Ever heard of a hippo suntanning its tummy? Neither have we.

Wild Card member Gilda Lewis writes: “I’ve recently returned from the Kruger National Park and wanted to share these photos and maybe ask your experts about this unusual hippo behaviour. It was in May, in the afternoon. As you can see, the water has some green algae growth.

“The one hippo kept rolling itself onto its back with its head under water often touching the hippo next to it. It was doing this the entire time we were there (about one hour) and had everyone in the hide thoroughly entertained.

“The pair were not very happy when a few elephants came down to the water. I have never in all the years seen such behaviour and wondered if something was wrong or was it just having fun?”

Our zoology expert, Rudi van Aarde, Professor of Zoology and Chair of the Conservation Ecology Research Unit at the University of Pretoria, says he has never seen such behaviour. At first sight it appears to him to be playful behaviour. “All animals, including humans, enjoy a game,” he commented. “Play is part of ordinary social behaviour, and hippos are very social animals.”

Hippos live in extremely sociable groups called rafts (pod is also used in some areas but is not species specific). Such rafts usually consist of a male with a number of females and young. Members of the group often stay in close contact, either on land when sunning, or in the water, especially when pools are drying up, writes Trevor Carnaby in Beat about the Bush – Mammals.

Being bottom dwellers, he explains, they wear pathways along the bottom and when ‘running’ along these, they sometimes have all four limbs off the ground, thanks to buoyancy. They cannot propel themselves along the surface in deep water. A habitat prerequisite is therefore slow-moving water that is deep enough for them to submerge, but not so deep as to make surfacing for air impossible.

Hippos can only float for short periods of time while actively treading water. They cannot swim at the surface, other than for brief periods after being propelled to the surface via the back legs with a jumping action. Hippos that are often observed lying motionless in water with their backs exposed are therefore guaranteed to be touching the bottom.