Seminar Two
Evolution of sexuality
Maternal investment

The biologist Robert Trivers, a pioneer in sexual selection research, argues that maternal investment is the single most important difference between the sexes. In most mammals maternal investment correlates positively with greater male-male competition than female-female competition, and also with a greater variance in male reproductive success.

Once you learn about this mammal's behavior you'll never forget it. In east Africa you'll see a hyena clan consuming a prey species it has chased down during the night, their special time for hunting. The carcass of the prey animal disappears so quickly, bones and all, that all you may see at 6:00 a.m. is the skull being finished off. Spotted hyenas eat fast, and they can crunch bones more effectively than lions.

Most spotted hyena females are larger and higher-ranking than males, and cubs share their mother's social rank, so that many cubs outrank adult males. There is intense intrassexual competition among females. Rank determines feeding order and access to other critical resources, like den sites. There is one highest-ranking female, called the alpha-female, whose cubs also carry highest rank status. All females are virilized, or masculinized, by exposure to high androgen levels in late gestation. Natural and sexual selection may have shared interocked roles in the evolution of this Rube Goldberg reproductive system.

The female carries a fetus which will be delivered through a canal continuous with the bladder outlet, passing through her pseudopenis. To my knowledge this is unique among mammals. Researchers who have studied captive spotted hyenas report that delivery is often obstructed, and if the mother is not assisted she may die. The pseudopenis also serves as entrance for sperm during copulation. What a traffic jam! This is bizarre anatomy for any animal.

Research on spotted hyenas is exposing flaws in our long-held beliefs, and only future studies will clarify the evolutionary meaning of the masculinized anatomy in females. For example, a pseudopenis develops even if androgen effects are artifically blocked during pregnancy. The point to keep in mind is that this animal breaks the mold, and we can't say why it is so odd. Natural and sexual selection must share interlocked roles, and we are trying to unravel the threads.

I cannot resist including an amazing paragraph from Out of Africa by Karen Blixen (Isak Dinesen), in which she quotes an authority in Africa in the early 1900s:

All hyenas, you will know, are hermaphrodites, and in Africa, where they come from,on a full-moon night they will meet and join in a ring of copulation wherein each takes the double part of male and female… Do you consider now… that it should be, on account of this fact, harder to a Hyena than to other animals to be shut up by itself in a cage? Would he feel a double want, or is he, because he unites in himself the complementary qualities of creation, satisfied in himself, and in harmony? In other words, since we are all prisoners in life, are we happier, or more miserable, the more talents we possess?