The ability to recognize and discriminate kin from non-kin is important in both the context of kin selection and optimal outbreeding. I describe both of these terms below. There are at least three ways to define kin recognition.
1) Differential treatment of related versus unrelated individuals.
2) Differential response to being subjected to a stimulus from related individuals versus a stimulus from unrelated individuals. This response is not necessarily observable, and the individual responding may not be cognizant of the response or of the difference between the stimuli.
3) Differential response to being subjected to a stimulus correlated with relatedness versus a stimulus not correlated with relatedness. Again, this response is not necessarily observable, and the individual responding may not be cognizant of the response or of the difference between the stimuli.
I prefer the second definition because it is closest to our English language definition of 'recognition'. One difference is that with the first definition, is it possible to test the hypothesis, and then conclude, that an animal did recognize kin or did not recognize kin. With the second and third definitions, it is possible to say that an individual has recognized kin if there is an observed difference between kin and non-kin treatments, but it is not possible to conclude that an animal does not recognize kin because the absence of a measured response may be explained either by the observer measuring the wrong variable, or the animal truly failing to recognize kin. The third definition allows incorrect discrimination between related and unrelated individuals to be called 'kin recognition' if unrelated individuals sometimes share characters. For example, Laurent Keller and Kenneth Ross (1998) showed that in the fire ant (Solenopsis invicta), queens with an allelic difference at a single locus were more likely to be attacked than queens without the difference. Because queens could share the same alleles but be unrelated, this may be called kin recognition under Definition 3, but not under Definitions 1 or 2.
In classical terms, there are four mechanisms by which an individual may recognize kin: by location, by recognition alleles, by familiarity, and by phenotype matching. In recognition by location, individuals use spatial cues that are reliably correlated with kinship, such as proximity to the nest, to discriminate between kin and non-kin. When kin recognition by recognition alleles is used, a single allele causes production of a phenotypic cue, recognition of that cue in others, and preferential treatment toward bearers of the cue. When kin recognition by familiarity is used, individuals learn phenotypic cues of conspecifics encountered during early development and remember these individuals as kin. When kin recognition by phenotype matching is used, individuals learn the phenotypic cues of their rearing associates (or its own cues) but instead use these cues to form a "kin template." Individuals later compare phenotypic cues of putative kin to the template and based on the similarity of the cue to the template, determine the degree of relatedness of the individual
There may be other means of helping kin or by avoiding kin as mates, but I hesitate to call these kin recognition. These are typically called 'decision-rules'. For example, male black-tailed prairie dogs (Cynomis ludovicianus) appear to follow a 'decision-rule' that if they are in an area the year after they mated with a female, they will not mate with any females a year old because they may be kin. As another example, the offspring of parental males in the bluegill sunfish (Lepomis macrochirus) may follow a decision-rule to remain with their neighbours as they leave their natal nest because they are typically very highly related to their nestmates.
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