Bloodsuckers or tick-pluckers? The case of the oxpecker
Birds have some awesomely descriptive names. Like the Yellow-bellied Sapsucker (Sphyrapicus varius), a North American woodpecker that specializes in drilling “sap wells” in trees to feed on their sugary phloem sap. Or the Brown Trembler (Cinclocerthia ruficauda), a Caribbean relative of the mockingbird that shakes its wings violently to communicate with other members of its species. But when it comes to quirky but descriptive names, the African oxpeckers are hard to beat.
You may not realize it, but you’ve probably seen oxpeckers in nature documentaries: small brown birds with festive red-and-yellow bills, traveling about the African savannah perched on the backs of large grazing mammals. As their name suggests, oxpeckers peck the animals on which they perch, harvesting ticks and other ectoparasites from the skin of their hoofed hosts. Their relationship is a classic example of a mutualism: an interaction in which each species benefits from the presence of the other. The hosts get rid of pesky ticks, while the oxpeckers get a protein-rich meal.
But the oxpeckers’ Latin name, Buphagus (literally “cow eater”), suggests a more sinister motive. Yes, oxpeckers eat ticks, but they have also been observed feeding directly on the blood of their hosts, opening new wounds or re-opening old ones. This behavior suggests that the relationship may be more asymmetric in its benefits: the birds might, in fact, be parasites.
Imagine being the biologist who first noticed this blood-feeding behavior. It would be like watching the guy washing windows on the apartment building next door as he suddenly stopped cleaning, looked around furtively, and took a bite out of the wall.
But is there any way to test whether the oxpeckers-ungulate relationship is fundamentally mutualistic or parasitic? In a paper published in Evolution last month, Charles Nunn and colleagues took a comparative approach to answering this question. They began with two alternative hypotheses, both consistent with existing observations:
Hypothesis 1: The relationship is essentially mutualistic – ticks are the primary food source – but the birds sometimes feed directly on blood if it is accessible
Hypothesis 2: The relationship is essentially parasitic – blood is the primary food source – but the birds also feed on ticks when they are readily available
If the relationship is a mutualism, the authors predicted, oxpeckers should prefer host species that tend to harbor more ticks; more ticks means more food for the birds. If the relationship is a parasitic one, however, the birds should prefer host species that have thinner hides, which would allow easier access to the host’s blood.
Luckily, most of the “grunt work” of counting ticks on African mammals had already been done; the authors simply consulted the Global Mammal Parasite Database (yes, there is such a thing) to gather data on tick abundance on each host species. They also searched the literature to determine oxpeckers’ host species preferences and the hide thickness of each potential host.
Next, they needed to test whether oxpecker preferences were related to tick abundance or host hide thickness. As it turns out, this step is not as simple as it might seem. To understand why, try this thought experiment: Suppose that oxpeckers preferred several species of antelope over other possible hosts. These preferred antelope hosts had thinner hides than many other possible hosts (zebra, water buffalo, rhinoceros, etc.). Would these data be evidence enough that oxpeckers prefer host species with thin hides?
No, they wouldn’t, because although the antelope hosts all have thin hides, they also share lots of other characteristics due to their common evolutionary history – i.e., simply because they’re all antelope. So you couldn’t be sure if it was the thin hides that attracted the oxpeckers to these species, or some other trait shared within the group. To account for this possibility, the authors needed to construct a phylogeny – an evolutionary family tree – that included all of the oxpecker’s possible host species. Nunn and colleagues obtained DNA sequences from GenBank (an online DNA sequence data repository) and used these data to reconstruct the evolutionary history of the African mammals in question.
With this phylogeny, the authors were able to use a statistical model that infers ancestral characteristics based on the branching pattern – or topology – of the tree, then asks whether evolutionary changes in one variable (e.g. hide thickness) are accompanied by changes in another (e.g. oxpecker preferences).
An aside: Remarkably, all the data required for this study were already available in other published sources. I used to work for Walter Koenig, one of the authors of this study (not this one)… In addition to his empirical research, which includes some really seminal work on the social behavior Acorn Woodpeckers, Walt has a special talent for breathing new life into old data. This paper is a great example of the latter!
So, what’s the upshot? The authors found a positive relationship between tick abundance and oxpecker preference: on average, the more ticks were found on each host species, the more oxpeckers liked that species. Oxpeckers also preferred larger hosts – which tended to harbor more ticks – but even among host species of similar size, oxpeckers preferred the species with the most ticks. On the other hand, oxpeckers didn’t seem to care much about the thickness of the host’s hide.
These patterns were identical between the two species of oxpecker, the Yellow-billed Oxpecker (Buphagus africanus) and Red-billed Oxpecker (B. erythrorhynchus). Taken together, these results favor the mutualism hypothesis over the parasitism hypothesis.
One way or another, oxpeckers are bloodthirsty. But this analysis shows that they tend to look for blood in convenient snack-sized packets – helping their hosts rid themselves of parasites – rather than collecting the blood for themselves. Personally, as much as I like a good paradigm shift, I find it reassuring to see a textbook “just so” story borne out with actual data. That said, it might not take much to change the nature of the relationship; if a host has readily accessible wounds, or if ticks are scarce (as in a captive situation) the birds can easily switch their tactics!
Nunn, C., Ezenwa, V., Arnold, C., & Koenig, W. (2011). MUTUALISM OR PARASITISM? USING A PHYLOGENETIC APPROACH TO CHARACTERIZE THE OXPECKER-UNGULATE RELATIONSHIP Evolution DOI: 10.1111/j.1558-5646.2010.01212.x