More on oxpeckers, on wound-feeding, and on the delightful habit of eating earwax…
In the previous post we looked at the behaviour of oxpeckers: the idea that they feed on blood and the other tissues of their hosts was introduced, and we can now doubt the idea that they are always symbiotic ‘friends’ of the mammals they clamber about on. As demonstrated by Paul Weeks in his several studies of the birds (Weeks 1999, 2000), oxpeckers spend a considerable amount of foraging time feeding – not on ticks – but on blood, ear wax, and on dead skin that they ‘scissor’ out of the fur. We also saw that, in cases, oxpeckers cause distress to their hosts, and that they apparently keep wounds open and can be a total menace. However, the point remains that they do eat ticks and other parasites: might it be that, while they can exact a cost on a host, their actions are still beneficial overall? It turns out that this is the hardest hypothesis to test and verify [adjacent image from busangasafari.co.uk].
Weeks (2000) split a herd of domestic cattle into a control group (where oxpeckers were allowed to visit as normal) and an experimental group (where all cattle were kept free of oxpeckers all of the time)… naturally there is a lot more to his study than this, but if you’re really interested you’ll need to obtain and read his paper. Anyway, after four weeks he found that the absence of oxpeckers had ‘no significant effect on the change in total tick load’ (p. 155). So there you go, good evidence that oxpeckers are not particularly useful to their hosts. It’s perhaps worth noting here as an aside that some large ungulates – the Impala Aepyceros melampus is the best example – have evolved unusual grooming mechanisms of their own (Impala have specialized lower jaw teeth that perform this function), and perform allogrooming* as a social behaviour: factors that may suggest that these mammals have no need for an allegedly helpful ‘cleaner’ belonging to another species. Furthermore, oxpeckers don’t eat all ticks, but show a definite preference for large adult female Blue ticks Boophilus decoloratus. Yet these are the ticks that have already gone through three life stages before growing big and fat on their hosts. Removing them so late in the game is not as beneficial for the host as it could be, had the oxpecker removed the tick at the start of its lifecycle.
* The grooming of other herd members.
Tick loads weren’t the only factors that Weeks (2000) recorded. He also tallied the number of wounds on the control and experimental cattle, and on the amount of earwax that the cattle had. The control cattle (those exposed to oxpeckers) ‘had significantly more wounds than experimental cattle [those without oxpeckers]‘ (p. 156), while the control cattle also showed a significant decrease* in earwax. So the oxpeckers apparently (1) didn’t help in controlling tick loads, (2) kept the cattle’s wounds open, and (3) reduced the amount of earwax. You might assume that reducing earwax would be a beneficial thing, but actually this isn’t clear. Earwax (properly termed cerumen) provides both physical protection for the sensitive ear canal, and antibacterial protection, so its removal by the birds might be a bad thing. Of course it could also be helpful, as impacted or unusually concentrated earwax creates problems for an animal’s hearing. [* corrected from earlier mistake]
It’s actually interesting and pretty bizarre that the oxpeckers should eat earwax anyway. Earwax consists of long-chain fatty acids and hence is presumably nutritious, but it’s almost certainly very difficult to digest. Indeed wax-eating elsewhere in birds (best known in honeyguides) is a highly specialized affair where the birds use either symbiotic bacteria or unique enzymes (last I heard this was still unknown). Honeyguides are so good at it that they can live off wax and nothing else for up to three months at a time, and will even eat candles if given the chance. Wax-eating has also been reported in bulbuls (Horne & Short 1990) and storm-petrels (Obst 1986) and might prove widespread in birds [the adjacent image of a Greater honeyguide Indicator indicator was borrowed from birdfinders.co.uk].
Sorry, I can’t move away from honeyguides without mentioning that their parasitic nestlings have special hooked bill tips that they use to kill the host’s nestlings (of the 15 honeyguide species, only one is not a parasite). Bizarrely, this fact was inspirational to a group of researchers who once intimated that Triceratops was a brood parasite. That’s, as they say, a story for another time.
Err, anyway, back to oxpeckers. The bottom line to all of this is as follows. It appears from their poor performance in tick control, and in their habit of keeping wounds open, that oxpeckers are not wholly beneficial to their hosts and may in fact exact a cost [in the adjacent image, borrowed from here, note that the oxpecker is feeding from the hippo's shoulder wound]. Conversely, the fact that their earwax-eating behaviour might be beneficial, and that their alarm calls function in alerting their hosts to danger, suggests that they do confer some benefits, sometimes. This fits well with the general feeling among those who work on mutualism that relationships of this sort operate on a sliding scale, with the various benefits and costs differing greatly between species pairs. Weeks has also emphasized the point that the relationships that oxpeckers have with large mammals might differ according to the species. As suggested by Olivier & Laurie’s (1974) report on hippos, oxpeckers might be truly parasitic on this species, but they seem to have a mutualistic relationship with rhinos, and a simple commensal relationship with impala. As usual, it doesn’t seem that there is one rule that applies across the board.
Yet more to come on this subject soon. Coming next: the path to parasitism!
Refs – –
Horne, J. F. M. & Short, L. L. 1990. Wax-eating by African common bulbuls. Wilson Bulletin 102, 339-341.
Obst, B. S. 1986. Wax digestion in Wilson’s storm-petrel. The Wilson Bulletin 98, 189-195.
Olivier, R. C. D. & Laurie, W. A. 1974. Birds associating with hippopotamuses. Auk 91, 169-170.
Weeks, P. 1999. Interactions between red-billed oxpeckers, Buphagus erythrorhynchus, and domestic cattle, Bos taurus, in Zimbabwe. Animal Behaviour 58, 1253-1259.
– . 2000. Red-billed oxpeckers: vampires or tickbirds? Behavioural Ecology 11, 154-160.