The results are presented of a 2-month study of the Mara River hippopotamus population in the Serengeti National Park and the Maasai Mara Game Reserve. A method for correcting aerial counts of hippos in turbid rivers and based on herd behaviour characteristics is described. There were an estimated 1927 hippos at an average density of 16-1/km in the area censused. The population became more dispersed and the mean group size decreased from thirty to fifteen after a rise in water level. The value of comparisons between populations in various terms of expression of density, and mean group sizes, are discussed. The amount of day living space, defined as areas preferred or utilizable by hippo during the day, and productivity of the food plants are seen to be very important factors in hippo habitat utilization, and as constraints on population increase.
Hippo use of the habitat and the effects of hippo on the vegetation at various distances from the river were quantified, and a cycle of vegetational changes suggested which may be associated with the effects of hippo grazing and fire in other parts of Africa.
Groups were found to consist of nearly equal numbers of males and females. There was no evidence of territorial behaviour either on land or in the water. A number of observations on social interactions and defaecating behaviour are described and indicate that olfactory communication is very important in interactions between hippos. Hippo society is unstable due to climatic influences, but a dominance hierarchy including males and females exists in the groups. Males appear less firmly attached to groups than females, but a male is normally the dominant animal in a group.