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 A105 Human Origins and Prehistory
Lecture 6

Primate Social Systems; Primate Evolution

I. Why be Social? Costs of Sociality
    A.  More competition for food, mates.
    B.  Larger groups noisier and more obvious to predators.
    C.  Must spend time on social interactions that could be spent getting food.

   There are also benefits, like safety in numbers and ease of finding mates.  Still, we want to consider how social behaviors and social organization may have evolved.

II. Sociobiology: Evolution applied to behavior.  Behavior, like bodies, can evolve.
    A.  Assumptions: Behavior has a genetic component, and behaviors must be varied in a population for evolution to occur.  If a behavioral variant increases fitness, it will become more frequent over time.

    B. Selection on behavior is toward maximizing reproduction of your genes; i.e., for increasing your inclusive fitness.
      -- Fitness: Average ability of a genotype to reproduce
      -- Inclusive fitness: Average ability of genotype to reproduce and help copies in relatives to reproduce.

    C.  Kin selection. Relatives more likely to help each other than nonrelatives. Age and health also factors -- aged or ailing individuals should be more willing to take risks for relatives than healthy individuals in reproductive years.

    D. How to maximize inclusive fitness?  Male and female strategies differ.  Recall:
       -- Females, limit on reproduction is energy/food.
       -- Males limited by number of copulations: more matings = more offspring.
       -- Thus, males should behave in ways that get them the maximum number of matings, and females should behave in ways that gets them the maximum amount of energy (food).  

    E.  What behaviors might evolve?  I.e., how can individuals maximize inclusive fitness?
       -- Alliances: Females can band together to defend food; males can band together to defend access to receptive females.  Social rank (i.e., dominance), complex communication also.

       -- Aggression:  A more aggressive primate may well do better in resource defense.  Can include displays (which are also communication), threats, and fighting.

       -- Territoriality:  One way to ensure access to food (or mates) is to defend a territory from neighbors; territoriality can thus be selected for.

       -- Affiliative Behaviors.  Include grooming, appeasement, etc.  Cements alliances/coalitions/ friendships.

       -- Infanticide:  Evolved because of its benefit to individual males, who get reproductive access to females in a group takeover immediately upon killing any existing infants.

       -- With any behavior, we should ask how it benefits the individual performing it, and how it benefits this individual's relatives. Can help understand how behaviors evolved.

III. Behavioral Ecology: Sociobiology and Environment.
  Adds sociobiological explanations for behaviors to ecological/environmental information.  Lets us make inferences about why primates have particular social systems, and about hominid ancstors.

Rule of thumb: Females adapt to food sources (distribution, etc.); males adapt to females.

There are five general primate societies, with variations within them.
 

    A.  Female bonded.  The classic monkey society.  
      -- Resource distribution. Usually eat fruits and other easily defensible resources -- defensible food sources that can provide meals for several individuals. (Monkeys can tolerate unripe fruits and other foods with toxins better than apes can, so they're more able to defend resources -- such foods are clustered such that there's enough to defend, and defense makes sense.)
     -- Group composition. Stable core group of related females band together to defend fruiting trees. Males transfer at maturity, must fight their way into new group and make friends with females.  Size: ~12 females, up to 5 males, ~20 subadult.
     -- Grooming and mating.  Very frequent to cement alliances/friendships. May spend up to 30% of their time grooming.  Mating is promiscuous; up to 1200 copulations per birth, 20 per day.
    -- Bonds? Females bond with each other.
    -- Defense. The group defends a territory from other individuals.
    -- Other.  Females lead the group, passing status on to daughters.  Females may reward males who participate in defending babies. Groups can last for generations; perhaps for hundreds of years.  Depending on fighting/defense particulars, can have moderate to high dimorphism.

