Exploring efficiency landscapes in the acoustic-morphospace of crickets reveals two alternative calling strategies

Abstract

Male crickets attract females by producing calls with their forewings. Louder calls travel further and are more effective at attracting mates. However, crickets are much smaller than the wavelength of their call, and this limits their power output. 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 112 cricket species across 7 phylogenetic clades. We found that all sampled crickets, in all conditions, could gain efficiency from tool use. Surprisingly, 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 an 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 feature in acoustic communication. Animals attracting mates or warding off predators are expected to make themselves as loud as possible. Two long-standing, seemingly unrelated unsolved problems regarding loudness in the field of animal communication are: the rarity of acoustic tool use, and animals that call from reflective ground-like surfaces, known to be an impediment to sound propagation. These two ideas are related; by refocusing analysis from sound propagation to sound radiation, we show that the ground is not an impediment, but rather an acoustic aid that can boost loudness more than tool use. We also show that calling from a reflective surface is an alternative strategy to maximize call loudness that is available to all animals.