Date of Award


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Biological Sciences


colour change, dynamic sexual dichromatism, hormones, Incilius luetkenii, Neotropical yellow toad, sexual selection


Stephanie Doucet


Daniel Mennill



Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.


Animals use colouration to serve diverse functions, including camouflage, thermoregulation, and communication. In most cases, animal colour signals are long-lasting or permanent. Yet some animals can change their colour on short time scales to adapt to their surroundings or to communicate with others. Anurans have long been known to moderately modify their colouration in response to light, visual background, and temperature, but recent research has revealed this group to be capable of a variety of pronounced colour changes, and growing evidence supports the idea that these changes are sexually selected signals. Male yellow toads, Incilius luetkenii, exhibit dynamic sexual dichromatism, changing from brown to yellow during explosive breeding events which typically last just a few days at the start of the rainy season in Central America. Males vary in their yellow colour, yet no study has investigated whether this variation could play a role in sexual selection or focused on determining the hormonal mechanisms responsible for this change in colour. The goal of my dissertation research is to investigate the proximate mechanisms and ultimate functions driving the evolution of dynamic sexual dichromatism in yellow toads. In my first data chapter (Chapter 2) I explore how social environment and hormones interact to influence male colour by placing male toads into one of four social treatments and measuring colour and serum corticosterone and testosterone before and after treatment. I found that males held with conspecific animals were the brightest and showed little or no change in their corticosterone levels; duller toads had the highest levels of corticosterone; and toads were duller yellow and exhibited greater levels of corticosterone post-treatment across subsequent treatment days at the onset of the rainy season. This study reveals that both conspecific interactions and corticosterone are involved in the dynamic colour change of yellow toads. In my second data chapter (Chapter 3) I test the hypothesis that acute stress impacts yellow colouration and corticosterone levels in males by confining toads to small terrariums for 4 hours and measuring colour and serum corticosterone before and after isolation. I found that toads grew darker during isolation, but that corticosterone levels did not change with colour, indicating that this hormone is not the main driver of colour change in yellow toads. In my third data chapter (Chapter 4) I explore whether females differentiate between males based on their colouration by quantifying their interactions with two hyper-realistic robotic model toads, with one model painted to match a bright yellow male and the other a dull yellow male. I found that females did not show a preference for bright or dull colouration, suggesting that female choice is not a primary driver of male yellow colouration in this species. In my fourth data chapter (Chapter 5) I investigate male-male competition strategies, in particular whether males selectively compete with rival males in amplexus with a female versus non-amplectant (single) males, and whether males selectively interact with rivals based on their colouration by conducting a two-choice arena experiment featuring hyper-realistic robotic model toads. I found that variation in male yellow colour, and the presence of a female, do not influence male-male competitive behaviour. In the final data chapter of my dissertation (Chapter 6), I describe the blood parasites found in yellow toads and examine whether parasites and body condition are correlated with yellow colouration in males. I detail four types of blood parasites found in this species, which appear to influence sexual traits in contrasting ways. Collectively, my research suggests that variation in colour is driven, in part, by environmental cues and parasitism, and my findings support previous work in this species that dynamic sexual dichromatism is a sex identification signal. Taken together, my data chapters provide new insight into the phenomenon of dynamic sexual dichromatism in anuran amphibians and reveal the complexities of this trait.

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