Calfs are born underwater and start swimming and at three weeks the calf stops nursing and starts eating grass. Hippos dont leave their mothers until they are adults (7-8 years old). At the age of 8, the hippos will start looking for a mate and start their own family. The male hippos reach sexual maturity by age 7, the females reach sexual maturity by age 5-6. Hippos walk along the slow moving rivers and lakes as an activity. Females but in a lot of work throughout the pregnancy, the male input ends after mating. Male hippos are larger with longer tusks.