Infanticide (zoology)

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Infanticide (from Latin infanticidium , "kindestkilling"; derived from Latin infans "child" and cidere "killing") is a technical term borrowed from English ( infanticide ) for the killing of offspring of one's own kind Generally used with reference to animals and people, in German it is used more in relation to animals and can therefore be distinguished from the term infanticide, which is exclusively related to people . The infanticide in the animal kingdom is by no means necessarily connected with cannibalism , i.e. with eating up the killed conspecifics. If the offspring are eaten, one speaks of Kronism , after the Greek titan Kronos , who ate his children.

Examples of infanticide

Infanticide occurs in almost all defensive styles on, not just when predators such as lions and brown bears , but also in many birds, possibly all rodents and primates , there probably with the exception of the largely monogamous living gibbons , marmosets and tamarins and the bonobos . Due to behave biological observations, he is, for example, rats , mice and meerkats also attributed as coat baboons , Hulmanen , chimpanzees , gorillas and even dolphins . There are also reliable reports from dippers and starlings .

Infanticide in the animal kingdom must therefore be viewed as a natural behavior.

Chimpanzees

Infanticide in chimpanzees was first documented by Jane Goodall in 1976 . In their observation area in Gombe , the female Passion and her daughter Pom had killed three young chimpanzees and partially eaten them within two years. At that time Jane Goodall could not understand whether this behavior should be classified as pathological or whether it was evolutionarily advantageous in the long term; only long-term observations could give a clue here. In the 1990s, also in the Gombe area, indications of infanticide in chimpanzees were found again, but mostly from males of foreign groups. In 2007, a group of researchers from the Ugandan Budongo Forest reported on three comparable killings. This time, however, it could be proven beyond doubt that it was young animals of females who had recently immigrated to the attackers' territory . In the journal New Scientist several researchers were quoted, according to which the killing of the strange young animals in the Budongo forest could be interpreted as adapted behavior , since it increases the chances of survival of one's own offspring in the face of scarce food resources.

In 2019, 30 aggressive actions against young animals documented in the Budongo forest over the course of 24 years were analyzed. Two thirds of the victims were younger than a week, the attackers were mostly adult males. The observations and analyzes of behavior in the chimpanzee group showed that the infanticide is most likely to be interpreted as a result of sexual selection : The mothers of the young animals killed became pregnant again much earlier than without the loss of the baby, namely by fathers of theirs Group.

Bengali Hanuman langurs

If the dominant male in a group of Bengali Hanuman langurs is ousted by a competitor, the successful male kills all unweaned pups in the group; pregnant females are harassed until a miscarriage occurs. Kinship analyzes confirmed that the males only kill the young of unfamiliar males.

Lions

From field observations of lions it is known that a new pack leader who has ousted his predecessor often kills all the young (i.e. the offspring of his predecessor). At the same time, the quarrels in the pack can lead to miscarriages - presumably due to social stress. Females who lose their young quickly come back into the oestrus , which means that the new pack leader can mate relatively quickly with the lionesses in his new pack and produce offspring of his own.

Rats

In laboratory rats , a Canadian study showed that nearly twice as many pups died if their cages were cleaned twice a week than if they were cleaned only every two weeks. The cause is probably u. a. the repeated destruction of the nests when cleaning. The two authors of the study also pointed out that young animals are occasionally killed by their mothers even if no comparable stressors are detectable.

Aquarium fish

Infanticide is quite common among aquarium fish (especially viviparous species such as platy , guppy or swordtail ), so breeders have to move the birthing females into a separate aquarium. After giving birth, the female must be separated from her newborn immediately. The same applies to sticklebacks. However, in this species the male tends the brood.

Birds

In which the African barbets belonging crested barbet has been observed so far infanticide in two cases. If one of the two partners died during the breeding season, the surviving crested bearded bird mated very quickly. The new mate either destroyed the eggs or killed the nestlings and then began brooding of their own.

About the origin of behavior in tribal history

From Sociobiologists particular three forms unterschieden of infanticide.

