Encyclopedia of Evolution.pdf - Online Reading Center

Other reproductive and related activities. Evolutionary
explanations are also available for reproductive systems that
seem very strange to human observers. In sea horses and some
other fishes and amphibians, males may carry the fertilized
eggs in their mouths or in special pouches until they hatch.
These examples of “fathers giving birth” are not simply weird
stories of nature. In aquatic animal species with external fertilization
(where females deposit eggs and the males deposit
semen on the eggs), there is at that moment no question of
paternity. However, if both of them leave, some other male
could deposit semen, which might affect the outcome of paternity
in at least some of the hatchlings. The male, by gathering
the eggs and carrying them, can prevent other males from
fertilizing them. Male brooding, far from being an example
of males showing care to their offspring, may actually be an
example of evolution in response to male–male competition.
Evolutionary hypotheses might also help to explain
homosexual behavior. Though not a reproductive system,
it is derived from reproductive systems that the animals
already have. Homosexual behavior, both between males
and between females, is widespread in the animal kingdom.
Examples include bonobos, penguins, dolphins, macaques,
baboons, and rhesus monkeys. In some cases, it is connected
with male aggressive behavior, as in razorbill birds. Animals
with homosexual proclivities can pass on their genes, if they
are homosexual only part of the time; or, their relatives can
pass on the genes if the genes are not expressed. Its widespread
occurrence among animals suggests that homosexual
behavior may have a genetic basis.
Concealed ovulation. Humans are one of 32 primate
species with concealed ovulation. In many animal species,
the female advertises ovulation, sometimes with conspicuous
swellings, scents, and behaviors. The males, whether
in a harem or a promiscuous breeding system, know which
females to mate with and when, to maximize their chances of
producing offspring. However, in humans, men do not know
when women ovulate—nor do the women. Therefore men
cannot choose to mate with women only during ovulation.
Two major explanations for the evolution of concealed
ovulation have been offered. One, proposed by evolutionary
biologists Richard Hamilton and Katharine Noonan, has been
called the “daddy at home” theory. Concealed ovulation keeps
the male home, because he cannot know when his mate is fertile
and has to keep copulating with her over long periods of
time in order to produce offspring. If he stays home he is more
likely to help provide for, and raise, the offspring. Concealed
ovulation might also discourage the man from finding other
mates for reproductive purposes, because he cannot know
which of them, at any given time, are fertile. This explanation
posits an association of concealed ovulation with monogamy.
The other explanation, offered by evolutionary biologist
Sarah Blaffer Hrdy, has been called the “many fathers”
theory. She suggests that concealed ovulation makes it impossible
for a male to know which offspring are really his, or,
more to the point, which ones are not, therefore which ones
to kill. The females never benefit from having their offspring
killed, therefore concealed ovulation might be the female’s
way of keeping males from killing her offspring. Male goril-
reproductive systems
las, in a harem system, know which offspring are theirs, and
they kill others; male vervet monkeys, in a promiscuous system,
do not know which offspring are theirs and seldom kill
them. This explanation suggests an association of concealed
ovulation with promiscuity.
Nothing could be easier, it would seem, than to simply
look at the data to see which explanation is correct. It turns
out not to be quite so simple. Swedish evolutionary biologists
Birgitta Sillén-Tullberg and Anders Møller gathered data on
primate breeding systems and the occurrence of concealed
ovulation. They found that nearly all monogamous primates
(10 out of 11) had concealed ovulation. This would seem to
confirm the daddy-at-home theory. However, the reverse is
not true. Of the 32 primate species with concealed ovulation,
22 are not monogamous. According to Sillén-Tullberg and
Møller, concealed ovulation may have evolved eight different
times in different primate lineages, and it may not be possible
to formulate a single explanation for it.
Sex Ratios
In most populations of animals, and of plants that have separate
sexes, there is an equal number of males and females.
The sex ratio (number of males per female) is about 1:1. This
occurs because if one of the sexes is less abundant, each individual
of that sex can have more offspring than an individual
of the other sex, which creates a selective advantage for the
less abundant sex. This is an example of frequency-dependent
selection. This occurs only when offspring of the two sexes
are about equally expensive for the parents to produce. In
some special cases, the sex ratio departs from 1:1.
• Among honeybees, the queen is related to each of her sons
and daughters by a relatedness of 0.5. The queen would
therefore benefit from producing an equal number of sons
and daughters. The workers, however, may be related to
one another by a factor of 0.75, but to their brothers by
only 0.5. The queen may lay equal numbers of male (haploid)
and female (diploid) eggs, but the workers kill many of
the males, resulting in a sex ratio considerably less than 1:1.
• In some insects, the larvae parasitize hosts by living inside
of them. When they become mature, the only other members
of their species that they may encounter may be those
that emerged from the same host, which are likely to be
their siblings. In such situations, it is advantageous for the
parent who lays the eggs to produce only enough males to
fertilize the females, resulting in a sex ratio considerably
less than 1:1.
Reproductive systems, in general, encourage the production
of genetically varied offspring. These systems can take
on many and varied forms over evolutionary time and can
be influenced by factors that are unrelated to reproduction.
Reproductive systems therefore defy complete explanation or
even adequate classification.
Further Reading
Asa, Cheryl S., and Carolina Valdespino. “Canid reproductive biology:
An integration of proximate mechanisms and ultimate
causes.” American Zoologist 38 (1998): 251–259.