Transcript Force favoring aggregation

Force favoring aggregation
protection from physical factors
hydrodynamic effects - birds & fish
reduce predation pressure: group
defense, vigilance, dilution, selfish herd
assembly for mate location
improve feeding efficiency
Reduce path overlap
Information transfer
Group foraging
Communal hunting
improve defense of resources
increase care and richer learning
environment for the young
division of labor among specialist
Factors against aggregation
competition for food, mate, etc
increase risk of infection
increase risk of exploitation of parental
care, brood parasites
increase risk of infanticide and cannabolism
increase risk of cuckoldry
attract predators' attention
Cooperation or mutualism - a mutually
helpful action
Reciprocal altruism (reciprocity) - a
helpful action that will be repaid in the
future by the recipient
Altruism - helpful behavior that ↑the
recipient's direct fitness while↓the donor's
direct fitness
Indirect fitness - the genes contributed by an individual indirectly by
helping non-descendant kin, in
effect creating relative that would
not have existed w/o the help
inclusive fitness - the sum of an
individual's direct and indirect fitness
B
1
----- > ----- , or rB - C > 0
C
r
Factors affecting reciprocal altruism:
length of lifetime
dispersal rate
mutual dependence
For reciprocity to persist
the pairs must live long enough to
permit reciprocity
the benefit to the receiver must
exceed the cost to the donor
donors must recognize cheaters
and not feed them
TIT for TAT as a model of reciprocity
The payoff matrix for one iteration of
Prisoner's Dilemma game
Individual 2 responses: cooperate or defect
Individual 1 actions: cooperate R=3 S=1
defect T=4 P=2
To be a PD, T > R > P > S & R > (T + S)/2
Always defect is best strategy in a finite
round game
Player B
Cooperate
R=3
Cooperate Reward for mutual
cooperation
Player A
Defect
Defect
S=0
Sucker's payoff
T=5
P =1
Temptation to
punishment for
defect
mutual defect
Iteration permits complicated strategies
TFT (cooperate on the first move and
thereafter mimic your opponent) is the
best strategy
Outscored all other strategies in computer
tournament (Axelrod)
Is an ESS if the probability of future encounter,
w, meets these criteria:
w > (T - R)/(T - P) and w > (T - R)/(R - S)
(Axelrod & Hamilton)
w – probability of meeting again
Payoff of tit for tat = R + wR =wR2 + …
= R/(1-w)
Payoff of all defect playing w/ TFT
= T + wP/(1-w)
Payoff of alternating defect:cooperate w/
TFT = (T + wS)/(1-w2)
TFT as an ESS, R/(1-w) > T + wP/(1-w)
and R/(1-w) > (T + wS)/(1-w2)
Cooperative breeding
a social systems in which some group
members defer their own reproduction,
even as adults, and help care for the
young of a few breeding individuals,
excluding cases of brood parasitism,
brood mixing, and extrapair fertilization.
Plural breeders
Singular breeders
Temporary or permanent breeding units
composed of two or more adults of the
same gender that engage in some form
of mutual reproductive activity at a
single nest.
Reproductive activity includes direct
genetic contribution to a clutch and all
forms of parental care, and “mutual”
implies that the joint activity is
sanctioned by same-gender individuals
Joint-nesting system (or communal
laying system) vs. helper-at-the-nest
system
Helpers are typically (but not always)
related to breeders and are often
individuals that do not disperse instead
aid in the rearing of their siblings
found in only about 3% of birds and
mammals (roughly 200-300 bird species
and about 120 mammal species)
Helper's duties--feeding, carrying,
huddling, babysitting, grooming, defense,
teaching, incubation, etc.
Do helpers really help?
