In most primate species, who represent our ancestors in more ways than not,the distribution of food in the environment
determines the distribution of females, and the distribution
of females determines the distribution of males. When food is so
dispersed that females do best by foraging on their own, males
disperse to pair up with the lone females. This gives rise to
monogamous couples. It is a fairly rare pattern among primates,limited to gibbons, some lemurs, and some African and South
American monkeys.When food comes in patches large enough for several females
to share, they tend to band together in small groups to find the
food, and to protect each other against predators, unwanted
males, and competing female groups. As long as the female band
is not too large, a single male can exclude other males from
sexual access to the band, which thus becomes "his." This
"harem system" of single-male polygyny is fairly common in
primates, being found in hamadryas baboons, colobus monkeys,
some langurs, and gorillas. The competition between males to
guard the female groups creates very strong sexual selection
pressures for male size, strength, aggressiveness, and large
canine teeth.
When food comes in still larger patches, female groups can
grow too large for any single male to defend them. The males
must then form coalitions, resulting in a complex multi-male,
multi-female group, as in some baboons, macaques, ring-tailed
lemurs, howler monkeys, and chimpanzees. Our hominid
ancestors probably lived in such groups, in which sexual selection
gets more complicated. Sometimes, females in multi-male groups
appear to use sperm-production ability as the main fitness
indicator. A chimpanzee female might mate with every male in the
group every time she becomes fertile. She lets their sperm fight it
out in her reproductive tract, and the strongest swimmers with the
best endurance will probably fertilize her egg.
In response to this sexual selection for good sperm, male
chimpanzees have evolved large testicles, copious ejaculates, and
high sperm counts. Female primates face a trade-off. They can
select for the best-swimming sperm by mating very promiscuously,
or they can select for the best courtship behavior by mating very
selectively. Or they can do a little of both, selecting a small group
of male lovers for their charm and then letting their sperm fight it
out.
In species that do not get completely caught up in runaway
sperm competition, females can favor various male behavioral
traits. Multi-male groups obviously allow greater scope for females to choose between males. If they favor dominant males,
males evolve through sexual selection to compete intensely for
social status by individual force or by forming coalitions. If
females favor kind males, males evolve through sexual selection to
groom females, protect their offspring, and guard them from other
males.
Given multi-male, multi-female primate groups, how does
mate choice work? Female primates can exercise choice by
joining groups that contain favored males, initiating sex with
them during estrus, supporting them during conflicts, and
developing long-term social relationships with them. Females
can reject unfavored males by refusing to cooperate during
copulation attempts, driving males away from the group, or
leaving the group. But female mate choice criteria remain
obscure for most primate species. In contrast to modern
humans, female primates rarely favor males who can provide
resources or paternal care of offspring. The sporadic male
care that is observed, such as watching, carrying, and
protecting infants, is better described as courtship effort than
as paternal care. The male is unlikely to be the infant's father,
but is simply trying to mate with the infant's mother by doing
her a favor.
Primate researchers still know little about what traits are
preferred by male and female primates. For example, we know less
about female choice in other apes than we do about female choice
in the Tungara frog, the guppy fish, or the African long-tailed
widowbird. Nevertheless, three kinds of female preference have
been reported in primates: preferences for high-ranking males
capable of protecting females and offspring from other males;
preferences for male "friends" that have groomed the female a lot
and have been kind to her offspring; and preferences for new
males from outside the group, perhaps to avoid genetic
inbreeding. Each sort of preference could be explained in terms of
female choice for good genes, or female choice for material and
social benefits. Although male primates have evolved an astounding
diversity of beards, tufts, and colorful hair styles, there has
been very little research on female choice for male appearance.Also, there has been virtually no research on primate sexual
choice for personality or intelligence. Female primates are
sometimes reported to show "irrational" or "capricious"
preferences that cannot be explained on the basis of male
dominance, age, or group membership. Sometimes two
primates just seem to like each other based on unknown features
of appearance, behavior, or personality. Female primates might
well be choosing males for their personalities and not just their
status, but we do not know.
Most primates follow the general animal pattern of male sexual
competition and female choosiness. But when the costs of male
sexual competition and courtship are high, males also have incentives
to be choosy When male mate choice becomes important,
sexual selection affects females as well as males. In monogamous
marmosets and tamarins, females compete to form pairs with
quality males and drive off competing females. In single-male
harem systems, the dominant male's sperm can become a limiting
resource for female reproduction, and high-ranking females
prevent low-ranking females from mating through aggression and
harassment. In multi-male groups, females sometimes compete to
form consortships and friendships with favored males. Such
patterns of female competition suggest some degree of male mate
choice. When the costs of sexual competition and courtship are
high, males have an incentive to be choosy about how they spread
their sexual effort among the available females. Males compete
much more intensely for females who show signs of fertility such
as sexual maturity, estrus swellings, and presence of offspring Like
females, some male primates also develop special friendships with
particular sexual partners. It may not be romantic love, but, at
least among some baboon pairs, it looks pretty similar.
Our closest ape relatives, the chimpanzees and the bonobos,
live in multi-male, multi-female groups in which sexual choice is
dynamic, intense, and complicated. Under these relentlessly social
conditions, reproductive success came to depend on social
intelligence rather than brute strength. Both sexes compete, both
sexes have dominance hierarchies, and both sexes form alliances.
