共食い
Cannibalism, ともぐい、同種間摂食行為
共食い
Cannibalism, ともぐい、同種間摂食行為
ここでは、人間以外のカニバリズムについて、ウィキ ペディア（英語）の"Cannibalism"から考える。
| Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded in more than 1,500 species.[1] Human cannibalism is well documented, both in ancient and in recent times.[2] The rate of cannibalism increases in nutritionally poor environments as individuals turn to conspecifics as an additional food source.[3] 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.[3] Other negative effects may include the increased risk of pathogen transmission as the encounter rate of hosts increases.[4] 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.[1][5][6] Cannibalism is prevalent in aquatic ecosystems, in which up to approximately 90% of the organisms[vague] engage in cannibalistic activity at some point in their life-cycle.[vague][7] Cannibalism is not restricted to carnivorous species: it also occurs in herbivores and in detritivores.[vague][5] Sexual cannibalism normally involves the consumption of the male by the female individual before, during or after copulation.[3] Other forms of cannibalism include size-structured cannibalism and intrauterine cannibalism. Behavioural, physiological and morphological adaptations have evolved to decrease the rate of cannibalism in individual species.[3] | |
| Benefits; 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.[8] 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.[9] 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.[3]
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.[10] 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.[10] 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.[10]
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;[8]
by lowering crowding effects.[11] 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;
Animals which have diets consisting of predominantly conspecific prey
expose themselves to a greater risk of injury and expend more energy
foraging for suitable prey as compared to non-cannibalistic species.[3]
Predators often target younger or more vulnerable prey.[12] However,
the time necessitated by such selective predation could result in a
failure to meet the predator's self-set nutritional requirements.[13]
In addition, the consumption of conspecific prey may also involve the
ingestion of defense compounds and hormones, which have the capacity to
impact the developmental growth of the cannibal's offspring.[10] 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.[3] 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.[14]
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.[4] |
|
| 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.[15]
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.[15]
However, this is only the case if the risk of disease transmission is
low.[4] 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.[4] 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.[15]
Some examples of diseases transmitted by cannibalism in mammals include
Kuru which is a prion disease that degenerates the brain.[4] This
disease was prevalent in Papua New Guinea where tribes practiced
endocannibalism in cannibalistic funeral rituals and consume the brains
infected by these prions.[16] 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.[16]
Bovine spongiform encephalopathy, or mad cow disease is another prion
disease which is usually caused by feeding contaminated bovine tissue
to other cattle.[17] 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 nematodes may also be facilitated by
cannibalism as eggs from these parasites are transferred more easily
from one host to another.[4]
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.[4] |
|
| 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.[3] 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,[18] confrontation, or cannibalism.[3]
Cannibalism rates increase with increasing population density as it
becomes more advantageous to prey on conspecific organisms than to
forage in the environment.[3] 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.[19] 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.[20]
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.[3] 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. |
|
| Sexual cannibalism;
Sexual cannibalism is present largely in spiders and other
invertebrates, including gastropods.[3] This refers to the killing and
consumption of conspecific 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 been
recorded under laboratory conditions.[3][21][22] Sexual cannibalism has
been recorded in the female redback spider, black widow spider, praying
mantis, and scorpion, among others.
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.[3] 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.[3] Other reasons for male consumption before mating may
include female choice and the nutritional advantages of
cannibalism.[23] 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.[23]
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.[3] 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.[24] 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.[25]
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.[3] 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.[3]
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.[3] Male orb weaving
spiders would often wait for females to moult or to finish eating
before attempting to initiate mating, as the females are less likely to
attack.[3] 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.[3] 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.[26] |
|
| Size-structured cannibalism;
Size-structured cannibalism is cannibalism in which older, larger, more
mature individuals consume smaller, younger conspecifics. In
size-structured populations, (where populations are made of individuals
of various sizes, ages, and maturities), cannibalism can be responsible
for 8% (Belding's ground squirrel) to 95% (dragonfly larvae) of the
total mortality,[1] making it a significant and important factor for
population[27] and community dynamics.[28]
Size-structured cannibalism has commonly been observed in the wild for
a variety of taxa. Vertebrate examples include chimpanzees, where
groups of adult males have been observed to attack and consume
infants.[29][30][31] |
|
| Filial
cannibalism; Filial cannibalism is a specific type of
size-structured cannibalism in which adults eat their own
offspring.[32] Although most often thought of as parents eating live
young, filial cannibalism includes parental consumption of stillborn
infants and miscarried fetuses as well as infertile and
still-incubating eggs. Vertebrate examples include pigs, where savaging
accounts for a sizable percentage of total piglet deaths, and
cats.[33][better source needed]
Filial cannibalism is particularly common in teleost fishes, appearing
in at least seventeen different families of teleosts.[34] 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.[32] This has been supported by experimental evidence, showing
that male three-spined sticklebacks,[32][35][36] male tessellated
darters,[37] and male sphinx blenny fish[38] 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
teleosts 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,[39] the negative
effects of accumulating embryo waste,[40] and predation.[40]
In some species of eusocial 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.[41] 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.[35][42]
It is not always the parent that cannibalises 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.[43]
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. |
|
| Infanticide;
Infanticide is the killing of a non-adult animal by an adult of the
same species. Infanticide is often, but not always, accompanied by
cannibalism. It is often displayed in lions; a male lion encroaching on
the territory of a rival pride will often kill any existing cubs
fathered by other males; this brings the lionesses into heat more
quickly, enabling the invading lion to sire his own young. This is an
example of cannibalistic behavior in a genetic context.
In many species of Lepidoptera, such as Cupido minimus and the Indian
mealmoth, the first larvae to hatch will consume the other eggs or
smaller larvae on the host plant, decreasing competition.[44][45] |
|
| Intrauterine
cannibalism; Intrauterine cannibalism is a behaviour in
some carnivorous species, in which multiple embryos are created at
impregnation, but only one or two are born. The larger or stronger ones
consume their less-developed siblings as a source of nutrients.
In adelphophagy or embryophagy, the fetus eats sibling embryos, while
in oophagy it feeds on eggs.[46][47]
Adelphophagy occurs in some marine gastropods (calyptraeids, muricids,
vermetids, and buccinids) and in some marine annelids (Boccardia
proboscidia in Spionidae).[48]
Intrauterine cannibalism is known to occur in lamnoid sharks[49] such
as the sand tiger shark, and in the fire salamander,[50] as well as in
some teleost fishes.[47] 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.[51] |
|
| Protection against cannibalism;
Animals have evolved protection to prevent and deter potential
predators such as those from their own kind.[3] 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.[3] In bees, worker
policing occurs to prohibit worker reproduction, whereby workers
cannibalize other worker laid eggs.[52] Queen laid eggs have a
different scent than worker laid eggs, allowing workers to
differentiate between the two, allowing them to nurture and protect
queen laid eggs rather than cannibalising them.[52] Parental presence
at nesting sites is also a common method of protection against
infanticide committed by conspecific individuals, whereby the parent
exhibits 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.[3]
Morphological plasticity helps an individual account for different
predation stresses, thereby increasing individual survival rates.[53]
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.[53] 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.[3] |
|
| Cannibalism in the media;
Cannibalism has been a subject for horror movies since the 1980s.[54]
This genre is called cannibal films.
Cannibalism has also been a subject of video games.[55] More recently,
cannibalism has been featured in a positive light, with video games
like Ark: Survival Evolved incentivizing players' virtual characters to
eat each other.[56] |
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