What is a chorus, what is an insect chorus, and why might we be interested in how and why singing insects create orchestral productions? To begin, chorusing is about timing. In a chorus, singers align their verses with one another in some non-random way.
When singing insects form a chorus, the alignment may only be a crude grouping of song during a given time of the day or night; e.g. a midday or evening chorus. But in the case of insect species that repeat discrete song units – calls, chirps, buzzes, etc. – in rhythmic fashion, the chorus may be much more refined. Here, neighboring singers may adjust the period and phase of their rhythms such that their song units are synchronized or alternate with one another. It is these high-precision choruses, particularly those that involve collective synchrony, that draw a lot of interest. Analogous phenomena occur in insects that generate visual displays, notably fireflies, and these too intrigue us. Perhaps our fascination is driven by an intrinsic appreciation for pattern in space and time.
Insect synchronies are about long-range advertisement songs that males broadcast to females. What makes male insects synchronize when females are within earshot? Three explanations may account for this cooperation among males. First, synchrony may preserve the clarity of rhythm or discrete song units within a local group that nearby females need to hear before moving toward any one male. Under these circumstances, a male who does not synchronize can reduce his neighbor’s attractiveness, but in so doing he will also jeopardize his own, a spiteful act that selection is not expected to favor. Second, synchrony may pose a cognitive problem for predators and parasites who listen to the advertisement songs of their prey and hosts before attacking. That is, when sound arrives from myriad directions, pinpointing any one source may be difficult. Third, synchrony would maximize the peak sound intensity that a local group of males broadcast, affording a group that synchronizes a longer radius of attraction than a comparable group that does not bother to. While these are seemingly cogent explanations based on sensory perception and ecology, it is worth noting that rather little supporting evidence exists. Neighboring males can achieve regular synchrony by making various call timing adjustments. A simple but effective mechanism recently found in European bushcrickets is to refrain from initiating a call for a 600-900 ms interval following the endings of neighbors’ calls.
An alternative to the adaptationist paradigm – which underlies the above propositions – is that complex synchronous displays may just emerge as an incidental byproduct of simple pairwise interactions between neighboring males. Importantly, these pairwise interactions are generally competitive, not cooperative. For example, hearing in acoustic animal species often entails attention to the first of two (or more) calls separated by a brief time interval and then ignoring the second call. ‘Leading call effects’ may originate as neural mechanisms that improve sound source localization, but when they occur in females evaluating potential mates they are expressed as a preference for the first male to sing.
Consequently, males come under intense selection pressure to play ‘timing games’ with each other that limit production of following calls and increase leading ones. A common game is deploying a ‘phase-delay mechanism’ that 1) resets the (central nervous system) rhythm generator regulating one’s calls to its basal level upon hearing a neighbor, 2) inhibits the generator by locking it at that basal level until the neighbor’s song ends, and then 3) releases the normal, free-running call rhythm. Simulations show that when males with comparable songs deploy this mechanism, a structured chorus emerges. The chorus is a synchronous one if rebound from inhibition approximates the normal rhythm, but alternation arises if the rebound is a bit faster. But even where nearest neighbors alternate, a considerable amount of synchrony occurs in the chorus: Males generally exhibit ‘selective attention’ to their nearest neighbor, which means that when males C and D each alternate with male B, C and D then synchronize. Some recent tests confirm that this type of structured chorusing may occur in the absence of any female preference for synchrony or alternation per se; i.e. the males who generate the chorus do not benefit from their overall production.
Because synchronies may arise from cooperation or competition, the study of insect choruses can offer some insight to the roles of these opposing forces in shaping behavior. Experiments with two closely related, chorusing bushcricket species, Sorapagus catalaunicus and Ephippiger diurnus, have yielded some surprising findings. S. catalaunicus males generate regular synchrony, and it was expected to reflect a necessity to maintain discrete calls for attracting females. E. diurnus males generate a mixture of alternation and synchrony, expected to emerge from competition to reduce the incidence of following calls, which females ignore. But tests showed that females in both species needed to hear discrete calls clearly separated by silent gaps, and females in both also had a strong preference for leading calls. It was inferred that call length, long in S. catalaunicus and short in E. diurnus, influenced the outcome by selecting for a specialized mechanism ensuring silent gaps in S. catalaunicus. These findings show how a small difference in a very basic trait can trigger a cascade of evolutionary events, ultimately influencing the emergence of rather dissimilar behavior characterizing social interactions at the level of animal groups.
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