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2011-10-16

霊長類行動学10. メス同士の関係パターン

霊長類行動学もくじ

http://news.bbc.co.uk/2/hi/science/nature/7934998.stm

Primate Female Social Behavior

  • why do females live in groups?
  • What determines female group size?
  • What determine the nature of female relationships with groups? between groups?
  • What determines which sex disperses?
  • Do females invest differently in male and female offspring?

Why do Females live in groups?

  • historically, primate females were viewed as a commodity
    • Partly male bias from researchers
    • Partly because first species studied (Baboons) are characterized by dramatic male behavior, and subtle female behavior
  • In 1960s, female primates began to receive attention
    • Female behavior is complex
    • the male-centered model was overturned
    • Now, it is recognized that females determine whether or not they group and the social system
    • Now many aspects of female social behavior are explained by a socio-ecological model.
  • about 80% of all diurnal primates are gregarious.
  • In an evolutionary context, we need to demonstrate a benefit to individuals for belonging to a group

Why live in a group at all?

  • predator defense
  • Resource defense
  • Mate acquisition

Predator defense
  • Advantages
    • "Selfish herd" effect
      • predator attack one at a time, so if you are with others the probability to be attacked decreased
    • alarm calls (like cebus capucinus)
    • mobbing
  • Disadvantages
    • Predator attraction
Resource defense
  • Advantages
    • Access to resources
    • defended by other groups
  • Disadvantages
    • Must share resources with your group members
What limits an individual's reproductive success?
  • Females
    • Time-interbirth interval
    • Survival of offspring
  • Males
    • Access to mates
    • Survival of offspring

Dichotomy of Male and Female Reproductive limitations

  • Food availability limits female reproduction output
    • Amount and type of food competition is what shapes the social relationships of females
  • Males limited by the number of females they can inseminate
    • Mate competition is more important.

In a Nutshell...

  • Females are avoiding predators and pursuing food, which is their limiting resource, and males are pursuing females, which is their limiting resources.

Variation in Female-Female relationships

  • Female philopatry is most common, through females disperse in some species
  • Some species have well defined female hierarchies, and some don't

The Essence of the Model

  • Predation risk forces females to live in groups
  • Group living leads to competition for access to resources, both within and between groups
    • The quantity, quality, and distribution of food should influences competition and, therefore, the types of groups that females form.

Competition

  • Two basic types of competition:
    1. Scramble competition
    2. Contest competition
Scramble Competition
  • Share a limited resource
  • Individual can limit the access of that resource to other individuals
  • Occurs when food is very dispersed.,
  • Can also occur at food sources that are very large relative to the group spread.
Contest Competition
  • Contest competition occurs when stronger individuals can limit the access to a resource to less powerful individuals
    • Individuals differ in competitiveness
    • Dominant individuals take a larger share of the resource
    • Contest competition occurs when a resource occurs in well-defined patches

Resource distribution and group size

  • Many small patches:
    • Small group size
    • Small home-range
  • Few large patches (apple)
    • Large group size
    • Large home-range.

Between group competition

  • Contest and Scramble competition can also occur between groups

Female perspective

    • Where food is clumped, females tend to form groups and defend a territory
    • Where food is dispersed, they may disperse
  • Where food is abundant, they may form groups, but engage in scramble competition within groups
  • Some females are solitary, but usually have relationship with other females

Male perspective

  • If females are dispersed
  • If females arr clumped..
    • one male can control access to many females
    • Higher potential polygynous systems
  • e.g., Gibon - females are evenly dispersed, so males are not possible to have more than one female at a time.
    • thus, pair bond

Summarized in a flow chart

f:id:astrobot:20111102222024p:image

Female social relationships

  • Female relationships in primates are complex
  • Simplify into two basic types of interactions:
    • Competitive (antagonistic)
    • Cooperative (affiliation)
      • e.g., grooming, allomothering, and etc.

Dominance

  • Liner Dominance Hierarchy

A → B → C

Competition and Evenly distributed resources

  • For example, leaves
  • Which groups, scramble competition dominates
    • Why?
      • Resources are evenly dispersed
      • Females cannot use aggression to get a larger share of a resource
      • Alliance won't help defend such a resource
  • Females may congregate to reduce predation risk
    • Groups are small and passive
  • Such relationships may be described as "egalitarian"
    • (i.e., no dominance hierarchy or difficult to detect)
  • Between group competition is also scramble
    • Inter-group relationships tend to be peaceful. Females intermingle.

