Toads prey upon scorpions and are resistant to their venom: A biological and ecological approach to scorpionism

Abstract

In recent years, SE Brazil, the most populous region in the country with an estimated population of 88 million, has been experiencing an alarming increase in scorpions accidents (scorpionism), mainly caused by the yellow scorpion (Tityus serrulatus), or "escorpião amarelo" in Portuguese. This species is considered particularly dangerous to humans and can reproduce by parthenogenesis favouring rapid dispersal and colonization of new environments. Since the 1940s, owing to the growing danger represented by scorpionism, public control policies have been developed, including active search for scorpions, together with the use of toxic substances applied in places most likely to serve as their refuges. Even so, the number of accidents is increasing year by year, presently at an alarming rate. It seems evident that the increase in accidents is directly (or primarily) related to the lack of predators that in healthy environmental conditions would naturally control scorpion populations. However, due to environmental changes, leading to a lack of predators, scorpions have been gradually invading the urban environment. Arachnids and insects in general, as well as some other invertebrates, are preyed upon by anuran amphibians (toads, frogs and tree frogs). Toads (family Bufonidae) are nocturnal, large, and highly voracious animals, capable of actively exploring extensive areas and consuming large numbers of insects and arachnids daily. One of the most common toad species in southeastern Brazil is Rhinella icterica. Both R. icterica and T. serrulatus inhabit the same nocturnal environment. The predatory action of toads, specifically on scorpions, is practically unknown from behavioural and toxinological points of view. Thus, we studied the predatory behaviour of this toad against the yellow scorpion and evaluated the resistance of the amphibian to scorpion venom. Our results show that R. icterica is a voracious predator of T. serrulatus and is extremely resistant to its venom. Human/toad relationship throughout western history has always been very conflicted and possibly one of the factors that most has contributed to human ignorance of the role of these amphibians in maintaining ecological balance. Presently, the control of scorpionism is being performed through active search and/or the use of chemical agents, although showing little efficacy in reducing human accidents. In the medium or long term, more effective actions taking into account the biology of scorpions and their predators have never been taken to reduce these accidents.

Keywords: Arms race; Predation; Rhinella icterica; Scorpion; Scorpionism; Tityus serrulatus; Toad.