Product Code Database
Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecology interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is also well documented, both in ancient and in recent times.
The rate of cannibalism increases in nutritionally poor environments as individuals turn to members of their own species as an additional food source.Elgar, M.A. & Crespi, B.J. (1992) Cannibalism: ecology and evolution among diverse taxa, Oxford University Press, Oxford England; New York. 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. Cannibalism, however, does not—as once believed—occur only as a result of extreme food shortage or of artificial/unnatural conditions, but may also occur under natural conditions in a variety of species.Elgar, M. A. and Bernard Crespi (eds) (1992) Cannibalism: Ecology and evolution among diverse taxa. Oxford University Press, New York. .
At the ecosystem level, cannibalism is most common in aquatic settings, with a cannibalism rate of up to 0.3% amongst fish. Cannibalism is not restricted to carnivorous species: it also occurs in and in . Sexual cannibalism normally involves the consumption of the male by the female individual before, during or after copulation. Other forms of cannibalism include size-structured cannibalism and intrauterine cannibalism. Behavioral, physiological and morphological adaptations have evolved to decrease the rate of cannibalism in individual species.
The nutritional benefits of cannibalism may allow for the more efficient conversion of a conspecific diet into reusable resources than a fully herbaceous diet; as herbaceous diets may consist of excess elements which the animal has to expend energy to get rid of. This facilitates 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.
In some insects, cannibalism is used to control population. In confused flour beetles, population density is lowered by cannibalism when crowding occurs.Mahfoud, Nawal & Kafu, Ali & Maghrabi, Hassan. (2016). Study of the Cannibalistic Cohorts Among the Various Life Stages of Confused Flour Beetle Tribolium confusum DuVal (Coleoptera: Tenebrionidae) Under Laboratory Conditions. 19. 11–27.
Cannibalism regulates population numbers and 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 will not 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.
Predators often target younger or more vulnerable prey. However, the time necessitated by such selective predation could result in a failure to meet the predator's self-set nutrition. In addition, the consumption of conspecific prey may also involve the ingestion of defense compounds and , which have the capacity to impact the developmental growth of the cannibal's offspring. Hence, predators normally partake in a cannibalistic diet in conditions where alternative food sources are absent or not as readily available.
Failure to recognize kin prey is also a disadvantage, provided cannibals target and consume younger individuals. For example, a male stickleback fish may often mistake their own "eggs" for their competitor's eggs, and hence would inadvertently eliminate some of its own genes from the available gene pool. Kin recognition has been observed in 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 facilitate trophic disease transmission within a population, though cannibalistically spread pathogens and parasites generally employ alternative modes of infection.
Some examples of diseases transmitted by cannibalism in mammals include the human disease Kuru which is a prion disease that degenerates the brain. This disease was prevalent in Papua New Guinea where tribes practiced endocannibalism in cannibalistic funeral rituals and consume the brains infected by these prions. It is a Cerebellum dysfunctional disease which has symptoms including a broad-based gait and decreased motor activity control; however, the disease has a long incubation period and symptoms may not appear until years later.
Bovine spongiform encephalopathy, or mad cow disease is another prion disease which is usually caused by feeding contaminated bovine tissue to other cattle. It is a neurodegenerative disease and could be spread to humans if the individual were to consume contaminated beef. The spread of parasites such as 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 sarcocystis and iridovirus in reptiles and amphibians; granulosus virus, chagas disease, and microsporidia in insects; stained prawn disease, white pot syndrome, helminthes and tapeworms in crustaceans and fish.
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. Over time, the dynamics within the population change 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 alternative 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.
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 is present in its web and may mistake the vibrations for those 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 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 males of certain 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 Moulting 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's legs in webs to reduce the risk of the female capturing him. Male choice is common in Mantidae whereby males were observed to choose fatter females due to the reduced risk of attack and were more hesitant to approach starved females.
Size-structured cannibalism has commonly been observed in the wild for a variety of taxa. Vertebrate examples include , where groups of adult males have been observed to attack and consume infants.
Filial cannibalism is particularly common in teleost fishes, appearing in at least seventeen different families of teleosts. Within this diverse group of fish, there have been many, variable explanations of the possible adaptive value of filial cannibalism. One of these is the energy-based hypothesis, which suggests that fish eat their offspring when they are low on energy as an investment in future reproductive success. This has been supported by experimental evidence, showing that male three-spined sticklebacks, male tessellated darters, and male sphinx blenny fish all consume or absorb their own eggs to maintain their physical conditions. In other words, when males of a fish species are low on energy, it might sometimes be beneficial for them to feed on their own offspring to survive and invest in future reproductive success.
Another hypothesis as to the adaptive value of filial cannibalism in is that it increases density-dependent egg survivorship. In other words, filial cannibalism simply increases overall reproductive success by helping the other eggs make it to maturity by thinning out the numbers. Possible explanations as to why this is so include increasing oxygen availability to the remaining eggs, the negative effects of accumulating embryo waste, and predation.
In some species of Eusociality wasps, such as Polistes chinensis, the reproducing female will kill and feed younger larvae to her older brood. This occurs under food stressed conditions in order to ensure that the first generation of workers emerges without delay. Further evidence also suggests that occasionally filial cannibalism might occur as a by-product of cuckoldry in fish. Males consume broods, which may include their own offspring, when they believe a certain percentage of the brood contains genetic material that is not theirs.
It is not always the parent that cannibalizes the offspring; in some spiders, mothers have been observed to feed themselves to their brood as the ultimate provision from mother to children, known as matriphagy.
The dinosaur Coelophysis was once suspected to practice this form of cannibalism but this turned out to be wrong, although Deinonychus may have done so. Skeletal remains from subadults with missing parts are suspected of having been eaten by other Deinonychus, mainly full-grown adults.
In many species of Lepidoptera, such as Small blue and the Indianmeal moth, the first larvae to hatch will consume the other eggs or smaller larvae on the host plant, decreasing competition.
In adelphophagy or embryophagy, the fetus eats sibling embryos, while in oophagy it feeds on eggs.
Adelphophagy occurs in some marine gastropods (, , , and ) and in some marine ( Boccardia proboscidia in Spionidae)..
Intrauterine cannibalism is known to occur in Lamniformes such as the sand tiger shark, and in the fire salamander,
as well as in some teleost fishes. The Carboniferous period chimaera, Delphyodontos dacriformes, is suspected of having practiced intrauterine cannibalism, also, due to the sharp teeth of the recently born (or possibly aborted) juveniles, and the presence of fecal matter in the juveniles' intestines.
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.
|
|
