The snake hiss: potential acoustic mimicry in a viper-colubrid complex

Examples of acoustic Batesian mimicry are scarce, in contrast to visual mimicry. Here we describe a potential case of acoustic mimicry of a venomous viper model by harmless viperine snakes (colubrid). Viperine snakes resemble vipers in size, shape, colour, pattern, and anti-predatory behaviours, including head flattening, false strikes, and hissing. We sought to investigate whether hissing evolved as part of, or separately to, the viper mimic syndrome. To do this, we recorded and analysed the hissing sounds of several individual asp vipers, viperine snakes, and grass snakes (a close relative of viperine snakes that hisses but does not mimic the asp viper). Frequencies consistently ranged from 40 to 12 000 Hz across species and individuals. All vipers (100%) and most viperine snakes (84%) produced inhalation hissing sounds, in comparison to only 25% of grass snakes. Inhalation hissing sounds lasted longer in vipers than in viperine snakes. The hissing-sound composition of grass snakes differed significantly from that of both asp vipers and viperine snakes; however, the hissing-sound composition between viperine snakes and asp vipers was not statistically distinguishable. Whilst grass snake hissing sounds were characterized by high frequencies (5000-10 000 Hz), both vipers and viperine snake hissing sounds were dominated by low frequencies . A principal component analysis revealed no overlap between grass snakes and vipers, but important overlaps between viperine snakes and vipers, and between viperine snakes and grass snakes. The likelihood that these overlaps respectively reflect natural selection for Batesian mimicry and phylogeny constraints is discussed.