Female gametes being large, metabolically expensive to produce,
and rare, constitute a limiting resource for males. Females, having made
a considerable parental investment in a particular offspring, will be selected
to be particular about the characteristics of the male that they allow
to fertilize their eggs. An unwise decision on the part of a female about
a mate results in loss of metabolic investment in gametes and could cost
the female her entire reproductive output for a particular breeding season.
The essence of intersexual selection is that mating by males is non-random,
and that the non-randomness is in part a result of female choice:
rejection by females of some males and acceptance of others;
some males are preferred as mates and others are avoided
Upon what criteria do females base choices about mates?
Material benefits:
Hangingfly bug story;
These insects are predaceous, but are also scavengers. They will find a dead insect to feed on if they cannot catch one. Males catch or find a prey, and may feed on it, but will not suck it completely dry. They then will use the dead insect as a visual display during courtship. They also have a pheromone that attracts females to their perch. A male hangs from the perch by his front legs, and dangles the dead bug from his hind legs, wafting the pheromone to attract females. A female will approach a male, and will check out his dead insect. If the insect is below a certain minimum size (if his bug isn't big enough) she rejects him utterly. If the bug is big enough, she will accept it as a nuptial gift and allow the male to begin copulating. The male must copulate for at least 20 minutes to be able to transfer his full load of sperm. If the female feeds and copulates for 20 minutes, she will become non-receptive and will lay eggs (fertilized with the male's sperm) for several hours. But, if the bug runs dry before she has fed for 20 minutes, she will terminate copulation and seek out another male with a bigger bug (another choice on her part).
If the male copulates for 20 minutes and the female is still feeding on his bug, what does he do? He takes it back, because he can use it to attract another female.
So females make choices about potential mates based on material benefits provided by the male. These directly affect the reproductive output of the female by providing nourishment to her of eventually to her offspring.
Male alternatives: secretion, stealing, rape
Genetic benefits:
In many species (especially birds) males possess sexual advertisement organs that are gross caricatures of similar organs in other species, and that are exaggerated beyond any possible functional necessity, so that these organs cannot have any possible use except in mate attraction. Examples include peacock tails, bellbirds' wattles and songs, tails of widowbirds, bowerbirds bowers, manakins displays, and the mandibles of dobsonflies.
How can we explain the evolution of these elaborate, exaggerated displays?
three possible explanations, based on the effect of these displays on female reproductive success (female choice: females prefer mates with elaborate displays). The choices of females affect the characteristics of her offspring and their eventual survival and reproductive success; indirectly affect the female's reproductive success.
Runaway selection hypothesis
In an ancestral population (say of african longtailed widowbirds -- this is a sparrow sized bird but the breeding males have a tail that is about 18 inches long) there was a normal distribution of size of male characteristic (in this case of tail length). Tail length (and presumably preference for tail length) is variable and we assume heritability.Females prefer to mate with males with longer tails - this can be demonstrated experimentally by shortening or lengthening males tails. Female preference is a behavior that can be inherited.
If males with slightly longer than average tails have some initial fitness advantage over other males (say because of better mobility for foraging or for escape from predators), the population mean will shift to longer tails.
But if longer male tails confer a fitness advantage, would females that prefer to mate with better endowed males also gain a benefit? Their male offspring would inherit longer tail length and would be at an advantage (which would benefit the female). What about the female offspring? They would inherit their mother's preference for males with longer than average tails.
These females with the initial preference for longer male tails would be at a reproductive advantage over other females (because of the initial advantage conferred by long tailed males) and so longer tails and the preference for them would become more frequent in the population.
In future generations, males with longer tails would be preferred as mates (because the majority of females carry the preference trait) regardless of the fitness advantage conferred by tail length due to natural selection. Now male tail length is selected for simply because females prefer males with longer than average tails. Females should continue to prefer males with longer tails because their sons will inherit longer tails and thus be preferred ? this is a self-reinforcing cycle of selection.
Male tail length and female preference evolve together ("run away") until the population reaches a point at which the disadvantages of tail length (inability to escape from predators, for instance) balance the advantages due to female preferences. Males with longer tails mate more often but survive less well; males with shorter tails survive better but mate less often. The increase in tail length stops when relative contributions of survival and mating success result in a balance of lifetime reproductive success.
There is evidence to support this hypothesis ? experimentally shortening and lengthening tails.
Parasite resistance hypothesis
Bright coloration and elaborate display are a signal on the male's part that he is free of or resistant to parasites or disease. Only males that are healthy can make the metabolic investment in displays.Parasites are assumed to reduce host fitness and reisitance is assumed to be heritable.
Females gain from exerting a preference for resistant males because her offspring would inherit the resistance
Data from studies on barn swallows (Moller 1990) and jungle fowl (Zuk) support this hypothesis:
Male (and female) barn swallows are parasitized by mites; parasitism reduces the ability of these birds to survive and reproduce (parasitized babies are smaller and less viable than unparasitized ones).
There is heritable variation among males in their resistance to parasitism. Experimental partial cross fostering: movements of offspring from parasitized parents to unparasitized parents demonstrate a partial genetic component to susceptibility to parasitism.
Males with greater resistance to parasitism have longer tail ornaments.
Females prefer as mates males with longer tails. The result of this preference is less parasitism of the female and her offspring, thus greater reproductive success for the female because of her choice.
Female choice affects both physiological and morphological characteristics of male population.
Increased viability hypothesis:
Bright coloration and elaborate display signal greater viability (ability to survive environmental rigors of various kinds) on the part of the male, which would be passed to the offspring of the female.Females benefit from choosing brightly colored males with elaborate displays.
Data from recent studies on peacocks support this hypothesis.
Males vary in the size of the eyespots on their tails.
This variation is heritable.
Females prefer to mate with males that have larger eyespots.
Eyespot size is positively correlated with body mass (of both male and female offspring) at 84 days of age. Body mass in young birds is a strong indicator of eventual survival.
Offspring of males with larger eyespots survived longer under more or less natural conditions (in a nature preserve) than did the offspring of males with smaller eyespots.
males with larger eyespots suffer less mortality due to predation than males with smaller eyespots
Females benefit from choice because their offspring are more viable. Female choice determines morphological and physiological characteristics of male population.
Handicap hypothesis
Males with elaborate ornamentation or displays are signalling simply that they can bear such characteristics and still survive and reproduce.Only males of high genetic quality can survive with the handicap; presumably there is heritable variation in genetic quality and degree of handicap, and that these are correlated.
Females should choose to mate with them because of their superior ability to bear hardships or handicaps
offspring inherit their father's genetic quality but also the handicap