  SEXUAL SELECTION: HOW FEMALES CHOOSE THE BEST MATE Sexual selection is the term used to define selection pressures not inflicted by the environment on the species as a whole, but rather selection pressure inflicted within the species as they compete for individual reproductive success (Daly and Wilson 1983). This competition for reproductive success within the species is composed of two elements: intra-sexual competition for mates; and mate choice, most typically made by the females (Daly and Wilson 1983). The fact that females tend to be choosier than males is reflective of the effects that a bad mating would have on each. It is the female who would suffer the most from one of these matings. Females have a limited amount of eggs available for insemination so it is important that they are not wasted, also the parental energy that they invest in the offspring usually surpasses that of the males. Conversely, males can produce sperm throughout the majority of their life; therefore they are not going to be in short supply (Daly and Wilson).
As a result of this it is important that females choose mates that will increase their reproductive success by either offering something to the female herself, or by simply contributing good genes to be passed to the progeny. How then is it that females choose the best males? Evidence suggests that the females choose males based on features that are indicative of their reproductive potential. One way that females evaluate the quality of the males is by morphological features known as secondary sex characteristics that seem to correlate with male fitness (Bateson 1983). An example of this kind of mate choice can be seen in the ring-necked pheasants, Phasianus colchicus (Mateos and Carranza 1995). In this species, the length of the male’s tail has been shown to be an attractive characteristic to the females.
The longer the tail, the more likely the female will choose him as a mate (Mateos and Carranza 1995). Other features found to be attractive to the females were tuft length, with the longer being the more attractive, and black spots in the wattle (Mateos and Carranza 1995). All three of these characteristics have also been shown to be indicative of male health and general quality. The males with the longer tails were in better physical condition than males with shorter tails (Mateos and Carranza 1995). Additionally, black spots in the wattle can be linked to testosterone levels because the hormone is involved in melanin distribution (Mateos and Carranza 1995). Therefore, the female choosing a male with black spots in his wattle would also be choosing a male with higher levels of testosterone.
Similarly, the “length of the ear tufts appears to be related to physical condition … [and] may reflect the ability of males to avoid injuries during agnostic interactions” (Mateos and Carranza 1995). This is because often times aggressive encounters between males result in pecks directed at the area behind the ears. Therefore, the males with the longer ear tufts would be the ones that were in a better physical condition. Thus, the morphological features that the females use to pick their mates are ones that can be used to judge general male fitness. Pheasants are not the only animals to use this system of mate choice. Female yellowhammers, Emberiza citrinella, also use morphological features to influence mate choice (Sundberg and Larsson 1994).
With this bird, plumage color is the phylogenic quality used to judge male quality. Males that were brighter yellow were picked as mates more often than the duller males (Sungberg and Larrson 1994). As it turns out, the yellow color of the plumage arises from carotenoid pigments ingested during the autumn moult (Sundberg and Larrson 1994). Thus it follows that the “more colorful males may signal good foraging ability, since coloration is a condition-dependent trait within a population” (Sundberg and Larrson 1994). Here again, the female chooses the male with the best features that will improve her reproductive success. Because both male and female yellowhammers feed their young (Sundberg and Larrson 1994), the foraging ability of the male becomes very important to the viability of the offspring.
It is also interesting to point out that carotenoids have been shown to stimulate immune response and to in some instances prevent cancer (Lozano 2001). Consequently, the males with the brightest yellow color are the ones less likely to succumb to disease. Other species besides the yellowhammer take advantage of using carotenoid dependent traits to choose their mates. One such species is the guppy, Poecilia reticulata, in which the female picks the male with the most and brightest orange patches (Houde and Hankes 1997). One study showed that if two populations of this fish species, the males from one having bright orange and the males from the other having a duller orange color, were mixed the females from both populations would all prefer the males with the brighter orange color (Houde and Hankes 1997). Therefore, the females undoubtedly make their choice of mate based on their color patter, the more orange the better.
