TDK conference 2017

Glyphosate-based herbicides are among the most widely used pesticides in agriculture. Many previous studies documented lethal or sub-lethal malign effects on larval anuran amphibians. However, an intriguing observation is that phenotypic changes observable upon exposure to the herbicide may resemble predator-induced responses. Our aim was to verify this observation and to test whether herbicide-induced changes may deliver an advantage to tadpoles of the agile frog (Rana dalmatina) when facing free-ranging predators. We raised tadpoles in outdoor mesocosms in the presence or absence of the herbicide (0 or 4 mg a.e./L glyphosate) and of predators (empty cage or caged larval dragonfly, Aeshna cyanea). After three weeks we observed hiding behavior of tadpoles, and subsequently removed individuals, photographed them, and transferred them to test boxes, where we performed bioassays using free-ranging predators. After seven days, we photographed survivors again, and identified them individually to determine the rearing environment they originated from. We assessed morphological changes attributable to the rearing environment by measuring six body shape variables based on the photographs. Our results showed that exposure to caged predators, but not to the herbicide, resulted in significant changes in several tadpole body shape measures. Nonetheless, tadpoles showed increased hiding behavior in the presence of the herbicide, even though this increase was not as high as in the presence of caged predators. Furthermore, when exposed to free-ranging predators, tadpoles that had developed in the presence of predators enjoyed elevated survival probability compared to predator-naive conspecifics, irrespective of herbicide presence. Finally, herbicide-exposure during development lead to enhanced survival of tadpoles in bioassays performed in herbicide-free water, while in tadpoles exposed to free-ranging predators in herbicide-contaminated water, it was the individuals that first came into contact with the herbicide upon entering the bioassays that had elevated survival. It appears that exposure to the herbicide can affect tadpole survival in complex and non-trivial ways, in which the effect on behavior may play an influential role. Therefore, further investigations are needed to clarify the mechanisms that underlie these results.