Increase breeding success
correlation approach
exp. removal of helper
Increase number of breeding free
females from caring fledgling
Increase breeder survivorship
Evolution of cooperative breeding
STEP 1 -- Potential helpers must decide
whether to disperse and attempt to breed
on their own or to remain in their natal
group and accept a non-reproductive
position
Why not disperse? Two hypotheses
Ecological constraints
Habitat or mate saturation hypothesis
-- the probability of successful dispersal
and breeding is low because of lack of
available habitat or mates, so may be
better off staying until chances of
successful dispersal improve
limited vacant breeding territories of
sufficient quality
occupants extensively modified their
territories and greatly increase their
quality
good quality territory attracts more
helpers and better labor
Benefit of philopatry
Group-living advantages hypothesis -- the
benefits of group-living may outweigh the
costs of foregoing reproduction
protection from predators
cooperative hunting & defense
reduce feeding time
may gain helpers from group members in
the future
STEP 2 -- If potential helpers decide to
remain in their natal groups, they must
decide whether or not to provide help
Why provide help? Several possible
reasons
Non-adaptive results of parental care
Increase own survival though benefit of
grouping
Increase own fecundity and/or breeding
opportunity
Increase own inclusive fitness - Requires that
helpers be relatives of breeders and predicts
that the degree of helping will covary with
the degree of relatedness of the helper to
the recipient
A. By improving the survival of breeders
B. By improving reproductive output of
breeders
To enhance own breeding
opportunities
A. Nonreproductive group members may be
more likely to inherit a territory
B. Helpers may be able to inherit a mate
when a former breeder dies
C. Helpers may recruit siblings they have
helped raise as coalition partners to
acquire mates or territories
In lions, brothers may form coalition to
take over a pride and share mating
opportunity
To gain parenting experience
A. In several species of callitrichid
primates, individuals that have helped
rear their siblings have a better chance
of having their own offspring survive to
weaning
B. Reproductive output (clutch size) in
some species of birds increases with
experience both as a breeder and as a
helper
Sex differences in cooperative
behavior in meerkats
Science 297: 253-256, 2002
More male than female helpers in bird
Male helpers may be more related to sibs
than to offspring if paternity certainty is < 1,
females are more related to their offspring
Male commonly remain and breed in their
group of origin
Meerkats – obligately cooperative, 16
habituated group (size 2-30 individuals)
Dominant male and female + equal number
of helpers of both sexes
Males and females similar in body weight
Approach adult weight and foraging success
by the middle of 2nd year
Some females breed as subordinates but
males rarely breed
Most females ejected by dominant female in
the 2nd or 3rd year and disperse in single-sex
parties of 2-6, males leave voluntarily
Helps’ cooperative activities
Babysitting pups of 1-3 weeks old
Feeding pups of 1-3 months
Social digging – clearing sleeping burrows
Raised guarding – sentinel duty when the
group is foraging
Individual contribution to most
cooperative activities increase during the
first 2 years of life in both sex
At most age, female helpers contribute
more to the care of young than males
No sex difference in the contribution to
digging
Male contribute more in raised guarding,
esp. in older male helpers
Differences between sexes are largest in
big groups
In the first year, individual contribution to
most cooperative activities are related to
their body weight
Effects of body weight on males and
females differ
Supplement feeding affects body weight
and behavior of helpers during 1st year
After the 1st year, behavior is more
affected by daily weight gain and foraging
success
Sex difference is unlikely due to differences
in relatedness
Females may benefit by raising young
Mortality declines w/ increasing group size
Females, not males, may breed in natal group
Recruit more and disperse in larger group
Females helpers show consistent
preference for feeding females pups, which
may be beneficial to raising female recruits
Males increase contribution to raised
guarding and decrease contribution to
raising young shortly before dispersal, and
reverse such contribution when entering a
new group
An association between sex differences in
cooperative behavior and philopatry
Females remain and breed in natal group –
female helpers contribute more to raising
young, e.g. dwarf mongoose, brown hyenas
Males remain and breed in natal group – male
helpers contribute more to raising young, e.g.
most cooperative birds, African wild dogs
Both sexes remain in natal group – no sex
differences in cooperative activities, e.g.
naked mole rat
Implication
pronounced sex differences in behavioral
development can occur in effectively
monogamous species w/ little sexual
dimorphism in body size
Comparison of cooperative behavior among
helpers need to control for the effects of age,
weight, sex, and nutritional status
Differences of male and female helpers in
their cooperative activities ~ direct cost and
benefits of cooperative in each sex
generated by sex differences in philopatry
Mutualistic, direct benefits play an important
role in the evolution and maintenance of
cooperative breeding