Oxpeckers and Ungulates




Oxpeckers (Buphagus africanus and B. erythrorhynchus) and ungulates form one of the only known cleaner relationships among vertebrates. This interaction is most noted in Africa where oxpeckers feed off large mammals such as rhinoceros, cattle, bison and giraffes. The relationship is obligate for the oxpeckers because whatever they find on the skin of the ungulates is their source of nutrition. Typically the birds consume ticks, dead skin, mucus, saliva, blood and other bodily fluids of the animals. Although the ungulates do not rely on the birds for survival it is believed that the mammals benefit from tick removal and having their wounds cleaned especially in areas where the animal cannot clean itself. If ticks were allowed to thrive on the ungulates they could become a source of disease and infection. At the same time, if the wounds caused by burrowing ticks were left open bacteria and flies could cause other infections.


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http://barryjnorthern.blogspot.com/2010/01/fable-of-rhino-and-oxpecker.html
The classification of the relationship between these two species has been the source of debate for many years. Since oxpeckers and their host are categorized as a “cleaning symbiosis” the relationship may not be as equally beneficial to both species as previously believed. Some researchers say that oxpeckers and ungulates are involved in a mutualism where the oxpeckers benefit by receiving nutrients and the ungulates benefit by having ticks removed and diseases prevented. On the other hand, other scientists argue that the ungulates are not really benefiting since there is no proof that the birds are reducing the hosts’ tick population. In reality the birds may actually be causing more wounds on the ungulates they populate in order to receive preferred nutrients.





Another discussion has arisen regarding whether the cleaner is taking advantage of the hosts’ enjoyment of tactile stimulation (Poulin and Grutter). It has been discovered that animals enjoy being touched and although the ungulates may feel some pain while the birds are cleaning their wounds, the overall sensation of having ticks removed or having the birds’ beaks in their mane may outweigh the short moments of pain. Researchers wonder if oxpeckers and ungulates are similar to marine cleaner symbioses where the cleaner “trains” the host to visit them. The cleaner provokes tactile stimulation in the other symbiont and as a result the symbiont remembers the pleasurable feeling and continues to return to the cleaner in order to relive the enjoyable experience. The sensation felt by the host ensures that they will be a constant source of nutrition for the cleaner. Based on the knowledge acquired from the two previously mentioned studies further studies have been conducted to find out more about the motives and effects of oxpeckers.


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http://c1.ecolocalizer.com/files/2009/02/file_red-billed-oxpecker-close-wikimedia-commons.jpg
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http://scienceblogs.com/tetrapodzoology/upload/2007/01/Yellow-billed%20Oxpecker.jpg


There are two types of oxpeckers which are described as either red-billed oxpeckers (RBO) or yellow-billed oxpeckers (YBO). Both species have body characteristics that were adapted for their role as “cleaners”. The major characteristics include small, sharp claws, long rigid tails, and flattened beaks that have a sharp edge for cutting. The claws and tails help the birds attach to their host and remain connected as the animal moves. The laterally flattened beaks are useful in removing ticks and piercing the skin of the host. The major difference between the two species other than beak color, is eye color and that YBO are slightly larger than RBO. These physical differences of the birds have not lead to drastic differences in what host each species prefers. Although the YBO have a slightly greater preference for larger ticks and flies, as discovered by stomach analyses, the two types of oxpeckers both tend to favor larger animals with manes. Researchers were most surprised to find that there was no noticeable competitive displacement of the two species and that often they were found in the same location and even on the same host. This demonstrates that the birds did not choose their host based on whether or not another bird was already present on that host.


Walter Koenig conducted a study in Lake Nakuru National Park and Masai Mara Reserve in Kenya in July and August 1990 to determine what host type oxpeckers preferred. Koenig recorded the time the birds spent on major activities, the number of birds present on each host and where they were located. From this experiment he noticed that RBO were present at both sites while YBO were only present at Masai Mara. This observation demonstrates that the RBO are more common and have a larger niche than the YBO. He also discovered that host preference was based on 3 important factors: tick density/ larger surface area of host, host behavior and the ability of the bird to hide from predators while on the host. The last factor may provide insight into why the birds prefer hosts with mane, since the hair provides a hiding place from potential predators (Koenig, 91-104). So, overall oxpeckers did not affect the symbioses of the other species with ungulates, however, following this experiment, further studies still needed to be performed in order to determine how oxpeckers actually impact their host.


One study wanted to test whether the presence of oxpeckers had a real affect on the density of ticks on ungulate species (Weeks, 154-160). To test this researchers in Zimbabwe performed a controlled field experiment on the Sentinel Ranch where there were two groups of domesticated cattle. The 22 cattle present were divided into 2 groups will 11 cattle in each group. One of the groups were allowed to interact with the oxpeckers while the control group was monitored and any incoming oxpeckers were scared away and thus unable to land on the cattle for the duration of the experiment. The two groups of cattle were located in two separate grazing areas but in order for the experiment to be consistent the location of each group was rotated every two days so that one group was not intentionally in an area that is sparsely populated by oxpeckers. This was also important so that the cattle were exposed to all of the possible tick populations. Another treatment was performed in which the 2 groups were switched so that the previous control group was now the experimental group and visa versa. Then a third treatment was performed where the cattle were randomly placed in either the experimental or control group. From this experiment it was discovered that the presence of oxpeckers had no real affect on the tick load of the ungulate. This was demonstrated because in the first and third treatment the experimental group (oxpeckers present) had more ticks but in the second treatment the control group, which lacked oxpeckers, had more ticks present. The inconsistency of the results made it difficult to definitively say that oxpeckers’ primary role is in tick removal (Weeks, 154-160).


