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霊長類行動学16. グループ形成の理由


Scientific study

  • create model
  • develop hypothesis of the explanation
    • has to be falsifiable
  • science cannot prove anything, but test the hypothesis
  • hypothetical deductive framework

at the beginning of Primatology

  • observational and deductive science
  • status of Primatology was how much they had data
  • people were interested in
    • 1. descriptive study:
      • details of behavior
    • 2. correlational study
      • ecological correlations of grouping patterns
  • findings were based on correlational study

Crook (1972)

  • provided intensive discussion
    • 1. "primate form groups because of predation risk"
    • 2. "sexual selection is very important"
  • brought those discussion

Wrangham (1980)

  • changed the way of study in Primatology
  • introduced a hypothetical deductive framework
    • 1st principle is
      • 1. prediction
      • 2. test
  • looked at relationship between behavior and ecology
  • Wrangham did
    • 1. changes the way we classify social systems of primates
      • groups are not fixed entities
      • instead, they are a product of underlying patterns of male and female behavior
        • response to environment
    • 2. emphasized importance of understanding the explanatory model
    • 3. focuses on female as a driving force behind
      • group organization
      • thought to be that males solely play the key roles.
      • his arguments were too biased, but the idea opened up the question

defined the question

  • "why they live in group?"
    • important question because it's odd among all other animals.
    • animals want to avoid competition so want to avoid grouping
    • for any animals, foraging by their own is better
      • group living should be disadvantages.
      • therefore, there should be some reasons
      • 1. living in group is actually an advantage
      • 2. cost stays, but getting other benefit more - less disadvantage
make the problem testable
  • two different kinds of groups of females
    • 1. Kin
    • 2. non-kin
  • Wrangham classified primates based on the type of group not based on group size
    • 1. "female bonded" - female stays in natal group
      • forms affiliative bonds with other females
    • 2. "not female bonded" - female leaves natal group
  • establish the criteria to distinguish "female bonded" from "not female bonded"
    • 1. do female breed in natal group?
    • 2. do female transfer out of natal group?
    • 3. do female engage in affiliative actions
      • e.g., grooming, haddling, dominate, assistance, etc
  • now, we can test hypothesis like if "Japanese macaque is female bonded"

why female live in group?

  • cooperation must be an Evolutionary Stable Strategy (ESS)
    • he was creating a model, the mechanism
  • rules:
    • suppose there is a fruit tree that can support only two individual
    • There exists a linear dominance hierarchy: A>B>C>D
  • Condition 1: within dyadic competition (two fight with no support from others)
    • A, B on the tree, C, D on the ground
  • Condition 2: any two individuals can cooperate, and they can defeat any other.
    • (how to form the cooperation tends to be based on kin.)
    • C+D on the tree, A, B on the ground, but soon
    • A+B on the tree, C+D on the ground
  • Thus, grouping is needed for intergroup feeding competition.
  • He argues that
  • => grouping patterns based on environmental condition
    • if one exploit patchy, high quality resources
      • females bond
    • if one exploit disperse, low quality resources
      • no bonding
  • => male attaches for resource defense
    • sexual selection is secondary for the reason to group
    • female behavior drives male dispersal
  • but his discussion about males were mostly wrong.

female-bondednon female-bonded
importance of food patches in inter-group interactions high low
frequency of female participation in inter-group interactions high low
role of female in group more merit active passive
frequency of female-female grooming high low
frequency of male and male grooming in multi-male grouplowhigh
shift in the mainstream of the theory
  • 1960's - 70's
    • female behavior is also important, but males are the most
  • 1980's - females determine

after Wrangham

  • Primatology is no longer a correlational study focusing on resource exploitation and resource availability
  • emphasis on the predictable model.
  • but where he failed was
    • he did not explain the primary question "why" primate form group, but only answerd "how".
    • idea about too much emphasis on sexual selection
    • he predicted that idea of predation drive grouping will be failed
  • but inspired people to go out to test, and built the foundation of science in this field.

Covel van Schails

  • falsifies food resource competition idea to explain the reason for grouping
  • very strong hypothetical deductive framework
    • existence of alternative model
      • have to test the opponent idea as well
    • Wrangham's model had worked well only in folivore paradox
3 hypothesis for why primate live in groups
  • 1. to avopid predation
    • increasing chance of defense
    • too costly for predator to attack
    • increase vigilance
  • 2. predation defense + advantages of group living
    • help in locating and defending resources
    • regulate resource use
  • 3. resource defense
    • predation is not an issue
    • regulation of group size

what makes group living bearable for individuals
  • 1. food is in large patches - if a lot of food
  • 2. individuals can only exploit limited amount of food
    • 1. ability to consume
    • 2. food itself is toxic (e.g., plant secondary compound), inadequate nutrition
  • 3. different age and sex classes
    • use different food items
      • niche separation, niche dimorphism
      • e.g., orangutan, males and females eat different food

alternative reasons
  • 1) animals regulate return time to patches
    • territory size increase, group size increase, defensibility decrease
    • => doesn't work
  • 2) information exchange (like bees)
    • benefit from sharing information of food
    • food is dispersed in space and time
    • they will benefit

what lacks in his approach
  • he made prediction and rejected all other ideas, but did not do for his model and predation model.

grouping v. population density
  • 1. predation model' predict group size goes up
    • then, fall off as group split
  • 2. intergroup feeding competition (IGFC)
    • group size should grow continuously
    • high population density -> more competition

groupsize v. fecundity (measured by #infants / female)
  • 1. Predation:
    • fecundity falls within increasing group
      • higher competition -> low number of infants
  • 2. IGFC
    • feeding increase with group size, then falls off as group get bigger

  • data fit both predation models
  • IGFC cannot explain why folivore is living in group



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