On the day that God made the Warthog there was a shortage of skin. As a result the body of the Warthog is a very tight fit. When it runs through the grass it closes its eyes to keep the seeds out. So tight is its skin that just closing its eyes is enough to make its tail stand up.
There are other tales to explain why it feeds on its knees and why its hair is so patchy. It seems to be a popular figure in African folklore.
bobmcgee 