On the left is Kai, age one year and two month, playing with new calf Max age four months.
Photo taken July 2020. Hippo calves play at open mouthed pushing games and often leap over each other in the water like dolphins. They will gape like this and joust with each other to see who is the strongest.
When they are young it is purely play but once they reach adulthood these games will have taught them how to defend themselves and even fight another bull to get their own habitat or family.