Estes, in his book ‘The Behaviour Guide to African Mammals’, briefly describes yawning as a threat display and includes a line drawing showing the behaviour where two rivals stand face to face with their mouths wide open. Another reference suggests that this is a way for two bulls to size each other up.
What is not mentioned in any of the references I have explored is the moaning or lowing sound that accompanies this behaviour. It was something that I observed and recorded on several occasions along the Luangwa River in Zambia’s South Luangwa National Park which holds the largest population of hippos in the world. It only took place at night when the animals were out of the water facing each other on the riverbank. The sound would go on for a very long time, often for more than an hour with no sign of physical contact as is regularly seen when they challenge each other.
This recording was made on one occasion when I was guiding at Bushcamp Company’s Bilimungwe Bush Camp in a remote section of the park and continued at various times during the evening. Dozens of Painted Reed Frogs call loudly from a nearby reedbed throughout the recording and at one stage a Puku gives a short alarm call.