Transcript Lecture 1

Proximate and ultimate questions
Two basic types of questions asked in
Animal Behavior
Proximate (focus on mechanics of
behavior).
Ultimate (focus on advantages of the
behavior)
Proximate questions
Questions about physical mechanisms.
Focus on the genetic-developmental aspects
and the sensory-motor aspects of the
behavior.
Focus on things that cause or enable the
animal to perform the behavior.
Proximate questions
Examples of proximate questions:
How mechanically does the behavior take
place?
What factors in the environment stimulate the
behavior?
How do the nerves and muscles generate the
behavior?
How do the animal’s genes affect the behavior?
Ultimate questions
Ultimate questions are questions about the
evolutionary reasons for a behavior. They
are questions about the selective processes
that shape the behavior.
Ultimate questions
Examples of ultimate questions:
What is the purpose of the behavior?
In what way does the behavior increase an
individual’s reproductive success?
Does the behavior increase an individual’s
prospects of survival?
How do we answer behavioral
questions?
Examples from Niko Tinbergen’s work.
A proximate question.
How do beewolves find their way home?
Beewolves (type of wasp) nest in sandy areas.
Beewolf homing
When leaving nest to hunt bees, female
beewolf covers nest entrance with sand.
How does she relocate it?
Beewolf homing
Beewolves circle next before leaving.
Do they remember landmarks? How would
we test this?
Beewolf homing
Tinbergen cleared objects from around nest
after beewolf left and found she struggled to
relocate it.
Tinbergen also set up landmarks around
nest for wasp to use.
Then moved landmarks. Result: wasp searched
where landmarks suggested nest should be.
.
Beewolf homing
Tinbergen’s second experiment is more
powerful than his first because it makes a
more specific prediction.
An ultimate question: Why do Black-headed
gulls remove eggshells from their nests?
Gull egg removal
Tinbergen hypothesized that broken
eggshells draw attention to the nest and
attract predators.
Removing the eggshells should reduce
predation risk.
Experiment
Place broken eggshells at different distances
from intact gull eggs.
Does presence of egg shells affect chance of
intact eggs being found?
Yes!
Distance from
eggshell to egg
Pct eggs eaten
15 cm
42%
100 cm
32%
200 cm
21%
Gull egg removal
Conclude that Black-headed gulls remove
egg shells because this behavior reduces
egg losses to predators (and ultimately
increase the bird’s reproductive success).
How did egg removing behavior
evolve?
Genetic mutation made a gull more likely to
remove egg shells from its nest.
This gull had higher reproductive success
than others in population, so gene became
more common and ultimately became fixed
in the population as alternative genes
became extinct.
This is the process of natural selection.
A couple of things to think about
Why haven’t gulls evolved eggs that are
comouflaged on the inside?
What color would you expect the eggs of
birds that nest in trees to be?
The logic of natural selection
Charles Darwin published the origin of
species in 1859.
Evolution by natural selection
In “The Origin”, Darwin outlined the
mechanism by which evolution mainly
occurs.
This mechanism is Natural Selection.
The process of natural selection
Natural selection is a simple process that is
based on a number of observations from
which a series of inferences can be made.
Population growth rate
Observation: Populations have high birth
rates and so populations have the potential
to increase exponentially, but they don’t.
Generally, populations remain stable over
time.
Observation: Resources are limited.
Struggle for existence
Observation: More young are born than
survive to reproduce.
Inference: There must be what Darwin
called “a struggle for existence” or more
appropriately “a struggle to reproduce” i.e.,
competition between individuals for limited
resources.
What determines the winners in the
struggle?
Variation.
Variation among individuals
Observation: There is variation among
individuals
Some birds remove eggshells from their
nests, but others don’t.
[or organisms differ in how well camouflaged
they are; or how quickly they can run, etc.)].
Variation affects reproductive
success
Inference: Variation (i.e., differences between
organisms) influences their survival and
reproduction.
Allele “for” egg removal exists. Organisms with
the allele on average produce more offspring.
In general, organisms that are better adapted to
their environment are more likely to survive and
reproduce.
Variation has a hereditary basis
Observation: Variation has a genetic basis
and traits can be inherited.
Allele for egg removal is inherited.
Remember: Variation can be affected by
both genes and environment, but for
evolution to occur the variation must be
heritable.
Differential reproductive success
results in evolution
Inference: Differences between individuals in their
reproductive success alter the frequencies of
alleles from one generation to the next.
Allele “for” egg removal causes organisms with
the allele to lose fewer eggs/young to predators.
Therefore, on average they produce more
offspring than those without the allele. That allele
then becomes more common in the next
generation.
Definition of evolution
Because of natural selection evolution is
inevitable.
What is definition of evolution?
Definition of evolution
Evolution is change in the genetic structure
of a population from one generation to the
next.
E.g. Allele for eggshell removal becomes
more common each generation. Eventually,
allele becomes fixed in population.
Always look for the advantage a
behavior confers
Natural selection underpins all ultimate
questions in animal behavior.
One should always look for the advantage a
behavior confers.
The advantage of a behavior is measured in terms
of how much it increases the organism’s
reproductive success.
Infanticide in Hanuman langurs
Langurs live in groups of one male, several
females and their young.
Males fight to control groups of females.
When new male takes over group, infants
often killed by the new male.
Why does infanticide take place?
Why does the male risk serious injury from
females to kill babies?
How can this behavior be favored by natural
selection? Shouldn’t it be selected against
because babies are killed?
Possible explanations for
infanticide?
Overcrowding. Males in high density
populations become hyperaggressive and so
attack babies.
But, infanticide occurs in low density
populations too.
Possible explanations?
Darwinian: Male kills young so females
come into oestrus (become fertile) sooner.
Male increases his reproductive success.
The behavior favors the spread of
“infanticide genes” even though it may have
a net negative effect on a langur population.
Infanticide in Hunuman langurs
Darwinian explanation is probably correct.
Infanticide found in other animals too under
similar ecological circumstances.
Lions: males taking over a pride try
to kill all young < 9 months old.
25% of cubs in some lion
populations killed by males.
Jacanas: Polyandrous (one female mates
with several males).
Female taking over territory kills any chicks of
previous female. Frees up male to tend new female’s
eggs.
Group Selection
In “Animal dispersion in relation to social
behaviour” Wynne-Edwards 1962 proposed
an alternative to
natural selection.
Vero Copner
Wynne-Edwards
(1906-1997)
Group Selection
Unrestrained reproduction and
overpopulation lead to starvation and
extinction of groups.
Groups that limit reproduction more likely
to survive than groups that don’t.
Traits that restrain reproduction and
increase survival of group favored.
Group Selection
Wynne-Edwards suggested that mass
assemblies of organisms e.g. at roosts
(“Epideictic Displays”) used by population
to estimate appropriate reproductive effort.
Claimed restraint shown by breeders.
Group selection explanation for
langur infanticide
Killer males reduce population growth
Population does not destroy food supply.
Therefore, less likely to die out.
Group selection explanation for
langur infanticide
Males risk injury by attacking babies.
But, groups with baby killers less likely to
become extinct.
Baby killing selected for by group selection.
Group selection explanation for
langur infanticide
But natural selection acting too.
Males that don’t kill babies less likely to be
injured and die.
“Cheater” genes for not attacking babies
spread through population.
Two major flaws of group selection
Cheater genes can invade and spread.
In nature too much gene flow between
populations for group selection to be
effective.
For next Friday week (the 3rd of February)
read chapters 1-3 of Dawkin’s The Selfish
Gene.
Be prepared to write brief synopsis/analysis
in class and discuss chapters in class.