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Evolutionary success is measured in offspring produced or genetic contribution to the next generation, but to reproduce it is necessary to survive long enough to do so.
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Consequently, organisms have evolved a diverse variety of strategies to enhance their ability to avoid or deter predators.

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Defensive adaptations include:
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Predator avoidance
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Hiding and camouflage
Group defense
Fleeing
Signal unprofitability
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Warnings, deception and honest signals

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Many organisms avoid predators by the use of cryptic coloration.
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A requirement of camouflage in many cases is that the individual choose an appropriate background.

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Classic example of evolution in action is that of the peppered moth, which occurs in two forms a typical white/speckled form and a melanic or black form.
In early 1800’s dark form very rare.
Dark form caused by dominant mutation that occurs spontaneously.

Peppered moths rest on trees and depend on camouflage for protection.

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In unpolluted areas trees are covered in lichens and the light form of the moth is hard to see.
In mid 1800’s air pollution in British cities covered trees with soot.
In cities dark form became common and light form rare.

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In mid 1950’s pollution controls were introduced in Britain and frequency of melanic form has declined since then.

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Kettlewell carried out famous experiment in which he placed moths on dark and pale tree trunks and showed that background strongly influenced survival.
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In wild, however, moths take much more care about where they settle and rarely settle on tree trunks.

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Instead moths usually choose to rest in shady areas where branches join the trunk.
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If moth’s choice of site is adaptive then moths in these positions should be taken less often by predators than those on tree trunks.

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In an experiment in which dead moths were pinned to open tree trunks or the underside of branches birds consumed fewer of those on the undersides of branches.

6.17

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Other moths also make very specific choices about where to rest.
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The whitish moth usually perches head up with its forewings covering its body.
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When given a choice of resting site these moths prefer birch trees.

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Pietrewicz and Kamil (1977) tested whether these chocies by moths were selectively advantageous.
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Trained blue jays to respond to slides of moths by pecking a button for a food reward whenever they spotted a moth.

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Results showed that blue jays spotted moths less often on birch trees and especially when moth was oriented with its head up.
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Thus, moths choices appear to reduce the risk of detection by visually hunting predators.

6.19

Costs and benefits of anti-detection behavior

Hiding from predators has costs.
If you’re hiding can’t be doing something else.

Belding’s Ground Squirrels, trapped six days running.
Held in trap and fed either peanut butter or lettuce.
Lettuce eaters lost weight.

Subsequently, lettuce eaters when foraging less likely to stop feeding when predator alarm call made.
Squirrels trade off risk of predation against need to feed.

Trinidadian guppies and predation risk
Males must display to attract females.
But, predators can spot them when they display.

A major predator is most active at high light intensities.
Male guppies risk is increased in bright light.
Expect males to reduce displays.

Big males most conspicuous and vulnerable.
Expect large males to be most likely to cease displaying in bright light.

Flocking and herding behavior widespread.
Several potential advantages.
1. More eyes increase chance of predator detection.
2. Better defense in a group
3. Dilution effect

1. More eyes increase chance of predator detection.

Experiments by Kenward using a trained
Goshawk showed that as flock size increased woodpigeons detected an approaching bird at greater distances.

2. Better defense possible as member of a group.

Many animals actively defend themselves against predators.
E.g. Musk oxen form defensive circle facing outwards with calves on inside when attacked by wolves.
Musk Ox

Wasps whose nest is disturbed swarm out and attack the intruder.

Sawfly larvae form clusters and defend themselves using drops of eucalyptus oil, which they regurgitate and apply to their enemy.

Many colonially nesting birds harass predators who enter the colony.
E.g. Gulls and terns dive bomb intruders.

Such attacks are effective at deterring intruders.
In experiment artificial nests placed in middle of colony less likely to be destroyed by predators than nests on the edge.

Noncolonial birds also “mob” predators.
In mobbing behavior perched hawks and owls are surrounded by groups of birds that call loudly and harass the predator.

Mobbed bird often flies away to avoid harassment.
Why does mobbed bird leave?

Probably because predator’s chance of catching prey is low once discovered by potential prey.

