Infanticide (zoology)
In
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.[5] 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.[5] 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
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 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
Infanticide not only reduces
Similar behavior is also seen in male
Male
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
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
Infanticide is also seen in
Female rats will eat the kits of strange females for a source of nutrition, and to take over the nest for her own litter.[20]
Resource competition
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.[3] 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.[21]
Other
In mammals, male infanticide is most often observed in non-seasonal breeders.[25] 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.[26][25] 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.[27] Other cases of seasonal breeding species where the infanticidal characteristic is observed has been explained as a way of preserving the mother's resources and energy in turn increasing the reproductive success of upcoming breeding periods.[28]
Costs and defenses
Costs of the behavior
While it may be beneficial for some species to behave this way, infanticide is not without risks to the perpetrator. Having already expended energy and perhaps sustained serious wounds in a fight with another male, attacks from females who vigorously defend their offspring may be telling for harem-polygynous males, with a risk of infection. It is also energetically costly to pursue a mother's young, which may try to escape.
Costs of the behavior described in prairie dogs include the risk to an individual of losing their own young while killing another's, not to mention the fact that they are killing their own relatives. In a species where infanticide is common, perpetrators may well be victims themselves in the future, such that they come out no better off; but as long as an infanticidal individual gains in reproductive output by its behavior, it will tend to become common. Further costs of the behavior in general may be induced by counter-strategies evolved in the other sex, as described below.
As a cost of social behavior
Taking a broader view of the black-tailed prairie dog situation, infanticide can be seen as a cost of
An avian example published in
Counter-strategies
Because this form of infanticide reduces the fitness of killed individuals' parents, animals have evolved a range of counter-strategies against this behavior. These may be divided into two very different classes - those that tend to prevent infanticide, and those that minimize losses.
Loss minimization
Some females abort or resorb their own young while they are still in development after a new male takes over; this is known as the
Preventive adaptations
Infanticide in burying beetles may have led to male parental care.[32] In this species males often cooperate with the female in preparing a piece of carrion, which is buried with the eggs and eaten by the larvae when they hatch. Males may also guard the site alongside the female. It is apparent from experiments that this behavior does not provide their young with any better nourishment, nor is it of any use in defending against predators. However, other burying bugs may try to take their nesting space. When this occurs, a male-female pair is over twice as successful in nest defense, preventing the ovicide of their offspring.
Female langurs may leave the group with their young alongside the outgoing male, and others may develop a false estrous and allow the male to copulate, deceiving him into thinking she is actually sexually receptive.[33] Females may also have sexual liaisons with other males. This promiscuous behavior is adaptive, because males will not know whether it is their own offspring they are killing or not, and may be more reluctant or invest less effort in infanticide attempts.[34] Lionesses cooperatively guard against scouting males, and a pair were seen to violently attack a male after it killed one of their young.[35] Resistance to infanticide is also costly, though: for instance, a female may sustain serious injuries in defending her young. At times it is simply more advantageous to submit than to fight.[36]
Infanticide, the destruction of offspring characteristic to many species, has posed so great a threat that there have been observable changes of behavior in respective female mothers; more specifically, these changes exist as preventive measures. A common behavioral mechanism by females to reduce the risk of infanticide of future offspring is through the process of paternity confusion or dilution. In theory, this implies that a female that mates with multiple males will widely spread the assumption of paternity across many males, and therefore make them less likely to kill or attack offspring that could potentially carry their genes. This theory operates under the assumption that the specific males keep a memory of past mates, under a desire to perpetuate their own genes [37] In the
Infanticide by parents and caregivers
Filial infanticide occurs when a parent kills its own offspring. Both male and female parents have been observed to do this, as well as sterile worker castes in some eusocial animals. Filial infanticide is also observed as a form of brood reduction in some birds species, such as the white stork.[44] This may be due to a lack of siblicide in this species.[45]
Maternal
Maternal infanticide occurs when newborn offspring are killed by their
Paternal
Paternal infanticide—where fathers eat their own offspring—may also occur. When young bass hatch from the spawn, the father guards the area, circling around them and keeping them together, as well as providing protection from would-be predators. After a few days, most of the fish will swim away. At this point the male's behavior changes: instead of defending the stragglers, he treats them as any other small prey, and eats them.[50]
Worker caste killing young
Humans and infanticide
Family structure is the most important risk factor in child abuse and infanticide. Children who live with both their natural (biological) parents are at low risk for abuse. The risk increases greatly when children live with step-parents or with a single parent. Children living without either parent (foster children) are 10 times more likely to be abused than children who live with both biological parents.[citation needed]
Children who live with a single parent that has a live-in partner are at the highest risk: they are 20 times more likely to be victims of child abuse than children living with both biological parents.[53]
Infanticide is a subject that some humans may find discomforting. Cornell University ethologist Glenn Hausfater states that "infanticide has not received much study because it's a repulsive subject [...] Many people regard it as reprehensible to even think about it." Research into infanticide in animals is in part motivated by the desire to understand human behaviors, such as child abuse. Hausfater explains that researchers are "trying to see if there's any connection between animal infanticide and child abuse, neglect and killing by humans [...] We just don't know yet what the connections are."[54]
Infanticide has been, and still is, practiced by some human cultures, groups, or individuals.[
Amongst some hunter-gatherer communities, infanticide would sometimes be extended into child cannibalism. This is documented in many regions, but particularly amongst pre-colonial Aboriginal Australian tribes. Infants and young children would often be killed, roasted, and eaten by their mother and sometimes also fed to siblings, usually during times of famine. In non-filial cases when a child was "well-fed" and in the absence of its mother sometimes a man or the whole community would kill and consume the child.[55][56]
See also
- Human infanticide
- Infanticide in carnivores
- Infanticide in primates
- Infanticide in rodents
- Parent–offspring conflict
- Parricide
- Paternal care
- Runt
- Sexual cannibalism
- Sexual selection
- Siblicide
Further reading
- ISBN 978-0-87893-009-8.
- Parmigiani, S.; vom Saal, F.S. (1994). Infanticide and Parental Care. Harwood: London. p. 493. ISBN 978-3-7186-5505-2.
- van Schaik, C.P.; Janson, C.H. (2000). Infanticide By Males And Its Implications. Cambridge University Press. p. 569. ISBN 978-0-521-77295-2.
References
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