Each one of us is result of a complex synergy among these and other variables. Our differences result from differences in our genetic information, personal experiences, and importantly for these factors -- small differences in hormones and other biochemical factors.
Variation of course is necessary for the diversity of life; imagine the effect of a viral epidemic on a cloned population where everyone has identical genetic makeup.
Similarly, variation enables occupation and creation of new niches and ultimately new species.
Another important function of sexual reproduction may be the redundancy of genetic material in each cell due to the two copies, one from each parent. Damage to DNA may not result in loss of crucial information.
This is certainly a recent development in primates but obviously important in bonobos and humans.
Darwin
suggested there were actually at least two selective forces--natural selection
and "sexual selection."
"This depends on the advantage which certain individuals have over other
individuals of the same sex and species, in exclusive relation to reproduction.
p.256...It is certain that with almost all animals there is a struggle between
the males for the possession of the female...hence the females, supposing that
their mental capacity sufficed for the exertion of a choice, could select one
out of several males. 259."[1]
This can lead to a genetic contest among males to develop the most attractive
behaviors, plumage, and whatever to attract the favor of females of their
species --developments that may have no other virtue and even be potentially
maladaptive on other dimensions, e.g. the peacocks tail.
This intra-sex competition is one aspect of sexual selection; the other is the
basis for the "female choice" in selecting one male over another who is
competing for her attention. This leads to a complex interacting process. As
Small (1993) puts it: "males have a trait, females like it and mate with them.
females then have sons with the trait and daughters with the preference. The
trait and the preferences are inherited together over and over, coevolving with
each other. p.93"
_______________________________
Genes
survive by constructing phenotypes which reproduce those genes and are not
"filtered" out by chance or inability to adapt to environmental circumstances.
Any gene contributes positive and negative effects, depending on circumstances,
to its phenotype's success in reproducing that gene.
See "evolution notes" for more details.
Previously we have seen this the case for everything from neurons to forelimbs. Even sperm cells carrying male or female chromosomes look different; the females are larger and somewhat slower moving.
Keep
in mind that all primates, both sexes, are built on the same basic body plan.
Variations are largely due to differential timing of growth patterns, extending
some dimensions, shortening or inhibiting others by genetically regulated
hormones. These include androgens, estrogens and other female hormones, as
well as the important human Growth Hormone (hGH).[2]
It is worth noting that these hormones are instrumental in body "construction"
as well as mediating differences in behavior. Both sexes respond, although
differently, to the same hormones.
Despite differences in function and anatomy; males and females share a common anatomical plan. Just as species are variants of that plan, so are the two sexes of each species variants of the species "plan."
All
mammals follow similar paths in developing the males and females of a species.
The process starts with a male or female chromosome-bearing sperm cell
fertilizing an egg cell, which triggers of a chain of events that builds a male
or female body governed by hormone levels. The resulting body and its pre- and
post-natal interactions determine the individual's behavior.
(See detailed notes.)
How do male and female primates differ from each other? What significance should be attributed to differences in overall body size or breast, penis, and testicle size? Even more interesting perhaps, how do the behavioral differences between the sexes emerge and contribute to the evolution of the species?
(review these differences in size, shape, and behaviors)
Fossil
remains indicate declining differences in body size and teeth and as brain size
increased. This suggests, perhaps, that competition among males declined and
closer interpersonal relationships between males and females developed. Both
males and females were reduced in size; males more so than females. (Borgognini
Tarli and Repetto (1997) in Morbeck et al (1997).
These differences are due to differing growth rates; males being slower and
reaching maturity later.(Actually measuring these differences is not trivial
due to allometry and subgroup variation.)
How do the male and female primates of the various species differ from one species to the next?
With
the very important exceptions of humans and bonobos, female primates primarily
mate when they are fertile, within a short period around ovulation.
(The female primate reproductive cycle is a pattern of ovulation, mating, and
menstruation if fertilization does not occur. Only a few species -- including
chimps and humans -- show traces of external bleeding during menstruation.
Estrus refers to a short period, generally around ovulation, when the female is
receptive to or even actively soliciting copulation.)
Female
NHPs typically signal when they might be interested in mating with visual,
behavioral, and/or olfactory cues. While common chimps and to a lesser extent,
bonobos have flamboyant pink swellings indicating estrus, most female primates
have little or no swelling. Only 22 of over 200 species have these swellings
--many of them macaques and baboons, as well as the chimpanzees. (Small,
1993).
