Comparing the Sexy Sons Hypothesis and the Pathogen Avoidance Models Effects on Sexual Selection
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Comparing the Sexy Sons Hypothesis and the Pathogen Avoidance Model’s Effects on Sexual Selection
Abstract
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.
Introduction
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.
Analysis of Sexy Sons Hypothesis
As I have stated before, polygamy plays a very crucial role in the sexy sons hypothesis as females will optimally mate with several males if “better sperm” is found. Females must be selective with the males they mate with and thus, if a more attractive male is found, it is better to mate with the new male instead of mating with previous less attractive males. A study was performed where rattlebox moths (Utetheisa ornatrix) were selected for either monandrous or polyandrous relationships and the offspring were then mated to test for ...
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Analysis of Sexy Sons Hypothesis
As I have stated before, polygamy plays a very crucial role in the sexy sons hypothesis as females will optimally mate with several males if “better sperm” is found. Females must be selective with the males they mate with and thus, if a more attractive male is found, it is better to mate with the new male instead of mating with previous less attractive males. A study was performed where rattlebox moths (Utetheisa ornatrix) were selected for either monandrous or polyandrous relationships and the offspring were then mated to test for increase in reproductive success (Egan, et al. 2015). The researchers acquired two size matched sister moths so that many factors in reproductive success may be accounted for. They then mated one moth with a single male three times and the other once with three different males. What they found was that sons produced from the polyandrous relationship had increased reproductive success. They produced around 81% of the total number of offspring including the monandrous line males. This translates into more than three times the amount of offspring than the monandrous offspring. This study shows that males who came from polyandrous lineage were more reproductively successful and were able to produce a larger quantity of offspring. There are limitations, however, to this study as the offspring of the sister moths were not tested for attractiveness. Instead, they were tested only for number of offspring produced through an arranged mating. It would have been more informative if research was done such as creating a choice chamber for the second generation sons to test why they had greater reproductive success. This study was helpful as previously stated, the purpose behind all life in an evolutionary perspective is to produce more offspring and the polyandrous lineage males were able to accomplish that goal.
A more complete analysis was done in 2013 by Adam Nelson and 8 other researchers. Mice release certain proteins in their urine call MUPs (Major Urinary Proteins) as social pheromones for attracting females. This process has been studied for a long time with more and more evidence confirming its reliability as a measure of attractiveness (Kumar, et al. 2014). It has been known that the higher the MUP concentration, the more attractive the male mouse is to females, so MUP concentrations were used as guidelines for the attractiveness of the males in the experiment. Nelson and his colleagues reintroduced them to free mate choice environments and allowed them to reproduce. On the other hand, he also mated monogamous non selective mice in a laboratory setting. What he found was that in the free mate choice scenario, the females produced offspring also had elevated MUP levels compared to the laboratory controlled mice. Another very interesting thing to notice is that those sons were not only sexier, but they also had shorter life spans on average. This shows a possible tradeoff between attractiveness and viability. The mice were able to make this tradeoff because they simply had better genes,and in the end, being more attractive is more important in an evolutionary perspective as it increases the likelihood of producing offspring.This study provides more support for the Egan study as it demonstrates in mice, the increased reproductive success was due to having better genes. It accomplishes what the Egan study did not because it goes a step further and demonstrates the reasoning behind the results and provides strong evidence for the sexy sons hypothesis.
A 2007 study performed by Kirsten Klappert and Klaus Reinhold, analyzed the mechanisms behind the female grasshopper (Chorthippus biguttulus) preference towards showy traits, in this case, the quality of the male song. This quality was determined in a series of experiments using choice chambers. Females were placed in chambers and a recording of the male song would be played. Based on the female reaction to the song, the males were given rankings of attractiveness and the moderately attractive males were removed from the experiment in order to analyze the extremes of attractiveness. The males were then placed independently with many females in enclosures in order to allow them to reproduce. Their larvae were then marked based on their father’s attractiveness and were allowed to grow up under constant conditions. Based on the sexy sons hypothesis, the offspring of the unattractive males should be less viable compared to those of the attractive males. Additionally, it should be expected that the offspring descended from attractive males should also produce songs that were more attractive than those of descendants of unattractive males. However, both of these hypotheses appeared to be false. In fact, there appeared to be a negative correlation between attractiveness of the father and viability of the offspring as fewer attractive descended larvae reached adulthood compared to the unattractive descended larvae. Also, attractive descended grasshoppers did not manage to have more attractive songs compared to the unattractive ones. This shows that the sexy sons hypothesis probably doesn’t apply to this particular species as the attractiveness of the first generation males did not translate into any particular benefits for the next generation. The major drawback to this study, however, was that the grasshoppers were not introduced into large community settings like the mice were in the 2013 Nelson study. In large communities, fitter individuals would most likely have dominated and produced the most offspring and possibly also the fittest.
