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The rate of cannibalism increases in nutritionally poor environments as individuals turn to other conspecific individuals as an additional food source. Cannibalism regulates population numbers, whereby resources such as food, shelter and territory become more readily available with the decrease of potential competition. Although it may benefit the individual, it has been shown that the presence of cannibalism decreases the expected survival rate of the whole population; and increases the risk of consuming a relative. Other negative effects may include the increased risk of pathogen transmission as the encounter rate of hosts increases.

Sexual cannibalism normally involves the consumption of the male by the female individual before, during or after copulation.

Behavioural, physiological and morphological adaptations have evolved to decrease the rate of cannibalism in individual species.

Benefits of Cannibalism
In environments where food availability is constrained, individuals can receive extra nutrition and energy if they use other conspecific individuals as an additional food source. This would in turn increase the survival rate of the cannibal and thus provide an evolutionary advantage in environments where food is scarce. A study conducted on wood frog tadpoles showed that those that exhibited cannibalistic tendencies had faster growth rates and higher fitness levels than non-cannibals. An increase of size and growth would give them the added benefit of protection from potential predators such as other cannibals and give them an advantage when competing for resources.

The nutritional benefits of cannibalism may allow for the more efficient conversion of a conspecific diet into reusable resources than fully herbaceous diet; as herbaceous diets may comprise of excess elements which the animal has to expand energy to get rid of. This facilitates for faster development; however, a trade-off may occur as there may be less time to ingest these acquired resources. Studies have shown that there is a noticeable size difference between animals fed on a high conspecific diet which were smaller compared to those fed on a low conspecific diet. Hence, individual fitness could only be increased if the balance between developmental rate and size is balanced out, with studies showing that this is achieved in low conspecific diets.

Cannibalism regulates population numbers and in turn benefits the cannibalistic individual and its kin as resources such as extra shelter, territory and food are freed; thereby increasing the fitness of the cannibal ; by lowering crowding effects. However, this is only the case if the cannibal recognizes its own kin as this won’t hinder any future chances of perpetuating its genes in future generations. The elimination of competition can also increase mating opportunities, allowing further spread of an individual’s genes.

Costs of Cannibalism
Animals which have diets consisting of mostly conspecific prey expose themselves to a greater risk of injury and expand more energy foraging when there are other alternate food sources available which entails less risk. To combat the risk of injury, a predator may target younger and more vulnerable prey, however, this may increase the time foraging for prey to meet their required nutritional targets. In addition, the consumption of conspecific prey may also involve the ingestion of defence compounds and hormones which may impact the developmental growth of the cannibal. Hence, predators normally partake in a cannibalistic diet in conditions where alternate food sources are absent or not as readily available.

Failure to recognize kin would also be a disadvantage if cannibals were to consume younger individuals. Male stickleback fish may often mistake their own eggs for their competitor’s eggs, and hence would inadvertently eliminate its own gene from the gene pool. Kin recognition has been observed in tadpoles of the spadefoot toad, whereby cannibalistic tadpoles of the same clutch tended to avoid consuming and harming siblings, while eating other non-siblings. The act of cannibalism may also encourage the spread of diseases within a population which could negatively impact on the cannibal’s fitness levels.

Diseases transmitted through cannibalism
Cannibalism can potentially reduce the prevalence of parasites in the population by decreasing the number of susceptible hosts and indirectly killing the parasite in the host. It has been shown in some studies that the risk of encountering an infected victim increases when there is a higher cannibalism rate, though this risk drops as the number of available hosts decreases. However, this is only the case if the risk of disease transmission is low. Cannibalism is an ineffective method of disease spread as cannibalism in the animal kingdom is normally a one-on-one interaction, and the spread of disease requires group cannibalism; thereby it is rare for a disease to have evolved to rely solely on cannibalism to spread. Usually there are different means of transmission, such as with direct contact, maternal transmission, coprophagy, and necrophagy with different species. Infected individuals are more likely to be consumed than non-infected individuals, thus some research has suggested that the spread of disease may be a limiting factor to the prevalence of cannibalism in the population. Some examples of diseases transmitted by cannibalism in mammals include Kuru which is a prion disease that degenerates the brain. This disease is prevalent in Papua New Guinea whereby tribes practice cannibalistic funeral rituals and consume the brains infected by these prions. It is a cerebellar dysfunctional disease which has symptoms including a broad-based gait and decreased motor activity control, however, the disease has a long incubation rate and symptoms may not appear until years later. Mad cow disease is another prion disease which is usually caused by feeding contaminated bovine tissue to other cattle. It is a neurodegenerative disease that causes neuronal loss; and could be spread to humans if the individual were to consume contaminated beef. The spread of parasites such as nematodes may also be facilitated by cannibalism as eggs from these parasites are transferred more easily from one host to another. Other forms of diseases include sarcocyst and iridovirus in reptiles and amphibians; granulosus virus, chagas disease, and microsporidia in insects; stained prawn disease, white potsyndrome, helminthes and tapeworms in crustaceans and fish.