Mobbing a predator potentially is dangerous.
Why do small birds take the risk?

Because mobbing may cause predator to move far away.
European kestrels after being mobbed moved on average a distance more than twice the territory diameter of birds doing the mobbing.

3. Dilution effect.
Increasing group size reduces chance that a particular individual will be chosen by a predator.
E.g. bird in flock of 100 has only 1% chance of being picked by predator.

Extreme example of dilution effect seen in
“swamping strategies”
Many prey synchronize behavior in attempt to overwhelm predators ability to consume them.

E.g. Almost all Wildebeest give birth in about a
2-week period.

Hyenas and other predators cannot eat all the babies, so most survive.

E.g. Mayflies emerge to breed over a period of only a few days.
Predation risk is lowest for those individuals that emerge with most others.

Most extreme example of emergence synchronicity is in periodic cicadas.
In some species all individuals emerge as adults to mate at intervals of 13 or 17 years .

Mating cicadas

Cycle of 13 or 17 years minimizes the chance of predators cycling their reproduction to match emergence pattern of cicadas.
Why?

13 and 17 are prime numbers . No shorter cycle can consistently match the emergence times.

Optimal group size and selfishness.
Many groups probably are “selfish herds”.
Individuals join groups for own benefit not that of group as a whole.

If for species X optimal group size is
10 individuals, would you expect to observe groups of 10 in the wild?
Why or why not?

Should expect groups to be larger than optimal size until they reach size at which benefit to an individual of joining a group is equal to that of remaining solitary.

Also see selfish behavior in cases where predator may or may not be present, but no one in group wants to be the one to find out.

E.g. penguins at edge of ice hesitate to enter sea (and sometimes push one another in) because of predatory leopard seals.

Major cost is food must be shared.
House Sparrows attract others by giving a
“chirrup” call to signal food availability.
When predation risk low sparrows don’t chirrup.

Defense by associating with a protective species
E.g. various tropical birds nest close to ants, bees or wasps.

Experiment: Polybia wasp nests moved close to rufous-naped wren nests.
Experimental nests: 50% produced young
Control nests: 10% produced young.

Many caterpillars attract ants who feed on sugary secretions “honeydew” produced by caterpillar.
Ants repel parasitic wasps and flies.

Fleeing from predators
Flight is an important means of escape.

The faster you can flee the more likely you are to escape.
Muscular, chunky butterflies fly fast.
Less likely to be caught by birds than thinner, less muscular butterflies.

Body shape, flight speed and escape probability in tropical butterflies.

If being fast is an advantage why aren’t all butterflies fast fliers?

Because there are costs to fast flight too.
Energy invested in muscle mass cannot be invested in other structures.
What tissue might be more important to invest in than muscle?

Reproductive tissue!
Fast flying butterflies have less ovarian tissue.
They produce fewer young.

Chemical defenses widely used to deter attackers
Many plants produce toxic/indigestible chemical compounds (allelochemicals) to reduce grazing.

Monarch butterfly caterpillars feed on milkweed.
Incorporate cardiac glycosides from plant into their bodies.
These provide protection against predators.

Adult monarch butterflies advertise their toxicity with bright colors.

Many organisms produce sticky substances to guard against marauding ants (e.g.
Asian honeybees and solitary paper wasps).

Lots of animals signal their chemical defenses/poisons with bright warning colors.
E.g. Monarch butterflies, bees, wasps, coral snakes, ladybugs all have bright warning coloration.

Coral Snake

Bright warning colors are mimicked by numerous non-toxic/non-dangerous species.
Such mimics are referred to as Batesian mimics.

Coral Snake and mimics. Which is the coral snake?

Some caterpillars mimic vine snakes.

Jumping spiders mimicked by a tephritid fly.

Fly has leg-like pattern on wings .
Fly Spider
When approached, fly waves wings mimicking territorial defense display of jumping spider.

Jumping spiders reluctant to approach displaying flies.
Effectiveness of display tested experimentally.
House flies and tephritid flies had wings surgically exchanged.

Tephritids with housefly wings and houseflies with tephritid wings were ineffective at deterring spiders.
Tephritids whose own wings were removed but reattached deterred 16 of 20 spider attacks.

