Transcript Optimal theory

Optimal theory
The theory used to generate hypotheses
about the adaptive value of characteristics
which analyzes the costs and benefits of
alternative decisions in terms of their
fitness payoffs
Decision variable – behavioral options
Currency – often related to fitness
Constraints – intrinsic vs. extrinsic
Advantages
Make assumption explicit
Generate testable prediction
Suggest new hypothesis if model does
not fit
Criticism
Behavior may not always optimal
Optimal diet
E1
E2
E2
E1
E1h2
----- > ----- , if ----- > ------- => S1 > -------- - h1
h1
h2
h2
S1+h1
E2
Always eat the most profitable prey type
Include less profitable prey only if S1 >
(E1h2/E2) - h1
Inclusion of the less profitable prey does
not depend on its abundance, only on the
abundance of the more profitable prey
Specialist on prey 1 will switch and
become generalist both suddenly and
completely when prey 1 become rare
Multiple prey choice
Rank all prey by profitability
To decide whether to include a prey
item when encounter, its profitability
must exceed the net profitability of all
higher ranking prey
E3 > (E1 + E2)/(h1 + s1 + h2 + s2)
Reasons for partial preference
Discrimination error
Lack of complete information
Variation in predator or prey
Simultaneous encounter of multiple prey
Short term sampling rule for estimating
encounter rate
Patch choice model
When is the optimal time to leave a patch?
e.g. hummingbird or bee visiting flowers
Constraints
Time spent searching in patches and traveling
between patches are independent
Perfect knowledge
Energy gain in patches show diminishing gain
Marginal value theorem –
patch residence time
Great tit
Meal worm hidden in sawdust in pots
hanging from trees
Long and short travel time achieved by
making lids hard or easy to remove
Actual patch resident time ~ prediction
Central place foraging
Starling travel between feeder and nest
Load curve shows diminishing return
because it becomes harder to probe as
bill fills
Observation fits MVT prediction
What if optimality fails
Nutrients, predation, competition, risk of
starvation, age, experience, etc.
Consider currency other than profitability
Efficiency, Egain/Espent
Nectar load of bee shows diminishing
return because larger loads take more
energy
Fit maximize efficiency model but not
maximize profitability
Selection on hives favor efficiency
Foraging in a variable environment
Immediate response
Risk sensitive foraging
Long-term response
Topor or hibernation
Fat storage
Caching or hoarding
agriculture
Risk sensitive foraging
Choose to forage in constant or variable
(unpredictable) environment
Risk averse vs risk prone
Foraging in Juncos
Two pans: one with a non-variable modest
reward and the other with a variable but
higher pay-off reward
The birds that were non-energy limited
chose the lower payoff pan over the higher
payoff, but risky pan. When food was
limited, they opted for the higher payoff
but risky pan.
Why hoarding instead of fat storage
Fat increase body mass and predation risk
If food is super-abundant, not all can be
stored as fat
Large store provide food supply for a
group over winter
Can be more easily transferred to offspring
Anti-predator strategies
making detection less likely
egg-shell removal
camouflage & cryptic behavior
industrial melanism in moth
Freezing
removing evidence of presence
broken-winged display
Cost of cryptic behavior
Time lost for other activities
Belding’s ground squirrels respond to alarm
call: hiding in underground burrows = time
not spent feeding
Cost of time lost for feeding varies among
individuals depending on their nutritional
status. Well-fed individuals should have
less to gain from additional feeding
Trinidadian guppies
Males court most vigorously at moderate
light intensity (low light, not visible to
females; high light, too visible to
predators)
Small males court more vigorously ,
especially at high light intensity
making attack less likely
physical and chemical repellents and weapons
warning coloration & behavior
bright color wings under dull color wings, big
eye-spots
hiss sound, inflation and increase body size,
tail of rattlesnake, stripes and hand-stand of
skunks
mimicry
stotting--hypotheses
alarm signal hypothesis
social cohesion hypothesis
confusion effect hypothesis
pursuit-deterrent (un-profitability
advertisement hypothesis)
anti-ambush hypothesis
handicap principle
startle effect
making capture less likely
vigilance, e.g. moth-bat
misdirecting a predator's attack
fleeing
Cooperative defense
vigilance
selfish herd
dilution effect
group mobbing
Key prediction of selfish herd hypothesis
Individuals should compete for access to safest
spots in the herd
Individuals in least safe spots in the herd should
be safer than are solitary individuals
e.g. blue-gill sunfish breeds in colony. Males
compete for central territory which is
preferred by females and lower in predation
risk. Solitary males experiencing higher rate
of infestation/predation than edge males
Cooperative defense: mobbing
Hypothesis: If mobbing protects eggs and
young, the degree of protection should be
proportional to the intensity of the mobbing
Test: placing eggs along a transect from
inside to outside the border of the colony
Results: mobbing rates increased toward
center of colony and predation rate
decreases as mobbing rates increased
Hypothesis: comparative method
Related species nesting in habitats less
vulnerable to terrestrial predation should
not exhibit the behavior – kittiwake
Unrelated species nesting in similar habitats
should demonstrate mobbing – swallow,
ground squirrels
Alarm call
Ideal free distribution
Animal sequentially fill available habitat
staring with best patches
Assumption
“ideal” by possessing perfect info about
resource quality
“free” to disperse appropriately
Expectation – animals disperse to equalize
energy intake or reproduction
Deviation from IFD
16/20 studies show too many in poor
habitat or too few in rich habitat
Perception error
Differences in competitive ability
Dominants exclude subordinates
Dominance – how?
Resource holding potential - ability of
control access to a resource
Correlate w/ body size, experience,
matrilineal relationship, fat reserves,
prior success or failure, etc.
Require recognition or status badge
Economics of territoriality
Resource must be defendable
Renewable, not ephemeral or super-abundant
Benefit > cost of defense
Energetic cost increase w/ # of intruders,
territory size
Benefit accrue by increasing energy intake
rate, reducing energy cost and starvation risk
If nectar level increase from 2 to 3 ul per
flower, the bird save 1.3 hr per day
foraging time and save
(1000x1.3) – (400x1.3) = 780 cal
But the bird spent 0.28 hr per day
defending and the cost of defending =
(3000x0.28) – (400x0.28) = 728 cal
 Economically defendable