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
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
Social organization of large primates
Dispersal of young
all species have a characteristic manner of dispersing their offspring as they
become sexually mature.
reduces chances for incest; conversely it increases genetic variation.
Monkeys vary in dispersal
many species, males leave their natal (birth) groups
males must join another group, or take over another group, in order to
Gorillas, orangs, gibbons
sexes leave family
Chimps, bonobos, humans?
Females leave natal
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
Degree/color of sexual
swelling (perinium) in F
Enlarged throat sac and
facial swelling M orang
Prominent breasts in
Sexual dimorphism among apes reflects type of competition for mates
Why gorilla dimorphism?
Behavioral diversity among hominoids
among primates is probably greater than for any other mammals. (Why isn't this
“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
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.
suggests differences may be exaggerated comparing captive bonobos with chimps
in their natural habitats. The
jury is still out as far as I know.
social behavior of chimpanzees and bonobos: Empirical evidence and shifting
assumptions. Stanford, Craig B.; Current Anthropology, Vol 39(4), Aug-Oct 1998.
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
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 –
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
• "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
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
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
Early experiences and mate choice
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?”)
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
Muiriqui monkeys: fertility and
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
Fifty percent of matings occur between
ovulations when probability of conception is low. See Fig.3.5.
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
Sex for food,
Not needed for
fertility -- surplus fat?