Thursday, February 12, 2015

Sexual Cannibalism: Putting 50 Shades of Grey to Shame

By Jackie Campbell 
With Valentine’s Day right around the corner, what better way to get into the spirit than reading about sexual cannibalism! Fifty shades of Grey can’t compare with these animal extremes that’s for sure.

Sexual cannibalism (SC) typically in nature refers to consumption of a male by his mate before, during or after copulation. SC is usually associated with females as they are the child-rearing individuals. In other words, it would not make sense evolutionarily for males to consume the individuals that will be rearing their children after insemination. SC is a common practice in many species of praying mantis; however, the rates of SC can vary among species. For example, some species can have a rate as high as 46% as in the Australian praying mantis Pseudomantis albofimbriata. Conversely, the genus Ciulfina lacks sexual cannibalism altogether. The practice of SC is not exclusive to species of praying mantis either. A wide array of species of spiders have been shown to demonstrate SC as well, sometimes even in the reverse direction (males cannibalizing females!). So what makes this practice of sexual cannibalism evolutionarily advantageous?

At first thought, it seems that SC would be disadvantageous as it would decrease the sex ratio of the species and eliminate genetic variability. Aka eliminating any potential future dates such as those needed for Valentine’s Day especially since the pickings are already slim enough as it is! *gasp* Thus, the advantage of SC lies in the nutritional benefits that can come from eating protein-rich and lipid-rich conspecifics. In fact, females in many species of praying mantis use the nutrients gained from cannibalism to increase body condition and reproductive output. Female body condition can improve by as much as 33% while fecundity (reproductive rate/success) can improve by as much as 40% after the consumption of a male conspecific. Additionally, this cannibalism can also provide more energy for offspring investment, increasing the survival of offspring. In a study by Welke and Schneider 2012, the effects of sexual cannibalism on clutch size and offspring survival success were observed in female orb-web spiders of the species Argiope bruennichi. The results of the study showed that female clutch size, a measure of fecundity, was higher in females allowed to consume mates compared to other females not allowed to consume mates (Figure 1). Eggs produced by female orb web spiders also had higher yolk density providing more nutrients to offspring and thus increasing their survival rate (Figure 1). After this description many of you may be thinking that the males are really drawing the short end of the stick in this situation. As with most males attempting to buy the “perfect” valentine’s day gift, the males of these sexually cannibalistic species appear to be in a lose-lose situation. However, males seem to know what they are getting themselves into and tend to discriminate when it comes to mate selection.  

Males of sexually cannibalistic species tend to discriminate when choosing potential female mates, especially when the risk of being trapped and eaten is a high possibility. A study conducted by Brown et al 2012 looked into the different mating approaches of males in sexually cannibalistic species. Males’ choice of mates was dependent upon two factors: proportion of risky females and frequency of mate encounters. The results of the study showed that males tended to discriminate more when the overall cost of approaching females was high. Males were tested under both high risk conditions (encountering starved females) and low risk conditions (encountering fed females). Overall, in the low risk conditions, males tended to discriminate less and approached females more frequently for copulation (Figure 2). Conversely, males that were paired with unfed females tended to approach the females slowly or avoided the females altogether.  In addition to the risk of the situation, another variable was also tested. The variable of frequency of mate encounters.

  Figure 2. Males with high female encounter rate approached high-risk females more slowly than they approached low-risk females, whereas males with low female encounter rate showed no difference in approach rate. (B) Males with high access to females approached remained farther away from high-risk females but males with low encounter rate approached equally closely to low- and high-risk females.

In a second experiment, risk and access to mates were compared to survival of males (Figure 3). In this case, males that had high access to mates exhibited the same pattern as in the previous experiment. However, males with lower access to mates and were exposed to unfed/high risk females expressed a different pattern. The low encounter males had decreased survival rate, as they tended to approach females regardless of their risk level (Figure 3). 

Figure 3. Mortality rate of male praying mantis compared to high and low risk females and accessibility to females.

It appears that these males were risk takers and the costs of not having the opportunity to mate during this breeding season out-weighed the risks of becoming cannibalized. These male praying mantids had their priorities straight that’s for sure.

In sexually cannibalistic species, the strategy of copulation is as follows: females remain sedentary and males go out and search for mates. In order to attract males, females release chemical signals known as pheromones to attract males to them for copulation. As was demonstrated above the males of these species of praying mantids have become quite discriminatory in their mate choice. With this in mind, it is no wonder that males of many species of praying mantids can detect differences in body condition and fecundity of females. Thus, males tend to choose females that have good body condition and high fecundity. These males want to make sure they are getting the most bank for their buck especially if there is a chance of having only one copulation event.  

What most males fail to understand is that with every “sneaky” strategy they think they are coming up with, there is a female that is always one step ahead of them. We females tend to be a wily bunch and know how to exploit sexually-motivated males as easy prey. These male praying mantids think they are smart and can detect the difference between poor condition females and good condition females but a study conducted by Barry 2014 begs to differ. In this study, females of varying body conditions were isolated with males and their success of attracting males was determined. According to Figure 4, females that were in good condition attracted the most amount of males but not as much as the very poor condition females. How can this be? Well, as was stated earlier, these poor condition females can improve the quality of their body condition by eating a nutrient-rich conspecific.

Figure 4. Representation of attractiveness of females (filled bars) and average body conditions (unfilled bars) and the amount of males attracted.

Instead of producing eggs, these females produce higher levels of pheromones compared to the good body conditioned females. Despite the fact that these pheromones are costly energetically to produce and their production in high amounts can affect both survival and future reproduction, females that are of already poor body quality have nothing to lose. The risks of potentially getting a nutrient-rich meal far out-weighs the costs of pheromone production. Poor males, bamboozled yet again. But in retrospect, do we really think these males care a whole ton? The plus side of sexual cannibalism is these males can avoid long-term commitments and future Valentine’s Day expectations. Win-win.


For more on the awesome praying mantis, check out this extremely scientifically accurate and respected video!

No comments:

Post a Comment