Population size, distribution, and small-scale seasonal variations in pod dynamics, habitat selection, and behaviour of hippopotamus (Hippopotamus amphibius) in the Okavango Delta, northern Botswana

Abstract

There is limited biological and ecological data on hippos, most of which originates from riverine/lacustrine populations, with none on the Okavango Delta population in its unique wetland habitat. This thesis aimed to investigate the Delta’s hippo population (size/distribution) and examine small-scale seasonal variations in hippo pod dynamics, habitat selection, and behaviour. The research provides baseline ecological and behavioural data on hippos in wetland habitats and gives insight into their adaptability to changes in water availability. This is particularly important as the Delta’s waters are threatened by climate change and human pressure, which will reduce habitat for hippos.

I utilised thirteen years of aerial survey data to examine temporal and spatial patterns of hippo populations in the Delta. Hippos preferentially occurred in large lagoons within seasonal swamps, avoiding the dense aquatic vegetation and deep water of the permanent swamp/main channels. Since the mid-1990s, Botswana’s hippo population has grown significantly, likely due to long-term increases in rainfall and inflow. The most recent survey (2018) emphasises Botswana as a stronghold for this species in Africa, however, the exceptionally small flood event of 2019 will likely have led to substantially reduced hippo numbers. This work demonstrates the value in long-term monitoring programs.

I developed a method to monitor (count/age) hippos using a drone, achieving more accurate counts than ground and aerial surveys. Using this method, I examined seasonal changes in hippo pods (size/density/demographic composition/distribution) related to varying surface water availability. In the low flood season, hippos occurred in larger pods in higher densities, aggregating on the minimal water still available. All seasons were characterised by near-constant changes in pods, emphasising their dynamic nature and challenging the notion of stable hippo groupings.

I conducted 24-hour activity budgets within the Delta and Chobe River to increase the currently limited knowledge of hippo behaviour and to investigate how behaviour changed temporally, spatially, and seasonally in variable landscapes. Large differences in behaviour between pods (even those within close proximity) indicate hippos are highly flexible, taking advantage of available resources, but also emphasising the effects of humans on hippo populations.