Killing of young offspring by an adult animal of the same species
In animals, infanticide involves the intentional killing of young offspring by a mature animal of the same species. Animal infanticide is studied in zoology, specifically in the field of ethology. Ovicide is the analogous destruction of eggs. The practice has been observed in many species throughout the animal kingdom, especially primates (primate infanticide) but including microscopic rotifers, insects, fish, amphibians, birds and mammals. Infanticide can be practiced by both males and females.
Infanticide caused by sexual conflict has the general theme of the killer (often male) becoming the new sexual partner of the victim's parent, which would otherwise be unavailable. This represents a gain in fitness by the killer, and a loss in fitness by the parents of the offspring killed. This is a type of evolutionary struggle between the two sexes, in which the victim sex may have counter-adaptations that reduce the success of this practice. It may also occur for other reasons, such as the struggle for food between females. In this case individuals may even kill closely related offspring.
Filial infanticide occurs when a parent kills its own offspring. This sometimes involves consumption of the young themselves, which is termed filial cannibalism. The behavior is widespread in fishes, and is seen in terrestrial animals as well. Human infanticide has been recorded in almost every culture. A unique aspect of human infanticide is sex-selective infanticide.
Background.
Infanticide only came to be seen as a significant occurrence in nature quite recently. At the time it was first seriously treated by , infanticide was attributed to stress causing factors like overcrowding and captivity, and was considered pathological and maladaptive. Classical ethology held that conspecifics (members of the same species) rarely killed each other. By the 1980s it had gained much greater acceptance. Possible reasons it was not treated as a prevalent natural phenomenon include its abhorrence to people, the popular group and species selectionist notions of the time (the idea that individuals behave for the good of the group or species; compare with gene-centered view of evolution), and the fact that it is very difficult to observe in the field.
Infanticide involving sexual conflict.
This form of infanticide represents a struggle between the sexes, where one sex exploits the other, much to the latter's disadvantage. It is usually the male who benefits from this behavior, though in cases where males play similar roles to females in parental care the victim and perpetrator may be reversed (see Bateman's principle for discussion of this asymmetry).
By males.
Hanuman langurs (or gray langurs) are Old World monkeys found in India. They are a social animal, living in groups that consist of a single dominant male and multiple females. The dominant male has a reproductive monopoly within the group, which causes sub-ordinate males to have a much lower fitness value in comparison. To gain the opportunity to reproduce, sub-ordinate males try to take over the dominant role within a group, usually resulting in an aggressive struggle with the existing dominant male. If successful in overthrowing the previous male, unrelated infants of the females are then killed. This infanticidal period is limited to the window just after the group is taken over. Cannibalism, however, has not been observed in this species.
Infanticide not only reduces intraspecific competition between the incumbent's offspring and those of other males but also increases the parental investment afforded to their own young, and allows females to become fertile faster. This is because females of this species, as well as many other mammals, do not ovulate during lactation. It then becomes easier to understand how infanticide evolved. If a male kills a female's young, she stops lactating and is able to become pregnant again. Because of this, the newly dominant male is able to reproduce at a faster rate than without the act of infanticide. As males are in a constant struggle to protect their group, those that express infanticidal behavior will contribute a larger portion to future gene pools (see natural selection).
Similar behavior is also seen in male lions, among other species, who also kill young cubs, thereby enabling them to impregnate the females. Unlike langurs, male lions live in small groups, which cooperate to take control of a pride from an existing group. They will attempt to kill any cubs that are roughly nine months old or younger, though as in other species, the female will attempt to defend her cubs viciously. Males have, on average, only a two-year window in which to pass on their genes, and lionesses only give birth once every two years, so the selective pressure on them to conform to this behavior is strong. In fact it is estimated that a quarter of cubs dying in the first year of life are victims of infanticide.
Male mice show great variation in behavior over time. After fertilizing a female, they become aggressive towards mouse pups for three weeks, killing any they come across. After this period however, their behavior changes dramatically, and they become paternal, caring for their own offspring. This lasts for almost two months, but afterwards they become infanticidal once more. It is no coincidence here that the female gestation period is three weeks as well, or that it takes roughly two months for pups to become fully weaned and leave their nest. The proximate mechanism that allows for the correct timing of these periods involves circadian rhythms (see chronobiology), each day and night cycle affecting the mouse's internal neural physiology, and disturbances in the duration of these cycles results in different periods of time between behaviors. The adaptive value of this behavior switching is twofold; infanticide removes competitors for when the mouse does have offspring, and allows the female victims to be impregnated earlier than if they continued to care for their young, as mentioned above.
Gerbils, on the other hand, no longer commit infanticide once they have paired with a female, but actively kill and eat other offspring when young. The females of this species behave much like male mice, hunting down other litters except when rearing their own.
Prospective infanticide.
