Biology Reference
In-Depth Information
mutant bull's relative fertilization success drops from one half to one third. Parker
(1982) has shown that a tiny amount of competition between the sperm of rival males
is sufficient to maintain anisogamy.
Differences in parental care
Anisogamous sexual reproduction, then, involves parasitism of a large egg by a
small sperm. Females produce relatively few large gametes and males produce many
small ones. In addition, females often invest more than males in other forms of care
(Chapter 8). In mammals, it is the female that takes on the burden of pregnancy and
lactation; males rarely contribute to offspring care (about 5% of all mammal species,
though male care is relatively common in three mammalian orders: primates, carnivores
and perissodactyls
the odd-toed ungulates). In birds, bi-parental care is the norm but
females often invest more in care. In other taxa, when parental care occurs female care
is commoner in both reptiles and invertebrates, male and female care are about equally
common in amphibians, while only in fish is male care more common than female care.
In fish, male care is particularly common in territorial species where males can continue
to attract new mates while guarding eggs, so male care involves fewer opportunity costs
(lost matings) than in other taxa (Chapter 8).
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Parental investment and sexual
competition
The theory of
Robert Trivers
Robert Trivers (1972) was the first to recognize the link between sex differences in
investment in resources for gametes and other forms of care (parental investment), and
sexual competition. He wrote: 'Where one sex invests considerably more than the
other, members of the latter will compete among themselves to mate with members of
the former'. The key point is that the sex with the least parental investment has a
greater potential rate of reproduction (Clutton-Brock & Parker, 1992). Thus, in general,
a male can potentially fertilize eggs at a much faster rate than a female can produce
them. Even in species where males temporarily deplete their sperm supply when offered
a surfeit of females, their potential for producing offspring is greater than that of
females (Nakatsuru & Kramer, 1982). This means that while a female can usually best
increase her reproductive success by increasing the rate of converting resources into
eggs and offspring, a male can best increase his success by finding and fertilizing many
different females.
The different effects of mating rate on reproductive success of the two sexes were
neatly demonstrated by A.J. Bateman (1948) with experiments on
Drosophila
and it
was his results in particular which helped to inspire Trivers's theory (Fig. 7.3). The
greater reproductive potential of males is graphically demonstrated by mammals,
such as man, in which a female spends many months producing a single child, during
which time a male could potentially fertilize the eggs of hundreds of other mates
(Table 7.1).
As we shall show in the rest of this chapter, the asymmetry in parental investment,
and consequent difference in reproductive potential of the two sexes, has far-reaching
Male reproductive
success is often
limited by access
to females …
… whilst females
are often limited
by resources
