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Chapter 6 Behavioral adaptations
for survival
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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.
Consequently, organisms have evolved a diverse
variety of strategies to enhance their ability to
avoid or deter predators.
Anti-predator strategies
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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
Costs and benefits of camouflage
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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
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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.
Peppered moth
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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.
Peppered moth
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In mid 1950’s pollution controls were
introduced in Britain and frequency of
melanic form has declined since then.
Peppered Moth
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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.
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.
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.
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.
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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.
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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.
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.
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
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.
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
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.
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
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.
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
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.
Avoiding consumption after capture
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
Optimality Theory
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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.
One way we can study such decisions is
by using optimality theory.
Optimality Theory
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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
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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 Bobwhites
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In Bobwhites individual daily survival rate
peaks at a covey size of about 10.
Fig 6.31 a
Optimality Theory and Bobwhites
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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.
Fig 6.31B
Optimality Theory and Bobwhites
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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
Game Theory
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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.