High frequency drone censuses reveal that habitat characteristics drive semi-aquatic megaherbivore fission-fusion social dynamics; considerations for freshwater and protected area management

Camille J. A. Fritsch, Marco Plebani and Colleen T. Downs, Life Sciences, University of KwaZulu-Natal, Pietermaritzburg, South Africa

Background/Question/Methods: Common hippopotamus Hippopotamus amphibious (hippos) are iconic ecological engineers in African aquatic and terrestrial systems. However, because of a lack of research, relatively little is known about hippo behavioral ecology. Fission-fusion societies are populations that experience changes in the size and composition of sub-groups over time as individuals separate (fission) or come together (fusion) while balancing the costs and benefits of group living. Varying levels of fission-fusion society dynamics have been described for large-bodied African herbivores like cape buffalo (Syncerus caffer), giraffe (Giraffa camelopardalis thornicrofti), and African elephants (Loxodonta africana cyclotis) however, changes in hippo population social structure remain unquantified. Therefore, we used the results of 47 drone-based population censuses conducted from 2016-2017 to identify sub-annual fission-fusion dynamics in the hippo population at Ndumo Game Reserve in South Africa.
Results/Conclusions: Here we show that, on average, the majority of the population (74.0%) occupied large nursery pods (≥ 20 indiv.) with 15.3% of the population occupying very large nursery pods (≥ 60 indiv.) when inundation area was below the mean where as when inundation area was above the mean, the majority (59.9%) occupied smaller pods (≤ 20 indiv.; isolated individuals, bachelor herds, family groups, and small nursery pods) with none in very large nursery pods (> 60 indiv.). Where other species like elephant and giraffe come together in periods of superior resource availability in the wet season, our findings suggest that hippos are forced into large pods in perennial wading areas in the dry season when food resources are thinnest; likely catalyzing natural cycles of population regulation. Hippos drive ecological processes in their environment and inundation area dependent spatial and temporal changes in hippo abundance and density influence the distribution and concentration of both aquatic and terrestrial ecological effects. Therefore, the results of our study provide additional information necessary for the conservation and management of populations of hippos and systems in which they exist.