A 1 REVIEW OF THE CHARACTERISTIC CRYPTIC COLORATION OF PENGUINS NOOR BECKWITH FRSEMR 21q: Biological Impostors Professor: Michael Canfield February 2008 Walking through Harvards Museum of Comparative Zoology one day, I came upon a case of organisms not unfamiliar to me, but which I had before only ever briefly considered: the maritime and littoral birds. As I scanned the skins for charismatic characteristics, I quickly observed a pattern. Where many of the other birds I had seen that day displayed brilliantly colored plumages, these species invariably displayed feathers of mainly black and white. The tuxedoed penguins in particular caught my attention, and, suspecting crypsis, Ive chosen to investigate this group further. Penguins (order Spenisciformes) are gregarious flightless birds evolved to live in frigid waters and on frigid lands (Burnie 2001). Their adaptations include a fatty torso and wings modified into flippers, but the most immediately recognizable characteristic of a penguin is its coloration. Being so conspicuous, and epitomizing a coloration pattern that is widespread, evolutionary biologists have considered its origin and function over a hundred years ago. Abbott Handerson Thayer, working towards the end of the 19th century, was one of the first to theorize and experiment rigorously on this general pattern. His conclusion was that dark colors on the dorsum of a given animal gradating towards light colors ventrally help to disrupt a potential predators perceptions of solidity and color harmony (Poulton 1902), and this cryptic pattern was dubbed ,,countershading.
2 Though other proposed factors that could select for specific bird feather colorations have been argued (appearing like the sky from below and the water from above, UV blocking, etc), the theory of protective countershading is widely accepted today (Bretagnolle 1993), and penguins serve as an obvious case study for its explanation in popular and academic settings alike. At the same time, while this theory has enjoyed dominance since its inception, empirical substantiation has been scarce, and it is only recently that studies are beginning to demonstrate quantitatively that countershading can confer a selective advantage. These studies, however, tend to indicate that countershaded targets enjoy only variable protection (Speed et 2004), al. suggesting that the specialized coloration of penguins may have been selected for by other forces. Two notable papers that I have come across each suggest a different selective for that could cause penguin plumage and coloration to evolve. One paper details an intraspecific mimicry apart from any aforementioned cryptic functionality. It describes juveniles that molt early in order to mimic the adults of their species and thus be allowed higher priority in communal feeding (Ryan et al. 1987). Another paper describes a strong correlation between facial coloration patch prominence and breeding success (Jouventin 2008). This first paper suggests for me the demonstrable selective forces of intraspecific competition and cheating on plumage, and the second paper shows that penguin color is affected by sexual selection. Despite these various selective forces, it is hard to argue that the tuxedoed appearance of penguins could not serve a cryptic purpose and confer a selective advantage. So what these other ideas ought to highlight is the integrated nature of organism. Looking at a penguin, one should not allow oneself to say, "Those colors are for this one function and they came about for the purpose of maximizing that one functionality." Instead, recognition of the manifold ecological
3 and chemical interactions that have sculpted the present should enable an appreciation of the manifold processes like cypsis, mimicry, and sexual signaling at work in a given trait.
References Jouventin, P., P. M. Nolan, F. S. Dobson and M. Nicolaus. 2008. Coloured patches influence pairing rate in King Penguins. Ibis 150:193-196. Speed, M. P., D. J. Kelly, A. M. Davidson and G. D. Ruxton. 2004. Countershading enhances crypsis with some bird species but not others. Behavioral Ecology 16(2):327-334. Burnie, D., Wilson, D. E., editors. Animal: the definitive visual guide to the worlds wildlife. New York: DK; 2001. 624 p. Bretagnolle, V. 1993. Adaptive significance of seabird coloration: the case of procellariiforms. American Naturalist 142:141173. Ryan, P. G., R. P. Wilson, and J. Cooper. 1987. Intraspecific mimicry and status signals in juvenile African penguins. Behavioral Ecology and Sociobiology 20:69-76. Poulton, E. B. 1902. The Meaning of the White Under Sides of Animals. Nature (London) 65:596-597.