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Some social animals are known for attacking odd members of their own group, for example, chicken:

If you have a large group of one type of chicken, they will usually gang up on the one or two others that do not look like they do. They recognize color differences the most and will beat up on the odd chickens in a group.

Of course the same behavior is most commonly observed in humans, where any difference can be a reason for aggressiveness.

Why this behavior developed and still exists and how it is called?

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    $\begingroup$ The same article also states that "being kept in too small of an area can also create aggressive situations in chickens of the same breed". This happens with many other animals, including humans. Perhaps the behavior you relate happened in an isolated area? In this case, kin selection theory predicts that the first attacked individuals will be those that do not look like relatives. $\endgroup$ – Rodrigo Apr 30 '17 at 3:54
  • $\begingroup$ @Rodrigo, as I understand it, space constraints just increase aggressiveness, not cause it. $\endgroup$ – contemplator Apr 30 '17 at 10:06
  • $\begingroup$ I couldnt find specific terms for aggression more than territorial disputes, hyerarchy disputes, persecution, xenophobia. It doesn't seem to have been studied as a general phenomenon. I found something about malawi fish attacking individuals of the same color. Wild chickens don't differ much genetically, and a persecuted albino/mutant can normally leave the flock. to the chickens, the mutant perhaps looks unwholesome and they are trying to chase it away, except it can't escape. Reasons why are very interesting and hyper complex and take several pages to list. $\endgroup$ – com.prehensible Apr 30 '17 at 14:09
  • $\begingroup$ @comprehensible, can you at least provide a summary of these reasons, so I would know what to search for? $\endgroup$ – contemplator Apr 30 '17 at 17:50
  • $\begingroup$ In ethology, aggression is part of what we call agonistic behavior. Yes, space constraints increase aggressiveness. Hens do attack each other in order to establish an hierarchy (the pecking order). Maybe the conditions in which the observations of your source were done help to explain what was going on. I think it may be a mix of territoriality, kin selection and restricted space. $\endgroup$ – Rodrigo May 1 '17 at 16:01
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Persecution is not a term often used in zoology, compared for example to the notion of social exclusion and hierarchy disputes/social animal hierarchy. That is because humans incarcerate animals which would otherwise flee to safe distance from the group if they are attacked. Wild Mammals, birds, fish, other animals with free movement can normally get safe distance from an aggressive group in order to be safe from persecution.

Fish and insects/island animals sometimes have restricted ranges, and "social exclusion of a mutant" - studies are difficult to find and perhaps won't give masses of information to know why it happens, for categorization and prevalence of different reasons.

I found only one research on social aggression based on appearance:

Aggressive biases towards similarly coloured males in Lake Malawi ... rsbl.royalsocietypublishing.org/content/4/2/156

Useful related terms for further research for your question are i.e.:

  • "social exclusion mutants traits aggression"
  • "social exclusion zoology"
  • "social exclusion injury zoology"
  • "social exclusion zoology mutation"
  • "hyerarchy disputes appearance zoology"

It's an interesting notion for research that you have found, for which a lot can be written, because it gives a complex range of potential reasons for the given behaviour: warding off a contagious individual, warding off a tribe from far away that could displace the current group, inability for the brain to average the mutant's appearance to associate with it, habituation of species to a type of social companion since their birth (i.e. birds sometimes think they are humans)

And so a research requires a broad understanding of juvenile establishment of appearance recognition of other individuals of the group, i.e. where humans search for an average appearance compared to the social group, and chimps recognize close relatives and mate with the most distant, chickens can adopt humans... and animal behaviour towards an ill individual, it's difficult and takes a while to gather actual sci documents for.

When you have found all the closest related research, the given write up would be very fascinating!!! i hope you can search the given topics and give a summary of some previous examples of vaguely related research.

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  • $\begingroup$ Thanks, I appreciate your answer. I've upvoted it, but I'll leave the question open for everyone to contribute, if you don't mind. $\endgroup$ – contemplator May 8 '17 at 21:03

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