9.15: Monogamy versus polygamy: mating behaviors can vary across human and animal cultures.

As we continue our tour of animal mating behavior, we turn again to the elephant seal. In December of each year, the males appear on islands off the coast of northern California, where they compete with each other for possession of the beach. Through bloody fights they establish dominance hierarchies, with the biggest males—which are 13 feet long and weigh more than 2 tons—generally winning.

In mid-January, the females arrive and are ready to mate. They congregate in large groups on just a few prime beaches. Because the females stick close together, the biggest males, who control the prime beaches, gain access to almost all of the females when they dominate the other males in the competition for sexual access to females (FIGURE 9-24). In one study that observed 115 males, the 5 highest-ranking males fathered 85% of the offspring. While nearly every female will mate and produce offspring, the majority of males never get the chance to mate during the 10–20 years of their lives.

The elephant seals’ mating pattern exemplifies polygamy, a system in which some individuals attract multiple mates while other individuals attract none. Polygamous mating systems can be subdivided into polygyny, in which individual males mate with multiple females, and polyandry, in which individual females mate with multiple males. Polygamy can be contrasted with monogamy, in which most individuals mate and remain with just one other individual. Polygamy and monogamy are two types of mating systems, which describe the patterns of mating behavior in a species. In this section, we explore the features of environments and species that influence mating systems and survey the range of mating systems observed in nature.

As we have seen, throughout the animal world, as a result of their relative parental investment, females are choosy about which males they mate with, and males compete for access to mating opportunities. Not surprisingly, multiple females often end up selecting the same male—usually a male on a territory rich in resources or a male with unusually pronounced physical features, such as antler size. Such female selection is the reason for the fancy ornamentation we see in male peacocks. Although the male peacock’s tail is like a giant bull’s-eye to predators, the number of eyespots on the tail is directly related to how well he can attract a mate: below 140 eyespots, he gets no mates; at 150, he gets two to three mates on average; with 160 eyespots, he gets six or more mates (see Figure 9-21).

Identifying a population’s mating systems is not as easy as the elephant seal example might lead us to believe. Three issues, in particular, complicate the task. First, there are often differences between animals’ mating behavior and their bonding behavior. That is, it may seem that a male and female have formed a pair bond—in which they spend a high proportion of their time together, often over many years, sharing a nest or other “home” and contributing equally to parental care of offspring in what appears to be a monogamous relationship. Closer inspection (often including DNA analysis of the offspring), however, sometimes reveals that the male and/or the female may be mating with other individuals in the population, and perhaps the mating system is better described as a variation on polygamy. A second difficulty in defining a species’ mating system arises because the mating system may vary within the species. That is, some individuals may be monogamous, while others are polygamous. And the mating system may even change over the course of an individual’s life. A third difficulty is that males and females often differ in their mating behavior. In the elephant seals described above, it could be said that the females are all mating monogamously, while the males are polygamous.

Examination of birds and mammals in general, however, reveals one sharp split. As we discussed in Section 9-10, the vast majority of female mammals make a greater parental investment than male mammals. In birds, females and males have a more equal parental investment. Does the difference in parental investment patterns in birds and mammals lead to different mating systems? Yes. In mammals, polygyny is the most common mating system across all large groups, from rodents to primates. Polygyny is a consequence of the significant female investment and lesser male investment. Males can generally benefit more by seeking additional mating opportunities, leading to male-male competition. In birds, the relatively equal parental investment by males and females has led to much less polygyny. More than 90% of the approximately 10,000 bird species we know about appear to be monogamous (although, in most cases, it is serial monogamy) (FIGURE 9-25).

And what of humans? Across a variety of cultures, males consistently have greater variance in reproductive success than females—that is, some males have very high reproductive success and many others have little or none. FIGURE 9-26, for example, presents data from one of the first investigations of this phenomenon. It is from a study of the Xavante Indians of Brazil. The Xavante were selected because they were a pre-industrial population subsisting primarily as hunter-gatherers, with no access to reliable birth control and with almost no contact with Western cultures. The average number of children (3.6) does not differ between men and women, but the range does: some men have very large numbers of offspring (as many as 23!), and many have none. There is significantly less variability in reproductive success from one woman to another. Interestingly, a 2010 study using data from 7,710 women and men living in the contemporary United States similarly found significantly greater variance in reproductive success among men than among women.

Some differences in the variance in reproductive success among males and females, which can be significantly influenced by socialization and other cultural forces, are generally associated with polygynous mating systems. However, the difference between the sexes found in the Brazilian study is quite small when compared with many other mammalian species, such as elephant seals, and is close to that seen in populations with a monogamous mating system. Humans, consequently, seem to have a mating system close to, but not completely, monogamous.