Sex and Evolution
Diverse Methods of Reproduction Have Evolved
I. Scientists have discovered nothing special about our position on the evolutionary tree; we are just one leaf out of many
A. Human sex lives are remarkably diverse but occupy only a fraction of the range of sexual behaviors that exist or have existed
B. Evolution states that individuals within a single species differ slightly from one another
1. In part, these differences are heritable, passed from parent to offspring
2. If situation favors survival & reproduction of animals with a certain heritable trait, that trait will be passed on to more offspring & will become more prevalent in succeeding generations
C. Heritable traits are encoded by genes strung out along the chromosomes in the nucleus of each cell in an organism
1. Human genome is estimated to contain 20,000 – 25,000 genes; any 2 people will show differences in many of these genes
2. The main ingredient in evolution is lots of time
II. Earliest organisms were microscopic & single-celled organisms without nuclei that reproduced by splitting in two
A. They also readily swapped DNA between individuals; this process was probably disorganized & may simply have involved the ingestion of DNA fragments released by dead organisms
B. Present-day bacteria also swap DNA, but they do so through a mechanism called conjugation
III. Eukaryotes (nucleated organisms) probably appeared between 3 and 2 billion years ago
A. Initially, they were all single-celled (like amoebas) followed by multicellular eukaryotes (animals
& plants) ~1.5 billion years ago
B. Eukaryotic animals diversified during the Cambrian explosion, 545 million years ago, when all the main kinds of body plans that now exist, first appeared
C. Eukaryotic cells usually divide by mitosis resulting in 2 new daughter organisms; this is a form of asexual reproduction; offspring carry the genes of a single parent
D. Nearly all eukaryotic organisms (even single-celled amoebas) have the capacity for sexual reproduction; offspring carry a mixture of genes from 2 parents
1. Mixing occurs through the fusion of 2 specialized cells (gametes), each of which contributes chromosomes to the new organism
2. Except for lab animals produced by cloning, mammals are the product of sexual reproduction; most vertebrates reproduce exclusively by sexual means

Explanations for Sexual Reproduction
I. Sexual reproduction must be adaptive (it must help an organism to perpetuate its genes in future generations) or so many multicellular organisms would not do it or rely on it exclusively
A. Asexual reproduction seems to be more adaptive on the face of it than sexual reproduction
1. An animal; that reproduces sexually devotes all its resources to passing on its own genes
& those genes are perpetuated in all of its descendants
2. An organism that reproduces sexually dilutes its genes with those of another, thus reducing representation of its own genes in future generations; this seems like pointless self-sacrifice
B. If a single parthenogenetically reproducing female arose in a population of a million sexually reproducing organisms, other things being equal…..
1. It would take <50 generations for her clonal descendants to replace the whole population
II. Sexual reproduction may limit harmful mutations – some believe that sexual reproduction is adaptive because it helps organisms deal with the problem of harmful mutations
A. Mutations are random changes in organism's genome caused by errors in DNA copying or by damaging chemicals, sunlight or radiation
B. When organisms reproduce asexually, harmful mutations accumulate over generations
1. Since all descendants of an asexually reproducing organism have exact copies of the parent's genes, there is no way to get rid of a harmful mutation short of eliminating that entire lineage
C. Most organisms carry numerous harmful mutations; thus each organism is likely to inherit some harmful mutations from each parent
1. Some offspring will receive a greater load of harmful & some will receive less
2. Natural selection will favor the survival & reproduction of the offspring with fewer harmful mutations
D. This advantage should outweigh the disadvantage of gene dilution but only under certain conditions
III. Sexual reproduction may generate beneficial gene combinations – this ignores the matter of harmful mutations & attributes the value of sexual reproduction to the novel combinations of genes it produces
A. Offspring with different combinations of genes might be useful because those offspring would have different ways of utilizing the resources available in the environment (like different foods)
B. Alternatively, the mixing of genes might be useful in dealing with environmental changes – evidence
1. British investigators compared growth rates of yeast cells from 2 different strains
2. In a normal, benign environment, the cells of both strains reproduced at an equal rate

