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Animal suicide is any kind of self-destructive behavior displayed by various species of animals, resulting in their death. Although contradicting the natural progression of life and an animal's evolutionary instinct for survival, some situations may lead to an animal inducing their own death. Animal suicide in the defense of the group could be instantaneous or altruistic self-removal once an animal becomes diseased. There are anecdotal reports of grieving pets displaying such behaviour after the death of their owner, or monogamous animals refusing to feed after the death of their mate.
Some parasites manipulate the behavior of their host, causing them to expose themselves to greater risks of predation to enable the parasite to proceed to the next life-cycle stage. Some carpenter ants and termites use autothysis, producing a sticky secretion to trap colony marauders, and pea aphids will sometimes explode, protecting other pea aphids from ladybugs.
It is generally considered that humans are the only known beings to use weapons when putting an end to their own lives. Some species might be driven to engage in suicidal behavior for the sake of others, which is an example of altruism in animals. These actions are performed when the reproductive fitness of others outweighs the reproductive fitness of the individual. There are examples of animals dying in defense of their family or colony, such as the case of pea aphids increasing their chances of death as a response to parasitism.
Many animals that appear to be depressed or grieving begin to exhibit self-destructive behavior that sometimes ends in death, but this is not considered suicide, as the achieving of death was not necessarily the purpose or objective of the behavior. In 1845, the Illustrated London News reported that a Newfoundland dog had been acting less lively over a period of days before being seen "to throw himself in the water and endeavor to sink by preserving perfect stillness of the legs and feet". Every time he was rescued he attempted to do this again before he finally held his head underwater until death.
Other dogs, as well as ducks, have also allegedly drowned themselves, although the veracity or certainty of the case is disputed in each of the known cases. In one of the alleged case, one duck did so after the death of its mate. Some dogs will refuse food from some unknown person after the death of their owner, a behavior that might lead to disease or death in severe cases. The death of mourning animals is likely to be caused by depression leading to starvation or drowning, instead of the intent of suicide. One deer leapt from a cliff to its death to avoid being captured by hunting dogs.
Another example of an alleged case of animal suicide is the case of the dolphin which most often portrayed Flipper on the 1960s television show Flipper. According to trainer Ric O'Barry in the film The Cove, Kathy, the dolphin, suffocated herself before him. The veracity or accuracy of this case hasn't been established in rigorous, scientific or objective terms. Similarly, a male bottle nose dolphin named Peter who was a subject in a series of experiments led by John C. Lilly, a neuroscientist, and Margaret Howe Lovatt, volunteer naturalist, apparently stopped breathing after it was moved to a lab in a different location and separated from Lovatt.
Animal suicide was long used to define human suicide; in the 1800s animal suicide was seen as an act of abuse, madness, love, or loyalty, the same acts human suicide was seen as. Although it is impossible to determine what drives animals to self-destruction, some specific traits associated with human suicide can be successfully transferred to animals. Scientists have been unable to explain whether animals are able to consciously end their own lives.
Aristotle described an unverified story involving one of the King of Scythia's horses dying by suicide after having been made to unwittingly impregnate its mother in his History of Animals.
Some species of social insects will commit suicide in an act of altruism through autothysis. These insects will sacrifice themselves if the colony is in danger, to alert the colony of danger, or if they become diseased they will sacrifice themselves to prevent the colony from becoming diseased. Carpenter ants and some species of termite will rupture glands and expel a sticky toxic substance thought to be an aliphatic compound in a process called autothysis. Termites will use autothysis to defend their colony, as the ruptured gland produces a sticky harmful secretion that leads to a tar baby effect in defense. When threatened by a ladybug, the pea aphid will explode itself, protecting other aphids and sometimes killing the ladybug. Another example is the Camponotus saundersi, or Malaysian worker ant, which is capable of committing suicide by exploding.
Some social Hymenoptera including bees, wasps, and ants, may use their stinger to deliver poisonous chemicals to their attacker, effectively killing both the predator and the insect in the colony's defense. This self-destructive and often altruistic defense is known as sting autonomy. The stinger is easily torn from the animal's body, allowing the hazardous stinger to be left stuck in the predator.
Certain types of parasites will cause their hosts to engage in suicidal behavior, through altering how the intermediate host acts, but this is not considered suicide (at least not considered suicide in a psychological or ethological sense). The change in the host's actions often benefit the parasite's search for a final host. A main example is the phylum Acanthocephala, which will direct its host to a predator so as to be eaten by the predator, their new definitive host. The parasitic worm Spinochordodes tellinii will develop in grasshoppers and crickets until it is grown, at which time it will cause its host to leap into water to its death so that the worm can reproduce in water. However, S. tellinii only causes its host to drown when the host is already close to water as opposed to seeking it out over large distances.
Infection with Toxoplasma gondii has been shown to alter the behavior of mice and rats in ways thought to increase the rodents’ chances of being preyed upon by cats. Infected rodents show a reduction in their innate aversion to cat odors; while uninfected mice and rats will generally avoid areas marked with cat urine or with cat body odor, this avoidance is reduced or eliminated in infected animals. Moreover, some evidence suggests this loss of aversion may be specific to feline odors: when given a choice between two predator odors (cat or mink), infected rodents show a significantly stronger preference to cat odors than do uninfected controls.
Suicide induction in intermediate hosts has been shown to help disperse the parasites to their final hosts. The intermediate host of Parvatrema affinis is the bivalve mollusc, Macoma balthica. The clams feed when in the sublittoral and tidal flat muds, and usually leave no conspicuous marks in the mud that could indicate their presence. However, infected clams are concentrated in the higher parts of the tidal flats, closer to shore, and leave conspicuous zig-zag markings in the sand. Visual and tactile cues have shown to be used by oyster catchers and other shore birds, the definitive hosts of the parasite.
A popular misconception is that the lemming will commit mass suicide during reproduction. This misconception was first popularized by media in the 1960s, such as a mention in the Cyril M. Kornbluth short story "The Marching Morons" in 1951 and the 1955 comic "The Lemming with the Locket", inspired by a 1953 American Mercury article. Perhaps one of the most influential factors in this misconception was the Academy Award-winning Disney film White Wilderness, which showed staged footage of lemmings jumping off a cliff during reproduction.
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