Short Signals The Duet And A Return To Long Complex Calls

Theoretically, the briefest piece of acoustic information is a single click formed by a transient created by a single tooth impact and the only context where such a brief signal may contain information is as a reply to another signal. This distinguishes it from other transient sounds within the habitat including those made by potential predators moving within the vegetation. Thus, for many insects the phonoresponse within a duet contains information for the receiver solely on the basis of its latency (Bailey, 2003). While male-male duets are possible among acoustically signalling insects, the commonest is for females to respond to the calls of males where the reply falls within a precise species' defined window (Heller and Helversen, 1986; Zimmermann et al., 1989; Heller, 1990; Robinson, 1990a, 1990b; reviewed by Bailey, 2003). In such cases, information provided by the female creates a brief pair-bond, which results in the male searching for the female. Call latency of the duet is therefore critical for mate attraction and males do not respond to female-created transient signals outside the species range (e.g. Zimmermann et al., 1989). But factors that determine this latency, as perceived by the receiving male, are not only the ability of the female to reply within the species' interval, but also the distance between sender and receiver. For very brief species' latencies, sometimes as low as 15 msec (Robinson, 1990), as the distance between the sexes increases, so the travelling time for sound from the male to the female and back again effectively increases the latency to a point where a female, over a certain distance, cannot make an effective response. This is clearly adaptive, in that it may be too costly for males to search females outside this range.

Females frequently take the less costly role of producing extremely short signals and require the male to search. Although this is not always so, in many species the role of searching is shared by both sexes (Bailey, 2003; Walker, 2004). Selection may now operate through female choice on the male call, so male signals that initiate a duet become increasingly ornate. Supporting this hypothesis is the observation that the most complex signals are exhibited by duetting species (Spooner, 1968; Heller, 1988, 1990; Tuckerman et al., 1993; Hammond and Bailey, 2003; Walker, 2004). In this context, while there are few biological data on the longest of calling species, Polysacus scutatus, Heller (1990) does provide behavioural information on the related P. dendticaudus, where at the beginning of the first phrase the male moves around whilst calling. But after about 2 min the male stops walking and then increases its calling rate and after several seconds produces loud distinct syllables. Females that are ready to mate will respond to these loud syllables and males then move to them. A secondary prediction is that, as selection through female choice increases with the associated increase in male call complexity because of the higher male costs, males should evolve defensive (or offensive) calling tactics. Complex signals have more than one component with a section of the call containing species' information, as well as masking signals that can best be described as mate guarding tactics.

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