Can Sexual Cannibalism Hurt the Population?

Dr. Tatiana covers female cannibalism behavior extensively. She gives several possible hypotheses as to why females are eating their mates. Females eat males in order to get the nutrients they need for their babies. Females eat males because they are a substantial meal. Females eat males because they are not fit enough to copulate with her and thus pass his genes onto a subsequent generation. Whatever the reason, usually in these populations have males that are great at escaping a female's hungry desires and males that don't make it out in time. Something that Dr. Tatiana did not cover was the affect the consumption of males could have on the population. 

'Sexual cannibalism and population viability' sought to answer how sexual cannibalism could possibly affect the population. It states that there are two negative consequences that sexual cannibalism can have on the population: 1) in every case of cannibalism, an adult male that was capable of mating was removed from the population, and 2) if a male is eaten before he is able to copulate, both sexes miss out on a reproductive opportunity. Both of these consequences can negatively affect the population, especially if the population is exposed to other factors that might put their numbers at risk. Two models were used to help assess the question in this study (a discrete-time mathematical model and an individual-based model). Both models displayed that an overall increase in sexual cannibalism does cause lower mating ratings (Figure 1). Both models also showed that in the presence of female fecundity benefiting from cannibalism, the population can exhibit growth. In the absence of female fecundity benefiting from cannibalism there was a consistent decrease in population growth rate. Also, when observing pre-copulatory cannibalism versus post-copulatory cannibalism there were noticeable differences. Pre-copulatory cannibalism negatively affected (Figure 2a), while post-copulatory cannibalism only had minor effects on population growth (it consistently increases the population growth rate) (Figure 2b). 

Figure 1. As cannibalism rate increase, the number of matings per individual decreases. Post and pre-copulatory rates are assumed equal, and "h" represents the average number of lifetime matings per individual. 

Figure 2. a) The pre-copulatory cannibalism rate when post-copulatory cannibalism is fixed. We can see a consistent drop below the extinction threshold (dashed line) for all models despite the difference in post-cop cannibalism. b) The post-copulatory cannibalism rate when pre-copulatory cannibalism is fixed. We can see a consistent increase for all models when the male has had the opportunity to copulate before he has been eaten. 

Based off of the findings in this study, we have evidence that supports both positive and negative effects sexual cannibalism can have on a population. There are the most benefits when there is a female fecundity benefit to cannibalism, and when the female consumes the male after they have copulated. This is consistent with Dr. Tatiana's statements. The ability for a male to fertilize a female before he has been eaten, and for a male to provide a female with a nutritious meal for her babies are reasoning that was given for sexual cannibalism in the book. But, when there is no real benefit to female fecundity with sexual cannibalism and when the female is eating the male before copulation, there can be negative effects on the population. 

Fisher, A. M., Cornell, S. J., Holwell, G. I., & Price, T. A. (2018). Sexual cannibalism and population viability. Ecology and Evolution, 8(13), 6663-6670. doi:10.1002/ece3.4155