SEX (continued)

Tennis anyone? Competitive advantages of sex change

Female to male: Whenever there is intense competition among males for females, usually the strongest males will have the greatest reproductive success. One way the weaker males can have some success is to adopt sneaky mating strategies (see below). Alternatively, individuals of some species start reproductive life as females and then change to males when large enough to compete successfully. This type of sex change occurs in some fishes and is called protogynous hermaphroditism. It will be favored whenever an individual reproduces best as a female when small and as a male when large, and when lifetime reproductive success is greater if an individual changes sex than if it stays one sex throughout life.

The blue headed wrass is an example. Males are brightly colored and defend territories on coral reefs. Females choose the largest, brightest males as mates; some males spawn up to 40 times a day at the peak of the breeding season. Sex change is socially controlled. If the territorial male is removed, the largest female changes sex, becoming male.

Male to female: More rarely, an individual may be a male when small and then change to a female when large (protandrous hermaphroditism). If male size has little effect on breeding success, and breeding success increases as female size increases (as it does in most fishes), this strategy may be an ESS. The anenome fish (Amphiprion alkallopisos), another coral reef fish, is symbiotic with anemones, and each anenome is large enough for only two fish. Thus, the species lives in monogamous pairs. The reproductive success is limited by the female's ability to produce eggs. A pair has the highest success if the female is large and the male is just large enough to be able to fertilize all of the eggs she lays. Sex change in the smaller individual (male) occurs if the larger (female) individual is removed and a newcomer that is smaller settles in her place.

In humans the most famous example of sex change is that of Renee Richards who changed from male to female to improve his (her) competitiveness on the professional tennis circuit.

As each sex attempts to maximize fitness or the benefit to cost ratio, i.e., to maximize the vertical difference between the straight line and the curve (handout, Fig. 6.1), males and females have different optima. As females are able to control the number of eggs produced, they, therefore, become a limiting resource for which males compete (see handout). The sex investing less will show little discrimination in mating preferences -- all potential mates are high quality, but the sex investing more will tend to evolve mechanisms of mate choice to insure mating only with high quality individuals of the other sex. Thus, a female that mates with a dominant male may gain a long-term genetic advantage, a short-term material advantage (if he controls key resources), or both.

Darwin invented the term sexual selection to describe the selection that arises when members of one sex, usually males, gain an advantage over other individuals of the same sex in obtaining mates. There are two distinct processes in sexual selection. First, intrasexual selection, results from competition among males for access to females, acting on those characters that influence a male's competitive ability in contests with other males. The second process is called intersexual selection, or more commonly epigamic selection, and acts on those characters in males which influence a female's choice. Thus, males may contest territories, some becoming floaters; intrasexual selection would act on those traits that influence a male's ability to compete for a territory. Secondly, females may choose mates on the basis of resources the male controls or some aspect of the male's courtship display, if females base their choice on characteristics of male, those characteristics would be under the influence of epigamic selection.

The intensity of sexual selection depends on the degree of competition for mates, which in turn depends on two factors: (1) the degree of difference in parental investment between the two sexes and (2) the ratio of males to females mating at any one time, i.e., the operational sex ratio. When parental investment is about equal for the two sexes, e.g., in monogamous birds where both the father and mother feed the young, sexual selection will be less intense (but not absent) than in species with very different levels of parental investment, e.g., those polygynous birds such as most populations of the Red-winged Blackbird in which only the female feeds the young.

In addition, the degree of synchrony in receptivity of the sex which is the limiting resource is important. Generally, if females come into breeding condition asynchronously, there is more chance for a few males to mate with many females, one after the other; consequently, sexual selection would be intense. Male bullfrogs (Rana catesbiana) defend territories in ponds and small lakes where females come to lay their eggs over a period of several weeks. Males that defend the most attractive spawning sites (low density of vegetation, which reduces susceptibility of eggs to leach predation) may mate with as many as six females in a season due to the asynchronous arrival of females. In contrast, the Common toad (Bufo bufo) is an "explosive breeder", i.e., breeding is highly synchronous; all the females lay their eggs in a period of about a week, and each male has time to mate with one female, or at most two, before the end of the breeding season. Thus there is stronger sexual selection acting on male bullfrogs than on male common toads.

How do sexually selected traits evolve? It's relatively easy to see how traits that improve male fighting ability and success in male-male competition would be favored by intrasexual selection. But what is the basis for epigamic selection of traits? Some sexually selected traits are certainly bizarre, e.g., the peacock's tail. Fisher's idea of "runaway sexual selection" was that if some females prefer a particular type of male and their male offspring also possess that trait, then other females should choose such males as mates because they would have sons that are attractive to many other females. This argument seems circular, but that's the idea; once a trait is preferred by some females, it will tend to be preferred by more and more females generation after generation. In Fisher's theory the success of strategy becomes greater as it becomes more common, and the system "runs away" until natural selection halts the process. For example, bright coloration might evolve due to female preference, but coloration that is so conspicuous that it results in predation before mating will be selected against no matter how irresistible it would be to females.

Hamilton and Zuk (1982. Science 218:384-387) suggested that traits such as a peacock's tail or bright coloration may function as signals of condition and that female preference for the most bizarre traits may actually be a way of assuring that their partners are the healthiest males. They argued that parasite loads and other types of infections result in power quality plumage (this is well-known to poultry farmers) and that a female that chooses a male with bright, elaborate adornments may be choosing the healthiest males. This is an alternative to Fisher's idea, which states that whim alone can select for attractive traits.

Amotz Zahavi suggested that bizarre traits such as the peacockís tail evolve because they are handicaps and only the ìbestî males can both survive and strongly express the trait. Recently several variations on this theme have been proposed. One idea is that sexually selected traits, e.g., long tails, might be "condition-dependent"; under this model males would grow a tail of a length that maximizes lifetime reproductive success, optimizing the balance between lowered viability and higher mating success associated with a longer tail. Another (closely related) idea is that handicaps might be "revealing", i.e., differences in traits like tail length may reveal genetic differences in male viabilities. These various ideas have merit and fit our discussion of Diamond's article "Kung Fu Kerosene Drinking" that signals used in intraspecific communication are typically reliable indicators of an individual's condition.

Some traits may function in both intrasexual selection and epigamic selection, and it may be impossible to differentiate the two effects. For example, a displaying territorial male bird both attracts a mate and excludes other mates from his area of exclusive use.

Several recent studies suggest that females may influence their choice of mates by inducing competition among males. This was first noted in the elephant seal. When females are mounted they give loud "protest" calls. If the courting male is the territory owner, he can mate without interference. However, if the male is a sneaky male, the territorial male responds to these calls and chases the intruder out. This behavior further insures that the female will mate with the most dominant male in her immediate vicinity.

Female mountain sheep aggregate with others when they are in estrus (see handout). It's also been noted that female red-winged blackbirds often prospect for nest sites on territory boundaries, causing intense agonistic interactions between neighboring males; she will then settle on the territory of the male that wins the encounter. All these examples show that females behave in ways to insure that they will most likely mate with the most dominant males.

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