Foraging Dynamics
Cannibalism may become apparent when direct competition for limited resources forces individuals to use other conspecific individuals as an additional resource to maintain their metabolic rates. Hunger drives individuals to increase their foraging rates, which in turn decreases their attack threshold and tolerance to other conspecific individuals. As resources dwindle, individuals are forced to change their behaviour which may lead to animal migration, confrontation, or cannibalism.

Cannibalism rates increase with increasing population density as it becomes more advantageous to prey on conspecific organisms than to forage in the environment. This is because the encounter rate between predator and prey increases, making cannibalism more convenient and beneficial than foraging within the environment. Overtime, the dynamics within the population changes as those with cannibalistic tendencies may receive additional nutritional benefits and increase the size ratio of predator to prey. The presence of smaller prey, or prey which are at a vulnerable stage of their life cycle, increases the chances of cannibalism occurring due to the reduced risk of injury. A feedback loop occurs when increasing rates of cannibalism decreases population densities, leading to an increased abundance of alternate food sources; making it more beneficial to forage within the environment than for cannibalism to occur. When population numbers and foraging rates increase, the carrying capacity for that resource in the area may be reached, thus forcing individuals to look for other resources such as conspecific prey.

Protection against Cannibalism
Animals have evolved protection to prevent and deter potential predators such as those from their own kind. Many amphibian eggs are gelatinous and toxic to decrease edibility. Often, adults would lay their eggs in crevices, holes, or empty nesting sites to hide their eggs from potential conspecific predators which tend to ingest the eggs for an additional nutritional benefit or to get rid of genetic competition. In amphibians, the development of non-aquatic egg deposition has helped increase the survival rates of their young by the evolution of viviparity or direct development. In bees, worker policing occurs to prohibit worker reproduction, whereby workers cannibalize other worker laid eggs. Queen laid eggs have a different scent than worker laid eggs, allowing workers differentiate between the two, allowing them to nurture and protect queen laid eggs rather than cannibalising them. Parental presence at nesting sites is also a common method of protection against infanticide committed by conspecific individuals, whereby the parent exhibit defensive displays to ward off potential predators. Parental investment in newborns are generally higher during their early stages of development whereby behaviours such as aggression, territorial behaviour, and pregnancy blocking become more apparent. Morphological plasticity helps an individual account for different predation stresses, thereby increasing individual survival rates. Japanese brown frog tadpoles have been shown to exhibit morphological plasticity when they are in a high stress environment where cannibalism between tadpoles and more developed individuals were present. Shifting their morphology plays a key role in their survival, creating bulkier bodies when put into environments where more developed tadpoles were present, to make it difficult for the individuals to swallow them whole. Diet shifts between different stages of development have also evolved to decrease competition between each stage, thereby increasing the amount of food availability so that there is a decreased chance that the individuals will turn to cannibalism as an additional food source.

Sexual cannibalism
Sexual cannibalism is present largely in spiders and other invertebrates; however, is also present in gastropods. This refers to the killing and consumption of sexual partners during courtship, and during or after copulation. Normally, it is the female which consumes the conspecific male organism, though there have been some reported cases of the male consuming the adult female, however, this has only occurred under laboratory conditions.

In most species of spiders, the consumption of the male individual occurs before copulation and the male fails to transfer his sperm into the female. This may be due to mistaken identity such as in the case of the orb weaving spider which holds little tolerance to any spider which are present in its web and may mistake the vibrations to be that of a prey item. Other reasons for male consumption before mating may include female choice and the nutritional advantages of cannibalism. The size of the male spider may play a part in determining its reproductive success as smaller males are less likely to be consumed during pre-copulation, however, larger males may be able to prevent the smaller ones from gaining access to the female. There exists a conflict of interest between males and females, as females may be more inclined to turn to cannibalism as a source of nutritional intake while the male’s interest is mostly focused on ensuring paternity of the future generations. It was found that cannibalistic females produced offspring with greater survival rates than non-cannibalistic females, as cannibals produced greater clutches and larger egg sizes. Hence, species such as the male dark fishing spider of the family dolomedes self-sacrifice and spontaneously die during copulation to facilitate for their own consumption by the female, thereby increasing the chance of survivorship of future offspring.

Sexual dimorphism has been theorised to have arisen from sexual selection as smaller males were captured more easily than larger males; however, it is also possible that sexual cannibalism only occurs due to the difference in size between male and females. Data comparing female and male spider body length shows that there is little support for the prior theory as there is not much correlation between body size and the presence of sexual cannibalism. Not all species of spiders which partake in sexual cannibalism exhibit size dimorphism.

The avoidance of sexual cannibalism is present in many male species to increase their rate of survival, whereby the male uses cautionary methods to lower the risk of his consumption. Male orb weaving spiders would often wait for females to molt or to finish eating before attempting to initiate mating, as the females are less likely to attack. Males which are vulnerable to post copulation consumption may gather mating thread to generate a mechanical tension which they could use to spring away after insemination, while other spiders such as the crab spider may tangle the female legs in webs to reduce the risk of the female capturing him. Male choice is common in mantids whereby males were observed to choose fatter females due to the reduced risk of attack and were more hesitant to approach starved females.