What would life be like without sex? It would be boring. What weird miss, besides the obvious, is the glorious diversity of shapes, colors, and sizes we see among individuals in sexual species, including of course human beings. Mutation and environmental factors cause some variation in asexual species, but nothing like the tremendous variety produced by genetic recombination in sexual species.
So let me change the question and ask you to consider what life would be like if our species required three sexes to reproduce. Never mind the anatomy, because I have no idea how it would work (make it up yourself). But one happy certainty is that relationships between any two of the sexes would be free of the worry of unintended pregnancy.
On the other hand, three-way dating would be very complicated. Imagine the difficulties involved in finding three mutually attracted people and getting them together. Does a couple made up of sex 1 and sex 2 first get together and then jointly ask sex 3 to join? Or does sex 1 invite sex 2, and then 2 invites 3? Or maybe 1 invites both 2 and 3 independently and hopes they hit it off. And then somehow all three have to agree on a restaurant and a movie. Good luck.
More seriously, we cannot expect that dating, or any norm or institution drawn from this (or any other) culture of a two-sex species, would occur in a triple-sex species. Nor can we predict triple-sex sexual, parental or larger-scale social arrangements with any confidence. Too much depends on the vagaries of history and culture (which would probably account for all the differences in social organization within the species), on the pattern of inheritance in reproduction, and on relative degrees of parental investment. However, we can make a couple of generalizations based on theory.
First, the inheritance pattern ought to have some consequences for parent-offspring ties. A parent has--evolutionarily speaking--an interest in each offspring that is proportional to the degree of kinship. In a two-sex species, the biological contribution of each parent is one- half, on average.
In a three-sex species, if each parent contributes only a third, we might expect natural selection to produce a bond that is less strong between parent and offspring.
Second, sex-role differences will depend partly on the division of reproductive labor among the sexes. The evolutionary principle is this: the sex that invests less in the offspring will tend to make that investment more casually (more promiscuously) and will compete for sexual access to the sex that invests more. In ordinary two-sex mammals, females typically invest much more.
Females produce larger sex cells, carry the offspring during pregnancy, and provide nourishment after birth. Males, in species that fit this pattern, typically compete for females. (In our own species and in some others, male investment is significant both during pregnancy and after birth, which changes the calculus considerably.) Likewise, the expected sex-role differences in triple-sex species would depend heavily on who does what. I leave it to you to fill in the specifics.
Suggesting a three-sex world naturally raises the question of what life would be like in a four-sex world. In a word, unmanageable.
Kemo D. (a.k.a. no.7) www.beyondgenes.com