Unlikely Friendships - An Exploratory Review into the Defining Criteria of Associative Behaviour between Vertebrate Species.

Abstract

Research on associations between individuals of different species has been conducted worldwide on a plethora of taxa and documented under multiple terminologies. This paper reviews the breadth of this complex social behaviour by collating relevant association papers across all vertebrates that use one or more of the four main terminologies to describe such associations (polyspecific (PSA), heterospecific (HSA), interspecific (ISA), mixed-species (MSA)). Over 400 vertebrate species from 6 phylogenetic classes exhibit a form of association with another species within the 592 papers reviewed. The definitions of eligible papers were extracted to discover the variety of associations between individuals of different species and how this correlates with the taxon or terminology used. Despite extensive variation and ambiguity across papers, a core set of definition segments has been found to appear in various combinations consistently throughout the dataset. Both multiple correspondence (MCA) and principal component (PCA) analyses were used to discover trends between the defining criteria/behavioural characteristics recorded and taxa/terminologies. Initial findings highlight a large overlap between taxa/terminologies and their respective definition segments, nevertheless, unique trends were found among primates, birds and polyspecific associations. To mitigate the low explanation of variance and further explore the observed trends, a subsequent targeted dataset was created containing studies focussed on primates in polyspecific associations (Primates:PSA) or birds in mixed- species flocks (Birds:MSA). The separation of defining components in the secondary dataset was seen across all multivariate statistics. Studies on birds in mixed-species associations (Birds:MSA) were likely to have a nuclear species, unequal benefits between the participating species, a lack of unified territoriality and little aggression towards each other. Primates in polyspecific associations (Primates:PSA) were likely to engage in coordinated activities together such as foraging, resting, alarm response and general interactions for a prolonged duration within proportionally close proximity of each other. Primates:PSA studies often record associative behaviour occurrence beyond the expected ‘chance encounters’ and describe strong seasonality as well as minor agonistic interactions. Birds:MSA studies recorded fewer defining criteria variables on average than Primates:PSA making true comparison challenging. A unifying framework for associations between different species regarding the appropriate usage of terminology has been created based on standardised defining criteria. The recommended framework and revised definition aim to be encompassing across all taxa and alleviate researcher ambiguity surrounding associative behaviour to minimise terminology misuse within this field.