The Lizard Log

The Langkilde Lab in Action

A Frog of a Different Color: Sexually Dimorphic Color in Wood Frogs

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Jett Peng is currently a senior majoring in Biology and minoring in German at Penn State. As an international student from Taipei, Taiwan, he has been embracing the American university life by learning all about American sports, food, and culture. He hopes to apply his laboratory experiences and knowledge to a pharmaceutical setting in the future. Jett describes his experiences in the lab below:

Last semester, I applied and received an Eberly College of Science Undergraduate Research Grant for my research project dealing with sexual dimorphism in wood frogs. Sexual dimorphism is a physical difference between males and females of the same species. These differences can include color, size, structure, shape, behavior, and much more.


Two examples of sexual dimorphism: On the left, a male mallard (bottom) has a green head while a female mallard (top) has a brown head. On the right, the size difference between a female (left) and male (right) Argiope appensa. Photos from

Humans, for example, are sexually dimorphic between males and females with differences in hair, height, and muscle mass. A fascinating thing about sexual dimorphism is the various functions it can have. Some sexual dimorphisms can be used as a defense mechanisms, such as warning signs or camouflage, or they can be used as breeding signals. Much research is being done into understanding what the exact functions of sexual dimorphisms can be and how they came about. One step into further understanding the origins of sexual dimorphism is to know when sexual dimorphisms first develop in an organism, which is what my experiment focused on.

The goal of my experiment was to determine if wood frogs (Lithobates sylvaticus) developed sexual dimorphism as juveniles. I worked with Lindsey Swierk and Brad Carlson, two graduate students in the Langkilde Lab, to help analyze data to see if juvenile frogs were sexually dimorphic. Previous research has shown that wood frogs are sexually dimorphic with female frogs being redder than males.

Male wood frogs are more brown (left) and female wood frogs are more red (right).

Male wood frogs are more brown (left) and female wood frogs are more red (right).

Knowing when the frogs begin to show this color change could give more information on the function of dichromatism; for example, it could serve as a function to prepare the wood frogs to become a perfect color for the breeding season.

To see if the juvenile wood frogs were sexually dichromatic, I analyzed photos of the frogs that were taken 1, 3, and 5 months after they underwent metamorphosis, the developmental process by which tadpoles change into frogs. The photos were analyzed and compared in the computer program, Photoshop, to obtain Red, Green, and Blue (RGB) values. The RGB values are of great importance for this experiment because those are the primary colors that are used to make an array of colors depending on the proportion of each color.  Color samples were analyzed from the back of the frogs because this was the area where the colors were most vibrant and noticeable.


Color sample was taken by using the circle shape tool in Photoshop to select the back area and analyze it for RGB values.

Finding the RGB values on the back of the frog allowed us to know the exact proportions of each color present; we would expect females to have a higher red value than males. The frogs were euthanized and then dissected to confirm the sex of each.

Preliminary results from the photographs (examples below) showed that wood frogs do display sexual dichromatism as juveniles and that they began to express sexual dichromatism about 3 months after metamorphosis.


Photographs of a female wood frog taken from July (left), September (center), and November (right) of the juvenile stage.


Photographs of a male wood frog taken from July (left), September (center), and November (right) of the juvenile stage.

Females tended to be redder and bigger than the males, with the males being more brown and smaller. These results are a basic foundation that can be further used to investigate why the frogs begin to experience color changes at around 3 months. Could the color changes be due to preparation for the breeding season or advantageous timing as a defense mechanism? What are the color differences between a juvenile and adult frog? These are some of the follow-up questions we can address to understand the development and function of sexual dichromatism in wood frogs.

Overall, this experiment was a very good experience for me where I learned many scientific research techniques. This individual research project allowed me to gain some experience in what it was like to do research. This project helped me become more organized as there was no set “due date” for the project, but I had to work at my own pace. I had to make sure I was doing all the work I needed to do for each week, so I could have enough data and time to work on a poster presentation for the Undergraduate Exhibition. The exhibition allowed me to communicate my scientific work in words for others to understand, which was very interesting. I really enjoyed doing this experiment because I learned how to obtain, record, and interpret results in a professional way. I am looking forward to help writing a manuscript next semester for this project.

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