Competition and Clumped Resources

  • Within groups, contest competition is important
  • Aggression can be used to compete for food
  • Alliances are useful, especially if you aid a closely related female (inclusive fitness)
  • Female rank should affect female reproduction success
  • Tendency to stay in natal group and form bonds with closely related females
  • Well established dominance hierarchies are common.
  • What if predation risk is low and resources are clumped?
    • No need for cohesive female groups
    • Between group competition can be more important

Which sex disperses?

  • The costs of dispersal
    • It's dangerous to disperse
    • Most commonly, males disperse
      • Male mortality spikes when they disperse
    • Other costs are associated with dispersal
      • Alliances may be important

  • Given the costs of dispersal
    • Females should attempt to remain philopatric...
    • ...if she can reproduce successfully at home
    • If not, then, she should take her chances elsewhere
      • female dispersal is therefore rare and complex

Primatological Examples of these principles

Pongo
  • Large-bodied and highly arboreal
    • Avoid terrestrial predators (e.g., tigers in Sumatra)
  • Resources are patchy
    • Large body size
    • Long inter-birth interval (high reproductive cost)
  • Densities are low
  • Female kin are scarce, may tolerate range overlap with kin
  • Females inhabit isolated territories, but show friendly relationships in a dispersed social network
  • They avoid each other through dispersal, but not an active avoidance mechanism.

Pan troglodytes
  • Females disperse within smaller territories
  • Maintain complex social relationships with other females
  • They avoid each other through simple dispersal, but regularly form parties
  • Party size is correlated with food availability.
    • There is some tendency to dominate by some females, but ranks are not rigid, and dominance can only be expressed in broad terms.

Hylobates
Galago
  • Females aggressively defend territories against other females

Macaca and Papio
  • Often rely on foods that occur in clumps, which can be monopolized
  • Contest competition both within and between groups
    • Females should be philopatric
    • Form alliances with kin in both within- and between-group contest competition
  • Higher ranking females should have higher reproductive success than lower ranking females
    • Low ranking females tend to get fewer resources
      • Most obvious effect when resources are scarce
        • Lose offspring first
  • Baboons and macaques from matrilineal dominance hierarchies
    • Females form linear dominance hierarchies based on agonistic interactions
    • Whole matrilines are dominant or subordinate to others
    • Females inherit their ranks from their mothers
    • Matrilines maintained by coalitionary aggression - mutual support
      • Size of the coalition determines the outcome
  • Females in groups tend to show a lot of reconciliation behavior
    • Groom one another or show other friendly behavior

Semnopithecus setellus (Hanuman langur)
  • vary in relationship to resources
  • Females don't form coalitions, competition is typically scramble, with some contest
    • Some female groups are characterized by age-granted hierarchies
      • In this case, females establish an agonistic dominance hierarchies.
      • Young females tend to be dominant over older females.
      • Classic case is Semnopithecus entellus
  • In species where females transfer, young incoming females may rise quickly to the top position, then decline in rank with age.

Marmosets
  • Young females are subordinate to their mothers
  • Cost of dispersal is high for young female, so she stays and helps mother raise offspring
    • Marmosets live in groups where adult females in same group as mother will not cycle until leave home
    • Beat up daughters to prevent them from cycling
      • ovulation is suppressed

Alouatta
  • Highly folivorous, expectation would be scramble competition and no female hierarchies, no barriers to transfer, no formal signs of submission
  • However, howler females often actively resist immigration of females from other groups. They also evict some females from their groups
  • why?
    • Population density and saturation of habitat
    • Female competition is more intense in small groups. If you are near the carrying capacity of your habitat, you stand to lose more by admitting a new individual
  • Live in fragmented forests
    • Within group competition is actually high
    • Infant survival correlated with maternal rank
    • However, no formal signs of submission exist
  • Is this a sign of howlers living in an environment that is different to the one they adapted to?

Do females invest differently in Male and Female Offspring?

  • Hypothesis: low ranking females invest heavily in sons, high ranking females in daughters
    • current data suggests it is not true
  • Why would you preferentially invest in one sex over the other?
    • Will doing so increase your genetic fitness?
  • Males can have higher potential lifetime reproductive outputs
    • If you are a high ranking female and raise a high ranking male, this should maximize you fitness
      • Therefore, dominant females should invest in sons
    • Low ranking males stand little change of reproducing; low ranking females should invest in daughters
      • Daughters are also potential allies in matrilineal hierarchies.

Estrus Asynchrony in with Estrous Synchrony?

Lemur catta (ring-tailed lemur)
  • Breed seasonally
    • Seasonal synchrony of estrus
      • All females in a group cycle within about a 20 day period
      • However, no two females cycle on the same day. Why?
    • Females are dominant to males and solicit matings
      • Asynchrony may circumvent this temporal conflict with other females
      • Many females can mate with the same "high quality" male.

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