MATING SYSTEMS The last item of comparison between male and female reproductive strategies in the table concluding the last topic was "preferred mating system." There exist four major types of mating systems used by different animal species. These are promsicuity in which there is no pair bonding after mating, polygyny in which a single male bonds with several females, polyandry in which a single female bonds with several males and monogamy in which a single female and male pair bond (sometimes for life). In this topic we will examine the male and female reproductive strategies in more detail and relate them to their preferred mating system. The Male Strategy: Promiscuity & Polygyny The adult populations of many species exhibit a 1:1 sex ratio, i.e., the number of sexually mature males equals the number of sexually mature females. Each male possesses sufficient sperm to fertilize a large number of females, although a single female need only mate with one male to be reproductively successful. Given these facts it is not surprising that males will compete among themselves for access to females which are the limiting resource for male maximization of fitness. The result of such competition will be a wide variance in male reproductive success with a few males mating with many females and others not reproducing at all. This is consistent with Bateman's observation in fruit flies. Females, on the other hand, will all find mates and so will exhibit little variance in reproductive success. Thus, the average number of offspring produced per female will vary little, while that per male will show wide variation. Trivers' theory of parental investment suggests that males must mate with more than one female to maximize their fitness, so in a population with a 1:1 sex ratio, competition for mates among males will be intense. The result of this competition is the evolution of behaviors (aggression) and structures (secondary sex characteristics) which distinguish males from females. This sexual dimorphism gives males an advantage in competing with other males (intrasexual selection leading to development of traits such as large size and antlers) or in attractiveness to females (intersexual or epigamic selection promoting the development of traits such as bright coloration and elaborate signalling devices, e.g., the peacock's tail feathers or the turkey's wattle). Some examples of sexual dimorphism might serve both functions, e.g., bird song used to attract females and warn other males away from territories. Note also that sexual dimorphism might simply be the result of adaptation to different modes of life between the sexes (e.g., larger male size simply to avoid food competition with females). Due to these three different potential selective pressures, caution must be exercised in interpreting the ultimate cause of traits which distinguish the two sexes. Because males must compete among themselves for access to females, they have little time to become involved with raising offspring. Hence, males favor any mating system which frees them from parental duties. Promsicuity is a form of mating which involves no pair bonding between the sexes. Parental care is either absent, or provided by the female. On rare occasions males will care for offspring and a number of theories have been advanced to explain which parent will provide parental care when only one is needed. These variations need not concern us here in our discussion of basic strategies, but they do greatly concern sociobiologists who attempt to explain not only basic strategies but also exceptions. Polygyny is a mating system in which several females bond to a single male. Males in this mating system provide little parental care and spend most of their time defending their territories from intrusion by other males (song birds, elephant seals and horses) or in direct combat with other males for access to females (mountain sheep and elk). There are two other mating systems which involve pair bonding: monogamy and polyandry. Males will accept monogamy when male parental care is essential for raising offspring successfully, e.g., in altricial birds like robins whose young are helpless at birth, or when there is a scarcity of females so that males who attempt to mate with more than one female will lose access to their first mate. Polyandry exists when one female bonds with several males. One might expect this type of mating system to be quite rare given the male strategy outline above, and in fact it is. Jacanas are birds which do engage in this form of mating system and the reason appears to be that males occupy smaller territories than females and a single female's territory, which she defends vigorously against other females, is large enough to encompass those of several males. She mates successively with each male in her territory and leaves parental care to the males. This is not, however, an arduous task for the male because the offspring are precocial, i.e., they can pretty much fend for themselves soon after hatching. These two exceptions indicate special environmental conditions in which male fitness maximization requires that they deviate from the most common male reproductive strategy. The Female Strategy: Monogamy & Polyandry Females stand to lose more in terms of fitness from a poor mating decision than males due to the fact that they produce fewer gametes over their lifetimes than do males. Consequently, females are more discriminate and selective in choosing mates than males. Female choice may be an important factor in the evolution of species-specific mating behavior because the worst mistake a female could make would be to mate with a male of a different species. Since species, by definition, are reproductively isolated from one another, i.e., they cannot interbreed successfully, any female who selects a male of a different species as a mate will leave no offspring. Males, on the other hand, need not be as discriminating since they possess sufficient sperm to mate with several different females. One bad mating decision will not impact as adversely on the fitness of males as on females. Evolutionary