How can cannibalism evolve by natural selection

Although there are benefits, there are also very significant costs. Eliminating the father means a definite loss of any potential paternal care. Additionally, cannibalism can occur before mating and, therefore, cause reproductive failure. In short, the evolution of sexual cannibalism comes with costs and benefits. The evolution of the big horned male rhinoceros beetle and sexual cannibalism epitomizes the imperfect nature of evolution. Both traits are not perfect in that they come with significant costs.

They are maintained because the benefits of each trait outweigh the costs. McCullough, Erin L. Wu, Lingbing, et al. After their initial grief, shock and revulsion, the families came to terms with the fact that, had all 45 survived the crash, none of them would have likely made it home alive. One father even expressed relief that his son's corpse had been granted a destiny other than simply decomposing: he compared the eating of his son's flesh to a life-saving blood transfusion.

The survivors also received forgiveness from the Catholic Church, which said the act had been committed in extremis to sustain life. Members of the Church also commended the sober and religious spirit in which the survivors had arrived at their difficult decision. In cases of survival cannibalism, our empathy is reserved for those who we believe had no other choice. We are less forgiving if we believe that there were other options that they could have pursued.

Again, we see this reflected in pop culture, such as the most recent season of The Walking Dead. Despite the desperation and suffering of Team Rick, they've never resorted to eating people. They've chosen the more difficult and nobler path: tilling the fields, hunting, and risking their lives on foraging missions.

By contrast, Team Terminus, "the Termites," have taken the easier path by surrendering their humanity and luring people to their settlement, where they are apparently slaughtered and eaten. Part of what makes the Termites so creepy is the practiced ease with which they commit their acts of cannibalism—including the tailgate party atmosphere of their outdoor grill. But, in truth, there are similarities between the Termites' behavior and those who resort to survival cannibalism.

When a group of people finally makes the fateful decision to violate the taboo of eating human bodies, they quickly grow accustomed to it. What had been considered a "person" just a short time earlier quickly becomes "food.

Again, the Andean plane crash offers an example. Initially, after tremendous soul-searching and with great reluctance, the survivors began eating the flesh of corpses. For most of them, those feelings of reluctance and shame quickly dissipated. A medical student among the group informed the others that they should begin eating the livers, as a source of vitamins. From there, the "menu" quickly expanded to include other internal organs—and not just as a source of nutrition.

After ten weeks, they began eating the lungs and intestines, craving the taste of food that had a stronger flavor. The tendency to quickly overcome feelings of shame is not the only behavior that is common among cases of survival cannibalism. The decisions made by the survivors concerning who should be eaten—and in which specific order—follow a curiously similar pattern.

There's a type of hierarchy that's been compared to triage. For instance, people on the verge of starvation will go to great lengths to avoid eating pets. Moreover, people will eat individuals who are foreign or members of a different race before they consume anyone else — even if another individual would provide more nutrition by virtue of their size.

And, while people often encourage family members to eat their corpses if they should die, there are no known cases of people committing suicide to feed their families. A unique case of reversed sexual cannibalism, in which the male amphipod Gammarus pulex consumes female conspecifics, lends support to this theory Dick In this species, males cannibalize newly moulted females, which simultaneously are ready for copulation and at their most vulnerable stage, at much lower rates than they cannibalize other conspecifics.

However, as the abundance of other prey organisms decreases, the incidence of reversed sexual cannibalism increases, as is predicted by the Newman and Elgar "Economic" model of sexual cannibalism All of the models thus far discussed consider sexual cannibalism in terms of its adaptive value to one or both sexes.

An obvious assumption of each is that sexual cannibalism evolved and is maintained because of fitness benefits conferred to reproductive adults. It has recently been suggested, however, that as attempts to generalize the evolution of this phenomenon, these approaches may be barking up the wrong proverbial tree. Recent theory suggests that sexual cannibalism in adult organisms may result indirectly from behaviors that are adaptive in previous life history stages, but nonadaptive or even maladaptive in adults Arnqvist and Henriksson Sexual cannibalism may carry over into the adult stage as a result of genetic constraints on life history aggression.

This model was developed to explain the evolution of sexual cannibalism in the fishing spider Dolomedes fimbriatus. In several members of this genus, males approach females very cautiously when courting, and flee rapidly after copulation. Consumption of males is a common behavior, yet it confers no fitness benefit to females and actually decreases fertilization rates Spence et al.

Sexual cannibalism in Dolomedes species thus does not conform to the assumptions or follow the predictions of any adaptive theory. Arnqvist and Henriksson suggest that sexual cannibalism need not be adaptive for either sex, and instead is a byproduct of strong selection for juvenile female rapacity. In Dolomedes species, food consumption is positively related to aggression, juvenile growth and adult female size depend on juvenile food consumption, female adult size is the major determinant of female fitness, and aggressive behavior is genetically constrained during development.

