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RESEARCH PRODUCT

Comments on Guimarães & Sawaya. Pretending to be venomous: is a snake's head shape a trustworthy signal to a predator?

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Several species of non-venomous snake are known to flatten their heads when disturbed, and this behaviour has been suggested to be a mimicry of vipers (Arnold & Ovenden 2002, Hailey & Davies 1986, Young et al. 1999). Using plasticine models, Guimaraes & Sawaya (2011) tested the antipredatory function of a triangular head shape in snakes. Their article presents the first published empirical experiment testing the adaptive significance of vipers’ triangular head shape. Guimaraes & Sawaya (2011) found no support for the viper mimicry hypothesis. Accordingly, they concluded that ‘the shape of [the] head seemed not to confer advantage itself’. Although the use of plasticine models is a generally accepted method of testing predation pressure on snakes, we argue that the experiment may have failed to find the antipredatory function of triangulation due to the pooling of attacks by mammalian and avian predators. Mammals generally rely on olfactory cues during foraging. Plasticine has a strong odour which does not resemble the odour of any prey species. It is thus unlikely that mammals would treat snake replicas as true snakes. We also conducted experiments testing the antipredatory function of the wide, viperid head shape in Coto Donana National Park in southern Spain (Valkonen et al. 2011a). In contrast to the findings of Guimaraes & Sawaya (2011), our results demonstrate that the triangular head shape typical of vipers can act as a warning signal to predators. We compared the predation pressure by free-ranging predators on plasticine snake replicas with triangular-shaped heads to the pressure on replicas with narrow heads. Plain snakes with narrower,