    B. Monogamous.  Seen in gibbons; some marmosets and tamarins.
    -- Resource distribution. May be patchy; clustered so as to be defensible.  Key issue, though, is that monogamy can occur when the ecology is such that females would have difficulty raising offspring on their own, but foods are not abundant enough for large groups.  
   -- Group composition.  Male and female with dependent offspring. 
   -- Grooming and mating.  Relatively infrequent. Gibbons spend about 1% of their time grooming, and mating occurs less than once a day, or 20 times per infant.
    -- Bonds?  Males and females form lifelong pair bonds.
    -- Defense. Male, female defend territory together; thus, low to no sexual dimorphism.  Youngsters must carve out their own territory with mate.  
    -- Other.  Males certain of paternity, help with infant care.  Group dissolves when one mate dies.  Polyandry, with one female and multiple males, is an extreme form of monogamy, often occurring when the bonded pair need even more help raising young.  Marmosets, tamarins most often practice polyandry.

    C. Solitary.  Fairly uncommon arrangement.  Some prosimians; orangutan. 
   -- Resource distribution. Patchy and not terribly defensible; widely distributed enough that individuals do better foraging alone.
   -- Group composition. Female and her offspring. 
   -- Grooming and mating. Mating slightly more frequent than gibbon. Little grooming.
   -- Bonds? Females do not bond with each other; females rely on males for protection.
   -- Defense.  Females have ranges but do not defend; males defend larger territories that encompass several female ranges.  Males fight aggressively to prevent marauding males from getting access to local females.  Extreme dimorphism.
   -- Other.  Spend up to 90% of their time alone.  

    D.  Female choice.  System seen in gorillas.  Has been called "harem."  
   -- Resource distribution. Food evenly distributed and abundant, so there's no point to defending it.  For example, gorillas eat leaves and other low-quality, abundant plant parts.
   -- Group composition. One male with up to five females and their offspring.  "Extra" males alone or in small groups.
   -- Grooming and mating. Some grooming; mating occurs only about once a year as the male has exclusive access to group females.
   -- Bonds? Females don't bond and can be aggressive toward each other; they are often unrelated. The only bonds are females with males.
   -- Defense. Males lead the group, protect females and infants from marauding males.  Significant dimorphism.  
   -- Other. Called female choice because females must attach themselves to the male perceived to be the best defender.  Males take over a group by stalking it, attacking and defeating the resident male, and killing the infants.  Females join established groups one at a time.  Groups last until the leader male dies.

    E.  Community/ Fission-Fusion.  Another rare society, seen in chimpanzees.  
   -- Resource distribution.  Again, food distributed so that individuals do better foraging alone or in small groups.  Thus, females don't bond -- one fruit tree not worth defending because only part of a meal for one ape (one tree can feed more than one monkey).  
   -- Group composition. Typically 5-50 in group, can go as high as 100, but all are rarely together at once.  Includes several adult males who are generally related, several females, and subadults.
   -- Grooming and mating. Lots of grooming and promiscuous mating.
   -- Bonds? Females don't bond with each other, but may form "friendships."   Males bond with one another, sometimes with females.
   -- Defense.  Males defend their core territory, forming border patrols and attacking unfamiliar individuals.  Because males defend cooperatively, dimorphism is moderate (females about 80% the size of males).
   -- Other.  Females spend up to 75% of their time alone, collecting food in a small, stable core area.  Females almost always transfer groups at maturity. Highly complex social system, with rankings.  (Bonobos have a similar system, but with less aggression, more sex.)

IV.  Ape Social Similarities
    A. No apes are female bonded.
    B. Females transfer groups at maturity (or form new group as with gibbons).
    C.  Males stay in group they are born into, defending against intruders.
    D. Males may form bonds with one another.
    E.  Females may bond with males, but rarely form lasting relationships with each other.

V. Primate Social Trends.
    A. Terrestrial groups tend to be larger than arboreal groups.
    B.  If males fight alone, dimorphism tends to be pronounced.  If males fight together, less dimorphism; if females help in defense, there may be no dimorphism (gibbon).