still asymmetrical because the way they run faster is different).

Prey have evolved different anti-predatory defenses, such as avoiding detection, chemical defense, mimicry,

escaping and group living.

Many preys escape predators by AVOIDING DETECTION, i.e. hiding. One way to hide is crypsis: the phenotype

and the behavior allow the animals to hide in their habitats. A particular kind of camouflage is

countershading, which is used by sharks and penguins: the upper surface of the body is darker to bland with

the background viewed from above and the lower surface is lighter to bland with the background viewed

from below.

Other animals use a CHEMICAL DEFENSE to repel or escape from predators, generally owned by small and

sessile animals that could not be protected in other ways. For example, bombardier beetles possess a pair of

gland near the anal opening, that both contains two compartments line with a protective cuticle: when in

danger, one chemical meet the other and they produce a chemical reaction with production of oxygen, that

leads to an “explosion”.

Other animals use MIMICRY as an anti-predatory defense: a nontoxic specie mimics a toxic species that uses

warning coloration. There are two types of mimicry

• Batesian mimicry: a nontoxic species (the mimic) looks like a toxic species (the model) and benefits

from the avoidance behavior learned by the predator in response to the toxic model species. Mimicry

is extended beyond physical appearance and many mimics also simulate distinctive behaviors of their

models

• Müllerian mimicry: several aposematic species converge on a common color pattern and all benefit

from providing a stronger recognition signal to predators.

In nature we also observe bad mimics, but why these species are not selected against? At least to a certain

level the mimic must be working but there are different hypotheses on how this occurs:

• the perception of the predator may not consider the mimic as we do, so the differences in the model

and in the mimic are not that visible to them

• speed-accuracy trade-off: if a predator encounters a conspicuous species maybe it does not make

sense for it to spend time distinguish the model and the mimic, considering that it could be wrong

in the interesting study “Why Batesian mimics are inaccurate: evidence from hoverfly colour patterns”,

the authors were able to measure the accuracy of the pattern in mimics with respect to the models !

they quantified the differences and similarities within and among species and they tested that:

phenotypic diversity does not change across accurate and inaccurate mimics, to model multiple species

mimics do not have to compromise over accuracy (some species could not be a great mimic of one species

but a good mimic to a series of species).

In addition to morphological defenses and crypsis many animals LIVE IN GROUPS: there are a variety of

benefits that may be accrued from group living but the most important is reducing the risk of predation. For

example, many fish species live in groups: some live in a shoal, which is a simple group, but some can live in

a school, which is a special sort of shoal in which the individual’s orientation is polarized in this species the

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presence of predators or even just the cue that there could be a predator induces the formation of a school

of prey. The risk of predation in shoals can be reduced by different mechanisms

• dilution effect: the risk of predation that an individual faces is reduced by the presence of other prey

if an individual lives in a group of x fish, rater then alone, the chances of being captured are 1 out

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of x. this strategy does not work this easily every time and it depends on the responses of predators

to prey density: if predators exhibit density-dependent behavioral responses, such as increase in

aggressiveness or many predators aggregate to face a large school of prey, then the dilution effect

may produce no reduction in risk of predation.

• enhanced vigilance: schools are more vigilant than individuals alone because there are more sensory

systems available to detect predators larger groups of fish detect predators at greater distances

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and also the predators are constantly seen. This early detection capability allows individuals within a

group to initiate anti-predator behavior earlier than solitary individuals.

• predator confusion effect: in a group, it is more difficult to focus on an individual within the group !

studies proved that with the increase in school size, the capture rate per strike declines due to the

confusion effect. The predator confusion effect is even more occurring if the animals behave in the

same way in addition to be morphologically similar.

Obviously, some predators have altered their hunting tactics in ways that overcome the predator-

confusion effect (Red Queen) they can have cooperative foraging (yellowtail), slap their preys out

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of the group (whale)…

Group living can be advantageous, but it also comes with costs: there is a higher potential of being see by a

predator as a group, increased rates of disease transmission and of competition for resources. Some studies

on coral-reef fishes have shown that large groups suffer higher mortality than small groups when the refuge

space is limited the problem of group living usually translates to a problem of competition between

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individuals in the group so there is a trade-off between the two.

When animals are confronted with a predator, natural selection might be expected to favor behavior that

minimizes individual predation risk, such as evasion of detection and capture. However surprisingly large

collection of data shown that some vertebrate prey will repeatedly approach and even physically attack

predator this behavior is called MOBBING, and it has been observed in fish, birds and mammals: it is an

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approach towards a potentially dangerous predator followed by frequent position changes with most

movements centered on the predator, associated also to stereotyped visual displays and loud vocal displays.

There are two major hypothesis that try to explain mobbing behavior:

• by-product mutualism: the cooperative behavior does not incur a net cost to the donor the donor

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is improving his fitness and also the receivers even though the receiver does not participate in the

act. For example, in the dilution effect, individuals come together and thereby reduce each other’s

individual risk of predation (mobbing is profitable for an individual even if no one else joins in).

• reciprocal altruism: the costly actions of a donor benefit a receiver in return for future reciprocation

by than individual the behavior must be contingent. This basically is focused on tit-for-tat

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strategies: reciprocity makes the pay-off from cooperation frequency dependent and stabilizes a

cooperative population against invasion by predators and cheaters. The problem with analyzing this

kind of strategies is that it is difficult to quantify the fitness value of altruistic behavior and the

reciprocation of that behavior and also there is a problem of considering the altruistic behavior is

species where there are dominant members and subordinates (how to quantify if the benefit from

dominant to followers and viceversa is the same).

In an experiment by Krama et al. they tried to prove wheatear in the species of pied flycatchers there is real

reciprocal altruism or the behavior is just given by by-product mutualism. Authors compared the mobbing

responses of 2 groups: pairs breeding closely and pairs breeding distantly from one another nestboxes

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were placed in pairs: close nestboxes were located 20-24 m apart and distant nestboxes 69-80 m apart;

different pairs of nestboxes were separated by at least 650 m. Close and distant nestbox pairs were further

assigned to either an experimental or a control group: 12 pairs experimental and 12 pairs control in the close

group; 14 pairs experimental and 14 control in the distant group.

In the experimental groups, for each pair, close and distant, they showed an owl to a nestbox and kept in

captivity both parents of the other nestbox for the 15 minutes of exposure of the owl at the first nestbox.

Also, during the attack, they played back alarm calls from the other nest that they had recorded previously

the individuals in the first nestbox are aware that the individuals of the other nestbox are aware of the

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presence of the predator and they choose not to come help. In the control group, on the other hand, when

the owl was presented to a nestbox, they left the birds free to assist their neighbors.

In the second phase of the experiment, they presented the owl at the other nestboxes and let free the

individuals of the first nestboxes to come and help.

In the close groups In the distant groups

Results: the pied flyca