Strier’s “Beyond Apes”

      Male cooperation, and engaging in non-reproductive sex is not exclusive to chimpanzees, bonobos, and humans.


     (And we know that close relatives like chimps and bonobos differ in tool use and hunting.)


Compare baboons and other monkeys on these dimensions.


     A major conclusion is that similarities in behavior may not be only due to a common ancestor.


reasons for similarities


    Due to similar genes, and/or

    Similar environments

    (Similar social environments)

Social organization of large primates

Dispersal of young

    Almost all species have a characteristic manner of dispersing their offspring as they become sexually mature.



    dispersal reduces chances for incest; conversely it increases genetic variation.


Monkeys vary in dispersal

    For many species, males leave their natal (birth) groups

    These males must join another group, or take over another group, in order to reproduce.

Gorillas, orangs, gibbons

    Both sexes leave family

Chimps, bonobos, humans?

     Females leave natal (birth) groups

     This is probably a "derived" feature, known as "male philopatry" i.e. males stay in the family territory.  This is important since it determines whether social groups are composed of male or female kin.  And this is important since biological kin have a stake in reproductive success of kin who share genes with them.

     male gibbons and gorillas may tolerate male kin intrusions on their territory contrasted with greater hostility to unrelated males.



Sexual diversity among these apes










     Anatomy adapted to  social-sexual environment

Adaptations relating to sex and mating

     Testes and penis size in M

     Degree/color of sexual swelling (perinium) in F

     Enlarged throat sac and facial swelling M orang

     Prominent breasts in human F?

     Sexual dimorphism (M>F?)




Sexual dimorphism among apes reflects type of competition for mates


Why gorilla dimorphism?

Behavioral diversity among hominoids

     Behavioral Diversity among primates is probably greater than for any other mammals. (Why isn't this surprising?)

      “The extremely closely related bonobos and chimps show considerable behavioral diversity."

     "Some of this diversity can be explained by the ways in which local ecological conditions such as predator pressures and the seasonal and spatial distribution of food resources, shape behavioral responses."


Bonobo vs chimp receptivity

Diversity 2

     "Even contrasts between the male-dominated hierarchies of chimpanzees, on the one hand, and the more egalitarian relationships of bonobos, on the other hand, come down to the degree to which individuals or groups can monopolize resources, such as food and mates, that are important to reproduction." 79



     (JL) This point is made also by differences in behavior of orangutans in various contexts – captivity, typical solitary living, and occasions where food supplies enable orangutans to congregate.

    Stanford suggests differences may be exaggerated comparing captive bonobos with chimps in their natural habitats.  The jury is still out as far as I know.

     The social behavior of chimpanzees and bonobos: Empirical evidence and shifting assumptions. Stanford, Craig B.; Current Anthropology, Vol 39(4), Aug-Oct 1998. pp. 399-420.




Diversity and life stages

Muriquis monkeys of Brazil—parallels to bonobos?

     Related to spider monkeys, these animals have teeth and guts adapted to processing more abundantly occurring foods such as leaves.

      In other primates, including bonobos, access to leaves and herbaceous vegetation allows for more cohesive grouping patterns than those found in specialized fruit eaters as spider monkeys and chimpanzees.


Parallels to bonobos 2

     Strier goes on to show muriquis parallels with bonobo life style – lesser dimorphism, eating leaves, avoiding direct contests over food, relatively low intra-group male aggression, little interference by males during other males copulation.

       Other features are similar to apes – slower growth and reproductive rates, females leave group (male philopatry).  Lesser mate competition is also expected "after all a male who misses a chance to fertilize a female has less to lose in terms of his genetic fitness when his competitor is a close relative." 92


Two other factors beyond phylogeny and male relatedness

Female grouping patterns – foraging preferences

Females with "patchy" food preferences adjust grouping to food availability (e.g. of patches, time availability).  Chimps resemble spider monkeys in this aspect more so than they resemble bonobos. 93

Female social influence

    Females select mates

     "males wait patiently until a receptive female favors them with chance to mates." 84


Comparative insights into social aspects of sex

"honest advertisement of female fertility”

For chimps, bonobos, and baboons, size and color of swelling indicates ovulation – and extent males might compete for chance to fertilize that female.


Female chimps vs. female bonobos

Bonobos swellings last more days per cycle than female chimps do.  There are two consequences:
1. bonobo females remain sexually active and sexually attractive to males longer.

2. the actual time of ovulation – maximum fertility – is concealed from males to a greater extent.  Male bonobos have fewer cues, less "reason", to risk competition or aggression in an effort to mate at the most critical time.


Bonobo vs chimp receptivity

Synergy of sex and other aspects of primate life

Mate choice

Early experiences and mate choice

    Harlow found early experiences impacted basic social-sexual and even cognitive processes in adults

    (Is it any surprise human raised apes may see themselves as “human?”)

    It remains unknown to what extent sexual identity and erotic imagery is “innate.”

Harlow’s monkeys - effects of “mother love”

Harlow’s curious peer group monkeys

Harlow’s findings- isolated monkey

Chimp erotica - :Sherman gets aroused

    Video clip

Muiriqui monkeys: fertility and mating

     Female hormones levels were assessed and  ovulation determined. This was correlated with observed copulations. It was found the annual rainy season triggers onset of ovulatory cycling.  Females tend to stop copulating once they conceive.

      Fifty percent of matings occur between ovulations when probability of conception is low.  See Fig.3.5.

Muiriqui matings

Benefits of concealing ovulation

     The possibility of paternity may act as inhibitor of aggression against infants.

     This may be especially true for dimorphic species where males compete with each other and where a competing male may kill off the infants and put the female back into cycling.



What about humans?

    What’s role of concealed ovulation?

  Sex for food, safety?

  Bonding and known paternity?

    What about prominent breasts?

  Not needed for nursing

  Indicator of child-bearing age?

  Indicator of fertility -- surplus fat?