The Repository @ St. Cloud State

Open Access Knowledge and Scholarship

Date of Award


Culminating Project Type




Degree Name

Biological Sciences - Ecology and Natural Resources: M.S.




College of Science and Engineering

First Advisor

Jennifer Y. Lamb

Second Advisor

Matthew P. Davis

Third Advisor

Sarah Gibson

Fourth Advisor

Lindsey Swierk

Creative Commons License

Creative Commons License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

Keywords and Subject Headings

American Toad, Anaxyrus americanus, Sexual dimorphism, color, paratoid, cranial crest


There are few published studies that have examined sexual dimorphism of either color or head morphology in temperate anurans. This has resulted in large gaps in our knowledge. In this study I ask whether there achromatic color, chromatic color, and head morphology are sexually dimorphic in American Toads (Anaxyrus americanus). Additionally, I ask whether achromatic and chromatic color vary seasonally and between the dorsal and lateral surfaces. To answer these questions I photographed the dorsal and lateral surfaces of 134 American Toads from populations at Lake Maria State Park, Wright Co., Minnesota (76 phenotypic females, 58 phenotypic males). I photographed these toads during the peak and the post-breeding seasons. I collected chromatic data including normalized red, green, and blue reflectance values, average Chroma, and average Hue, from multispectral images. I also collected achromatic data including average brightness, mean mottling score, and percent area covered by dark pigment. Chromatic and achromatic data were analyzed with multiple linear regression models that included phenotypic sex, season, and the interaction of phenotypic sex and size (SVL) as predictor variables. To examine head morphology, I placed 25 landmarks on the dorsal surface of phenotypic females and males. I used a Procrustes Analysis of Variance (ANOVA) to compare the average head shape of phenotypic females with that of males. I also compared the distribution of head shapes with a principal component analysis and calculated the lengths and widths of paratoid glands. There was no evidence of sexual dichromatism of achromatic or chromatic color on either the dorsal or lateral surfaces. Larger toads, regardless of sex, had higher mottling scores and thus more complex dorsal patterns. There is evidence of seasonal variation in achromatic color on the dorsal surface. Toads inside the breeding season are brighter and have less percent area covered by dark pigment. There is an interaction between snout-vent length (SVL) and achromatic variables on the lateral surface. Larger toads are less bright and have a greater proportion of their lateral surface covered by dark pigment compared to smaller toads. Although these surfaces were not dimorphic in color, head morphology was sexually dimorphic. Phenotypic females have longer and wider paratoid glands compared to males, and phenotypic males have more pointed rostrums. There are multiple hypotheses which could explain the patterns observed in these data. The variation observed in dorsal and lateral patterns may help individuals avoid predation, identify conspecifics, or have better visual perception in certain environments. Sexual dimorphism of head morphology may be due to niche partitioning or male to male competition. My studies were not designed to identify the causal factors behind these patterns, and each of these potential explanations warrants further study. This is one of the first studies to test for sexual dimorphism of color on the lateral surface of a toad. It is also one of few which examines sexual dimorphism of head shape in a temperate bufonid. This study lays the groundwork for future investigators to address questions related to variation in achromatic and chromatic color, as well as head morphology, in temperate anurans.


I would like to thank my primary advisor Dr. Jen Lamb (SCSU). Dr. Lamb has supported me throughout this project and helped to ensure that this thesis was completed in a way that I could be proud of it. I am grateful for the endless conversations that we had about toads and the never-ending potential questions to be asked. I have grown tremendously from this experience and through her support was able to ask questions that I am passionate about. I would like to thank my committee members Dr. Althea Archer (USGS), Dr. Matt Davis (SCSU), Dr. Sarah Gibson (SCSU), and Dr. Lindsey Swierk (Binghamton University). My committee members were all essential in supporting me throughout this project and brought their own skills to the table to make this project what it is. Dr. Archer ‘s knowledge of statistics was vital and I am appreciative of how much I have learned on that front. Dr. Davis’ knowledge of geometric morphometrics and as an expert outside of the field of herpetology were important in bringing up details I might not have considered. Dr. Gibson’s editing abilities and knowledge as an expert outside of the field of herpetology was immensely appreciated. Dr. Swierk’s endless knowledge of color and herpetology were crucial, I am infinitely grateful that I can continue to learn from someone who is so curious.

I would like to thank all Lamb Lab members, current and past, who assisted on this project, including Kerri Berrs (SCSU) and Bailey Richards (Fond du lac Reservation). To Simon Everest who supported me throughout this project, the accomplishments and the obstacles. I still cannot believe that you helped out in the field, even if it was only one day. Thank you to my family for your support throughout my education, it has always been bigger than me. To my mentor Dr. Fredric Janzen, thank you for believing in me and my ability to take up space in this field.

This work was funded by the St. Cloud State University’s Student-Mentor Collaboration Grant program and a 2022 Minnesota Herpetological Society Grant. Specimen collection and handling were permitted by SCSU IACUC Permit (17-130, Appendix A) and Minnesota Parks and Trails Permit (202239).

Available for download on Wednesday, April 24, 2024