Even in common chimps, some copulation may occur outside of the estrus period.
Gibbons, orangutans, and gorillas have only very marginal visual changes, unlikely to be of much significance. Humans have none and may engage in intercourse more or less anytime.
(hair, face, breasts, lips, behavior, etc.?)
(hair, face, muscles, height, etc.?)
The
obvious function is to better pass on the genes of the participants! Humans,
while not greatly different in size, have specialized their reproductive
processes for great efficiency, enabling reproductive rates two to four times
that of our large ape cousins.
(Note here and below synergistic factors here of extreme neonatal immaturity,
MPI and food provision, nursing and fertility, loss of hair, etc.)
The large apes are characterized by single births, slow weaning and maturation, with extensive maternal care invested in each offspring.
(See table 8.1 overhead for apes).
Group size of 40-60, 4.7 children, 44.1 birth interval, suckling frequent and an important part of diet until 3. Compared to urban women, they have 15 years suppressed ovulation while nursing vs little or none?, later menstuation and earlier menopause and fewer menstrual cycles -- compared with 35 years of menstrual cycling in urban females today.
Young apes in the wild may become sexually mature in eight to ten years. A female chimpanzee may have her first infant somewhere between 13 and 15 years; considerably sooner in captivity where diet and body weight may speed up maturation.
Because
females invest so much more in their offspring (see below) than males, it is
usually argued that they must be extremely careful in mate selection if they
are to succeed in passing on their genes[3].
They need to get both quality genes and when relevant, protection and resources
for themselves and adequate male parental care.
(Males in contrast with many cheap sperm to spread around, have no need to
exercise such care. Darwin seems not to have considered this specific point
(Trivers, 1972)).
In primates females exercise varying degrees of choice including making sexual
advances, responding to some males but not others, and in species like humans
and bonobos, maintaining sexual relationships beyond their periods of
fertility. Small (1993) suggests, however, that the reality may be somewhat
different in that female NHPs may --like some human females -- mate simply for
pleasure or other non-reproductive reasons.
She points out that female-female sexual interactions are somewhat common,
especially among bonobos, and these cannot be reproductive in nature. Some
serve a bonding function; others perhaps just a result of hormonal motivation.
(p.146).
"the human female is encumbered to an extent not seen in other species.
Because of the extremely dependent state in which young are born and because of
their slow development.....the human mother must continue to invest high levels
of parental care in several young simulataneously....she adds to her
encumberment and goes farther and farther into "debt" in the sense as her
dependents multiply, but her physical reservior of energies remains the
same....nearly all human groups attempt to regulate access to the reproductive
capabilities of women by designating a mate (husband) and making him and hiskin
share in the work of rearing or defending the children. (p. 226 Draper (1997)
in Morbeck et al, 1997)
The duration varies somewhat among the large primates, eg. 7 months for chimpanzees, 9 for humans.
Nutrition
demands double when nursing.
A reduction in suckling is primarily responsible for the restoration of
ovulation sometime after birth.
This is contribution of sperm and seeking a mate at the minimum -- providing protection, food, and child care resources at the maximum.
In most nonhuman primates this is largely a maternal function.
Language in humans, food skills in non-human primates are obvious examples.
Probably all primate groups, including humans, have "dominance" -- high status with certain privileges, e.g. preferential access to food, mates, etc. Dominance may be inherited or acquired due to size or other characteristics. Physical aggression may play a role but is not the driving force in primates.
Pusey,
Williams, and Goodall (199x) show that female chimpanzees in Gombe National
Park, Tanzania, vary in their fitness and that fitness is correlated with
dominance. If it seems odd that it has taken 37 years for Jane Goodall's Gombe
studies to yield data on female reproductive success, consider that the
chimpanzee social community contains only 10 to 15 females at any one time,
with a mean interbirth interval of 5 years between surviving offspring. Half
of the breeding females are immigrants, often of unknown age. Many females meet
each other rarely, and even then clear-cut dominance interactions are
infrequent, so that it can take several years for observers to detect the
dominance relationship of any given pair. But by indexing the dominance ranking
of the females over 2-year periods, Pusey et al. found that status correlated
with at least three measures of fitness for breeding females. (from the
Wrangham, 1998 Science paper)
(See aggression notes)
(See Table 2.2 in Bard (1995 "Parenting...")
