Topics: College of Sciences and Humanities, Research

August 17, 2020

A woman hangs a camera in a tree

When the temperatures rise to 105 degrees in the middle of the day on the savanna in West Africa, nursing chimpanzee mothers will seek out a nearby cave more frequently than other females and males, says a Ball State University primatologist.

“We knew some chimpanzees were using the cave more often than others,” said Kelly Boyer Ontl, an assistant professor of anthropology at Ball State. “To find that it was nursing mothers made complete sense.”

The professor came to the conclusion that these females were using the living room-size cave as a cooling center during the heat of the day after she set up a camera facing the cave entrance for 325 days between 2011 and 2013. 

The camera captured nearly 29,720 images of chimpanzee visits, consisting of 35 individuals: 17 adult females, including 10 nursing mothers, three adult males, five juveniles, and 10 infants.

The area surrounding the cave is not populated by people and includes a preferred nesting site near the cave and three Cola cordifolia trees, a favored fruit tree during the late dry season within a few hundred feet.

The results suggest the importance of thermoregulation in extreme heat for all individuals, especially for lactating females with nursing infants.

“We had heard about the use by various animals of this cave, known as Drambos Cave in Senegal,” Boyer Ontl said. “We looked at the images and it was quite obvious that among these chimps, it was the lactating mothers who were visiting the caves most often. Lactating females are more susceptible to dehydration and energy costs and counter that with time spent in the cave.”

Boyer Ontl and co-author Jill D. Pruetz, a Texas State University anthropology professor, recently published their findings, “Mothers Frequent Caves: Lactation Affects Chimpanzee (Pan troglodytes verus) Cave Use in Southeastern Senegal,” in the July issue of the International Journal of Primatology. Boyer Ontl has been studying the animals since the mid-2000s.

The study provides evidence that the animals’ reproductive state affects cave use frequency in West African chimpanzees, which are critically endangered.

A chimpanzee with her baby in the trees

Boyer Ontl notes that specific groups in the population, such as females, infants, and the elderly, are at greater risk of thermoregulatory problems, including dehydration and heat-related stress. Access to cooler cave environments may help buffer the effects of the hot and dry environment for susceptible individuals.

Lactating females are particularly susceptible. They experience the most energetically demanding phase of reproduction, which necessitates increased water intake to support milk production. Female reproductive success – specifically fertility and fitness – varies with access to resources and energy expenditure.

The professor also noted that camera images revealed various behavioral activities of the chimpanzee party in the cave.

The apes entered predominately in small groups and occasionally brought fruit to the cave and fed. They spent the majority of their time resting, socializing, and grooming. Juveniles and infants often played with one another. Juveniles performed camera directed behaviors such as peering at the camera lens or touching the camera, more frequently than other age groups at the onset of the study and continued to do so on occasion throughout the study.

 “Understanding why chimpanzees use caves is important to help conserve the declining species, but the results of this study can also shed light on our own human ancestry,” Boyer Ontl said. “Early humans may have also benefited from cool caves to provide much needed refuge to lactating mothers and their infants.”

Boyer Ontl began studying chimpanzees in West Africa in 2008 at the Fongoli Savanna Chimpanzee research site in Senegal. Her research led her to establish the Falémé Chimpanzee Conservation Project to promote chimpanzee conservation in Senegal through children’s education programs.

Her current research looks at the effects of gold mining activities, particularly the use of mercury in the mining process, on chimpanzees and their habitat. Boyer Ontl also serves as a key member in writing Senegal’s Chimpanzee Conservation National Action Plan due out in 2021.