For a biologist in the making, the most exciting aspect of traveling is observing the huge diversity of organisms found on earth. However, making sense of this diversity—why it exists, how it is organized, and how to remember the name of that bizarre orange spider you saw just three days ago—is a central challenge in biology. A first step in the process of grappling with huge volumes of biodiversity is realizing how similar two organisms from different corners of the world can be. Traveling in the southern U.S., South Africa, India, and Mexico over the last year, I have seen more cool creatures and funky biological phenomena than I could have hoped for. The pattern that struck me most forcefully, however, was how the similarities kept cropping up, despite a conscious effort on my part to appreciate different ecosystems for their uniqueness.
Consider the example of fish-eating birds of prey. The African Fish Eagle (Haliaeetus vocifer) is a fairly common sight around the waterholes of Kruger National Park in north-east South Africa. The striking contrast between its black-and-brown body and bright white head makes this fish eagle easy to distinguish from other raptors. When, upon returning home to India, I saw a similarly patterned and colored bird-of-prey by a large pond, I dismissed it as yet another fish eagle. This turned out to be a mistake, since there are no fish eagles in the interior of India. It turned out that I had seen a Brahminy Kites (Haliastur indus), and my mistake was justifiable, for Brahminy Kites are also brown-bodied, white-headed birds of prey that are found mostly near large bodies of water and whose diet includes fish. And then in Baja California, Mexico, a few weeks later, I saw two ospreys (Pandion haliaetus) perched on sand dunes by the beach—brown-bodied, white-headed, fish-eating birds of prey yet again! And if I tell you that bald eagles (Haliaeetus leucocephalus) are also fish-eating raptors, I bet you can guess exactly what they look like, even if you haven’t seen one.
The observant reader would have noticed that at least two of the four raptors discussed above are closely related: the African Fish Eagle and the Bald Eagle share the first part of their scientific name, denoting that they belong to the same genus, Haliaeetus. This observation points to one reason that two species can look similar—the two species might have inherited the properties they share from their common ancestor. Thinking about shared ancestry is essentially like thinking about why you and your brother might look and behave similarly, except on an evolutionary time scale.
So does that mean that all species of fish-eating raptors are closely related, and both the behavior of eating fish (or piscivory) and the color patterns of a white head and brown body evolved just once? How does one go about answering such a question? One turns to a phylogeny—a branching “tree” that represents a hypothesis for how a group of species are related to each other. These hypothesis trees are constructed from data that describe the species of interest in some way. One could collect morphological data (do these species of butterflies have spotted or striped wings?), behavioral data (are they active at dawn or dusk?), or, as is now ubiquitous, DNA sequence data (do they have the nucleotide adenine or nucleotide guanine at a particular spot in a particular gene?). One would then compare these data for different species, and group species by similarity, inferring that more similar species are more closely related.
It is easy to see that if we created a phylogeny of raptors using either preferred diet or coloration to group species, we would readily conclude that the fish-eating birds-of-prey are all closely related to each other. Why is this a problem? Consider the fact that both fish-eagles and kingfishers eat fish; I am certain you will agree that fish-eagles are likely more closely related to other eagles than to kingfishers. This implies that piscivory evolved independently in these two groups of birds. Similarly, it is conceivable that the evolution of piscivory occurred independently in the African Fish Eagle and the Brahminy Kite, and the two raptors may not be closely related at all. But, you counter, these two species both look AND behave similarly (while kingfishers look nothing like fish eagles), so maybe they are closely related after all. Suppose, however, that raptors with white heads are more successful at catching fish than raptors with brown heads. One would then expect this color pattern to evolve by natural selection (perhaps independently) in all fish-eating birds of prey. One would therefore not be surprised if our examples of white-headed fish-eating raptors were in fact not closely related to each other. This reason for two species looking similar—that they experience similar pressures from natural selection leading to morphological or behavioral similarity—is known as convergent evolution.
It turns out that the genus Haliaeetus comprises eight species of fish-eating eagles, seven of which have brown bodies and white heads. The Brahminy Kite, Haliastur indus, is not very closely related to these fish-eagles, but is not too far either. Look at the phylogeny, derived from DNA sequence data1 and trace the branch back from the Brahminy Kite: the first intersection we reach branches off to the Whistling Kite, Haliastur sphenurus, which means that the Brahminy Kite is most closely related to the Whistling Kite. Tracing further back, one discovers that the genus Haliastur is most closely related to the genus Milvus, which includes the Red Kite (Milvus milvus) and the Black Kite (Milvus migrans). None of the three species most closely related to the Brahminy Kite feeds mostly on fish or has a white head.2 Going back even further, the group of species that include the genera Milvus and Haliastur is most closely related to the genus of fish-eagles, Haliaeetus.
