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1. Interspecific relationships, termed symbiosis, are considered very diverse, while intraspecific relationships have just two categories.
2. The present classification fails to define all interactions among members of the same species, as demonstrated by observations of suckling behaviour in pigs.
3. Supporting latest findings on the suckling behaviour of piglets, I propose an extension of the classification for existing intraspecific animal relationships by adding a new distinct category: commensalism.
Version 3: In this version only a bibliographic details for Skok (2016) have been supplemented.