Abstract
With their striking predilection for perching on African ungulates and eating their ticks, yellow-billed (Buphagus africanus) and red-billed oxpeckers (B. erythrorhynchus) represent one of the few potentially mutualistic relationships among vertebrates. The nature of the oxpecker-ungulate relationship remains uncertain, however, because oxpeckers are known to consume ungulate tissues, suggesting that the relationship between oxpeckers and ungulates may also be parasitic. To examine this issue further, we obtained data on oxpecker preferences for different ungulate species, the abundance of ticks on these ungulates, and ungulate hide thickness. In support of the mutualism hypothesis, we found that both species of oxpeckers prefer ungulate hosts that harbor a higher abundance of ticks. We found no evidence that hide thickness-a measure of the potential for parasitism by oxpeckers-predicts oxpecker preferences for different ungulate species. Oxpeckers also prefer larger-bodied ungulates, possibly because larger animals have more ticks, provide a more stable platform upon which to forage, or support more oxpeckers feeding simultaneously. However, the preference for ungulates with greater tick abundance was independent of host body mass. These results support the hypothesis that the relationship between oxpeckers and ungulates is primarily mutualistic.
Highlights
If oxpeckers function as parasites by feeding primarily on host tissues, we predicted that oxpecker preferences should covary negatively with ungulate hide thickness because thinner hides should be easier to pierce, resulting in more wounds and making it easier for the birds to access host flesh
Our analyses further revealed that λ varied widely but is on average greater than zero and less than one. This finding argues strongly for using phylogeny-based methods when testing this prediction. We found that both species of oxpeckers tend to prefer the same ungulate host species
We investigated whether ungulates living in larger groups have higher tick abundance
Summary
With their striking predilection for perching on African ungulates and eating their ticks, yellow-billed (Buphagus africanus) and red-billed oxpeckers (B. erythrorhynchus) represent one of the few potentially mutualistic relationships among vertebrates. If oxpeckers function as parasites by feeding primarily on host tissues, we predicted that oxpecker preferences should covary negatively with ungulate hide thickness because thinner hides should be easier to pierce, resulting in more wounds and making it easier for the birds to access host flesh. We investigated whether oxpeckers prefer larger-bodied ungulates, possibly because larger-bodied ungulates offer a larger or more stable environment for foraging or because larger ungulates have greater tick abundance (Ezenwa et al 2006), and whether they prefer ungulates that live in more open environments, which would facilitate perching by the birds
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