Pleasure

painting by marguerita
(section from Le Bonheur d'etre Aime)

The hippos, far from passive participants, splay their toes, gape open their mouths and spread their legs to assist the fish in their cleaning services.These three examples of animal behaviour share a common, central element: pleasure.
In each case, the motivation for the behaviour is the reward of a pleasurable experience.
"So what?" you might ask.
Hippos are extremely graceful
in the water, despite their clumsy appearance on land. Their specific gravity allows them to sink to the bottom of rivers and literally walk or run along the bottom.
The central core of social groups appears to be females with their dependent offspring. Adult males vie for control of these herds. Aggression between males is intense. The hippos use their long canine teeth as weapons, and death often results from fighting between males. Most adult male hides are covered with scars from injuries incurred during such fights. Losing males are often relegated to a solitary existence.
An open mouth signals that the hippo feels threatened.
Hippopotami appear to communicate verbally, through grunts and bellows, but the purpose of these vocalizations is unknown.Mating occurs in the water with the female submerged for most of the encounter, her head emerging periodically to draw breath.
http://en.wikipedia.org/wiki/Hippopotamus
http://www.guardian.co.uk/science/2006/may/29/animalbehaviour.evolution
http://animaldiversity.ummz.umich.edu/index.html