Sexual relationships develop over weeks and years rather than minutes. Many primatologists and anthropologists believe that
our earliest hominid ancestors probably lived under similar social
and sexual conditions. Constant sociosexual strategizing in mixedsex
groups was the legacy of our ape-like ancestors. It was the
starting point, not the outcome, of sexual choice in human
evolution.
determines the distribution of females, and the distribution
of females determines the distribution of males. When food is so
dispersed that females do best by foraging on their own, males
disperse to pair up with the lone females. This gives rise to
monogamous couples. It is a fairly rare pattern among primates,limited to gibbons, some lemurs, and some African and South
American monkeys.When food comes in patches large enough for several females
to share, they tend to band together in small groups to find the
food, and to protect each other against predators, unwanted
males, and competing female groups. As long as the female band
is not too large, a single male can exclude other males from
sexual access to the band, which thus becomes "his." This
"harem system" of single-male polygyny is fairly common in
primates, being found in hamadryas baboons, colobus monkeys,
some langurs, and gorillas. The competition between males to
guard the female groups creates very strong sexual selection
pressures for male size, strength, aggressiveness, and large
canine teeth.
When food comes in still larger patches, female groups can
grow too large for any single male to defend them. The males
must then form coalitions, resulting in a complex multi-male,
multi-female group, as in some baboons, macaques, ring-tailed
lemurs, howler monkeys, and chimpanzees. Our hominid
ancestors probably lived in such groups, in which sexual selection
gets more complicated. Sometimes, females in multi-male groups
appear to use sperm-production ability as the main fitness
indicator. A chimpanzee female might mate with every male in the
group every time she becomes fertile. She lets their sperm fight it
out in her reproductive tract, and the strongest swimmers with the
best endurance will probably fertilize her egg.
In response to this sexual selection for good sperm, male
chimpanzees have evolved large testicles, copious ejaculates, and
high sperm counts. Female primates face a trade-off. They can
select for the best-swimming sperm by mating very promiscuously,
or they can select for the best courtship behavior by mating very
selectively. Or they can do a little of both, selecting a small group
of male lovers for their charm and then letting their sperm fight it
out.
In species that do not get completely caught up in runaway
sperm competition, females can favor various male behavioral
traits. Multi-male groups obviously allow greater scope for females to choose between males. If they favor dominant males,
males evolve through sexual selection to compete intensely for
social status by individual force or by forming coalitions. If
females favor kind males, males evolve through sexual selection to
groom females, protect their offspring, and guard them from other
males.
Given multi-male, multi-female primate groups, how does
mate choice work? Female primates can exercise choice by
joining groups that contain favored males, initiating sex with
them during estrus, supporting them during conflicts, and
developing long-term social relationships with them. Females
can reject unfavored males by refusing to cooperate during
copulation attempts, driving males away from the group, or
leaving the group. But female mate choice criteria remain
obscure for most primate species. In contrast to modern
humans, female primates rarely favor males who can provide
resources or paternal care of offspring. The sporadic male
care that is observed, such as watching, carrying, and
protecting infants, is better described as courtship effort than
as paternal care. The male is unlikely to be the infant's father,
but is simply trying to mate with the infant's mother by doing
her a favor.
Primate researchers still know little about what traits are
preferred by male and female primates. For example, we know less
about female choice in other apes than we do about female choice
in the Tungara frog, the guppy fish, or the African long-tailed
widowbird. Nevertheless, three kinds of female preference have
been reported in primates: preferences for high-ranking males
capable of protecting females and offspring from other males;
preferences for male "friends" that have groomed the female a lot
and have been kind to her offspring; and preferences for new
males from outside the group, perhaps to avoid genetic
inbreeding. Each sort of preference could be explained in terms of
female choice for good genes, or female choice for material and
social benefits. Although male primates have evolved an astounding
diversity of beards, tufts, and colorful hair styles, there has
been very little research on female choice for male appearance.Also, there has been virtually no research on primate sexual
choice for personality or intelligence. Female primates are
sometimes reported to show "irrational" or "capricious"
preferences that cannot be explained on the basis of male
dominance, age, or group membership. Sometimes two
primates just seem to like each other based on unknown features
of appearance, behavior, or personality. Female primates might
well be choosing males for their personalities and not just their
status, but we do not know.
Most primates follow the general animal pattern of male sexual
competition and female choosiness. But when the costs of male
sexual competition and courtship are high, males also have incentives
to be choosy When male mate choice becomes important,
sexual selection affects females as well as males. In monogamous
marmosets and tamarins, females compete to form pairs with
quality males and drive off competing females. In single-male
harem systems, the dominant male's sperm can become a limiting
resource for female reproduction, and high-ranking females
prevent low-ranking females from mating through aggression and
harassment. In multi-male groups, females sometimes compete to
form consortships and friendships with favored males. Such
patterns of female competition suggest some degree of male mate
choice. When the costs of sexual competition and courtship are
high, males have an incentive to be choosy about how they spread
their sexual effort among the available females. Males compete
much more intensely for females who show signs of fertility such
as sexual maturity, estrus swellings, and presence of offspring Like
females, some male primates also develop special friendships with
particular sexual partners. It may not be romantic love, but, at
least among some baboon pairs, it looks pretty similar.
Our closest ape relatives, the chimpanzees and the bonobos,
live in multi-male, multi-female groups in which sexual choice is
dynamic, intense, and complicated. Under these relentlessly social
conditions, reproductive success came to depend on social
intelligence rather than brute strength. Both sexes compete, both
sexes have dominance hierarchies, and both sexes form alliances.
Sexual relationships develop over weeks and years rather than minutes. Many primatologists and anthropologists believe that
our earliest hominid ancestors probably lived under similar social
and sexual conditions. Constant sociosexual strategizing in mixedsex
groups was the legacy of our ape-like ancestors. It was the
starting point, not the outcome, of sexual choice in human
evolution.
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