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 Reference ARNOLD and OVENDEN2002, Hailey & Davies Reference HAILEY and DAVIES1986, Young et al. Reference YOUNG, LAOR and SOLOMON1999). Using plasticine models, Guimarães & Sawaya (Reference GUIMARÃES and SAWAYA2011) 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. Guimarães & Sawaya (Reference GUIMARÃES and SAWAYA2011) 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 Doñana National Park in southern Spain (Valkonen et al. Reference VALKONEN, NOKELAINEN and MAPPES2011a). In contrast to the findings of Guimarães & Sawaya (Reference GUIMARÃES and SAWAYA2011), 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, colubrid-like heads suffered significantly higher predation by raptors than snakes with triangular-shaped heads. The number of attacks by mammalian predators was generally higher than raptor attacks but there was no difference in attack rate between treatments. During the experiments, we often observed mammalian predators (e.g. red fox, Vulpes vulpes Linn.) following our tracks along the transect lines and non-selectively biting almost all snake replicas in the area.
Based on experience and observations from experiments with similar methods (Niskanen & Mappes Reference NISKANEN and MAPPES2005, Valkonen et al. Reference VALKONEN, NOKELAINEN and MAPPES2011a, Reference VALKONEN, NISKANEN, BJÖRKLUND and MAPPES2011b), we argue that the ‘attacks’ by mammalian predators on plasticine snake replicas do not reflect true predation behaviour on snakes but rather curiosity and/or mineral- or fat-seeking. Thus, it is important not to pool data from different types of predators but to analyse mammalian and avian attacks separately. Although many mammals are likely important predators of snakes and they have played an important role in the evolution of snakes' antipredatory strategies, the use of plasticine may not be a valid method for testing the survival value of visual signals against mammalian predators.