1. Often a male kills one or more young animals that were not conceived by him in order to be able to mate more quickly with the mother animal. Lion mothers z. B. try to defend and "save" their young, but if unsuccessful they are quickly prepared to mate with the male who killed their children. This behavior is consistent with the principle of Darwin fitness : the adaptive value of a trait is measured by successful reproduction. Therefore the infanticide is an evolutionarily stable strategy as long as it does not become a disadvantage (because of the increased risk) due to changed selection conditions (union of the females to defend the offspring). This interpretation considers the ultimate causes of behavior (the causes that can be deduced from the tribal history ), not the so far hardly explored, immediate external triggers and internal physiological causes.

According to the evolutionary biologists and ethologists van Schaik and Dunbar, monogamy has developed as a protection against this type of infanticide , in which the male is not confronted with children of his sexual partner conceived by others. Also, the man could have been protected by the establishment of monogamous lifestyles before infanticide. However, according to more recent studies, many animal species previously assumed to be strictly monogamous do change partners. In the meantime, infidelities in males and females have also been observed in gibbons , for example in the white-handed gibbon . However, since there is primarily a couple relationship and the male cannot know whether a child is from another, this is still protected from infanticide.

Also, promiscuous sexual behavior, so the frequent change of sexual partners, as is common in chimpanzees, protects evident before infanticide. In any case, Sarah Blaffer Hrdy’s statement applies not only to her nine-year observation of Hanuman langurs: "Young animals were only attacked by unfamiliar adult males, but never by males who could themselves be the fathers of these children."

2. If the survival of the entire offspring is at risk due to acute food shortages, it can be evolutionarily advantageous to sacrifice the weakest offspring in order to keep at least the strongest of them alive. In 2007 the journal The American Naturalist published the results of a computer model that was used to analyze the environmental conditions under which infanticide and boy care can develop. The researchers first modeled an egg-laying species whose young grew up after hatching without being cared for or killed by adults. They later introduced “ mutants ” into the virtual population who were either cared for by their parents, killed or exposed to both influencing factors. The result of the model calculations showed that when there was a shortage of food, either one or the other behavior spread in the population. However, when food was in abundance, both behaviors spread at the same time. According to the researchers, different causes could be proven: In times of food shortage, at least some of the offspring survived if individual eggs or young animals were eaten, while otherwise all offspring would have died due to inadequate care. In other simulations, it ultimately proved to be advantageous for the reproduction rate if individual weak or sick young animals were killed. This modeling thus supports the basic sociobiological assumption that the greatest possible number of one's own offspring is evolutionarily relevant, but not the survival of each individual offspring.

3. Another form of infanticide occurs when individuals kill strange offspring, not to enable immediate reproductive success, but to generally eliminate competitors for food and reproduction. This should be a meaningful interpretation of both the behavior of the chimpanzees Passion and Pom described above and the behavior of chimpanzees who patrol the border and, if the opportunity arises, kill foreign children (as well as adults).

Ecological and behavioral triggers

Infanticide can have a variety of, mostly largely unknown, triggers. Attempts at an explanation are often mere guesswork.

Most often, infanticide is described as the result of struggles for hierarchy , turf wars , overpopulation and food shortages.

  • Young leading or suckling females are often not ready to mate; a male who is victorious in the ranking or turf war increases his reproductive success if he eliminates the offspring of the defeated rival.
  • Overpopulation can lead to more frequent aggressive confrontations with conspecifics, which can lead to a general increase in the willingness to attack. Young animals are less able to defend themselves against attacks and are therefore more likely to be victims than adult individuals. The infanticide can also be an unintended by-product of rivalry or re-mating (e.g. crushed pups in elephant seals ).
  • Laypeople in particular occasionally justify infanticide in domestic animals with a developmental disorder in the young or with an unrecognized disease. However, behavioral studies on house mice and rats have only found such a connection in exceptional cases. In rodents in particular, it happens again and again that well-fed and mobile nestlings are killed, while significantly weaker individuals survive.
  • Scarcity of food or other resources can lead to the killing of some or all of the offspring. Even if the connection is obvious, the exact causal connections are usually unclear, since animals in general cannot be assumed to act with foresight in the long term.
  • In addition, “social stress ” is frequently cited as a trigger for infanticide. This trigger mechanism is difficult to prove, however, since the observer cannot always name specific, external stimuli (so-called stressors ). However, evidence has to be given, for example, of the killing of young animals or their abandonment (also giving up the nest or any further feeding means their certain death) in some wild animal species by the mothers after repeated disturbance.