Prospective infanticide is a subset of sexual competition infanticide in which young born "after" the arrival of the new male are killed. This is less common than infanticide of existing young, but can still increase fitness in cases where the offspring could not possibly have been fathered by the new mate, i.e. one gestation or fertility period. This is known to occur in lions and langurs, and has also been observed in other species such as house wrens. In birds, however, the situation is more complex, as female eggs are fertilized one at a time, with a 24-hour delay between each. Males may destroy clutches laid 12 days or more after their arrival, though their investment of around 60 days of parental care is large, so a high level of parental certainty is needed.
By females.
Females are also known to display infanticidal behavior. This may appear unexpected, as the conditions described above do not apply. Males are not always an unlimited resource though—in some species, males provide parental care to their offspring, and females may compete indirectly with others by killing their offspring, freeing up the limiting resource that the males represent. This has been documented in research by Stephen Emlen and Natalie Demong on wattled jacanas ("Jacana jacana"), a tropical wading bird. In the wattled jacana, it is exclusively the male sex that broods, while females defend their territory. In this experiment Demong and Emlen found that removing females from a territory resulted in nearby females attacking the chicks of the male in most cases, evicting them from their nest. The males then fertilized the offending females and cared for their young. Emlen describes how he "shot a female one night, and ... by first light a new female was already on the turf. I saw terrible things—pecking and picking up and throwing down chicks until they were dead. Within hours she was soliciting the male, and he was mounting her the same day. The next night I shot the other female, then came out the next morning and saw the whole thing again."
Infanticide is also seen in giant water bugs. "Lethocerus deyrollei" is a large and nocturnal predatory insect found in still waters near vegetation. In this species the males take care of masses of eggs by keeping them hydrated with water from their bodies. Without a male caring for the eggs like this, they become desiccated and will not hatch. In this species, males are a scarce resource that females must sometimes compete for. Those that cannot find a free male often stab the eggs of a brooding one. As in the above case, males then fertilize this female and care for her eggs. Noritaka Ichikawa has found that males only moisten their eggs during the first 90 seconds or so, after which all of the moisture on their bodies has evaporated. However, they guard the egg masses for as long as several hours at a time, when they could be hunting prey. They do not seem to prevent further evaporation by staying guard, as males that only guarded the nest for short periods were seen to have similar hatching rates in a controlled experiment where there were no females present. It seems rather that males are more successful in avoiding infanticidal females when they are out of the water with their eggs, which might well explain the ultimate cause of this behavior.
Female rats will eat the kits of strange females for a source of nutrition, and to take over the nest for her own litter.
Resource competition.
Black-tailed prairie dogs are colonially-living, harem-polygynous squirrels found mainly in the United States. Their living arrangement involves one male living with four or so females in a territory defended by all individuals, and underground nesting. Black-tails only have one litter per year, and are in estrous for only a single day around the beginning of spring.
A seven-year natural experiment by John Hoogland and others from Princeton University revealed that infanticide is widespread in this species, including infanticide from invading males and immigrant females, as well as occasional cannibalism of an individual's own offspring. The surprising finding of the study was that by far the most common type of infanticide involved the killing of close kin's offspring. This seems illogical, as kin selection favors behaviors that promote the well-being of closely related individuals. It was postulated that this form of infanticide is more successful than trying to kill young in nearby groups, as the whole group must be bypassed in this case, while within a group only the mother need be evaded. Marauding behavior is evidently adaptive, as infanticidal females had more and healthier young than others, and were heavier themselves as well. This behavior appears to reduce competition with other females for food, and future competition among offspring.
Similar behavior has been reported in the meerkat ("Suricata suricatta"), including cases of females killing their mother's, sister's, and daughter's offspring. Infanticidal raids from neighboring groups also occurred.
Other.
Bottlenose dolphins have been reported to kill their young through impact injuries. Dominant male langurs tend to kill the existing young upon taking control of a harem. There have been sightings of infanticide in the leopard population. The males of the Stegodyphus lineatus species of spider have been known to exhibit infanticide as a way to encourage females to mate again. There is at least one documented case of infanticide among Asian elephants at Dong Yai Wildlife Sanctuary, with the researchers describing it as likely normal behavior among aggressive musth elephants.
In mammals, male infanticide is most often observed in non-seasonal breeders. There is less fitness advantage for a conspecific to carry out infanticide if the interbirth period of the mother will not be decreased and the female will not return to estrous. In Felidae, birthing periods can happen anytime during the year, as long as there is not an unweaned offspring of that female. This is a contributor to the frequency of infanticide in carnivorous felids. Some species of seasonal breeders have been observed to commit infanticide. Cases in the snub-nosed monkey, a seasonal breeding primate, have shown that infanticide does lessen the interbirth period of the females and can allow them to breed with the next breeding group. Other cases of seasonal breeding species where the infanticidal