3. When yeast cells were stressed by placement in a high-temperature environment , however, the sexually reproducing cells reproduced faster than those that reproduced asexually
C. Many organisms engage in an endless war with parasites, in which the parasites are constantly evolving new ways to outwit the host's defenses
1. Gene mixing may be an effective way for hosts to rapidly deploy new defenses against these ever-changing attacks
2. Called the Red Queen Hypothesis of sex; it is named for the Red Queen in Alice Through the
Looking Glass, who had to keep running to stay in the same place
IV. Which of these two explanations is true, we do not know, maybe both are; research may clarify the issue
A. Observation of host-parasite interactions in natural, isolated environments like lakes – these have lent support to the Red Queen Hypothesis
B. Another line of research is in silico evolution – the evolutionary process is modeled by a computer
C. Worth studying the few species that never engage in sexual reproduction
– if analyze how these species survive without sexual reproduction, we may learn why sex is so essential for rest of us
Why Are There Two Sexes?
I. Males & females – males are individuals with small gametes (sperm); females are individuals with large gametes (ova)
A. Sex without sexes might well be an ideal arrangement for a species
1. But natural selection operates at the individual level & not at species level, so it does not necessarily produce arrangements that are ideal for the species as a whole
2. Rather, it produces compromises — states in which the conflicting interests of countless individuals are in dynamic equilibrium
3. Sex without sexes is not generally an equilibrium state
B. Hypothetical example of sex without sexes
1. Gametes of all individuals are roughly the same size & nutrient content; even so, there will be some natural variation
2. Some individuals will make slightly larger gametes & some slightly smaller ones
3. Over time, natural selection will favor individuals that produce larger gametes (more nutrients) because they stand a better chance of developing into offspring
4. Individuals with smaller gametes will also be favored, because they require smaller investment

C. Other questions come into play
1. Mobility – to fuse gametes need to come together, which means that at least one gamete must be motile; it is much easier for small gametes to be mobile so male gametes are usually motile
2. Also numbers – since small gamete requires such a small investment to produce, males can produce many more gametes than females
II. Hermaphrodites combine male & female reproductive functions – they combine male & female reproductive functions within a single body
A. In some species, including most flowering plants & trees as well as some invertebrate animals
(worms, snails), all the individuals are hermaphrodites; there are no pure males or females
B. One might imagine that hermaphrodites would fertilize themselves but such self-fertilization would nullify much of the genetic advantage that sexual reproduction is thought to confer
C. In hermaphroditic animals, there are often behavioral mechanisms that prevent self-fertilization
1. The animals simply do not inseminate themselves, even though they are physically capable of doing so
2. Some hermaphroditic mollusks engage in a bizarre behavior called penis fencing
III. Can there be more than 2 sexes? – there is no known species in which individual organisms have >2 genetic parents; 2 sexes seem to be the limit on Earth
A. However, many simple species have multiple mating types
B. There are also many species in which individuals of the same sex can have different reproductive roles
1. Some individuals of one sex may be temporarily or permanently sterile or reproductively inactive
2. Well known examples are found among bees, ants & termites, in which only a single female in a colony (the queen) is fertile; the rest are sterile workers
IV. Why are there equal numbers of males & females?
A. At first glance, the desire of most humans to form sexual partnerships with a member of the opposite sex, imbalance in the numbers of males & females would leave many without partners
B. Roughly equal numbers of males & females are also seen in species in which males & females don't partner up or do so in very unequal proportions
1. Among langur monkeys, a dominant male controls a harem of 12 or so female partners

2. Among elks, a single bull controls up to 60 females
3. In species like these, might it not be better to have more females than males?
C. But one must understand that evolution does not necessarily lead to what we might consider to be the ideal arrangement for the species
D. Imagine that the "ideal arrangement" really existed, so that among langur monkeys, a female typically gave birth to 10 daughters for every son born
1. There would then be 10 times as many female langurs as males
2. For awhile, things would be idyllic; each male will have his harem, every female will have all the offspring she is capable of & resources will be distributed efficiently
3. This would not last; if a particular female underwent a genetic change such that she produced more sons than daughters, she would gain a tremendous advantage
4. Conversely, if there were a species in which females had 12 sons for every female born so there was vast excess of males in population, genes for producing females would be an advantage
E. This general principle is liable to be modified by a number of factors, however, so one does not always get a sex ratio of exactly 1:1
1. Among humans, males are more vulnerable than females; at every phase of life from conception on, males are more likely to die
Evolution Has Led to Diverse Methods of Sex Determination
I. Given that so many species in animal kingdom have settled on two-sex system, one might expect that mechanisms of sex determination would also have been fixed early in evolution & now be universal
A. Sex determination is the mechanism that controls whether an embryo becomes a male or female
B. However, a variety of sex determining mechanisms have evolved
II. Sex may be determined by chromosomes
A. In humans & most mammals, an embryos sex is determined by the chromosomes it possesses
1. 44 of our chromosomes are known as autosomes; they come in 22 homologous
(corresponding) pairs, regardless of a person's sex
2. The remaining 2 chromosomes are sex chromosomes & the situation is more complicated
B. Gametes are produced by meiosis, a process of cell division in which the number of chromosomes is halved