theory assumes that female choice of male characters is under genetic control and thus is subject to genetic variation. Females who are genetically programmmed to make the right choice will leave more offspring than females who are genetically incapable of selecting the proper mate given the particular environmental circumstances under which the species lives. What then is the basis of female choice? In species with no parental care or in which the female can raise the offspring alone, females would be selected for choosing males which are genetically superior. The only contribution by the male to a female's fitness under these circumstances would be his genes. But genes cannot be observed; how then might females evaluate male genetic superiority? Females obviously would select phenotypic characters which are indicative of "good genes" according to sociobiologist David Barash. These traits could be ones which indicate success in avoiding mortality, e.g., age and large size, or success in competing with other males, e.g., antlers or exclusion through combat of other males from a common breeding ground. In some species of grouse males gather during the breeding season in a communal display ground or lek and compete with one another for the prized center area. Females enter this ground and make their way to the center and mate with those males. On their way the females encounter the less fit males on the periphery, but ignore their advances. The assumption underlying this female strategy is that the male traits they select are under genetic control and so will be passed on to their male offspring. In polygynous mating systems females prefer to mate with the dominant males who either control territories or occupy the highest positions in a dominance hierarchy (pecking order). To determine which males are dominant, females often incite males to combat and then mate with the winner. Females may be prevented from mating with dominant males by other females who are themselves dominant; hence, dominance hierarchies may exist for females as well as males. Another object of female choice is the resource controlled by a male. In this instance females are not choosing the male because he is dominant, but rather are choosing the territory controlled by the male because of its quality. High quality habitats provide the shelter and food the female requires to raise her offspring. Again in this case, females may exclude other females from this area and force them to mate with males controlling less suitable territories. Some females will accept secondary mate status on a high quality territory rather than mate monogamously with a male controlling a very poor territory and the difference in resource quality is the determining factor. This model is known as the polygyny threshold model which enjoys empirical support in studies of some bird species. Finally, females can choose males on the basis of their ability to assist the females in raising offspring. In Barash's terms these females are choosing "good behavior" and such choice is characteristic of many monogamous mating systems. Monogamous species of birds generally lack the sexual dimorphism which serves to distinguish males and females in species which engage in promiscuous and polygynous mating systems. Instead, mating is characterized by elaborate courtship rituals during which females attempt to assess the extent to which the male will remain with her and assist in raising offspring. Males, on the other hand, benefit from such prolonged precopulatory behavior by determining if the female has already mated with another female. From the male's point of view, the cardinal sin is to invest time and energy in raising the offspring of another male. Females can evaluate male commitment by exacting tasks from the male, e.g., nest building or providing her with food, before mating with him. Female choice is the selective pressure which causes intense male competition and the evolution of both intrasexual and epigamic phenotypic characters. This is especially true for males in promiscuous and polygynous mating systems in which males provide little, if any, parental care. Females are not exposed to these pressures and so compete among themselves only for resources with which to raise offspring. Whereas some males will have a high fitness payoff by mating with more than one female while others won't mate at all, all females will be able to find mates and so vary much less in reproductive output. What is of greater concern to females than finding a mate is obtaining the resources for successfully raising young. In summary, the difference in gamete production sets the stage for different male-female reproductive strategies. In general, males are promiscuous and combative among themselves in finding mates, whereas females are choosy and nurturant and compete among themselves only for resources. This greater male aggressiveness, mediated by the hormone testosterone, results in a higher mortality rate for males than females as they are exposed to more risks in reproducing. There is evidence that testosterone itself (a proximate cause of male behavior) reduces longevity which may also explain why males tend to have shorter lives than females. Males are much more exposed to both forms of sexual selection than are females and they have a greater variance in reproductive success. Other than gamete size and number, the contrasting characteristics of the two reproductive strategies listed in Table 1 are not intrinsic to either males or females. Thus, they can vary between the sexes and in fact do so in the polyandrous jacana mentioned above. In the jacana, those traits listed as components of the male strategy apply to the female, with those of the female applying to the male. The reason appears to be that in this bird species males provided a valuable resource for which females compete, namely, parental care, and so male parental investment is greater than that of females. Note that this difference is due to the parental care component of PI and not to the gamete component (females still have a larger gametic investment than males).