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Mating Systems in Mammals
A. Anisogamy.
1. Female mammals produce few, large, sessile, and energetically expensive eggs, and
males produce many, small, motile, and energetically cheap sperm.
2. This difference in gamete size, referred to as anisogamy, sets the stage for a host of
differences in reproductive behavior patterns of males and females.
3. Because the relatively small number of expensive female gametes is likely to be a
limited resource for which males compete, a male’s reproductive success is likely to be
a function of how many different females he can inseminate. A female’s reproductive
success depends on how many eggs and young she can produce.
4. Several features of mammalian reproduction further promote differences in
reproductive behavior between the sexes.
a. Fertilization occurs in the oviducts, so it is certain that the female is the biological
parent of offspring born to her, but paternity is not as certain.
b. Only female mammals gestate and lactate, thus limiting the investment males can
make in offspring.
5. One of the results of the differences in reproductive behavior between the sexes is that
the sexes are likely to follow different reproductive strategies.
6. In this lecture, we will discuss why males of some species provide little care for young
but those of other species share parental responsibility with the female, and why males
and females of some species form pair bonds while in others 1 sex has multiple mates.
B. Mating systems in mammals.
1. Classifications of mating systems traditionally have been based on the extent to which
males and females associate (bond) during the breeding season.
2. Monogamy.
a. Monogamy refers to an association between 1 male and 1 female at a time and
includes an exclusive mating relationship between pair members.
b. In monogamous systems, neither sex is able to monopolize >1 member of the
opposite sex (the term “monopolize” here refers to having >1 member of the
opposite sex all to yourself).
c. If there is no opportunity to monopolize mates, an individual benefits (i.e.,
increases its reproductive potential) from remaining with its initial mate and
helping to raise the offspring. The formation of long-term pair bonds also seems
advantageous because less time is needed to find a mate during each reproductive
cycle.
d. What situations preclude monopolizing mates? When the habitat contains
scattered, renewable resources or scarce nest sites that are not defensible.
e. Monogamy is relatively rare in mammals; it is found in <5% of mammal species.
f. Monogamy is reported among most mammalian orders, with the bulk of the
examples coming from Primates, Carnivora, and Rodentia. The monogamous
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mammals tend to be the hunting mammals, in which the male is useful for
obtaining meat for the female and young.
3. Polygamy incorporates all multiple-mating, non-monogamous mating systems. The
following mating systems are subsets of polygamy.
a. Polygyny.
1) In habitats that have clumped resources that are defensible, polygyny is likely.
2) In polygynous systems, individual males monopolize >1 female.
3) In resource defense polygyny, males defend areas containing the feeding or
nesting sites critical for reproduction, and a female’s choice of a mate is
influenced by the quality of the male and his territory.
4) In female defense polygyny, females are gregarious for reasons unrelated to
reproduction, as when females herd for protection against predation or gather
around resources such as food or nesting sites. In this case, males will
monopolize females and exclude other males from their harems.
5) If males are not involved in parental care and have little opportunity to control
resources or mates, male dominance polygyny may develop.
a) If female movements or concentrations are predictable, the males may
concentrate in such areas and pool their advertising and courtship signals.
b) Females then select a mate from the group of males. These areas are called
leks, and males congregate and defend small territories within them in
order to attract and court females.
c) Leks do not contain resources (food or nesting sites) but are purely display
sites for mate choice and copulation.
d) Male hammer-headed bats lek as do ungulates such as Uganda kob and
topi.
6) In the absence of territory or dominance, scramble polygyny may operate, as
males try to obtain copulations. Where females are widely dispersed, as with
13-lined ground squirrels, males become highly mobile during the breeding
season.
b. Polyandry.
1) In polyandrous systems, females monopolize >1 male.
2) Because female investment in gametes and offspring exceeds that of males,
polyandry is expected to be rare, especially in mammals.
3) Several species of larger canids show most of the features of true polyandry.
Although mostly monogamous, the African wild dog sometimes exhibits
polyandry; females are occasionally mated by several males, males provide
extensive care of pups, and females are the dispersing sex.