Behavioral Adaptations for Survival Chapter 5

Download Report

Transcript Behavioral Adaptations for Survival Chapter 5

Behavioral Adaptations for
Chapter 5
Adaptationist Approach –
Assumes a
behavior is adaptive (trait confers greater
reproductive success than an alternative trait)
• If one assumes traits are just random, arbitrary
outcomes of development, there are no hypotheses
to test. If one figures that natural selection would
select for adaptive traits, one can question why the
trait is adaptive and what is working to shape the
• Example: Kruuk’s work with mobbing in blackheaded gulls. His hypothesis was that mobbing
distracted predators and decreased their ability to
locate the mobbers’ young.
Adaptationist Approach (con’t)
• Hypothesis: Given the cost to mobbing and the
potential danger to the one mobbing, there must
be benefits to the behavior that out way the cost.
Testable Predictions would be:
Mobbing should result in the predator having to
spend more effort finding the young. (If you’ve
ever watched a cat being mobbed, you know that
they often have to keep an eye on the bird(s) and
move to avoid getting hit.)
The number of young surviving should be directly
proportional to the amount of mobbing.
Test: Crow predation on
chicken eggs set every 10 ft.
from outside Black-headed
gull colony to inside colony.
Comparative Method:
1) Compare unrelated species that have similar behavior and
look for common selective factors that may have brought
about this convergence;
2) Compare related species that have different behavior and
look at the selective factors that may have resulted in this
 If mobbing is adaptive in the black-headed gulls, then it should be
adaptive in other birds with similar nesting behavior.
 Conversely, mobbing should not be present in species that have
low risk of predation at the nest. Mobbing is costly and would not
be worth doing if there were no fitness gains and possible loss due
to cost.
 Cliff nesting gulls like kittiwakes do not mob
Principle of Parsimony
• Use simplest explanation with the fewest
evolutionary changes.
• Cliff nesting in Kittiwake is derived from ground
nesting, so mobbing was probably lost as a trait.
• Other species that mob: other birds - hawks and
cats, CA ground squirrels and prairie dogs rattlesnakes
Cost-Benefit Approach –
Optimality Theory
 With natural selection, animals should behave as to
maximize the benefit to cost ratio. If they do this, they
are said to behave optimally.
Example: CA ground squirrel mothers mob less as their
young grow and become better able to avoid snakes
themselves. A prediction one would make based on a
cost –benefit analysis.
Northern bobwhite covey size – 2-22.
Coveys of 11 most common. Can ask what
the costs and benefits are to being in a
group of this size and ask why 11 seems to
be optimal. Predator pressure and food
competition – stabilizing selection.
Game Theory
• Another way to study the costs and benefits of
different phenotypes to understand their
evolution. Theoretical approach from which
predictions can be made and tested.
• Some terms:
Fitness - The ability of an allele or genotype to be
replicated in next generation. Compare
reproduction from one generation to the next.
Terms con’t
• Relative genetic fitness - The average
contribution of one allele or genotype to the next
generation or to succeeding generations,
compared with that of other alleles or
Range = 0-1 Calculated as your fitness relative to
that of the most fit individual in population,
which is set at 1.0
Game Theory (con’t)
• When studying animal behavior, one can
consider the activities of animals as all part of an
evolutionary game.
Since behavior has been shaped by natural
selection, the game animals play is about
increasing survivorship and reproduction (=
relative fitness).
The winners (= genotypes) get to stay in the game
for another round
Not all individuals of a species are dealt the
same cards, some have slight differences in
habitat, morphology, color patterns, etc.
Grasshoppers hatching in a marginal habitat – may not be able to
start reproducing at as early an age as those in better habitats.
Ex. Large and small male grasshoppers. = alternative strategies for
different sizes. If there is behavioral flexibility = Conditional
Large - hold territories and sing
Small - don't sing, intercept females as possible
Ex. Centris pallida = desert bee
Males defend the burrows where females are
about to emerge. Mated females, will struggle and refuse to mate
again. Thus the best time for a male to attempt to mate with a
female is just as she emerges from her burrow.
Centris pallida example:
• Size differences in males are fixed at emergence, some
are smaller than others
• Large males - have no trouble defending burrows –
maximum fitness benefit
• Small males - can’t give up - no reproduction - Some
females do leave burrows without being mated
(defending male is distracted) - Thus if males hang out at
flowers, they might encounter unmated females.
This is not the best strategy for the entire male
population, but it is for small males
Thus in a population with large and small males, you
might see alternative mating strategies
Evolutionary Stable Strategy (ESS)
• A strategy that if adopted by most members of a
population cannot be bettered by an alternative
strategy. The behavior will not change in the
near future. However, sometimes there are
alternative strategies that stay in balance as in
the above examples.
Consider a species that defends
Alternative strategies:
• Hawk: animal does not use threat displays, but
rather engages in all-out fighting until its
opponent is killed, injured, or flees, or until the
hawk itself is injured.
• Dove: animal never escalates its behavior
beyond mere displaying. It continues in a
contest until the opponent leaves or attacks. If
attacked, the dove flees immediately.
Payoffs for each strategy:
Some parameters for hawk vs dove interactions
• Winning = +50; losing = 0
pts = resource
gain for reproduction
• If seriously injured, = -100
• A long display, with wasted time = -10
Payoff matrix:
½(50) + ½(-100)
= -25
½(50-10) + ½(-10)
= +15
Is either 100% hawk or dove an Evolutionary Stable Strategy?
No, neither is always best. In a population of only doves, a
hawk would have the advantage. In a population of only
hawks, doves would do better. This is frequencydependent selection.
• An ESS that is best with the parameters as set is a mix:
7/12 hawks, 5/12 dove individuals or a pop of individuals
that behave as hawks about 7/12 of the time and doves
the rest of the time.
• Conclusion: all-out aggression is not the expected
evolutionary outcome, rather a mix of attacks and
displays (as observed in nature).
Frequency Dependent Selection
Right- and left-jawed cichlids
• Bite off chucks of host fish
• Rt and left jawed trait is inherited
• If more of one type of cichlid is
present (more frequent), host fish
protects that side
• Selection favors other form
Conditional Strategies
• Feed along shores in flocks
• Dominant relationships are
▫ Dominants feed on and
around seaweed patches
▫ Subordinates feed by poking
through sand and in less
invertebrate rich areas.
• Distinct foraging behavior that
is based on rank
▫ Not genetic and fixed
▫ Conditional on social rank
Summary of concepts & methods used
in study of ultimate causation
• Adaptation perspective = framework for forming
questions and hypotheses
• Methods used to study these:
▫ Use Optimality Theory (cost-benefit analysis) or
Game Theory to develop predictions about what
should be observed in nature and then look to see
if it does
▫ Use comparisons among related and unrelated
species to understand what evolutionary forces are
acting on behavior (Principle of Parsimony)
▫ Experiments with controls to test hypotheses