The researchers involved in this experiment had also recorded the time oxpeckers allotted to removing ticks from hosts. They found that more than 85% of the time that the oxpeckers were perched on the ungulate was spent feeding on blood, earwax or running their beak through the ungulate’s hair. Although it may be assumed that looking through the hair would be in search of ticks they found that the more likely explanation was that the birds were looking for dead skin. Only 5% of their time was spent looking for ticks (Weeks, 154-160).


Although the hypothesis that oxpeckers reduce tick load was not supported many other interesting findings were discovered about this symbiosis from the study. It was noted that the birds did not appear interested in eating ticks that were out in the open. Instead the oxpecker would use its beak to peck at an area of skin where a tick had attached and make the wound, initially caused by the tick, larger. Surprisingly ungulates with oxpeckers had more wounds than those without, however, it was hard for them to tell whether the wounds were purposely inflicted by the oxpeckers or not.
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http://www.scenicreflections.com/ithumbs/Red-billed-Oxpecker-On-Impala_Wallpaper__yvt2.jpg


Oxpeckers also appeared to have a preference for the large blue female ticks. A possible hypothesis for this was that the females were full of blood and thus feeding on them would provide oxpeckers with the source of nutrition (blood) that they actually wanted. Being choosey for engorged ticks may provide the birds with the nutrition they want but it does not help the ungulates because at this point they have already been bitten and infected with any bacteria or disease that the tick may have carried. Also by the time the tick was eaten it had already lived its whole life on the host. This gives the first insight into the fact that the oxpecker-ungulate symbiosis may not be as mutualistic as it once appeared (Weeks, 154-160). Since the mammals have no real say in whether or not they interact with the birds it is hard to classify the symbiosis as a real mutualism.


The fact that oxpeckers eat the tissue and fluids of their host is still considered a reason for why these birds appear as cheaters in the symbiotic system. Charles Nunn and fellow researchers wanted to see if oxpeckers preferred ungulates with more ticks or if hide thickness determines what host the bird populates. From the results they hoped to acquire they believed that they would be able to tell whether ticks or body tissue were the oxpeckers’ primary food source. It was hypothesized that if the birds choose the animal with thinner hide then it meant that they wanted a host with skin that was easier to penetrate and create wounds in and thus easier to feed on blood, mucus, etc. Through stomach analysis it was determined that the main food source of the birds were ticks of varying sizes and life stages as well as flies that also suck blood. Through the study they found that tick density on the hosts were not actually reduced by the presence of the oxpeckers. As a result it was speculated that although the stomach analysis demonstrated high tick and fly levels, it could be assumed that the bloodsucking flies and ticks were probably favorable because of the blood that they carry in their stomachs after biting the ungulates. Even though the number of ticks was not drastically reduced in the presence of oxpeckers there did appear to be a correlation between tick density and the oxpecker’s host preference, whether this is coincidental or not, the results supported the hypothesis that oxpeckers and ungulates participate in a mutualism.


Numerous studies have been performed in order to determine what hosts oxpeckers prefer and whether or not ticks or body tissues are their main source of nutrition. It is difficult based on the experimental findings to definitively determine the motives of the oxpecker however, it very likely that environmental conditions play a role in what the birds prefer to eat. For example: if cattle have more wounds, whether they are from ticks or scratches from trees or other environmental factors, it may be more beneficial for the oxpeckers to feed on the wounds instead of ticks. Because the wounds would be already present on the ungulate it would require a smaller energy expenditure from the oxpeckers in order to receive the preferable found source. Another possibility is that the birds feed on wounds or create new ones when there are fewer ticks available to eat. If ticks are scarce there is no reason for the birds to go hungry when they are able to feed off ungulate wounds. For the most part the most likely reason for oxpeckers eating a certain way is that they are taking advantage of the most favorable situation as it arises. If there are a lot of ticks they might as well eat the ticks, but if ticks are few or wounds are abundant they should eat from the wounds. So, although the symbiosis between oxpeckers and ungulates remains a mystery it appears that the relationship is predominantly commensalistic if not mutualistic, however, similarly to many other symbioses when certain circumstances arise it may result in a shift parasitism. (Nunn et. al)



References

Koenig, Walter D. "Host Preferences and Behaviour of Oxpeckers: Co-existence of Similar Species in a Fragmented Landscape." Evolutionary Ecology 11 (1997): 91-104. Print.


Nunn, C. L., Ezenwa, V. O., Arnold, C. and Koenig, W. D. (2011), MUTUALISM OR PARASITISM? USING A PHYLOGENETIC APPROACH TO CHARACTERIZE THE OXPECKER-UNGULATE RELATIONSHIP. Evolution, 65: 1297–1304. doi: 10.1111/j.1558-5646.2010.01212.x


Poulin, Robert and Alexandra S. Grutter. “Cleaning Symbioses: Proximate and Adaptive Explanations.” Vol. 46, No. 7 (Jul. - Aug., 1996), pp. 512-517. Published by: University of California Press on behalf of the American Institute of Biological Sciences Stable URL: http://www.jstor.org/stable/1312929

Weeks, Paul. "Red-billed Oxpeckers: Vampires or Tickbirds?" Behavioral Ecology 2nd vol. 11 (2000): 154-60. Online.