Anthropogenic factors, including the spread of endocrine disrupting chemicals, have been linked to alterations in the reproductive physiology, morphology, and behavior of wildlife. Although records of gonadal abnormalities in vertebrates have been increasing, we have a limited understanding of the effects of pollutants on secondary sexual traits. The larynx of many anurans (frogs and toads), for example, is larger in males than in females and is crucial for the production of mating calls. The only evidence that laryngeal development is impacted by chemical pollutants, however, comes from controlled laboratory experiments with captive bred frogs. Animals living in human-altered landscapes are frequently exposed to a varying cocktail of chemicals, limiting the ability of laboratory-based studies to predict changes in wild populations. Our study examined laryngeal morphology and a suite of other sexual traits in wild-caught cane toads (Rhinella marina) from sugarcane fields, which have high pesticide inputs, as well as from nearby urban areas in South Florida, USA. We aimed to determine if the male toad larynx is susceptible to the demasculinizing effects previously observed in sugarcane growing areas, and to investigate how laryngeal morphology is associated with the expression of other sexual traits.
Results/Conclusions
We found evidence of demasculinization in both primary and secondary male sexual traits, and revealed clear impacts of living in sugarcane fields on the cane toad larynx. Relative to body size, the laryngeal mass, vocal fold length, and dilator muscle thickness are all reduced in male toads from sugarcane growing areas compared to their urban counterparts. In the sugarcane areas, these three measures of larynx size are also significantly correlated with each other as well as with other primary and secondary sexual traits, such as the testes mass and nuptial pad size. These correlations among male sexual traits indicate that demasculinization occurs in concert both within and across diverse tissue types. Our results provide compelling evidence that anthropogenic disruption of reproductive systems extends to secondary sexual traits like the larynx, potentially altering mating signal production in wild animals.