Saturday, July 30, 2011

Raptors Avoid Snakes With Triangular Heads

Vipera latastei gaditana. Photo courtesy of Harold van der Ploeg.
Looking like a venomous species can have advantages for mimics, and a triangular-shaped head similar to that found in many vipers may convey a message to predators that says I am dangerous. Janne K. Valkonen, Ossi Nokelainen, and Johanna Mappes tested this idea using snake modles made of non-toxic grey plasticine. Four different kinds of artificial snakes were used: 1) zigzag-patterned snakes with triangular (viper type) heads and 2) with narrow (colubrid type) heads and 3) plain (grey) snakes with triangular heads and 4) with narrow heads. The use of plain snakes was crucial to separate the effect of the head shape from the overall appearance of vipers. The length and diameter of artificial models were identical in all treatments and in correspondence to the size Vipera latastei gaditana. The experiment was conducted in Coto DoƱana National Park, southern Spain. Six transects were conducted in 2009 and thirteen in 2010 and a total of 595 snake models were used. The replicas were placed on the natural background in random order at approximately 15 metre (15 paces) intervals following features of the terrain. The authors found a a significant difference in the number of raptor attacks among treatments. Plain snakes with narrow heads were attacked significantly more often by raptors than were plain snakes with triangular heads. There was no difference in the number of raptor attacks on patterned, triangular-headed snake replicas and on patterned replicas with narrow heads. When attack data was pooled on snake replicas based on their patterns, plain snakes were attacked by raptors significantly more frequently than were zigzag patterned replicas. 8.2% of the 595 snake replicas were attacked by raptors and 18.5% by mammalian predators. Attacks by mammalian predators did not differ between treatments.

Citation: Valkonen JK, Nokelainen O, Mappes J (2011) Antipredatory Function of Head Shape for Vipers and Their Mimics. PLoS ONE 6(7): e22272. doi:10.1371/journal.pone.0022272

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