Jumping spiders also are mimics. Mimic non-dangerous species and inanimate objects.
Ant mimic Beetle mimic

Bird dropping mimic.

An acoustical Batesian mimic.
Burrowing Owls live in prairie dog burrows.

Burrowing Owls make sound like a rattlesnake’s rattle.
Deters animals from entering owl’s burrow.

Mullerian mimicry
In Mullerian mimicry several toxic or dangerous species all display same or similar warning colors. Convergent evolution.

Mullerian mimics on left of red line
Batesian on right of line

Advertising unprofitabilty to deter pursuit.
Cheetahs hunt Thompson’s gazelles.

Cheetah
Thompson’s gazelle

Gazelle that spot cheetahs frequently stot.
They bounce in a stiff-legged gait and display their white rump to the cheetah.

Display apparently advertises that predator has been spotted and prey is too quick so a chase would be pointless.

Stotting appears to be an honest signal of uncatchability as cheetahs fail to catch stotting individuals and usually abandon the hunt

A similar honest signal is given by Anolis lizards which perform pushups when they spot an approaching snake.

The number of pushups an Anole performs closely matches the lizard’s endurance in running and so appears to honestly signal its ability to flee.

Because signal is honest it appears to benefit both prey and predator to exchange information.

As a last-ditch defense captured animals may attempt to force the predator to release them.

Several approaches tried.
(i) Chemical deterrence.
(ii) Misdirection of attack.
(iii) Startle predator
(iv) Attract competing predators

(i) Chemical deterrence.
Many insects spray defensive chemicals such as formic acid when gripped.

Some salamanders release toxic secretions when grabbed by garter snakes.
In one California population arms race between salamanders and snakes has produced salamanders so toxic that snakes are paralyzed for hours after eating.

(ii) Misdirection of attack
Common defensive tactic is to divert attack to non-critical part of the body.
Examples include:

False eyespots on fish.
Direct attention away from vulnerable head.

Prominent detachable tails in lizards.
Tail often held high above body to induce an attack there.

Tail can be shed if grabbed and regrown.
Regrown
Tail.

Dark tail tip in stoats

In experiments predatory birds strike at dark tail tip rather than stoat’s head.

Also, some butterflies have fake heads on their wings.
False head has been bitten off

(iii) Startle predator.
Underwing moths flash bright hindwings when pecked.
Many animals scream. Loud cries may
Induce predator to let go.

(iv) Attract competing predators
Fear screams may also attract other predators which may interfere with attacking animal allowing prey to escape.

Minnows use chemicals to “scream”
Fathead minnows release skin chemicals when bitten.
These attract predatory fish.

In presence of extra predators handling time is longer. Prey sometimes escapes.
One pike Two pike

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Cost-benefit ratios are important and when costs and benefits can be measured accurately we can make precise predictions about the behavioral choices we would expect organisms to make.
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One way we can study such decisions is by using optimality theory.

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Optimality theory assumes that organisms attempt to maximize their benefits while simultaneously minimizing their costs.
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Thus, we predict organisms should behave in such a way that the benefit to cost ratio is maximized.

Fig 6.30
Four different phenotypes X,X,Y and Z). Phenotype X has largest benefit:cost ratio and should increase in frequency as a result.

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Bobwhite Quail form flocks (called coveys) in winter.
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Coveys appear to provide anti-predator benefits.
Percentage time at least one individual is scanning for predators increases with covey size up to a flock size of about 10 and then levels off.
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Increased competition for food among flock members is likely cost of increased flock size.

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In Bobwhites individual daily survival rate peaks at a covey size of about 10.

Fig 6.31 a

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Mean daily movement of coveys is minimized for coveys of 10 or 11 birds.
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Small coveys may move a lot trying to find another covey to join and large coveys move to find more food.

Fig 6.31B

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Benefit:cost ratio is maximized for coveys of 10-11 birds and these are the commonest covey size found.

Fig 6.31C

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Game theory is another way of analyzing behavior.
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Game theory focuses on the strategies organisms choose and the best strategies depend on what other individuals are playing. Recall the Hawk-Dove model from Dawkins.