Polygyny and Sexual Selection

Polygynous mating systems are characterized by several traits. Male-male competition for access to females involves the establishment and defense of territories (blue gourami fish) or

Reproductive and Social Behavior 419 leks, body ornamentation (peacocks), large male body size (elephant seals), the construction of sophisticated bowers (bowerbirds), and other similar features. These are usually associated with sexual dimorphism in secondary sexual characters (e.g., body size, feather ornamentation, fur coloration, etc.) and behavior. Many of these features make males more vulnerable to predators (e.g., bright coloration) and are cosdy to develop and maintain (e.g., antlers). Male-male aggression can be vicious in some species, often leading to sickness and even death for the defeated animal. It is in these characters that the importance of reproductive success is fully appreciated, since they evolve despite compromising the bearer’s survival. Such a success is based on their ability to supersede other males and/or attract females. In some species, females may actively select their mates and even promote male competition, a process called female choice. Intrasexual competition and mate choice are the central aspects of sexual selection, a special case of natural selection that illustrates the coevolution of reproductive behavior and morphology among males and females (Darwin, 1981/1871).

Sexual selection highlights the fact that males and females within a species can be subjected to different selective pressures and exhibit different patterns of reproductive success (for lions, see Figure 1.13). For example, the reproductive success of male red deer (Cervus elaphus) is more dependent on age than that of females. For any given season, there is greater variability in reproductive success for males than for females. Reproductive success peaks at a particular age for males, whereas it is relatively constant for females. Over the entire lifetime of the animals, reproductive success may balance out, but only for males capable of surviving long enough (Clutton-Brock, Guinness, & Albon, 1982).

The fact that polygynous males and females are subject to different selective pressures explains sexual dimorphism. For example, males and females often differ in their parental investment, defined as anything a parent does for an offspring at the expense of the parent’s ability to invest in other offspring (Trivers, 1972). The sex displaying the greatest amount of parental investment is usually the one with the least variability in reproductive success— Bateman’s principle. Generally speaking, females need just a few copulations to fertilize most of their eggs, but they are limited by the number of eggs they can produce in their life. By contrast, males are primarily limited by the number of copulations with different females, given that sperm cells are relatively more economical to produce. In addition, females usually invest more in the offspring than males in terms of parental care (e.g., milk production in female mammals).

Polyandry

In polyandrous species, in which several males mate with a single female, females (rather than males) tend to compete for mates, to show more conspicuous secondary sexual characters, and to exhibit greater variation in reproductive success. These generalizations apply to some species, including the spotted sandpiper (Actitis maculatin'). Field studies at Leech Lake, Minnesota, show that males are in charge of providing parental care, including incubation of the eggs. Females can lay up to five clutches (clutches of four eggs each are usual) by copulating with different males, but each male can raise a single clutch per breeding season. As expected, females compete aggressively for territories and mates. The reproductive success of females is correlated with their age, the size of the territory they are able to defend (a patch of beach in the lake), and the number of mates (Oring, Colwell, & Reed, 1991). In siamangs (Symphalangus syndactylus), a small ape from Southeast Asia, multi-male groups with a single female can defend territories that provide higher-quality food, such as large figs. However, there is no evidence, for example, that infants reared in one-male groups receive less care than those reared in two-male groups. Multi-male groups can provide other reproductive benefits, including reduced infant mortality and predatory pressure (Lappan,

Andayani, Kinnaird, Morino, Nurcahyo, & O’Brien, 2017). A major problem is to determine which group members are reproductively active. In Weddell’s saddleback tamarins (Saguinus weddellt), a monkey from South America, genotyping tests at a variety of DNA loci showed that most groups have a single reproductive female, but multiple reproductive males (Garber, Porter, Spross, & Di Fiori, 2016). Although occasionally, tamarins form monogamous pairs, the norm appears to be polyandrous groups.