C. If one studies humans with odd chromosome combinations (X, XXX, XXY, XYY), one sees that a person with at least one Y chromosome will be male & all others females
1. This indicates that there is a sex-determining gene on the Y chromosome
III. Sex may be determined by temperature
A. Many species of reptiles lack sex chromosomes – the sex of the offspring is determined by the temperature at which the eggs are incubated
1. In a turtle known as the red-eared slider ( Trachemys scripta ), clutches of eggs incubated at 26°C (79° F) develop as all males
2. Among lizards & alligators, it's the other way around - low temperatures produce females; high temperatures produce males
B. This mechanism of sex determination, though strange, must have some value to species using it
1. It allows animals to determine the sex of their offspring, by laying eggs under a shady tree or on a sunlit bank or earlier or later in the season
2. Just having offspring of all one sex could be beneficial — by eliminating inbreeding among the offspring, for example
Sexual Selection Produces Anatomical & Behavioral Differences Between Males & Females
I. In many ways, natural selection acts similarly on females & males it ensures that both men & women are adapted to life on land & that male & female fish are both adapted to life in water
A. Yet marked differences can develop between males & females of a single species (ex.: peacocks & peahens)
B. Such differences in appearance & behavior of males & females result from competition for mates; Charles Darwin called this process sexual selection
II. Males & females follow different reproductive strategies – 2 common though not universal features of sexual selection among nonhuman animals are competition among males & choice by females
A. These features result from the differing strategies adopted by males & females at the origin of the 2 sexes
1. Females commit themselves to a nurturing strategy by virtue of their investment in large, nutrient-rich ova
2. Males commit themselves to an exploitative strategy by virtue of their production of small, nutrient-poor sperm
B. Female mammals carry the burden of internal fertilization followed by a prolonged period of gestation (pregnancy), which may last from 2 or 3 weeks (rodents) to 22 months (elephants)
1. After delivery of young, female mammals continue to nourish them through lactation (milk production) & usually provide most or all of care & protection needed by mammalian infants

2. This prolonged investment results in offspring that have a greater chance of surviving to adulthood than young of other vertebrates, but…..
C. Males, however, can often get away with a very small investment in reproduction, a few drops of semen containing sperm
III. Females & males are exposed to different reproductive risks
A. For a female, the maximum number of potential offspring is rather low, but her chances of having close to this number are quite good, since there will usually be many males willing to mate with her
1. The variation in the number of offspring that females produce is quite limited
2. The risk for a female is not so much that she will produce few offspring, but that her offspring will fail to survive & reproduce in their turn
B. Males can father numerous offspring, but they can easily end up having none
1. In langur monkeys & elks, dominant males control large harems or females, leaving subordinate males without mates
2. Although there are wide differences among species, males typically face the real possibility of having few or no offspring
IV. Males often compete for access to females
A. Because of the differences in reproductive risks experienced by males & females, males often compete with one another for access to females, while females often choose among males
B. What traits are influenced by sexual selection?
1. Competition among males naturally leads to selection for traits that confer success in that competition
2. Competition among males also favors traits that assist males in locating receptive females before other males do
3. Another trait favored by male-male competition is sexual endurance – the ability to remain reproductively active for a long time

V. Females often choose among males - females can choose to mate with healthy, genetically favored males, but how can they identify such males?
A. Beating other males into submission speaks volumes about his health & fitness, but not all species engage in such male-male contests
B. Choice based on appearance
1. Females often pay attention to males' bodily symmetry
2. Females often look for other anatomical characteristics in males
3. The preference for bigger & brighter seems to be open-ended
a. If one exaggerates features females pay attention to (giving male barn swallow an artificial tail longer than any male normally possesses), such males will be preferred over any natural males
b. Thus, sexual selection can lead to a runaway process in which the display characteristics become highly exaggerated as with the peacock
4. It is precisely the fact that these displays have a cost that makes them attractive
C. Choice based on behavior – females also choose on the basis of courtship behavior
1. Sometimes this behavior is of practical use to the female in producing young
2. Besides the direct value of such gifts to female, there is another more subtle benefit
3. There are many instances in which courtship by males seems to involve useless make-work
a. Male bowerbirds must build elaborate thatched structures (bowers) & decorate them with hard-to-find items (colored shells, berries, bottle tops) before females will pay attention to advances
b. Courtship song is another example: singing for hours at a time offers no direct reproductive benefit, but by doing so, a male bird advertises that he is not foraging

