Trans-generational effects of ivermectin exposure in dung beetles

Ivermectin is a powerful antiparasitic drug commonly used in cattle. Ivermectin residues are excreted in dung, threatening non-target coprophagous fauna such as dung beetles. This can have severe ecological and economic consequences for dung degradation and soil fertility. Even though the negative effects of direct ivermectin exposure on dung-degrading organisms are well known, effects could extend across generations. Here, we tested the effects of paternal or maternal exposure to ivermectin on offspring in the dung beetle Euoniticellus intermedius. This species is a classic study subject in ecotoxicology and sexual selection because males have a cephalic horn that is under intense selection via male-male competition. After confirming a negative effect of ivermectin on the number of emerged beetles, we found trans-generational effects of ivermectin exposure on the horn size of male offspring. Surprisingly however, this trans-generational effect only occurred when only the father was exposed. We detected no trans-generational effects of ivermectin exposure on offspring number, sex ratio or body size. Our results confirm that ivermectin not only has a strong effect on exposed individuals but also in their progeny. Our study opens new questions about the mechanisms responsible for parental effects and their long-term fitness consequences in contaminated habitats.