Lynne A. Isbell has loved animals ever since she was little. She grew up with dogs, cats, rabbits, geese, and even snakes. But for the past 48 years, Isbell has been focused on primates. 

“Observing primates can feel like watching a favorite long-running soap opera.” said Isbell, a Distinguished Professor in the Department of Anthropology and a past president of the American Society of Primatologists. 

Isbell studies primate socioecology, which is how ecological factors like competition and predation, among others, influence primate social organizations. 

“With their active social lives, primates make it easy to fall for them,” she said. 

In her recent book Primate Socioecology: Shifting Perspectives, Isbell presents a new way of classifying primate social organizations. Primates are unusual among mammals in having a wide diversity of social organizations, including living alone, in pairs, in small cohesive groups, in large cohesive groups, and in groups whose members split up and come back together repeatedly. Figuring out what drives this diversity has been an ongoing challenge since the 1960s when socioecology first developed as a field of study.   

The book is both an introduction and examination of the field, challenging traditional research and assumptions about primates. 

Tackling current assumptions

Many researchers assume primate social organizations are determined by both their access to food and the impact of predators. 

“That view has been accepted with very little evidence, I think in part because of the absence of a viable alternative” Isbell explained.

But researchers have failed to recognize the impact daughters can have on their mothers’ reproductive success. This is because researchers often study land occupancy and dispersal from the offspring’s perspective, leading them to forget about the mothers. 

Primate socioecology from a mother’s perspective

The land individual primates live on, also known as their “home range,” is central to understanding primate social organizations. Female primates forage for food in their home range, so a potential factor impacting their foraging efficiency is how much of their home range they share. Female primates are extremely dependent on food to improve their chances of having and raising offspring. As a result, their home range partially influences how many babies they can have and raise to adulthood.

“Reproductive success is the name of the evolutionary game,” Isbell said. 

Isbell’s book categorizes primate social organizations largely by how much females share their home ranges with other females. This basis in home range sharing allowed her to develop seven types of female social organizations, including “stingy” females, who keep their home ranges all to themselves; “generous” females, who share parts of their home ranges with other females who are often daughters; and “promoting” females, who share their entire home ranges with many other females. The names are meant to make the extent of their home range sharing more intuitive and memorable, not to suggest personality traits or psychological motivations. 

Building off this, Isbell wondered what ecological conditions determine the extent of home range sharing. 

A new view of the primate world

Isbell hypothesizes that the relative need for certainty in knowing where food is makes all the difference. At the one extreme, if primate mothers depend heavily on certainty, they will not be able to share their home ranges with their daughters at all. At the other extreme, if they do not need certainty, they can expand their home ranges enough to let their daughters stay. 

Isbell hopes her book serves as a viable alternative and better explanation for how ecology feeds into primate social organizations.

By challenging common assumptions and explanations on primate socioecology, Isbell welcomes researchers to question what they know about the field. 

“I hope researchers will walk away from the book with a strong desire to test my predictions.” Isbell said. “I don’t see myself as a contrarian, but one could get that impression.”

YOU MAY ALSO LIKE THESE STORIES 

Documenting the Monkey Stone Age in Real-Time 

For the last six years, graduate student Meredith Carlson has studied the ongoing development of stone tool use in white-faced capuchins on the islands of Coiba and Jicarón, both located within Panama's Coiba National Park.

How Pilgrimage Sites Arise

How do pilgrimages and rituals arise? How do people become convinced to try something new? What makes a place so special that it persists through time, drawing people to it again and again? UC Davis anthropologists Cristina Moya and Nicolas Restrepo Ochoa are answering these questions at the ground floor. 

Stories Archive