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GPS Tracking Show Why Primates Need to Build Fences

We humans are highly aware of boundaries. We demarcate our homes with fences, our cities and states, with signs, and our countries, with passport control stations. If people without permission still trespass and get caught, they may get a gentle warning, a notice to appear in court, or even a war. It turns out that we are not alone: many other animals are also very sensitive to their own boundaries and don’t like it when others intrude.
GPS Tracking Show Why Primates Need to Build Fences

Lynne Isbell and a patas monkey

Vervet monkeys are cat-sized primates that live cohesive groups of multiple adult females, their young, and multiple adult males. They are often found walking on the ground or clambering in trees along riverine woodlands throughout eastern to southern Africa. They are also territorial, with both sexes defending the boundaries of their home ranges against incursions by their neighbors.

Using Global Positioning System technology that recorded the locations of three contiguous vervet groups every 15 minutes around the clock for one year, researchers were able to map the groups’ home ranges, the extent of spatial overlap between groups, and the locations of all encounters between groups.

Range map of Vervet monkey groups

The research, titled “Absentee Owners and Overlapping Home Ranges in a Territorial Species”, was published in January, 2021 in the journal Behavioral Ecology and Sociobiology by UC Davis Department of Anthropology faculty member Lynne Isbell and four others associated with UC Davis or Mpala Research Centre in Laikipia, Kenya. They found that maps alone cannot determine how vervets perceive ownership. Intergroup encounters were also important because they revealed territorial boundaries well within overlap areas. But the location of the intergroup encounter zones within overlap areas also revealed that groups frequently intruded beyond their territorial boundaries. How were they able to intrude so far onto their neighbor’s land if vervets are territorial?

It turns out that the shape of the home range makes a difference. Vervet home ranges are often centered around rivers, which makes them narrow and highly elongated. Most of the time when neighbors intruded, the owners were too far away and likely unaware of the intrusion. Like humans, vervets cannot always patrol the boundaries of their homes, but unlike humans, these absentee owners cannot build fences.