4. Females often initiate courtship
a. Caterwauling of cat females advertising that they are sexually receptive
b. Females may also approach individual males & show proceptive behavior, behavior designed to elicit reciprocal courtship
VI. Sometimes males make significant investments in reproduction – if females make males invest in reproduction, they may restore some balance between female & male reproductive strategies
A. The more resources males invest, the more interest they will have in ensuring their investment is not wasted
1. If male has to spend days or weeks wooing female or if he has to provide expensive nuptial gifts or do difficult tasks, he may get as committed as female to seeing that offspring survive
B. Thus, while there are many species in which males make little or no contribution to care of their offspring, there are other species in which males make contributions as great as those of females
C. In a few species, males take on the entire responsibility of caring for eggs or young
1. A male stickleback fish constructs an underwater nest in which females lay eggs; after fertilizing them, the male spends about 2 weeks guarding the eggs & the newly hatched fry
VII. If males invest, sexual selection may work differently
A. If males & females invest about equally in reproduction, sexual selection may not lead to any marked anatomical or behavioral differences between the sexes
B. If males invest more than females in reproduction & thus are limited in how many offspring they can produce, the effects of sexual selection on the 2 sexes can actually be reversed
C. Sometimes males choose among females because individual females vary in how much they can invest in reproduction
1. This variation is most obvious in species in which individuals continue to grow after reaching reproductive maturity
D. Among some primate species, males invest considerably in reproduction, even if not to the same degree as the females; for that reason, there may be competition in & choosing by both sexes
1. Male baboons are larger than females & compete intensely for mates as is true in so many other species

2. In addition, however, female baboons compete for the sexual attention of dominant males
a. They do this by means of sexual swellings, patches of pigmented genital & perianal skin that swell around the time of ovulation
E. It turns out that competition among females is much more common than might be imagined on the basis of the traditional "males compete, females choose" concept of sexual selection
Diverse Relationship Styles Have Evolved
I. Evolution has led to a bewildering variety of sexual relationships, from sexual free-for-alls to lifelong, sexually exclusive pairings
A. Can start with the basic assumption that evolution is always at work
B. Animals' genes are likely to promote sexual behavior & relationships that offer them the best prospects for leaving copies of those genes in future generations
II. Social & sexual relationships are not necessarily the same – in looking at animal liaisons, we must distinguish carefully between 2 phenomena: social arrangements & sexual reality
A. In the past, people have tended to take animals' social arrangements at face value, as if these arrangements tell us unambiguously who is having sex with whom
B. In many species, individuals are essentially solitary or belong to same-sex groups, & they reproduce by mating with strangers (either one or many) whom they never see again
1. This pattern is seen in the majority of invertebrates, fishes, amphibians & reptiles
III. When reproduction does involve lasting relationships, we see 2 basic patterns: monogamy & polygamy
A. Monogamous relationships – 2 animals (usually of the opposite sex) form a pair bond for the duration of the breeding season or even for their entire lifetime
B. Polygamous relationships – one animal forms stable bonds with several individuals of the other sex; in most such species, single males form relationships with multiple females
1. Like harem arrangement in langur monkeys; it is technically called polygyny (many females)
2. Opposite arrangement, a single female with a harem of males, called polyandry (many males)
C. That polygyny is more common than polyandry is consistent with the greater investment in reproduction by females

