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Step 3. As the next-to-last step, Sillén-Tullberg and Møller combined steps 1 and 2 to ask: is there any tendency for more or less conspicuous ovulations to be associated with a particular mating system? Based on a naive reading of our two competing theories, concealed ovulation should be a feature of monogamous species if the daddy-at-home theory is correct, but of promiscuous species if the many-fathers theory holds. In fact, the overwhelming majority of monogamous primate species analyzed—ten out of eleven species—prove to have concealed ovulation. Not a single monogamous primate species has boldly advertised ovulations, which instead are usually (in fourteen out of eighteen cases) confined to promiscuous species. That seems to be strong support for the daddy-at-home theory.

However, the fit between predictions and theory is only a half-fit, because the reverse correlations don’t hold up at all. While most monogamous species have concealed ovulation, concealed ovulation in turn is no guarantee of monogamy. Out of thirty-two species with concealed ovulation, twenty-two aren’t monogamous but are instead promiscuous or live in harems. Concealed ovulators include monogamous night monkeys, often-monogamous humans, harem-holding langur monkeys, and promiscuous vervets. Thus, whatever caused concealed ovulation to evolve in the first place, it can be maintained thereafter under the most varied mating systems.

Similarly, while most species with boldly advertised ovulations are promiscuous, promiscuity is no guarantee of advertisement. In fact, most promiscuous primates—twenty out of thirty-four species—either have concealed ovulation or only slight signs. Harem-holding species as well have invisible, slightly visible, or conspicuous ovulations, depending on the particular species. These complexities warn us that concealed ovulation will prove to serve different functions, according to the particular mating system with which it coexists.

Step 4. To identify these changes of function, Sillén-Tullberg and Møller got the bright idea of studying the family tree of living primate species. They thereby hoped to identify the points in primate evolutionary history at which there had been evolutionary changes in ovulatory signals and mating systems. The underlying rationale is that some modern species that are very closely related to each other, hence presumably derived recently from a common ancestor, turn out to differ in mating system or in strength of ovulatory signals. This implies recent evolutionary changes in mating systems or signals.

Here’s an example of how the reasoning works. We know that humans, chimps, and gorillas are genetically about 98 percent identical and stem from an ancestor (“the Missing Link”) that lived as recently as nine million years ago. Yet those three modern descendants of the Missing Link now exhibit all three types of ovulatory signal: concealed ovulation in humans, slight signals in gorillas, bold advertisement in chimps. Hence only one of those descendants can be like the Missing Link in ovulatory signals, and the other two descendants must have evolved different signals.

In fact, most living species of primitive primates have slight signs of ovulation. Hence the Missing Link may have preserved that condition, and gorillas may have inherited it in turn from the Missing Link (see figure 4.1). Within the last nine million years, though, humans must have evolved concealed ovulation, and chimps must have evolved bold advertisement. Our signals and those of chimps thus diverged in opposite directions from the cues of our mildly signaling ancestors. To us humans, the swollen derrières of ovulating chimps look like those of baboons. However, the

Eigure 4.1

ancestors of chimps and baboons must have evolved their eye-catching derrières quite independently, since the ancestors of baboons and of the Missing Link parted company around thirty million years ago.

By similar reasoning, one can infer other points in the primate family tree at which