In most organisms, there are small gametes and there are large gametes
(anisogamy). The small gametes compete for access to the large gametes,
which provide the bulk of energy for the embryo. Producers of small and
large gametes are defined as males and females, respectively.
The eggs of the flightless Brown kiwi are each 25 percent of a female's body
size. Females should protect their investment in large and expensive eggs,
e.g., by carefully choosing a social mate and by carefully choosing the sperm
to fertilise their eggs (not always by the same mate).
Natural selection: Differences among individuals in their ability to
transmit copies of their genes to the next generation.
Sexual selection: The component of natural selection that refers to
differences among individuals in their ability to compete with others for
mates (usually male-male competition) or in attracting mates (usually due to
female choice of mating partners).
The ornaments and displays favoured by sexual selection are often those
that reliably reflect the superior condition of certain males, enabling females