霊長類行動学10. メス同士の関係パターン
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
- 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
- "Selfish herd" effect
- 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:
- Scramble competition
- 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
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
- Little change for polygyny
- 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
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
- Why?
- 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
- Females disperse to territories, which are defended against other females.
- Usually, contact is avoided through dueting.
- Scramble for territories, then contest
- http://www.arkive.org/siamang/symphalangus-syndactylus/video-13.html
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
- Most obvious effect when resources are scarce
- Low ranking females tend to get fewer resources
- 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
- Some female groups are characterized by age-granted hierarchies
- 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.
- If you are a high ranking female and raise a high ranking male, this should maximize you fitness
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.
- Seasonal synchrony of estrus
