Seminar Two
Evolution of sexuality
Limiting resource in sexual selection

Are male animals natural philanderers, and females chaste and choosy? This misleading generalization was proposed in 1948 by the researcher A. J. Bateman. It is now considered wrong as a generalization. The closer one looks at sexual behavior in animals, the more one finds so-called philandering among females as well as males. On the surface, though, it seems that Bateman was right: most of the time we see females being more choosy than males. For a long time we rationalized this conclusion as reasonable, arguing that “sperm are cheap” and eggs are not.

But a more prominent pattern stands out: with heavy maternal investment, as in most mammals, females are the limiting resource, and are usually competed for by males more than males are by females. That sentence was a mouthful—what does maternal investment mean? Just what it says: for most mammals, the female invests more time and resources in offspring than the male does.

Just think of the ways she invests: pregnancy, childbirth, breast feeding, and—in most mammals—rearing the offspring without the help of the male. And what kind of intersexual selection is more obvious? Male-male competition for access to mates. (Female-female competition occurs too but is usually less obvious.) The difference can be seen in in the graph below: