PART 2 -
Many animals, indeed, do not even sort neatly into two sexes at all. If you go snorkelling on a coral reef, about one-third of the fish you see make both eggs and sperm at either the same time or different times during their lives. These are called simultaneous or sequential hermaphrodites respectively, and are said to "change sex" when they
switch from making eggs to making sperm or vice versa. In fact the most common body plan among multicellular organisms, including plants, is for a single individual to make both male and female gametes at some time during their life. So the condition whereby an individual can be unambiguously classified as either male or female
should not be considered the norm.
Species may also feature more than one type of male and female. The multiple morphs of males in such species all produce sperm, but otherwise differ in body size, colour, morphology, behaviour and life history so much that an inexperienced naturalist might be tempted to classify them as different species. The same is true for multiple kinds of females that have nothing in common except that they all
make eggs, such as yellow-throated and orange-throated side-blotched lizards, which lay eggs of different sizes.
I have termed these distinct morphs as "genders", and this
terminology allows one to say there are more genders than sexes. The bluegill sunfish of the north-eastern US and Canada, for example, has three male genders that I term controllers, cooperators, and endrunners. The large, orange-breasted controllers and medium-sized cooperators, whose dark, barred colour pattern resembles that of females, court females jointly. The controller fertilises most of the eggs, but allows the cooperator a limited role as well. The small,pale endrunner males lurk in the weeds waiting to dash in while a female is laying her eggs and deposit some sperm of their own.
The second problem with Darwin's notion of sexual selection is that in relatively social species, such as most birds and mammals, sexual contact - mating - is not necessarily, indeed not even often, about the transfer of sperm. Mating is mostly directed at forming and managing relationships that may ultimately result in the successful production and rearing of offspring. A simple count of how many times
mating takes place relative to the number of young born illustrates the point. In humans, for example, suppose Ozzie and Harriet have two children, have been married for 50 years, and make love regularly each week - say, Thursday night. After 50 years they will have mated over 2500 times, and produced two children, thus mating 1250 times per offspring produced. Sounds inefficient? Not if we suppose that regular mating allows the couple to stay together to successfully rear their two children. Similarly, in birds, primates, indeed everywhere, lots of mating occurs at times and places that cannot possibly result in immediate offspring production.
By this stage of my research I was beginning to suspect that Darwin might be all wrong about sex. It seemed to me that social organisation in animals revolves around the control of access to reproductive opportunity, which includes all the things that animals need to reproduce: food and nest sites, for instance, as well as
mates. Animals make direct use of the resources they control, but may also use them as bargaining chips to buy the help of others.
Furthermore, the dynamics of animal societies also involve decisions about where to allocate friendship and cooperation among animals of both the same sex and the other sex. Different arrangements of cooperative effort lead to the emergence of different structures for
families and small groups.