Certain physical characteristics are sexually selected in animals; certain behaviors are as well. For example, a peacock’s feathers and mating dance are characteristics and behavior that were selected for by females over evolutionary time. Certain characteristics of human anatomy give us some powerful tools for understanding how our ancestors behaved as well as explaining some of our modern behaviors.

Three of these characteristics are sexual dimorphism, cryptic ovulation and concealed estrus, and penis and testicle sizes. Sexual dimorphism refers to average size differences between males and females of a species. Cryptic ovulation and concealed estrus means that it is very hard, if not impossible, to know when a female is ovulating and thus fertile. Penis and testicle size refers to.. well, I think you know what that refers to.

Why are these things relevant to humans? The first, sexual dimorphism, bears directly on the number of female mates a male member has, or more precisely, on the degree of polygamy in a species. For most animals in general, the hypothesis is that if males compete for females, larger bodies are more likely to prevail in male-male competition and larger males are able to monopolize several females. This idea is supported in nature: the more females in a group, the larger the difference between the body sizes of males and females. Animals characterized by harems, such as gorillas and elephant seals, exemplify this. A male mountain gorilla may have a harem of perhaps 10 or more females led by a single silverback male who is the sole impregnator of the females in the troop and outweighs the females by a factor of 2 or 3. Male elephant seals may outweigh their female couterparts by a factor of 10, and they may mate with over 100 females in a breeding period.

Conversely, in species where the males and females are the same size, monogamy is the usual form of reproduction. There is no competition between males trying to get more than a single mate and thus no pressure to select for larger, stronger males.

Sexual dimorphism is thus a reliable indicator of the degree of polygamy that happens under natural conditions. Same body size = monogamy. Extreme differences in male-female body sizes = extremely polygamy. Moderate differences in body sizes usually indicate a moderate degree of polygamy. This is exactly what we see in humans. Males are on average larger than females and tend to cheat or have multiple wives.

Cryptic ovulation and concealed estrus was a big change for our human ancestors as well. In most primate species, when a female starts ovulating, glands around her genitals swell up and turns red, a big sign that she is fertile. In humans however, there is no similar sign. This changed sexual relations in three major ways. One, it gives women more control over mate choice, two, it obscures paternity, and three it induces greater sociality. In a polygamous species, being able to identify when the female is in heat allowed the dominant males to, well, dominate the females and keep other males from getting a chance to inseminate them.

With concealed estrus, however, no one knows when anyone is fertile or not. All males benefit from treating females nicer at all times and some of the smaller (i.e. weaker) males have a chance to get in with the females. Also, since no one knows who the father is, it opens the door for greater offspring survival. In lions, when a new male takes over the pride, he kills as the babies. The mothers then go into heat and he has the opportunity to reproduce with them. But if no one knows who the father is, males are less likely to kill offspring like this for fear of killing their own progeny. (By the way, genetic paternity tests indicate that as many as 1/3 of human babies are born to fathers other than the husband.)

Penis and testicle size is an interesting indicator of social structure as well. How big do you think a gorilla's penis is? On average about 1.2 inches with 1 ounce testicles. Humans have 5.12 inch penises and 1.42 ounce testes, but chimpanzees have 3.15 inch penises and massive 4.17 ounce testicles. The explanation? Gorilla males have no competition for females once they establish themselves at the head of a harem. That takes physcial size and strength and is unrelated to genitals. Once they are established with a receptive group of females, there is no need for a large penis or testicles: their sperm is all there is.

Humans and chimpanzees however, traditionally lived in larger social groups with sexual competition. With cryptic ovulation and concealed estrus in humans, there is no way to know if you are copulating at the right time. Even if you are at the right time, of course it is optimal to be the first to fertilize the egg. A longer penis gets the swimmers that much closer at blastoff, shortening the total distance they have to travel as well increasing the probability that some may live longer.

From the chimpanzee perspective, large testicles are useful for the hit-and-run approach to reproduction. Chimpanzee culture has a pretty strict male hierarchy and the males at the top tend to have more copulations. But the little guy on the bottom has devised a strategy to get some though: We know it as Wham! Bam! Thank you ma'am! strategy. Chimps have very short copulation periods, about 15 seconds or so. They also have been known to have up to 20 or more copulations in a day. The large testicles make it possible to send a fruitful package with every copulation as well as provide large numbers of swimmers that can compete with other males' sperm.

All of these issues are relevant to human society and behavior, but we have to keep in mind that this type of analysis in no way suggests that humans HAVE to act a certain way. Just because our sexual asymmetry indicates we are a mildly polygamous species does not give men the right to defend infidelity or to think that it is their natural perogative to sleep around. The behaviors that we project on to humanity from anthriolutionary perspectives describe inclinations that humans may have for acting in a certain way. Biologically predicted behaviors are expected when people act without thinking, but culture and symbolic cognition interact to create systems of thought and meaning that change the whole scenario from a non-human natural setting to a human-specific setting. We have systems of meanings and expectations, as well as memories, morals, and manners that we can be expected to adhere to.

Conscious thought and language are the major characteristics that set the human animal apart from other animals. Some people oppose evolutionary explanations of behavior because they fear that those explanations would be used to argue that innate behaviors are "natural," "excusable," and "desireable." This is-ought fallacy shouldn't worry us though, because exploring some of our natural tendencies give us the ability to identify behaviors that may occur unless we are aware of our natural inclinations towards them.