Rich in phenomena-lacking in terms. A classification of kairomones

Summary.

Although the usefulness of the term kairomone was discussed controversially after its introduction, it is now widely accepted in chemical ecology. It is commonly used to describe a chemical that is pertinent to the biology of an organism (organism 1) and that when it contacts an individual of another species (Organism 2) evokes in the receiver a behavioural or physiological response that is adaptively favourable to organism 2 but not to organism 1. A look at the chemoecological literature reveals that chemicals classified by the mere term kairomone may have completely different biological functions for the receiving organism. Chemicals meeting the definition of a kairomone are used for the location of food sources and sexual mates, or may be used by potential prey or host organisms to decrease the negative impact of natural enemies. Thus, by describing those diverse mediators merely as kairomones, no information on the actual function of the chemical is given. When considering the terminology of pheromones, another diverse group of infochemicals mediating intraspecific interactions, further subdivision is common practice and useful to describe the multitude of different functions and thus, to prevent terminological confusion.¶The present paper demonstrates the diversity of kairomone-related ecological phenomena by several examples and proposes a further classification of kairomones according to the function for the benefiting organism, by introducing the terms foraging kairomone (used in the context of food location), enemy-avoidance kairomone (used to reduce the negative impact of natural enemies), sexual kairomone (used for sexual purposes), and aggregation kairomone (attracting/arresting both sexes of an organism). Additionally, discrimination of two groups of kairomones according to the effect on the benefiting organism is proposed leading to the terms primer kairomone (inducing physiological responses) and releaser kairomone (inducing behavioural responses). The intention of the proposed classification is to allow a more precise description of kairomones and thus, to aid the discussion of these compounds and to improve the readability of kairomone-related papers.