All AbouT SEx 249
What Are Females and Males?
In plants, animals, and some other eukaryotes, the gametes of a species come in
two sizes. One, the egg, is large and immobile. The other gamete, the sperm, is
small and mobile, either actively or passively. This dimorphism in gamete size,
which is called anisogamy, likely evolved because there are two ways that a gam-
ete can have high fitness. It can succeed by being large and well provisioned,
which inhibits movement. Alternatively, it can succeed by being small and mobile,
which enhances its ability to find and fertilize a large immobile gamete. This dif-
ference in size leads to a difference in numbers: females typically make far fewer
eggs (or ovules) than males make sperm (or pollen). The difference in gamete size
is so basic that we use it to define the two sexes. Males are the sex that makes
small gametes, and females the sex that makes large ones.
As you saw earlier, some species are hermaphroditic, while others have separate
sexes. Ecological conditions can favor one or the other of these mating systems. An
important factor favoring hermaphroditism is reproductive assurance, which is the
increased chance of successful reproduction when potential mates are rare (or even
absent). When finding a reproductive partner is difficult, there is an advantage
for individuals to be simultaneously male and female. This condition allows them
to mate with any other individual they encounter, or (in some species) even with
themselves. The need for reproductive assurance is one reason why plants, which
cannot actively search for a mate, are hermaphroditic more often than animals.
In species with separate sexes, a variety of mechanisms determine which
embryos develop into females and which into males (FIGURE 10.3) [4]. In humans
and other mammals, sex determination is genetic: females have two X chro-
mosomes, while males have one X and one Y. In birds, the sex chromosomes are
reversed: it is the females that have two different sex chromosomes (called the Z and
W chromosomes) and the males that are homozygous (with two Z chromosomes).
In yet other groups, sex is determined not by chromosomes but by the physical
or social environment [4]. Cool temperatures cause the eggs of many reptiles to
develop into males, while in warm temperatures they develop into females [23].
Males and females of some animals are so similar that the only way they can be
distinguished is by their genitalia. But in other species, including humans, many
traits are sexually dimorphic, meaning they are expressed differently in males and
females. One of the most spectacular examples of sexual dimorphisms is found inFIGURE 10.2 Many animals include chemical signals called
sex pheromones to attract and court mates. In tropical America,
male orchid bees (tribe Euglossini) collect scent from certain
flowers and use it as their sex pheromone. Here a brilliantly
colored male Euglossa igniventris collects scent from a Coryan-
thes orchid flower in Panama. (Courtesy of David Roubik.)10_EVOL4E_CH10.indd 249 3/22/17 2:25 PM