Evolution alarm pheromones

Williams (1964, 1992), however, argued that there are considerable problems in explaining the evolution of an alarm pheromone. It was assumed that individuals produced alarm substance to warn their school or species of danger, but schools of fish are not composed of closely related individuals (Naish et al. 1993). Magurran et al. (1996) further demonstrated that fright responses in fish were elicited in a context-dependent manner. The alarm responses were likely exaggerated in the laboratory condition where the opportunities for escape were largely reduced. In the natural environment, alarm substances did not produce adaptive behaviors. In crustaceans, behaviors similar to the alarm response in fish can be elicited by the reception of injured conspecifics (Hazlett, Chap. 18). 

The aim of this paper is to provide some speculative discussion and propose some hypotheses about the multiple possible functions and interactions, both within a species and between species, that could influence the evolution of chemical alarm signals in fishes. Fish alarm pheromones have been reviewed by Smith (1992) and Chivers Smith (in press) but I will provide basic background information with a few examples. 

The substances that function as alarm pheromones for conspecifics are detectable to heterospecifics and can influence other prey species and alter the behavior of predators. These interspecies effects may have had some influence on the evolution of alarm signal systems. 

What might have been the starting point in the evolution of injury-released alarm pheromones As a starting point, assume that there was a compound in the skin that was distinctive and easily detected, perhaps a wound healing or antipathogen secretion. Then, conspecifics and heterospecific prey guild members could benefit if they responded to that... 

Costs. The benefits of sending the signal must exceed the costs for the alarm signal function to drive the evolution of the ASCs. In the case of the fish alarm systems the cost will be the production and storage of the alarm pheromone. Injury or death during the release process would presumably occur anyway and are not costs of the signal system. There is no... 

Chemical alarm signals operate when interacting individuals are aggregated or in close proximity. It is not unexpected, then, that such signals are well-developed in the social insects and relatively uncommon in pre-social species. Outside the social insects, chemical alarm systems are best developed in two homopteran groups aphids and treehoppers. In this chapter, we discuss alarm pheromones and their social context in these and other pre-social insects and suggest modes of their evolution. 

Johnston (1998) gives several examples of how combinations of neurophysiology and behavioral analysis can sort out the differences between classical pheromones, pheromone-like signals and odor quality signals composed of a mixture of several active compounds. Application of these techniques to fish alarm substances might reveal patterns in the evolution of these chemical signals. 

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