Each species has its own practices in forming groups. Generally either the females or the males of the species leave their birth group when they become sexually mature. Females gorillas and chimps leave "home" and fall in with another group. The males orangs and gorillas typically must establish their own group. All these practices function to increase genetic variability in the populations.
In addition to the need to prevent excessive inbreeding, social structure of groups probably reflects individual species reproductive adaptations. Male gorillas, for example, may not compete like chimps in sperm production but do quite well in "hand to hand" combat!
Food resources are an important factor in group structure. For example, from one perspective orangs and gorillas have similar social structures except for the great distance between the individual females and the dominant male in orangs. This seems to be a result of availability of the preferred foods, easily obtained leaves and shoots for gorillas and widely dispersed fruits for orangs.
These may be reflected in the size differences between males and females.
Despite the superficial structure observed in the various species, the actual breeding patterns remain somewhat unknown without an accurate account of paternity. This is only likely with DNA analysis or implausibly close and continuous observation. (See Small, 1993, for discussion.) Recent DNA studies of gibbons and monogamous birds have demonstrated a certain amount of infidelity exists. Very recent DNA studies of chimpanzee matings revealed that a certain percentage of offspring were fathered by males not in the group. See "Furtive matings.......")
the
"Coolidge effect" (Small, 1993, p.179)
"More than any other variables that stand out to our human eyes, novelty and
variety, appear to be the preferences of female nonhuman primates...183"
There are varied opinions on the importance of these temporary pair-bonds between a female in estrous and a male of her group. They appear to increase chances of a male's fertilizing a female as he can more or less control access to her around ovulation. DNA studies are necessary to confirm this.
Theoretically females might want to mate with high ranking males, both for their presumably "quality genes" as well as protection and in some cases, increased sharing of resources. However the evidence is mixed. (Small, 1993).
This seems like a good idea but there's not much evidence for any species.
W&P summarize three patterns of male violence in each of orang, gorilla, and chimp societies. These are not random acts of violence, but specific patterns that make sense in the particular social context and further the genetic interests of the violent males.
Fertilization or dominance? "The life histories of gorillas - and bonobos ...show that rape is not inevitable if you are an ape. (Wrangham & Peterson, 1996, p.142)
Perhaps the best predictor of violence toward a primate is the number he or she is traveling with at the moment.
"In
a typical interaction, he (a young adult male chimp) might charge at the
female, hit her, kick her, pull her.. jump on top of her huddled and screaming
form, slap her, lift her and slam her to the ground, and charge off again.
This never becomes the all-out sort of wild gang violence directed against ...
other communities. The female is not killed, rarely injured. W&P 143"
Goodall notes that male chimps are not very "nice" to females and that unless
he old or crippled, a male can always coerce a female in copulating with
him.
There are some scenes of intimidation in the Family of Chimps video at the
Arnhem Zoo.
Infantcide
is a part of a number of species typical behavior --including humans in some
cultures. In humans it occurs deliberately with unwanted children, often
females. It may occur less obviously in extreme child abuse that may have more
commonality with gorilla infantcide.
W&P estimate 1 of 7 gorilla infants is killed by adult males; most of these
unprotected by a silverback (in all likelihood the infant's father). By
killing infants, males that take over a group put the females into a new
reproductive cycle and further their own genetic advantage by impregnating
them.
More surprising, are cases where the father silverback successfully defends
himself but the invading male kills one of the infants. W&P report that
this may serve to draw the mother of the slain infant to the killer!
"When another male does break through the defenses and kills her infant, she
responds in a way that violates all our assumptions about attachment, loss, and
revenge. It may take a few days before the female leaves her troop, but the
evidence is clear. Infantcide draws a female to a male. She leaves her old
mate and joins the killer....The female's choice is imposed by the logic of
violence, by the threat to her next infant. 151"
"The hugs and kisses and embraces of the apes are as elaborate as their use of brute force...Intelligence turns affection into love and aggression into punishment and control. WP, p.152."
WP
answer their rhetorical question as follows:
"...the curse stems from our species own special party-gang[4] traits: coalitionary bonds among males, male domination
over an expandable territory, and a variable party size. The combination of
these traits means that killing a neighboring male is usually worthwhile, and
can often be done safely. 167"
Bonobos are only about 2 million years separated from the common chimps and bear a number of even closer morphological and behavioral similarities to humans. Yet they apparently have little of the violent aggression common to their other cousins: orangs, gorillas, chimps, and humans.
W&P
offer a complex explanation for this unique species.