A study performed in 2005 claims to have acquired results that do not support the sexy sons hypothesis like the 2013 Nelson study did. The study performed by Lars Gustafsson and Anna Qvarnstrom showed that collared flycatchers did not inherit sexiness from their fathers. A male collared flycatcher’s attractiveness is determined largely by the size of its white forehead patch. Larger patches correlates to greater reproductive success for males. A major problem that the research team attempted to analyze was the problem with the tradeoff between reduced fecundity (ability to produce an abundance of offspring) and the sexiness of their sons. What they discovered was that the flycatchers who were in polygamous relationships actually produced sons with smaller forehead patches than those of monogamous relationships. Additionally, because the monogamous sons had larger forehead patches and were thus more attractive, a higher proportion of their sons actually went into polygamous relationships contrary to what their fathers had done. One important point concerning the reason behind this discrepancy is found. The paternal care for the polygamous lineage offspring was significantly lower than that of monogamous relationships due to the spread of time among the many offspring. Males who were indeed more attractive produced less attractive sons because there was an apparent tradeoff between offspring sexiness and reproductive success. This brings up an important point: when the attractiveness of the offspring is dependent on both environmental factors and the attractiveness of the parent, then the costs of polygyny are unlikely to outweigh the initial genetic advantage. This can be tied to the Nelson study which demonstrates that many epigenetic factors also play a role in the development of sexy offspring. In this case, the flycatchers were raised with little paternal care which led to underdevelopment of their potentially attractive features. Paternal care’s importance is further backed up in the Aguilar study for pathogen avoidance.
Overview of Pathogen Avoidance Model
The pathogen avoidance model was created in response to the sexy sons hypothesis to acknowledge some apparent flaws. Popularized by Craige Loehle in 1997, the theory states that females can avoid parasites by choosing showier males and that the benefit of choosing showier males is to benefit the female’s reproductive success rather than the fitness of offspring. 4 main problems Loehle points out about the good genes model are that secondary sex traits don’t signal fitness levels, assumptions must be made about gene linkages between female preference and secondary sex traits, females must prefer showier trait when it first occurs which is highly unlikely, and females must show preference for an unusual trait. Good genes aren’t always signaled by showy or ornamental traits as is shown in the fact that males with greater expression of secondary sex traits likely live shorter lives (Nelson, et al. 2013). Thus, it is far more likely that over the course of many years, females will recognize this fact rather than viewing more ornamental males to be better. The second point adds on to this fact as female preference to the trait must be linked to the presence of the trait otherwise the trait would not even be viewed as “ornamental” or “showy” which is highly unlikely. Additionally, the third and fourth point tie together in that showier traits must be prefered which is also unlikely as when the trait first emerges, it is viewed as radical and different which should signal a defect in the male and thus counteracts the interactions between individuals in the sexy sons hypothesis. The pathogen avoidance model can be inferred in many examples in nature such as in satin bowerbirds (Borgia and Collis, 1990). In these birds, females can be seen to shake after intercourse in attempts to rid themselves of parasites. This can be explained by the fact that females are the primary caretakers of most animal species on the planet and that a parasite on the female will likely also transmit itself to the young; thus females must select for more healthy males in order to protect themselves and eventually their offspring from pathogens. The primary difference between this model and the sexy sons model is that safety from disease is the primary focus rather than the fitness of the offspring. This can only hold true if the degree to which an individual is ill can be seen through their expression of secondary sex traits. The traits are what are called honest indicators as when infection status increases, the expression of the traits decreases. It has been found that males infected with a pathogen usually received less physical contact from females which backs up this model of sexual selection.
Analysis of Pathogen Avoidance Model
A 2016 study by Pedro Vale and Michael Jardine analyzed female fruit fly (Drosophila melanogaster) behavior when presented with the possibility of becoming infected with a virus. They gathered fruit flies that had previously been infected with a virus and those without an infection history and placed them in chambers with two vials of food, one infected with a virus and the other clean. What they found was that flies which had previously been infected showed avoidance towards the vial that was infected with the virus and that flies that had not been previously infected showed no preference. This is interesting as it shows that the pathogen avoidance mechanism in the flies is inherited through life events. More importantly however, was the discovery that only female flies exhibited this avoidance behavior. The gender bias of avoidance demonstrates that females behave in a way in order to keep their offspring safe from infection. This provides strong evidence for the pathogen avoidance model as it shows females selecting for environments where they and their eventual offspring are less susceptible to infection. The major limitation to this experiment however, was that it would have been a lot more informative if the researchers also used the same choice chambers for mates as the pathogen avoidance model deals mainly with finding mates, not foraging for food. Looking past this limitation, the results show convincing evidence that females select for pathogen avoidance as a major consideration for their future offspring as only females exhibited the selective behavior.
