Oscar deserving performance: death-feigning in dice snakes

Abstract

Dice snakes (Natrix tessellata) display an extensive repertoire of antipredator behaviors. They flee when they detect an attacker, and if captured, they vigorously struggle to evade. They spin their body, while spraying the foe with a malodorous mixture of cloacal content and cloacal gland secretions. Ultimately, dice snakes can switch to death-feigning (DF, thanatosis), i.e. tonic immobility in inverted position; usually opening their mouth, protruding their tongue, and sometimes producing bubbles with blood and saliva. However DF is a dangerous option since vital organs, like the liver, are exposed to the predator. We examined antipredator behaviors triggered by human observers, comparing capture-induced DF among five populations. DF prevalence ranged from 6.4% to 31.5%, probably due to different local predator pressures. We found no effects of dorsal coloration pattern or sex on DF frequencies. In Golem Grad population, a large sample (N=1,909) showed that DF frequency increased with body size. This might indicate to protective effect of DF from gape-limited predators. DF was three times less frequent in gravid than in non-gravid females. In another experiment we compared duration of DF in water and on land, after half a minute long standardized manipulation. Snakes released in container with water significantly decreased DF duration comparing to those released on land. Overall, different factors influence the occurrence and duration of DF, suggesting that this “desperate” attempt is under selection.