"Females form the core of bonobo parties, so that in small parties there are
more females than males.. Females spend their time closer to each other than
males do. Females are more likely to be in the center of the party, with males
on the outside. And they form alliances that effectively protect them against
male aggression. 227"
This development was made possible, in their theory, by a cool drought some 2.5
million years ago, driving gorillas out of a small region around the Zaire
river. When the drought ended, the herbs that are the primary diet of gorillas
returned and local chimps adapted to that diet. Without competition from
gorillas, they prospered and no longer had to forage in small groups as did
their recent ancestors and today's chimps. Traveling together in large
parties, the arboreal bonobos, male and female, were able to protect themselves
from violent aggression from their own males.
One might speculate that sexuality and neotenous characteristics of bonobos
evolved in response to these circumstances, while the violent traits were
submerged --perhaps a simple adjustment in hormones and neurotransmitters as
the more violent males were filtered out. This would be quite an achievement,
though, since aggression is such a successful reproductive strategy.
Even though bonobos are not closer genetically to humans than chimpanzees, they may share more characteristics because of their neoteny which may influence everything from bipedal ability to face to face sexual intercourse. (See Gould, 1977 for characteristics of humans perhaps neotenous.)
This may work in curious ways -- even modifying hormone levels.
Many cultures have been polygamous; many nominally monogamous. Yet fidelity is varied and alternatives are commonplace.
Polygamy was widespread in early historical times and probably before. Financial resources remain important considerations for both males and females in regard children.
Males everywhere prefer "attractive" younger women and females prefer "older" men with resources. (No kidding!) (See his BBS article.)
"Why are tough guys popular? Most of the guys ..."
"On
theoretical grounds, it is clear that dominating mechanisms -- whether
aggressive or non aggressive in form -- would have evolutionary advantage in
helping an individual acquire valued resources, especially in competition for
mates. This is not simply a matter of a dominant man taking what he wants;
women regard men who look dominant as attractive (Townsend 1993). Teenage men
rated by naive judges as having "dominant looking" faces (often with prominent
chins, heavy brow ridges, muscular rather than fleshy or skinny faces) report
copulating earlier than their submissive-looking peers, presumably in part
because they have an easier time finding willing partners (Mazur et al. 1997).
"
(Perhaps not coincidentally, these characteristics are also consequences of
taking hGH to enhance athletic performance.)
It is not obvious why there would be selective advantage in aggressiveness per
se, apart from its dominating function.
Females
of at least 20 of 200 primate species signal ovulation by rear-end swellings
and skin color changes, oflactory cues, or specific behavioral gestures.
Only chimps, among apes, have obvious swellings. The bonobos are apparently
permanently "swollen" and (continuously, 3 of 4 days? See de Waal plot.)
receptive to sex unlike the common chimp female who may be receptive 10-15 days
each cycle.
Small (1993) following Hrdy & Whitten, 1987) notes that chimps may NOT be
such a good model for human female sexual behavior and that contrary to lots of
conjecture, human ancestors may have NEVER had overt sexual swellings as do
chimps. p. 129
Gorillas (1-4 days)
Orangutan (5-6 days)
This probably depends on nursing patterns and diet of both mother and infant. Chimps and gorillas may resume in 3-4 years; bonobos much sooner and orangs longer. See Eliason reading for humans where it clearly depends on nursing patterns.
In many primate species, female infants stay with their mother and in her group, while males leave or are forced out. In others the reverse is true; chimp females for example, generally leave the natal (birth) group. Gorilla young of both sexes typically leave their natal group.
(see
overhead from deWaal (1995) Sci. Am.)
The apes certainly illustrate this; each species has a different arrangement.
Even among a given species there may be variation. For example gorillas are
described as having a haren arrangement, yet in 40% another younger male--often
the brother or son of the silverback-- may mate with the 6 to 8 females of the
"harem."
Mating behavior itself is extremely variable; gorillas copulate infrequently
with a copulation lasting 80 seconds while female bonobos mate continuously and
engage in lots of sex activity, including homosexual, as a means of gaining
food and forming alliances.
Orangs present another system altogether (though it may be seen as a
wide-ranging harem in which the females are not in contact.)
Female chimpanzees come into estrus once a month. They have a 35-day menstrual
cycle and can breed at anytime of the year. The gestation period is more than
seven months long, and a single offspring is produced.
