Naphthenic Acids Disrupt Courtship in Silurana tropicalis

Abstract

Neuroendocrine processes coordinate the behavioural, physiological, and seasonal aspects of reproduction. Some chemicals can disrupt the hypothalamus-pituitary-gonadal axis, impacting reproductive health. Naphthenic acids (NAs), the carboxylic acids in petroleum, are of emerging concern as they contaminate coastlines after oil spills and aquatic ecosystems of the Athabasca oil sands area. They are acutely toxic in fish and tadpoles and possibly endocrine disrupting at sublethal levels. I characterized courtship behaviours and disruption by NAs in the Western clawed frog, Silurana tropicalis. Courtship primarily consists of males producing low trills and achieving amplexus, a mating position where a male clasps a female. Adult frogs were exposed for five days to 20 mg/L NA, a dose low enough to not affect physical activity. In males, absolute calling activity was reduced. Other acoustic parameters such as dominant frequency, click rate, and trill length were not affected. Injecting human chorionic gonadotropin had a slight rescue effect. Vocalization and amplexus were both inhibited after exposure and restored after 2 weeks of recovery. However, calling behaviour did not predict competitive ability or mating success. In females, NA exposure reduced mating success, possibly through decreased attractiveness or receptivity. Receptivity can be indicated by attraction towards the sound of mating calls (phonotaxis), which is cryptic and subjective. I created an apparatus that measures phonotaxis by placing speakers inside traps with infrared lights to detect the time of entry. This novel method is widely applicable for low-visibility observations and studies of choice and preference. This work shows that an aquatic contaminant can reduce mating success in otherwise healthy frogs, and provides a detailed foundation for further investigation.