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ISSUE: q306
01/08/2006
By Glenn Feldhake and Derek Solomon
All mammals support external parasites. The forms of what is called ectoparasitism vary considerably from parasite to parasite and host to host. However, two ectoparasites are particularly notable and unique to hippos. One, Oculotrema hippopotamus, attaches itself to the hippo’s eyes; the other, Placobdelloides jaegerskioeldi, is associated with the hippo’s rectum.
In his book The Hippos, S.K. Eltringham states that hippos are widely infected with the fluke, Oculotrema hippopotamus, which lives on the outer surface of the eye. In studies, typically eight but as many as 41 of these tiny parasites may be found in a hippo’s eye. Although they do not appear to cause serious harm to the eye, they do appear to produce local irritation judging from the increased frequency with which the hippo blinks. The adult flukes attach themselves to the inner edge of the nictitating membrane of living hippos but they are found under the eyelids of dead animals, so they possibly occur there in living animals as well.
A possible observation of the effects of these flukes was made in the Luangwa Valley in 2003. The Luwi Lagoon in South Luangwa National Park normally holds sufficient water for most of the year to accommodate three separate groups of hippos. In September 2003 the water level had dropped sufficiently to expose the branch of a fallen tree that ended with a relatively sharp point. On 13 and 14 September, 2003, Derek Solomon observed an adult female hippo, with well-grown calf in tow, approach the branch and carefully place the sharp point inside the lid of the eye and rub it backwards and forwards. This rubbing was repeated several times each day and lasted for five to ten seconds at a time. The immediate thought was that this animal could be infected with a fluke, Oculotrema hippopotamus, living on the outer surface of the eye and causing irritation.
A very different type of ectoparasites is known by scientists as Placobdelloides jaegerskioeldi, or to the rest of the world simply as the “hippo leech”. The hippo leech is aptly named, as it will attach itself to nothing else. It is coloured the same brownish-red as a hippo’s skin, which may make it perfectly camouflaged from hungry oxpeckers. In laboratory tests, even the hungriest hippo leech will have nothing to do with fish, toads, frogs, terrapins or, thankfully, humans. Such species specific tastes are very rare for leeches.
For such a specialised lifestyle, the hippo leech has evolved some amazing adaptations. One example is its mechanism for locomotion. Most leeches have suckers on either end of their bodies that allow them to loop end over end as they travel from place to place. Hippo leeches, however, flatten themselves out and they move by wriggling along like earthworms. By keeping a low profile as they travel, they are able to remain attached to the hippo’s body even when it is travelling through fast-moving water. In order to move from hippo to hippo, the leeches have also had to become outstanding swimmers. They are even able to pursue their prey upstream in strong currents. Perhaps the most curious, if not disturbing, habit of the hippo leech is its choice of venue for mating - the rectum. Researchers in South Africa who collected and studied over 2,200 hippo leeches have provided the evidence. Of all the sexually mature leeches found in the rectum, over half had spermatophores or sperm cases attached to their bodies. As these sperm cases would not have survived the snug journey from outside the hippo’s body, and spermatophores were never found in leeches outside the rectum, the researchers concluded that reproduction must occur in the rectum.
References
Eltringham, S.K., 1999. The Hippos. T & AD Poyser Natural History, pp 116.
Oosthuizen, J.H. & Davies, R.W., 1994. The biology and adaptations of the hippopotamus leech, Placobdelloides jaegerskioeldi, to its host. Canadian Journal of Zoology, 72. pp 418-422.
Contacts
Glenn Feldhake, 3871 Deer Lake Dr. Medina Twp, OH 44256. USA Susanglenn@aol.com
Derek Solomon, PO Box 100, Mfuwe. solomon@zamnet.zm.
Photograph: Jim Pugh, National Zoo, Washington.
Date: q306
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