Carotenoid coloration as a visual signal

Darwin (1871) hypothesized that elaborate, showy traits evolved to communicate quality in the sender, and that individuals with the most conspicuous of these ornaments benefit by early pairing and increased reproductive success.  Some of the most famous and striking ornaments involve the brightly colored plumage of birds, which may advertise a low parasite load and perhaps enhanced immune function.  In collaboration with the Fernbank Science Center, we are conducting a field study in Northern Cardinals, which are known for their bright red plumage.  Red coloration in animals comes from pigments called carotenoids, which also play an important role in the immune system in all animals. We tested the hypothesis that bright red males use their striking carotenioid-based plumage to advertise their superior health and low stress levels.

To assess the health status of these birds, we use a commonly used index of stress, called the “heterophil to lymphocyte ratio”. This measure is the ratio of two different kinds of white blood cells, and predicts the levels of stress hormones in all vertebrates, including humans. A high ratio indicates a high stress level. Contrary to our expectations, we found that males with more highly saturated red coloration had higher ratios, not lower ratios, than the duller males. In other words, the bright red males had the highest stress levels. So, the red coloration does not in fact advertise superior health and low stress. We now hypothesize that bright red males may experience more stress because they defend larger territories and may have to sing and fight more than duller males.

This project was funded by the Center for Behavioral Neuroscience and has provided hands-on research experience to high school students in the DeKalb County Public School system.

Scanned image of the breast of a male Northern Cardinal.  We are collecting data with a flatbed scanner as well as a spectrophotometer in order to test for correlations between plumage brightness and immune parameters.

All cardinals in this research are released unharmed.

Research