MATING SYSTEMS (Concluded)

Polygyny (Concluded)

An alternative hypothesis to the polygyny threshold model for the evolution of territorial polygyny is that relatively few males may hold all the territories such that the number of females seeking to breed is much greater than the number of available territories. Consequently, a female can have some success mating with an already mated male but would have no success if attempting to mate monogamously and all males are already paired. If the operational sex ratio (number of reproductively active males:number of reproductively active females) is skewed in favor of females, polygyny may be the only option of breeding for many females. Of course, in such situations we should also ask why the sex ratio would become so skewed in the first place. Generally, male survivorship is depressed in polygynous species because of the major emphasis on male-male competition.

Red-winged Blackbirds appear to fit the polygyny threshold model because reproductive success of females in large harems is typically equal or higher (recall our earlier discussions of the ideal-free and ideal-despotic distributions) than that of females in small harems or in monogamous breeding units (see Alcock). In most redwing populations the male provides no parental care, and success of all females on a territory is therefore similar.

The bobolink is another species in the blackbird subfamily, but bobolink males, unlike redwing males, aid in rearing the young of their first mate (but generally not the second mate). Second mates even on good quality territories typically have lower success than monogamous females. Is this a contradiction of the polygyny threshold model? Jim Wittenberger studied bobolinks and accumulated evidence that the quality of territories of unmated males is relatively poor compared to that of territories with females. Thus apparently a female is better off as a second mate on a good quality territory than as the only mate on a poor quality territory. Apparently she gains more by being on a good territory than she loses by having no male help in incubation and brood-rearing.

Unlike most territorial polygynous birds, polygynous Yellow-bellied Marmot females have poorer reproductive success than monogamous females. Marmots are diurnal, burrowing colonial rodents living in alpine meadows. Males pair with 1-3 females; monogamous females have greater success than bigamous females who likewise have higher success than females sharing a male's territory with 2 other females. Thus the polygyny threshold model doesn't seem to fit these data (also see figure in Alcock). Polygynous females aren't as successful reproductively as are monogamous females. Why don't females mate monogamously? Several years ago, it was thought that a cooperation model may apply: females could benefit through mutual vigilance, and all territorial members hibernate together; however, survivorship of females appears to be independent of harem size. Female marmots do not tend to disperse. Perhaps females have higher inclusive fitness by tolerating female relatives than if they were to try to drive each other away. Alternatively, polygyny may simply be the only option available to some females, and they may be making the best of a bad lot.

In harem polygyny or female defense polygyny males gain access to females by directly defending them against rivals. Such a system is facilitated if females have an independent tendency to be gregarious but at least somewhat unpredictable in their movement patterns. Red deer males control harems in prime feeding areas where females tend to aggregate. The distribution of reproductive success of Red deer males is primarily a function of male dominance relationships.

In some polygynous species, rank in male dominance hierarchies confers access to single females as they enter estrous, e.g., Dall sheep, olive baboons, and dominant males consort with only one female at a time, i.e., they don't defend harems. In these systems, the variance in male reproductive success will depend to a considerable extent on the reproductive synchrony of females: higher synchrony will result in more mating opportunities for subdominant males. You might ask yourself how these species fit into Alcock's framework.

In lek polygyny, males defend small mating territories on communal display areas. Typically, females of lek-breeding species are too dispersed for males to economically defend, and females and their young feed on non-defensible resources. Females choose males on the basis of their display characteristics (strong epigamic selection) and/or the location within the lek.

Lek polygyny perhaps evolved from systems of resource defense polygyny. The hartebeest may be a good example of a precursor stage for the type of lek polygyny exhibited by the Uganda kob. Male kob defend small display territories on leks on raised ground in open country. Male hartebeest defend territories with a wide variety of habitat types from streamside to upland areas. However, males tend to rest near each other in upland, grassy areas where they can best view predators and females tend to feed. If males were to stop defense of lowland portions of their territories, the result would be a lek system.

Most lekking species do live in open country; predator vigilance may be an important factor in male grouping and predator avoidance (selfish herd effect) could play a role in female preference for mating in central territories. In Uganda kob and fallow deer females prefer to join female groups whether or not males are present.

A controversial question about leks is called the lek paradox. Because males are only providing genes and relatively few males are successful, it seems that after several generations all males would be offspring of highly successful males, and, therefore, they should be approximately equal in genetic quality. Why do females continue to be highly selective? This is the "lek paradox." The current idea is that even if genetic variability is reduced, some variability is present due to recombination (and perhaps mutation) and some variants may result in better condition, e.g., resistance to parasites (recall the ìRed Queenî hypothesis). Thus females should remain choosy because even slight differences in male genotypes may influence fitness of their offspring.

Polyandry

Polyandry is a rare mating system primarily studied in shorebirds. In several species females are emancipated from incubation duties and parental care; these become the responsibility of the male only. This is advantageous to males only if the future reproductive condition of the female (ability to produce eggs) is directly important to him. For example, if rates of egg loss are high and a female can replace a lost clutch much easier if she is not incubating, then female emancipation from incubation would be advantageous to both the male and the female. Also, in some shorebird species (and some other birds with precocial young), the female lays a clutch of eggs which the male incubates and then a second clutch which she incubates. Typically, but not always, the original male fathers the second clutch.

Male incubation preadapts a species for possible evolution of polyandry. To the degree that females can produce more clutches than can be incubated by males, males become a limiting resource. True polyandry is rare among birds. In all three phalarope species, males perform all the parental care. Females compete among themselves for males to incubate each of their clutches. At high densities, some females are prevented from breeding. However, as females must retain males until a clutch is completed and the mating system is an explosive breeding assemblage, the operational sex ratio (number of breeding males and females) typically is not particularly skewed.

Spotted Sandpipers exhibit resource defense polyandry. Females defend territories and provide clutches for males settling on their territories. Some females are unsuccessful in obtaining mates; others may pair with as many as three males. The "polyandry" threshold model applies to this polygamy situation because there is considerable variability in quality of the female territories, and females on the best quality territories can, therefore, attract more mates.

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