Small (1993) argues "orgasm for human females is the necessary feature of our reproductive biology which evolved to to take the place of estrus...anticipated pleasure for BOTH sexes is most likely an initiator to begin mating behavior. p.148"
While
size, dominance, fathering skills may play a role in a given species, the
evidence is sparse that this really matters in general. Small (1993)
summarizes research on rhesus monkeys--the best studied primate--and reports
the females prefer a variety of mates, especially outsiders, new to their
group. This of course is a natural protection against excessive inbreeding.
"Their interest in novelty, therefore, might be a primitive urge to find mates
lurking at the periphery of their group with genes different, but not
completely different from their own. p.171" Thus choice on this basis
increases the genetic diversity of the species, hence its overall
fitness. (Of course this may not always be a virtue if successful adaptations
are LOST; cf role of birdsong dialects)
A variety of mates with a promiscuous female also ensures that enough sperm are
available at the moment of ovulation, since repeated matings with the same male
will not provide the same number of sperm each mating.
Small says, while these are speculative explanations, they "pull these females
out of the negative light of promiscuous sex-crazed harlots into the arena of
strategizing creatures trying to improve their individual reproductive success.
p.182"
Small suggests the wild orangs support her idea that in general female nonhuman
primates show a preference for fully mature, sexually able males by resisting
efforts by younger males to mate with them. p.155
"Status, hierarchies, and rank are certainly part of our primate heritage, but
unlike ourselves, nonhuman female primates seem to focus less on status then we
do. A fancy car won't do a male ape much good.....p.160"
Much of this fits with the recent findings based on DNA analysis of chimp
infants (Furtive mating...1997). Females to a certain extent bear offspring of
males NOT in the group, presumably of their "choice."
Male
chimps are usually unkind to females and Goodall suggests females prefer the
"nicer" males.
"Bonobos have sex more often and in more combinations than most people of any
culture, and most of the time bonobo sex has nothing to do with reproduction.
p.175"
de Waal relates many instances of tension reduction apparently the primary
function of a sexual encounter. Generally these involve genital stimulation
but apparently no orgasm or ejaculation.
(Another way to look at this is to treat ideas as shaping the brain along with other such factors as genes, nutrition, hormones, etc. Note the parallel with language.)
These all act synergistically in human evolution; for example nutrition speeds up sexual maturity hence reproductive rate.
A normal 2000 calorie woman requires double that when nursing.
Virtually
all humans have a marriage ritual in which men tend to marry one woman at a
time.
"Female humans, like chimpanzees, tend to initiate sexual activity", e.g. 2/3
of pickups in bars! Compare with studies of captive chimps showing 85% of
sexual acts initiated by females.
Adultery is widespread.
"Marriage has a cross-cultural pattern of decay. 112"
"The human animal seems built to court, to fall in love, and to marry one
person at a time; then, at the height of our reproductive years, often with a
single child, we divorce; then a few years later, we remarry once again."
She attributes pair-bonding to need for successful raising of infants. "What
made me think of this was a remarkable correlation between the length of human
infancy in traditional societies, about four years, and the length of many
marriages, about four years. .153" (high mobility, weaning of young).
(She gives a lot of caveats on the generality of these comments immediately
afterward!)
David Buss has summarized cross-cultural evidence on human mate choice and preferences. Some (not surprising) findings are:
[1]Of course it can be that a certain trait
like "size" is ambigous --possibly selected for by female choice or as result
of male-male competition. Only observation can determine if there is extensive
male competition. (See Boinsky, 1987).
[2]This hormone, hGH, has recently been
synthesized and is used to enhance athletic performance, with some negative
consequences. "The hormone of the moment, hGH, can cause disfigurement by
encouraging growth not only of muscles but also of bones, especially in the
feet, hands and face. Some hGH users develop jutting foreheads, prominent
cheekbones and an elongated jaw. (In Olympic circles an athelete with a
pronounced jaw is sometimes said to have a hGH jaw. (Sports Illustrated,
p. 70, April 14, 1997).
[3]Obviously females have no direct way to
assess genes other than indirectly through signs of health and relevant skills
and possession of resources.
[4]Their definition of "party-gang" species are
those species in which there are "parties"--as in war parties -- or temporary
groups, from lone individuals to an entire community. Chimps probably are the
prototype here, varying from 10 or less to 20 or more. The coalitionary bonds
refer to the relationships among the same-sexed adults used in aggression
against others of the same species, usually same sexed. See W&P p. 165.