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.