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Chapter 6 Behavioral adaptations for survival. 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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chapter 6 behavioral adaptations for survival
Chapter 6 Behavioral adaptations for survival
  • 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.
  • Consequently, organisms have evolved a diverse variety of strategies to enhance their ability to avoid or deter predators.
anti predator strategies
Anti-predator strategies
  • Defensive adaptations include:
    • Predator avoidance
      • Hiding and camouflage
    • Group defense
    • Fleeing
    • Signal unprofitability
      • Warnings, deception and honest signals
costs and benefits of camouflage
Costs and benefits of camouflage
  • Many organisms avoid predators by the use of cryptic coloration.
  • A requirement of camouflage in many cases is that the individual choose an appropriate background.
peppered moths
Peppered moths
  • 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.

peppered moth
Peppered moth
  • 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.
peppered moth8
Peppered moth
  • In mid 1950’s pollution controls were introduced in Britain and frequency of melanic form has declined since then.
peppered moth10
Peppered Moth
  • Kettlewell carried out famous experiment in which he placed moths on dark and pale tree trunks and showed that background strongly influenced survival.
  • In wild, however, moths take much more care about where they settle and rarely settle on tree trunks.
Instead moths usually choose to rest in shady areas where branches join the trunk.
  • 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.
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.
Other moths also make very specific choices about where to rest.
  • The whitish moth usually perches head up with its forewings covering its body.
  • When given a choice of resting site these moths prefer birch trees.
Pietrewicz and Kamil (1977) tested whether these chocies by moths were selectively advantageous.
  • Trained blue jays to respond to slides of moths by pecking a button for a food reward whenever they spotted a moth.
Results showed that blue jays spotted moths less often on birch trees and especially when moth was oriented with its head up.
  • Thus, moths choices appear to reduce the risk of detection by visually hunting predators.

Hiding from predators has costs.

If you’re hiding can’t be doing something



Belding’s Ground Squirrels, trapped six days


Held in trap and fed

either peanut butter

or lettuce.

Lettuce eaters lost



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


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.


Vigilance and groups.

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


Experiments by Kenward using a trained

Goshawk showed that as flock size increased

woodpigeons detected an approaching bird at

greater distances.


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


Sawfly larvae form clusters and defend

themselves using drops of eucalyptus oil,

which they regurgitate and apply to their



Many colonially nesting birds harass predators

who enter the colony.

E.g. Gulls and terns dive bomb intruders.


Such attacks are effective at deterring


In experiment artificial nests placed in middle

of colony less likely to be destroyed by

predators than nests on the edge.


Non-colonial 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


Why does mobbed bird leave?


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



3. Dilution effect.

Increasing group size reduces chance that

a particularindividual 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



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 17years.


Cycle of 13 or 17 years minimizes the

chance of predators cycling their reproduction to match emergence pattern

of cicadas.



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 anindividual 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.


Costs of flocking

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



Defense by associating with a protective


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


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.


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.

signaling unprofitatbility
Signaling Unprofitatbility

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


E.g. Monarch butterflies, bees, wasps, coral

snakes, ladybugs all have bright warning



Bright warning colors are mimicked by

numerous non-toxic/non-dangerous species.

Such mimics are referred to as Batesian



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


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


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.



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



A similar honest signal is given by Anolis

lizards which perform pushupswhen 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



Avoiding consumption after capture

As a last-ditch defense captured animals

may attempt to force the predator to release



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



(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



Prominent detachable tails in lizards.

Tail often held high above body to induce

an attack there.


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

optimality theory
Optimality Theory
  • 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.
  • One way we can study such decisions is by using optimality theory.
optimality theory114
Optimality Theory
  • Optimality theory assumes that organisms attempt to maximize their benefits while simultaneously minimizing their costs.
  • 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.

optimality theory and bobwhites
Optimality Theory and Bobwhites
  • Bobwhite Quail form flocks (called coveys) in winter.
  • 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.
  • Increased competition for food among flock members is likely cost of increased flock size.
optimality theory and bobwhites117
Optimality Theory and Bobwhites
  • In Bobwhites individual daily survival rate peaks at a covey size of about 10.
optimality theory and bobwhites119
Optimality Theory and Bobwhites
  • Mean daily movement of coveys is minimized for coveys of 10 or 11 birds.
  • Small coveys may move a lot trying to find another covey to join and large coveys move to find more food.
optimality theory and bobwhites121
Optimality Theory and Bobwhites
  • Benefit:cost ratio is maximized for coveys of 10-11 birds and these are the commonest covey size found.
game theory
Game Theory
  • Game theory is another way of analyzing behavior.
  • 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.