literature

Individual evidence

  1. ↑ Kronism. On: Spektrum.de , accessed on June 23, 2016.
  2. ↑ Mother brown bears protect cubs with human shields. On: sciencemag.org from June 21, 2016.
  3. ^ A b C. P. van Schaik, RIM Dunbar (1990): The Evolution of Monogamy in Large Primates: A New Hypothesis and some Crucial Tests . Behavior 115 (1/2), pp. 30-62.
  4. ↑ Frans de Waal: Bonobos - The tender apes . Birkhäuser Verlag , Basel 1998, p. 193. Translated from the English by Monika Niehaus-Osterloh [Orig .: Bonobo - The Forgotten Ape , University of California Press, Berkeley 1997].
  5. ↑ Jane Goodall: Beyond Innocence. An Autobiography in Letters: The Later Years. Houghton Mifflin Harcourt, Boston 2001, ISBN 978-0618125203 .
  6. ↑ Jane Goodall: Infant killing and cannibalism in free-living chimpanzees. In: Folia Primatologica. Volume 28, No. 4, 1977, pp. 259-282, doi: 10.1159 / 000155817 .
  7. ↑ Marcus Anhäuser: Child murders in the realm of the apes. In: Süddeutsche Zeitung. No. 111 of May 15, 2007, p. 20, full text of
    cruel child murder in the jungle. On: welt.de from May 18, 2007.
  8. ↑ Simon W. Townsend, Katie E. Slocombe, Melissa Emery Thompson and Klaus Zuberbühler: Female-led infanticide in wild chimpanzees. In: Current Biology. Volume 17, No. 10, 2007, pp. R355-R356, doi: 10.1016 / j.cub.2007.03.020 .
  9. ↑ Mairi Macleod: Female chimps can resort to infanticide. In: New Scientist. Vol. 194, No. 2604, from May 19, 2007, p. 19, introduction to the article .
  10. ^ Adriana E. Lowe et al .: Intra-community infanticide in wild, eastern chimpanzees: a 24-year review. In: Primates. Advance online publication of May 27, 2019, doi: 10.1007 / s10329-019-00730-3
    Research identifies key driver for infanticide among chimpanzees. On: eurekalert.org from June 13, 2019.
  11. ↑ Volker Storch , Ulrich Welsch and Michael Wink : Evolutionary biology. Springer, Berlin and Heidelberg 2001, p. 358, ISBN 978-3-540-41880-1 .
  12. ↑ John Alcock : The behavior of animals from an evolutionary perspective. G. Fischer, Stuttgart, Jena and New York 1996, pp. 11-12, ISBN 978-3-437-20531-6 .
  13. ↑ Charlotte C. Burna, Georgia J. Mason: Effects of cage-cleaning frequency on laboratory rat reproduction, cannibalism, and welfare. In: Applied Animal Behavior Science. Volume 114, No. 1, 2008, pp. 235-247, doi : 10.1016 / j.applanim 2008.02.005
  14. ↑ Lester L. Short and Jennifer FM Horne: Toucans, Barbets and Honeyguides - Ramphastidae, Capitonidae and Indicatoridae . Oxford University Press, Oxford 2001, p. 121, ISBN 978-0-19-854666-5 .
  15. ↑ Frans de Waal: Bonobos - The tender apes . Birkhäuser Verlag , Basel 1998, p. 193. Translated from the English by Monika Niehaus-Osterloh [Orig .: Bonobo - The Forgotten Ape , University of California Press, Berkeley 1997].
  16. ↑ Klaus Wilhelm (2010): Cheating is the rule. Behavioral researchers are surprised: More and more animal species are turning out to be polygamous. Is man also born to have an affair? Bild der Wissenschaft 9/2010, p. 16.
  17. ↑ Sarah Blaffer Hrdy: Mother Nature. The feminine side of evolution . Berlin Verlag, Berlin 2000, p. 53.
  18. ↑ Hope Klug and Michael B. Bonsall: When to Care for, Abandon, or Eat Your Offspring: The Evolution of Parental Care and Filial Cannibalism. In: The American Naturalist. Volume 170, No. 6, 2007, pp. 886-901, doi: 10.1086 / 522936 .