No gain from pain: lack of nociceptor activation suggests no adaptation for defence in the evolution of European viper venoms

• Snake venoms show variability in composition at all taxonomic levels, both within and between species, and the selective pressures underlying this variation have long been debated.
• Since snake venoms are used for both foraging and defence, we hypothesise that venom composition may represent a trade-off between lethal/immobilising predatory toxins and pain-inducing defensive toxins. Snake species feeding on weakly defended prey, such as insects, may experience relaxed selection pressure on the ability of their venoms to immobilise prey and thus may be able to invest more resources into developing a defensive function against their own predators, compared to snakes that feed on well-defended prey, such as rodents.
• Here, we use the trophically diverse Eurasian viper genus Vipera to test this hypothesis. Using Ca²⁺ imaging of mammalian sensory neuron cells, we tested the nociceptor-activating (pain-inducing) capacity of the venoms of populations of the largely insect-eating Vipera ursinii complex, and of the primarily mammal-eating V. ammodytes, V. aspis and V. berus.
• Although we found considerable variation in venom composition among the tested species, none of the venoms caused activation of sensory neuron cells, irrespective of diet.
• We conclude that selection for defence is unlikely to have played a significant role in driving the evolution of venom composition in Vipera, and that therefore there is no trade-off between a predatory and a defensive function in their venoms.