ANTIPREDATOR DEFENCE IN AN EVOLUTIONARY CONTEXT

Question 1

Predators versus prey: an evolutionary arms race

Answer 1

We might expect natural selection to result in predators with increased efficiency in finding
and capturing prey. However, we would also expect selection for prey’s ability to avoid
detection and capture. In fact, we would expect that each improvement in the predator will
put pressure on the prey to evolve counter-adaptations, and vice versa; an example of an
evolutionary arms race.

Question 2

Selection pressure is greater

Answer 2

on prey (life or death) than on predator (meal or no meal)

Question 3

Examples of predator adaptations and counter-adaptations by prey:Searching for prey

Answer 3

Prey adaptations=Crypsis
Polymorphism
Predator adaptations=Visual acuity
Search image

Question 4

Recognition of prey

Answer 4

Mimicry Learning

Question 5

Catching prey

Answer 5

Escape flights, startle response Motor skills (speed & agility)

Weapons of defence Weapons of offence

Question 6

Handling prey

Answer 6

Active defence, tough integument,
spines
Subduing skills

Toxins Detoxification ability

Question 7

Defenses of underwing moths against predation by birds

Answer 7

Underwing moths (Catocala spp) have cryptic forewings resembling the bark on which they
rest. Their hindwings are often strikingly coloured; these are not visible when at rest but are
suddenly exposed when the moth is disturbed.

Question 8

This suggests that the

Answer 8

forewings are designed to avoid detection and the hindwings are used
to avoid capture once detected.
Variation in colour both of the cryptic forewings and the bright hindwings hinders learning
by the predator and helps the moths to escape:

Question 9

1) Polymorphism in crypsis impedes the formation of a search image by the predator

Answer 9

Different species of underwing moth choose different backgrounds that maximise the
cryptic effect. Experiments with jays showed that camouflage is effective in protecting
moths: the jays detected fewer moths on cryptic than on contrasting backgrounds.

Question 10

In many of these moths the forewings are polymorphic; i.e. there are different varieties
within the one population. How could this help avoid detection?

Answer 10

If all of one morph are
presented to a jay in an experiment it improves its detection rate with successive trials,
indicating that it has formed a “search image” for prey of that particular type. If two morphs
are presented in random order the jay’s detection rate did not improve with successive trials.
So, polymorphism is adaptive for the prey, by impeding the predator’s learning.

Question 11

(2) Variation in “startle” colour impedes learning by predator

Answer 11

Experiments with models confirm that the hindwings startle jays.
However, again the predator learns: jays habituate to a particular pattern after repeated
presentations, reducing the efficacy of the adaptation. However, they were still startled by a
different hindwing pattern. Sympatric species of Catacola have different hindwing patterns,
which would reduce the probability of habituation by the predator.
So, again, it is beneficial to prey if they do not resemble their neighbours too closely.

Question 12

Advantage and evolution of warning colouration

Answer 12

Mostly, predators learn to associate bright colouration with distastefulness. It is suggested
that conspicuous patterns are more easily learnt, or that eating a certain number of prey in a
short time (easy to find) may be a more powerful learning stimulus than eating even more
prey over a longer period. Furthermore, the predator is less likely to make mistakes.

Question 13

Which came first, conspicuousness or unpalatability?

Answer 13

In species that feed on toxic plants
and store the toxins (such as the monarch butterfly) unpalatability probably evolved first,
and the bright colours evolved later to advertise this.

Question 14

How could this happen?

Answer 14

Any bright-coloured mutant would be rare, but more likely to be
spotted by a predator. If bright coloured insects are more likely to be eaten than other
insects, the mutation causing bright colour would not persist in the population. One solution
is that the individual may survive the attack and be rejected by the same predator later.
Alternatively, the adaptation could arise by kin selection: close relatives with similar
colouration are saved from predation and genes for the colour pattern flourish in the gene
pool.

Question 15

Trade-off between conspicuousness and crypsis

Answer 15

The colouration of animals may represent a compromise between different selective
pressures: e.g. crypsis as a defence against predators versus conspicuousness for territorial
defence and mate attraction.

Question 16

One solution is to

Answer 16

be conspicuous only during the breeding season and more drab the rest of
year (e.g. many birds). Work on guppies illustrates the trade-off. Males are more brightly
coloured than females, and brighter males have a mating advantage; orange spots are
particularly attractive.