D. Some animals (lions, chimpanzees) practice a more balanced polygamy in which social groups contain >1 sexually mature adult of each sex
1. Both males & females mate with multiple partners of the other sex
E. The terms monogamy & polygamy refer to bonding relationships, not to sexual behavior as such
1. Some pair-bonding species mate only within the pair bond & are therefore called sexually monogamous
2. More commonly, pair-bonded individuals will mate not only with their partners but also, on some occasions, with strangers; such species are called socially monogamous
3. Individuals of polygamous species may also mate with strangers
a. Willingness to engage in sex outside animal's established relationship(s) is promiscuity , without negative connotation the word sometimes carries when applied to humans
IV. Male promiscuity offers obvious evolutionary benefits - from an evolutionary standpoint, male promiscuity is more or less to be expected
A. Investment in mating outside pair bond (extra-pair sex) is usually so slight that it is worth it for male, even if the chances that the mating will lead to viable offspring are not very great
B. In some species, sexual monogamy may be imposed on males by females; females may simply refuse to engage in extra-pair sex; this is true for some species of birds
1. Sometimes males mate with only one female, simply because they only mate once – period; ex.: deep-sea angler fishes
V. Why are females promiscuous? – at first glance there seems to be no reason for it, since females can usually produce all the offspring they are capable of producing with the aid of a single male
A. In fact female, promiscuity is fairly common, even in species that have long been considered sexually monogamous
B. The best evidence for female promiscuity comes from DNA analysis
C. The results
1. In some socially monogamous birds, up to 3/4 of a female's offspring are fathered by males other than her social mate
2. Chimpanzees
D. Possible reasons for female promiscuity; they may differ among species
1. In species in which males provide resources such as food or protection, obtaining these resources from multiple mates may make promiscuity worthwhile
2. Socially monogamous females may be promiscuous in order to obtain sperm from higherquality males than their social partners
a. If males vary in quality, most females will not be partnered with the very best males, so they may seek a higher-quality male

3. A truly devious reason for female promiscuity may also exist
a. When the dominant male in a polygamous species is displaced by a new male, the new male may kill the young born to harem females over the following few months
E. Female promiscuity leads to adaptive responses by males – while female promiscuity benefits females, it harms the reproductive success of the males who are their social mates
1. Thus, males of many species have developed behavioral strategies to prevent their mates from engaging in sex with other males
a. A common behavior of this type is called mate guarding – a male remains close to a female throughout period when she is fertile & attempts to keep other males away from her
b. Demonstrated in an experiment on red-winged blackbirds – David Westneat
2. Males may also respond to promiscuous females by making large numbers of sperm; by sheer force of numbers, these sperm compete with sperm of other males in female's reproductive tract
a. Chimpanzees & gorillas – female chimps mate many times with many different males for every time that she gets pregnant; female gorillas mate with only 1 or 2 males per pregnancy
b. In humans, testis size is intermediate between that of chimps & gorillas,
3. No primate can compare with the pig in terms of sperm statistics
Males May Copulate With Females by Force
I. Reproductive interests of males & females can be in conflict with the result sometimes being forced copulation, which is seen in a wide variety of animals from insects to primates
II. In some of these animals, it is clearly an adaptive behavior; this means that it persists because those animals that engage in it have more offspring that they would otherwise have had
A. Most detailed such study done on scorpionflies by evolutionary psychologist Randy Thornhill
(Univ. of New Mexico); in these insects, a male is able to mate with a female by 1 of 2 strategies
1. He offers the female a nuptial gift (like a dead insect); the female approaches the giftbearing male & they mate
2. If the male approaches the female without a gift, the female will attempt to flee, but the male may grasp female & hold her immobile with a special appendage called a notal organ

3. Only a couple of other species (also insects) have anatomical adaptations that facilitate forced copulation
B. Quite a few species have the opposite, arrangements of the female anatomy that make copulation impossible without her active collaboration
1. A female rate's vulva is situated on the underside of her rump & is inaccessible to males unless she exposes it by arching her rump upwards, a behavior called lordosis
C. Most animals do not have anatomical specializations that either facilitate or prevent this behavior, but attempts at forced copulation by males, & resistance by females, are seen in many species
Sometimes Helping Relative Reproduce Is a Good Strategy
I. Genes act within individual who possesses them; thus, it seems that a genome should make its owner focus 100% of its efforts on reproducing itself; there is a lot of selfish sexual behavior in the world
II. In some cases, altruistic behavior can be adaptive, so that genes promoting it are favored by natural selection
A. Alternatively, it may be an evolutionary byproduct or, in the case of humans especially, it may result from cultural processes that have little direct connection with evolution
III. Examples of altruistic behavior with adaptive value
A. Parental care – in evolutionary terms, it is no use having offspring if those offspring don't have offspring in their turn
1. Thus, it may pay to help one's offspring survive & become sexually mature, even if that limits the number of offspring one can produce
2. Mammals & birds, in particular, have followed that strategy, but some insects also devote considerable resources to producing their young
3. Genes promoting parental care (or at least maternal care) are evidently widespread
4. Genes promoting altruistic behavior toward one's offspring survive because the offspring have a good chance of possessing those same genes
B. Parents & offspring are not the only kinds of relatives who inherit the same genes
1. Siblings also co-inherit one half of their genes, on average
2. First cousins co-inherit ~1/8 of their genes, etc.
C. The logic behind this theory was laid out by British evolutionary theorist W. D. Hamilton in 1960s
1. He proposed that natural selection causes individuals to devote resources to helping their relatives reproduce, to an extent determined by the degree of relatedness

