Feathers at a fine scale

Abstract

In 1668, Antoine von Leeuwenhoek improved the crude microscopes that were being produced in Europe to better study small biological objects (Madigan et al. 1997). Although von Leeuwenhoek 's microscope revolutionized biology, helping to support among other things cell theory and germs as the cause of disease, the potential of the microscope escaped the attention of most ornithologists. A small group of researchers has used microscopes to study feathers since the late 19th and early 20th centuries (for historical reviews, see Fox 1976, Prum 1999), but it was not until very recently that a consideration of feathers and the organisms on them has been united with traditional studies of macro colors and structures. The result is a new appreciation of the importance of the bacterial flora of feathers and their potential to serve as a selective force that can affect the colors of feathers. Microbes were isolated from feathers more than 40 years ago (e.g. Gierløff et al. 1961, Pugh and Evans 1970a,b), but the study of feather bacteria remained largely dormant until Burtt and Ichida (1999) isolated feather-degrading Bacillus spp. from the feathers of several species. Shawkey et al. (2003a) subsequently cultured thirteen distinct isolates from the feathers of house finches (Carpodacus mexicanus). More comprehensive surveys using both culture-based and culture-independent methods (see Amann et al. 1995 for a review of these methods and their importance in detecting microbial diversity) have revealed even greater microbial diversity on feathers (Shawkey et al. in review). While it is now clear that feathers are capable of harboring a diverse microflora, the ecological role(s) of that microflora remain largely a mystery. In this issue of the Auk, Goldstein et al. (2004) take us forward in our understanding of those roles. Using standard microbiological methods, they demonstrated that feather-degrading bacteria degrade unmelanized white feathers more quickly and completely than melanized black feathers in vitro. These data, along with those in Burtt and Ichida (in press) suggest that melanin may protect feathers against such bacterial degradation and that many patterns of melanin-based coloration might have evolved in response to bacterial parasitism. Melanized feathers have previously been shown to be harder and more resistant to abrasion than unmelanized feathers (e.g. Burtt 1979, 1986, Bonser 1995, but see Butler and Johnson 2004), but this study is the first to explicitly demonstrate an enhanced resistance to bacterial degradation in melanized feathers. Melanin-based plumage is used in social signaling (e.g. Rohwer and Rohwer 1978) and may also be involved in thermoregulation (Walsberg 1983) and crypsis (Wallace 1889, Zink and Remsen 1986). The findings of Goldstein et al. (2004) suggest that resistance to the degrading effects of bacteria is another important function of melanin, and this observation may have important implications for the evolution of plumage color.