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