The ground offers acoustic efficiency gains for crickets and other calling animals

Abstract

AbstractMale crickets attract females by producing calls with their forewings. Louder calls travel further and are more effective at attracting mates. However, crickets are small, and therefore inefficient, dipole sound-sources. Only a small group called tree crickets make acoustic tools called baffles which reduce acoustic short-circuiting, a source of dipole inefficiency. Here, we ask why baffling is uncommon among crickets. We hypothesize that baffling may be rare, because like other tools they offer insufficient advantage for most species. To test this, we modelled the calling efficiencies of crickets within the full space of possible natural wing sizes and call frequencies, in multiple acoustic environments. We then generated efficiency landscapes, within which we plotted the positions of 111 cricket species across 7 phylogenetic clades. We found that all sampled crickets, in all conditions, could gain efficiency from tool use. Surprisingly, however, we also found that calling from the ground significantly increased efficiency, with or without a baffle, by as much as an order of magnitude. We found that the ground provides some reduction of acoustic short-circuiting but also halves the air volume within which sound is radiated. It simultaneously reflects sound upwards, allowing recapture of a significant amount of acoustic energy through constructive interference. Thus, using the ground as a reflective baffle is a very effective strategy for increasing calling efficiency. Indeed, theory suggests that this increase in efficiency is accessible not just to crickets, but to all acoustically communicating animals whether they are dipole or monopole sound sources.Significance StatementLoudness is a crucial functional feature of calls in acoustically communicating animals. Animals attempting to reach prospective mates or ward off predators are expected to make themselves as loud as possible. Given the importance of loudness, there are two long-standing, seemingly unrelated paradoxes in acoustic communication. The first is the rarity of acoustic tool use. The second is the high number of animals that call from reflective surfaces, like the ground, known be an impediment to sound propagation. We resolve both paradoxes and show them to be related. By refocusing analysis from sound propagation to sound radiation, we show that the ground is the opposite of an impediment and can, in fact, boost sound loudness more than tool use. Thus, we show that calling from a reflective surface is an alternative strategy for maximizing call loudness, and one that is available to all animals.