Research Interests

Methods

Projects

Publications

CV

RESEARCH INTERESTS

My research focuses on understanding how elaborate animal colors have evolved. I use a variety of techniques including ancestral state reconstruction, reflectance spectrometry, and visual modeling. 

I am currently using New World orioles (genus Icterus) as a model system to investigate color evolution. Most orioles range in color from bright yellow to scarlet, although a few orioles have rust or gold plumage. New World orioles also have a variety of patterns and a range sexual color dimorphism (dichromatism). Most importantly, orioles have a well-resolved molecular phylogeny that is supported by both mitochondrial DNA and nuclear introns. With this phylogeny, I am able to investigate and compare how color has changed across the entire genus. 

Currently, my major foci are: investigating the evolution of melanin-based colors (specifically phaeomelanins) in orioles, reconstructing male and female color evolution, and investigating sexual dichromatism. 

METHODS

In order to investigate color evolution in New World orioles, I use a reflectance spectrometer to quantitatively score color plumage. Reflectance spectrometers provide several advantages over visually based methods of scoring color in that they are quantitative and unbiased by any particular visual system. The quantitative data from the spectrometer allows me to detect subtle color differences between closely related orioles, differences that might have otherwise remained undetected.

Since my research uses a phylogenetic approach to study color changes across large numbers orioles, most of my measurements are taken from museum specimens.

RESEARCH FOCI

Melanin Coloration in Orioles

In passerine birds, two major pigment classes contribute to elaborate plumage: carotenoids and melanins. These two different pigments are produced by very different physiological mechanisms. Carotenoids cannot be synthesized by any animals (although they can be modified, e.g., a yellow pigment can be converted to a red one), while the two forms of melanins, eumelanin and phaeomelanin, can be synthesized. In many cases, carotenoid and melanin-based colors signal different things such as health (carotenoids) vs. status (melanins). Although recent research has suggested that these different roles are not as clear cut as initially thought. 

Within the oriole genus, most males have bright yellow, orange, or scarlet coloration. However, a few orioles have chestnut, or ochre colors. Male Orchard Orioles (I. spurius spurius) are one example. Not only do adult male Orchard Orioles look different than most other male orioles, they also look different from lemon yellow female and yearling male conspecifics. 

Using a combination of spectrometry and biochemical analysis, we were able to determine that color in adult male Orchard Orioles was produced by a high concentration of phaeomelanin pigments. Yearling male and female Orchard Oriole color is produced by a single carotenoid pigment (lutein). Surprisingly adult male Orchard Orioles had similar concentrations of lutein in their feathers, which were being masked by the high concentration phaeomelanin pigments. Also surprising was the finding that some orioles, such as the closely related Fuertes’s Oriole (I. fuertesi) use a combination of both pigments to produce a unique ochre color. When we examined color across the entire genus we found that there appeared to be several independent gains of these melanin based colors. However, these melanin colors only occurred in one of three major clades (groups) within the oriole genus.

Reconstructing Male Color Evolution

Most phylogenetic studies that address color evolution have focused on the presence or absence of discrete ornaments, such as carotenoid-based bills or epaulets, rather than differences in the coloration of particular structures across taxa. The few studies that have attempted to reconstruct evolutionary changes in color define character states in very broad terms, such as red and yellow or carotenoid and phaeomelanin. In orioles, color appears to vary continuously from yellow to scarlet, suggesting that such discrete measures of color coding might not be appropriate. Thus, we have quantitatively measured oriole color across the entire genus. We then used these measurements to reconstruct evolutionary changes in color.

Our findings suggest that oriole color is extremely labile, and that large changes in color can occur between closely related orioles. However despite this lability, there is still an overall tendency for closely related orioles to resemble each other more than would be expected due to chance. Also, we did not find any evidence for directional evolution in color. Instead, our results also suggest that the ancestral oriole had a middle-wavelength (orange) color with multiple changes to longer- (redder) and shorter- wavelength (more yellow) colors.

Evolution of Sexual Dichromatism

Many birds display differences in color between males and females (sexual dichromatism). Historically, studies of sexual dichromatism have focused on the elaboration of male traits rather than changes in females. However, a phylogenetic perspective reveals that several different pathways may lead to dichromatism, including the loss of elaborate female coloration. Previous research has suggested that such a loss may have occurred in orioles. 

To address this issue, we are scoring female color plumage using the same methods that we used to score and reconstruct male color plumage. We will then reconstruct female color evolution and investigate whether female color has the same degree of lability and tendency for closely related species to resemble each other that was found in male coloration. We will also investigate how carotenoid- and melanin-based (eumelanin) plumages differ between monochromatic and dichromatic species and examine which components of color (e.g., those corresponding to hue vs. saturation or brightness) change in species that are dichromatic. Addressing these questions should provide a better understanding of the evolution of sexual dichromatism. 

Male (L) and female (R) Altamira Orioles (I. gularis), a monochromatic species with elaborate female coloration. 

PUBLICATIONS

Hofmann, C., W.-S. Lo, C.-T. Yao and S.-H. Li. 2007. Cryptic sexual dichromatism occurs across multiple types of plumage in the Green-backed Tit. Ibis: In Press PDF

Hofmann, C. M., K. J. McGraw, T. W. Cronin, and K. E. Omland. 2007. Melanin coloration in New World orioles I: carotenoid masking and pigment dichromatism in the orchard oriole complex. Journal of Avian Biology: 38:163-171 PDF

Hofmann, C. M., T. W. Cronin, and K. E. Omland. 2007. Melanin coloration in New World orioles II: ancestral state reconstruction reveals lability in the use of carotenoids and phaeomelanins. Journal of Avian Biology: 38:172-181. PDF

Hofmann, C. M., T. W. Cronin, and K. E. Omland. 2006. Using Spectral Data to Reconstruct Evolutionary Changes in Coloration: Carotenoid Color Evolution in New World Orioles. Evolution 60:1680-1691. PDF

Omland, K. E., and C. M. Hofmann. 2006. Adding color to the past: ancestral state reconstruction of bird coloration in G. E. Hill and K. J. McGraw, eds. Bird Coloration. II. Function and evolution. Harvard Univ. Press.

We are supported by the National Science Foundation CAREER DEB-0347083. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
Last updated: 04/01/2006.