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Ornamental traits have always been known to be important in attracting mates. However, it has been unclear for a long time why this is the case. The sexy sons hypothesis proposes that females choose showier males and engage in polygamous relationships in order to maximize the viability of their offspring. The pathogen avoidance model states that females select for showier males in order to protect themselves and eventually their offspring from infection as these males are overall more healthy.
Both theories fall short in that they do not acknowledge the importance of parenting in the health of offspring as sometimes, theoretically more viable offspring don’t thrive due to having low amounts of paternal care. I analyzed many different studies that have been performed throughout the years in an attempt to unravel this clash of views among many respected evolutionary biologists. Though it is inconclusive which theory of sexual selection is more representative of nature, this paper demonstrates the complexities involved in both theories and why potentially, both may be correct in one way or another.
Sexual Selection is a very important driving force in nature and even in our everyday lives. However, it is important to note that it is only prevalent if it goes against mechanisms of natural selection. Essentially, secondary sex traits are costly to the individual and are therefore not optimal to have. Instead, one must acknowledge that the goal of an organism’s existence in an evolutionary standpoint is to reproduce. Having these costly ornaments is only selected for because they must in some way increases an individual’s likeliness to produce offspring—even at the cost of survivorship and viability.
Two very important mechanisms of sexual selection that are known are the sexy son hypothesis and the pathogen avoidance model. The sexy son hypothesis states that females choose males with showier or more attractive traits in order to select for genes for pathogen resistance in order to pass those “good genes” onto their offspring. The pathogen avoidance model states that females select for males with secondary sex traits in order to distinguish between the diseased and the healthy males in order to stay safe from pathogens. Both theories are widely used to explain many evolutionary trends, but are they mutually exclusive? It is possible that both theories are correct in certain scenarios. However, one must look into the mechanisms of both in order to find out.
Overview of Sexy Sons Hypothesis The sexy son hypothesis which was popularized by Ronald Fisher is currently stated as a possible explanation for the great diversity of ornamental traits in animals. These ornamental traits are known to negatively affect the longevity of the individual but benefits its reproductive success. Essentially, females prefer to choose attractive and showier mates in order to produce attractive sons who are selected for by females, which leads to greater amount of grandchildren and so on. On top of that, through this theory, the idea is presented that females don’t simply select for the trait itself, but instead for the possession of the trait.
The difference, no matter how subtle, is important to acknowledge because theoretically, a male may possess a trait that is actually harmful to the mating relationship but is still desirable. As long as the trait itself signals “better genes” in terms of offspring health, it doesn’t matter what the trait itself is–the possession of that trait is enough. One such example of this is the trait of infidelity in a monogamous relationship. Though it is “better” to remain in that single mate relationship for the female because it allows her genes to be passed down instead of another member of the same species, the trait of infidelity may actually signal vigor and health in males and thus, the female will still be attracted. This leads to children who are also non-monogamous and through many generations, the non-monogamous, “good genes” are passed down to multiple grandchildren, great grandchildren, etc.
Another aspect of the sexy son hypothesis involves female selection for male genes through the selection of “good sperm”. In terms of long term success in reproduction (offspring which also produce many offspring), individuals should encounter many potential partners and when better partners are found, they should re-mate in order to maximize health of offspring. It is apparent the sexy son hypothesis argues heavily against monogamy and in favor of polygamy due to polygamy’s ability to maximize mate quality and quantity. The current social structure in many species today including humans revolves around monogamy, and the sexy son hypothesis proposes that monogamy is simply not optimal due to not maximizing reproductive success. The question arises: why does monogamy exist in many social constructs if it is contradictory to the single goal of evolution–to reproduce? Thus, it is important to justify the presence of monogamy in order for the sexy son hypothesis to hold true.
It is a prevalent question in the scientific community to this day why monogamy exists. Most families in nature involve heavy emphasis on maternal care which leads the male to provide for and invest in less than the female parent. Natural selection has so far favored this relationship where females invest a lot of energy into caring for offspring while males compete with other males of the same species for females. What current evolutionary biology supports is that that optimal mating protocol involves the pursuit of many partners to maximize offspring.
Fit males are defined by the statement that If x is fitter than y, then probably x will have more descendants than y (Pence, C., and Ramsey, G., 2013). Essentially, a fit individual is one whose probability of spreading their genes to the next generation is relatively high which only seems to back up the theoretical prevalence of polygamy where monogamy currently exists. Many theories for the justification of the presence of monogamy exist but I will be going over the process of male mate guarding. Male mate guarding is defined as the close association between a male and female prior to and/or after copulation for paternity assurance (Brotherton, et al. 2003).
One thing that a group of evolutionary biologists discovered was that the availability of partners plays a large role in this discrepancy (Schacht, Ryan, and Adrian V. Bell 2016). What this means is that in communities where males are valued over females (male-biased gene pool), for example in humans, females to mate with may be a scarce resource. This may lead the males to act in their best interests and achieve paternity with one female. This is possibly because with a lack of females, each female is viewed as more “valuable” and thus paternity with multiple females may have too big of a time expended/risk to reward ratio as competition will be extreme.
This leads to males that are more fit to mate with the limited number of females and the males that are not as fit will be left out as there are such limited resources. This shows that in the sexy sons hypothesis, the concept of limited female availability is not accounted for as the primary goal is to maximize offspring fitness and survivability by mating with numerous different mates. Though I have talked about male mate guarding, the same concept applies to females in the context of sexual selection as females must be choosy with males (limited males) and expending too much time with non fit males (time expended vs reward) takes away from potentially producing offspring.