These relationships give rise to several possibilities regarding when and in which species piscivory and white heads evolved. A common way of deciding among these different possibilities involves counting up the number of evolutionary changes that must occur for a given possibility to be true. Under the reasonable assumption that the occurrence of more changes is less likely than the occurrence of few changes, one concludes that the scenario necessitating the fewest changes is probably true. For example, it is possible that these eight species of fish-eating, white-headed raptors in Haliaeetus and Haliastur evolved these traits separately, a scenario involving eight evolutionary events. A far more parsimonious possibility, requiring just four evolutionary changes, is that piscivory and white heads evolved in the common ancestor of Haliaeetus, Haliastur, and Milvus, and that these traits were subsequently lost in the Whistling, Red, and Black Kite. The most parsimonious explanation, however, would involve two evolutionary changes—piscivory and white heads evolved once in the ancestor of all Haliaeetus fish-eagles and once, convergently, in Haliastur indus.
A more convincing case for the convergence of fish-eating behavior and white-and-brown coloration is made by the Osprey. Pandion haliaetus is not even in the same family as hawks and eagles—scores of other eagles that do not eat fish or have white heads are more closely related to fish eagles than the osprey. Looking across the family of hawks and eagles, Accipitridae, there is one more example of a fish-eating, white-headed raptor—the Black-collared Hawk, Busarellus nigricollis, from South America2—whose behavior and coloration have almost certainly evolved convergently. However, there are several white-headed raptors (the Black-and-white Hawk Eagle, the pale morph of Wahlberg’s Eagle, and several hawks in the genus Leucopternis) that do not eat fish, and several fish-eaters (in the genus Buteo) that have dark heads2, suggesting that there is more at work than a simple association between eating fish and having a white head. For instance, perhaps all white-headed raptors live in environments with similar light conditions in which having a white head is helpful for catching any sort of prey.
Nevertheless, the association between white heads and piscivory is compelling enough to warrant further investigation. But what sort of investigation? The most definitive evidence either for or against any hypothesis explaining the association would come from an experiment. For instance, consider the hypothesis constructed earlier—that raptors with white heads are more successful at catching fish than raptors with dark heads, because white heads are better camouflaged against the sun when viewed from the water by a fish than brown heads. One could test this hypothesis by catching, say, 50 African Fish Eagles, painting the heads of 25 of them brown and the other 25 white, releasing them and then measuring how many fish they catch. The group with white paint on their heads act as a control, since it is quite possible that painting a bird’s head, irrespective of the color, might affect its abilities to catch fish. If our hypothesis were true, then birds whose heads have been painted brown would catch significantly fewer fish than birds with white paint on their heads or unpainted birds. Let me know if you ever get around to carrying out this experiment; personally, the thought of catching these 3 kg birds with a 2 m wingspan and undoubtedly sharp talons and beak, let alone painting their heads, makes me slightly nervous.
This little investigative journey is precisely the sort of reasoning that being outdoors (“in the field” in biologist-speak) often prompts. And what makes field biology so much fun is that the sparks that set this reasoning into motion are everywhere around you and are usually entirely unrelated to whatever you are meant to be studying, making field work really good brain exercise. So the next time you’re outdoors, pay special attention to how the organisms around you look and behave, and you might end up following a path of reasoning that concludes with needing to paint an eagle!
Thanks to Professors Jill Miller and Rachel Levin for a discussion about why white heads might be adaptive in fish-eating raptors and how to test this hypothesis (which occurred when we were in the field collecting leaves and flowers from plants in the tomato family). Thanks to Professor Ethan Temeles for suggesting the phenomenal Handbook of the Birds of the World as a source of descriptions of raptors and information on their foraging behavior.
1. Griffiths, C.S., G.F. Barrowclough, J.G. Groth, L.A. Mertz. 2007. Phylogeny, diversity, and classification of the Accipitridae based on DNA sequences of the RAG-1 exon. Journal of Avian Biology 38: 587-602.
2. del Hoyo, J., A. Elliott, J. Sargatal (eds.). 1994. Handbook of the Birds of the World, Volume 2. New World Vultures to Guineafowl. Lynx Edicions, Barcelona.