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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

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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
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 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
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
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
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
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
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
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
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
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
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
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
    • 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