Working in the Great Ruaha River in Tanzania, these researchers hit the animals with tranquilizer darts, then followed the drowsy beasts until they passed out. They then attached waterproof tracking bracelets to their legs. (The bracelets wouldn’t work around the neck, because hippo necks are actually fatter than their heads.)
The researchers divided the tranquilized hippos into three categories: large, dominant males that run things in the river; large sub-adult males that are coming up in the world but still answer to the dominant males; and small males even lower in the pecking order. By tracking the movements of each group, they could see not only how the animals’ meanderings changed with water availability, but how each group affected another. The researchers didn’t work with females because their movements are well known and predictable: Males always want them around, so they don’t tend to wander much.
The trackers revealed that dominant males tend to find a pool and stick to it, even as it begins drying up. Small males would hang out peacefully nearby. “They showed a very similar pattern to the dominant males,” says UC Santa Barbara wildlife ecologist Keenan Stears, lead author on the new paper. “Even when water becomes very limited, dominant males don't see these really small males as a threat and allow them to stay in these pools, as they would with females.”
But the large sub-adult males, which are approaching sexual maturity, were a whole other story. They’d lose fights with dominant males and be cast out of the pod, wandering upriver, where water availability is best, in search of territory. This infighting would only grow more contentious during the dry season, as the hippos squabble over less and less water. Food becomes scarcer, and if the large sub-adult males are forced to wander, they’re using up valuable energy, brawling with more dominant males as they move upstream.
The fate of these non-dominant males is important to the species—they’re the next generation of leading males who will someday replace the bullies currently running the river. “So if they're experiencing a lot of additional stress it can have important consequences for the population,” Stears says. “Looking at the two different patterns is really interesting and important because when it comes to conservation, not all individuals can be managed in the same way.”
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