Aggressive juvenile females therefore consume more prey, obtain larger sizes, and produce more offspring than non-aggressive juveniles. The result is intense selection for high female aggression toward both conspecific and heterospecific prey during juvenile development. Because of genetic constraints on aggressive behavior, adult females will exhibit high aggression and low discrimination i.

Arnqvist and Henriksson posit that aggression has likely evolved as a balance between conflicting selective pressures on aggression in juveniles and adults. They suggest that the taxonomic distribution of sexual cannibalism should reflect differences in the relationships between aggression, food consumption, adult size, and fecundity.

A comparative study of sexual cannibalism across insect and arachnid taxa may thus yield insight into the general applicability of this model in exploring the evolution of sexual cannibalism in different taxa. In light of the diversity in form of sexual cannibalism and the conflicting implications of empirical studies exploring its evolution, it is likely that sexual cannibalism evolved independently multiple times Elgar and is maintained in different species by different selective forces.

Development of an all-encompassing model for the evolution of sexual cannibalism is thus an unrealistic endeavor. We should attempt instead to explain sexual cannibalism on a case-by-case basis. Instead of using certain models and empirical data to refute others, we should assess the applicability of each existing model to the diverse interactions we see in nature.

When we are unable to reconcile a natural pattern with any of the existing models, we should explore mechanisms and possibilities that may have been overlooked in existing explanations. To this end, I present a hypothesis for the evolution of sexual cannibalism in the jumping spider Phidippus rimator Araneae: Salticidae. Similar to the Dolomedes species studied by Arnqvist and Henriksson , in many species of Phidippus , food consumption is positively related to aggression, juvenile growth and adult female size depend on juvenile food consumption, and female fecundity is positively related to size Edwards ; Jackson I carried out extensive behavioral observations on P.

Through this work I learned that juvenile and adult females exhibit high levels of aggression toward conspecific and heterospecific prey. Females commonly attack and consume males before, during, and after copulation. Males approach females cautiously, and with an elaborate courtship display, and retreat rapidly after sperm transfer, usually by curling into a ball and rolling to the litter below.

Furthermore, males often locate a female, but do not approach to court her until the female has successfully captured a prey item. In 32 observed courting approaches, fours of six successful copulations occurred when the female was consuming another prey item one of these males was killed during copulation when the female dropped her lepidopteran prey and attacked her mate; another was killed after copulation in a similar scenario.

Similarly, I have rejected the Newman and Elgar "Economic" model for the evolution of this interaction because females attack males regardless of their feeding status.

Another possible explanation for sexual cannibalism is that the female attacks the male because she mistakes his identity as a prey item rather than a conspecific reviewed in Elgar This hypothesis was not discussed because there has been no empirical evidence to support it Elgar A case of mistaken identity certainly does not explain sexual cannibalism in Phidippus spiders.

Salticids have the highest visual acuity of all arachnids Forster , and are thus unlikely to attack or approach anything by accident. Furthermore, the occurrence of cannibalism during and after copulation further precludes the possibility of mistaken identity. It is more likely that the ontogeny of aggression explanation invoked by Arnqvist and Henriksson accounts in part for the evolution of sexual cannibalism in Phidippus rimator.

However, while Arnqvist and Henriksson developed their hypothesis as a fitness trade-off between aggressive behavior in juvenile and adult fishing spiders, I posit that this ontogenetic conflict of interest is absent, or at least less important, in P. It is likely that aggressive behavior is selectively advantageous in juvenile P. However, while this behavior is thought to confer negative fitness to adult fishing spiders by decreasing fertilization rates of adult females Arnqvist and Henriksson , it may have a less deleterious effect on jumping spiders.

Male P. Suttle, personal observation. Thus a population of P. Furthermore, the increase in female survival conferred by high levels of adult aggression may offset fitness costs to females that remain unmated. Female P. Because aggressive females will better defend their eggs and foraging space from competitors and potential predators, high levels of aggression may confer survival and fitness advantages to adult females.

I hypothesize, therefore, that sexual cannibalism in Phidippus rimator may have evolved along a similar pathway to that in Dolomedes fimbriatus , but does not cause the same conflict of interest between the adults and juveniles that it does in D.

Furthermore, sexual cannibalism may be maintained in the species through a selective advantage of aggressive behavior to both juveniles and adults. Andrade, M. Sexual selection for male sacrifice in the Australian redback spider.

Science Female hunger can explain variation in cannibalistic behavior despite male sacrifice in redback spiders. Behavioral Ecology Arnqvist, G. Sexual cannibalism in the fishing spider and a model for the evolution of sexual cannibalism based on genetic constraints.

Evolutionary Ecology Birkhead, T. Young, and P. Sexual cannibalism in the praying mantis Hierodula membranacea. Behavior