As has been discussed, this increased orange would be indicative of the increased health of the male. Sight is not the only sense that females can utilize to choose their prospective mates; smell and sound are what is used by some species. An example of this is the meadow vole, Microtus pennsylvanicus. Females of this species choose their mate based on their scent (Ferkin et al 1997). The scent that the females prefer is indeed one that responds with the health of the male. This occurs because diet can influence body odor, therefore the males that are preferred by the females are the ones that have the optimal diet (Ferkin et al 1997).
One aspect of the diet that seems to most specifically affect body odor and female preference is the protein content in the diet. “Protein content is an important determinant of diet quality for mammals; it affects reproductive maturation, … and growth of young” (Ferkin et al 1997). Hence, by picking the males by their smell, the females are choosing males that will have the greatest reproductive success and pass on their genes to the next generation. Sound can also be an important indicator of male fitness. Females from species as different as toads and red deer utilize this sense to choose their mates (Daly and Wilson 1983). Female toads pick males with the deepest croak (Daly and Wilson 1983).
Not surprisingly, the deepness of the males’ croak correlates with body size; the deeper the croak, the larger the body (Daly and Wilson 1983). Similarly, the female red deer chooses the buck with the loudest roar, which is indicative of chest capacity (Daly and Wilson 1983). By picking these males, the females are increasing the fitness of further generations. Another way in which females select their mates is by using certain male behaviors as indicators of male fitness much in the same way morphological features are used. The wolf spider, Hydrolycosa rubrofasciata, is a species that does just that. When courting a female, the male wolf spider uses his stomach to drum on dry leaves (Kotiaho et al 1996); the female then picks which male she will mate with.
Female preference is for the male that is most actively drumming, which seems to also correspond with the most viable males (Kotiaho et al 1996). Drumming ability and metabolic rates are positively correlated, thus the greater the drumming activity, the greater the metabolic rate of the male (Kotiaho et al 1996). Males with a higher metabolic rate are also the males that have higher energy expenditure. By choosing these males, the females ensure that their offspring will inherit genes that promote fitness and viability. The Australian bowerbird, Ptilonorhynchus violaceus, is another species where females choose mates based on some male behavior, specifically in their ability to build bowers (Cronin 1991). Bowers are built by males and are not used for nesting, but solely for mating (Cronin 1991).
Females choose the males with the most decorative bowers (Cronin 1991). The reason for this lies in the fact that males tend to steal decoration from other males, so the male with the most is not only the best at stealing, but also the best at defending his territory (Cronin 1991). Bower decorating is therefore an honest indicator of male quality and once again the females are choosing the most advantageous mates. In contrast to the bowerbird, the penduline tit, Remiz pendulinus, bases her choice of mates on nests built by males for the purpose of nesting (Hoi et al 1994). The males preferred by the females are the ones with the largest nests (Hoi et al 1994). Nest size is very important for this bird because the larger the nest, the better insulated it is and therefore, the eggs are well incubated (Hoi et al 1994).
Furthermore, increased insulation would allow for the female to spend more time away from the nest looking for food (Hoi et al 1994). Consequently, she could gather more food at one time and decrease her chances of being spotted by predators because she doesn’t have to leave the nest as often (Hoi et al 1994). It has thus been shown that females do chose mates based upon qualities that are indicative of their overall fitness such as color, smell, sound, and behavior. The question now arises of how is it that the females know which traits to look for. The answer to this lies in realizing that it is not a conscious choice that the female makes, but that her choice us a product of evolution. Helena Cronin (1994) explains thus, “females with one particular gene make a particular choice of mate and also mate more successfully than females with a different allele”.
The result of this would be that the female with the gene that allowed her to make the best mate choice would be the one with the most genes going into the next generation. Natural Selection would therefore select for the females that possess this gene thereby eventually changing the allelic frequency of the population by making that allele the normal one. In short, females are choosier than males because their reproductive success is hindered more than the males if a bad mating should occur. Because of this, females able to choose males with the greatest fitness are selected for. Thus, eventually females of the species would be able to pick the most fit male on the basis of morphology, sound, smell, or behavior of the males. This then, is how females choose their mates.
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