A 2015 study performed by Patricia Lopes and Barbara Konig was very similar to the 2013 Nelson study as it analyzed males house mice (Mus musculus domesticus) and female choice. The difference in this experiment, however, was that the males were infected with a virus and the difference in attractiveness between control mice and infected mice were analyzed using choice chambers. In this experiment, darcin, a protein found in mouse urine, was used as a measure the degree to which the mice were sick. As these secondary sex traits are known to be honest signals of quality, meaning lower quality mice (sick) express lower amounts of it. Mice infected with the virus exhibited significantly lower darcin levels as well as testosterone levels. Female mice were also able to identify this drop in viability and when presented with choice chambers, they tended to select the uninfected mice. In fact, in a previous study, injecting mice with high levels of testosterone was actually effective in maintaining attractiveness in infected males and in stabilizing urine protein levels (Litvinova, et al. 2005). Thus, it may concluded that darcin levels dropped as a result of decreased testosterone. In fact, other studies have shown that testosterone actually plays a role in inhibiting the immune response (Bouman, et al. 2005), which is important to this study as it shows that mice which were infected had to accept a tradeoff between their ability to fight the infection and their ability to attract mates, and in the end, fighting off infection was more important. Looking back at the 2016 Vale study, females tend to avoid pathogen rich environments and thus, males accepted this tradeoff until testosterone levels could be uninhibited. This study presents strong evidence for the pathogen avoidance model as it demonstrates female mice tend to avoid infection by selecting healthy males.
Another interesting study was performed in 2007 by a group of researchers which showed that in blue-black grassquits, parasites decreased male displays but did not change mate preference (Aguilar et. al). They infected blue-black grassquits with parasites and compared them to a control group. The parasites did indeed have negative impacts on secondary sex traits but when presented with choice chambers, females’ choices were random in terms of selecting against parasitism. What this shows is that ornamental traits alone cannot always show health and vigor. Instead, paternal behaviors such as parenting and nest building also play an essential role in a female’s choice. This ties back to the 2005 Gustafsson study which showed that flycatchers which had descended from polygamous relationships actually exhibited lower levels of secondary sex traits because this study demonstrates that greater initial success (lack of parasitism/polygamous relationship with fitter males) doesn’t always translate into success later on. Other factors play roles such as parenting.
The most fundamental and most groundbreaking study was one of the first studies performed relating to selection against pathogens. The Hamilton-Zuk hypothesis was published in 1982 by William Hamilton and Marlene Zuk, and it analyzed the role of ornamental features in the metaphorical arms race against parasites. This arms race is essentially what drives great genetic variation because with little to no variation, parasites can adapt easily to its hosts’ genes. However, in order to avoid this, hosts must evolve just like the parasites do, and the individuals who do not adapt will fall victim to the parasites. This is called a coadaptation cycle. Hamilton and Zuk discovered that in many species of birds, increased showiness (plumage and song) led to increased resistance to parasites. In the context of evolution, birds which are more resistant to parasites and are therefore more viable are able to demonstrate this viability via secondary sex traits such as showier plumage. Contrary to the 2015 Lopes study however, this study demonstrated that individuals that were more showy were also more viable as seen in their greater resistance to parasites. In the Lopes study, mice which became sick downregulated their testosterone and darcin levels, indicating a tradeoff between immune response and secondary sex traits. In the Hamilton-Zuks study, however, it was discovered that instead of a tradeoff, the individuals which exhibited greater ornamental traits could afford to do so because of being overall more healthy.
Discussion
Through analyzing both the sexy sons hypothesis and the pathogen avoidance model, it is inconclusive whether one is more viable than the other as both have their justifications and their weaknesses. For the most part, both theories were backed up by numerous studies. However, instances where certain sons should have been more viable indicated that the sons were in fact less healthy due to other influences such as paternal care and other environmental factors. Additionally, it should be noted that the nature of the secondary sex traits is still unknown. There is either a tradeoff between the ability to exhibit ornamental traits and the ability to remain healthy, or there is no tradeoff and instead, males who are showy are actually overall more healthy. This difference distinguishes between risk vs reward in attracting mates and overall higher quality (survival of the fittest). The value of these traits in attracting mates is undisputed, but to what extent evolution has stressed species into balancing risk and reward is still unclear and is still open to research. Also, the discrepancy of many species who partake in monogamy and those which are polygamous is interesting because it shows that the same theory has many small details branching from it, requiring more specific analysis. In conclusion, both of the theories are backed by strong evidence but what needs to be realized is that there are many factors not encompassed by the theories that have been discovered, with more to likely come.
Works Cited
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