Post on 12-Jan-2016
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, etcincrease risk of infectionincrease risk of exploitation of parental care, brood parasitesincrease risk of infanticide and cannabolismincrease risk of cuckoldryattract 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 contri-buted 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 defectIndividual 1 actions: cooperate R=3 S=1defect 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
Cooperate Defect
CooperateR = 3
Reward for mutualcooperation
S = 0Sucker's payoff
DefectT = 5
Temptation todefect
P =1punishment formutual defect
Player A
Player B
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 againPayoff 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 breedinga 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 constraintsHabitat 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 experienceA. In several species of callitrichid primates, individuals that have helped rear their siblings have a better chance of having their own offspring survive to weaningB. 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