D. Hamilton's theory is known as kin selection & central concept of kin selection is inclusive fitness
1. Kin selection theory says that in considering an individual's fitness, one has to consider not only how many offspring that individual produces, but …….
2. This is a little different from survival (& reproduction) of the fittest
IV. Kin selection explains some altruistic animal behavior – it explains quite a lot of social & sexual behavior in the animal kingdom
A. Subordinate males in lion prides & other groups may have few or no offspring of their own, at least as long as they are subordinate
1. If the dominant male is their brother or other close relative, however, it may still be worth it for them to remain in the group & help him reproduce
2. Kin selection also favors the development of "aunting" behavior in primates, the tendency of females to share maternal duties,…….
B. Probably the most successful application of kin selection theory has been to social insects, like bees & ants
1. Many individuals in these species are sterile worker females; they work tirelessly to help their fertile sister, the queen, produce enormous numbers of offspring – why do they do this?
C. Kin selection doesn't explain all altruistic behavior, however
1. Females may adopt orphans who are completely unrelated to them
2. Such behavior is not predicted by kin selection theory
Sex May Acquire Other Functions Beyond Reproduction
I. Reproduction is the primary function of sexual behavior, not the only function
A. We know this because individuals of many species engage in sexual behaviors that cannot generate offspring, like sex between 2 males or between 2 females – why do they do so?
B. Engaging in non-reproductive sex for pleasure alone would not be adaptive in evolutionary sense
1. But some species have found functions for nonreproductive sex that are truly adaptive; notable among these species is our close relative, the bonobo ( Pan paniscus )
II. Female & male bonobos engage in nonreproductive sex
A. Like most female mammals, female bonobos advertise when they are willing to copulate
(estrus) – bonobos, like baboons, do so by means of their genital swellings
1. In striking contrast to mammals like mice, however, the estrus of a bonobo extends over almost 2/3 of her entire ovarian cycle, which lasts ~2 months
2. In fact, the bonobo is out of estrus only for a few days around the time of menstruation
(the periodic shedding of the uterine lining)

B. Not only are female bonobos sexually receptive for most of their ovarian cycle, but they are also receptive when they are not cycling at all
1. Bonobo mothers nurse their young for several years after they are born, & lactation suppresses ovulation
C. Bonobos have 2 basic positions for heterosexual copulation
1. In one, the male mounts the female from the rear, as in nearly all other mammalian species
2. In the other position, the two animals face each other; this again is an evolutionary novelty
D. Further accentuating the nonreproductive nature of most bonobo sex, both female & male bonobos engage in frequent homosexual encounters
1. When 2 females have sex, they embrace face to face, & each rubs her swollen clitoris sideways against the other's until both females reach orgasm
2. Sexual encounters between males occur in one of two ways
III. Bonobos use sex for conflict resolution & alliance formation – all this nonreproductive sexual activity raises one question — why?
A. Close observation of bonobo colonies in captivity & in the wild indicates that bonobos use sex for the prevention & resolution of conflicts
1. 2 bonobos who are faced with a conflict in food allocation will engage in sex & then divide the food peacefully
B. A related function of sex in bonobos is the cementing of social relationships & the formation of alliances; this function is particularly important for females
1. Female bonobos leave their natal (birth) groups & join new ones, whereas males stay in their natal groups
2. Females joining a group are initially unwelcome, but they solidify their position by forming close alliances with high-ranking females
C. Because bonobos, like many humans, see more to sex than making babies, we might imagine that the bonobo's sexuality is ancestral to our own, but such an assumption would be risky
1. Chimpanzees, equally closely related to us, are far more restricted in use of nonreproductive sex
2. What is remarkable about the Hominoidea , the superfamily that includes gibbons, orangutans, gorillas, chimpanzees, bonobos & humans, is diversity of their sexual & social arrangements