The Significance of Digesta– an Overlooked Source of Ice Age Carbs


Bison illustration

ANN ARBOR – Until about 10,000 years ago, all people relied on hunting and gathering for subsistence, and many scholars believe they divided that labor by sex– with men doing the bulk of the hunting and women gathering plant foods. Raven Garvey– an associate professor of Anthropology at the University of Michigan and an affiliate of the Research Center for Group Dynamics at the Institute for Social Research– has written a new study just out in Evolutionary Anthropology that complicates the animal/plant dichotomy that is common in the “foraging theory” literature, and explores new possibilities regarding sexual division of labor in the past. 

She introduces an important consideration: early foragers may, in some contexts, have consumed their required portion of “vegetables” in the form of digesta– the semi-digested vegetal matter present in the stomachs or digestive tracts of large game herbivores such as bison.

“This underappreciated resource has important implications for several major questions in evolutionary anthropology,” Garvey says. “Accounting for digesta in our foraging models– as a source of both kilocalories and carbohydrates –leads to very different predictions from models that do not include this resource.”

The study explores the significance of digesta in two contexts: sex-divided subsistence labor and archaeologically observed increases in plant use and sedentism, or the transition to more permanent settlements. 

Subsistence Hunting

Using estimates of available protein and carbohydrates in the native tissues and digesta of a large ruminant herbivore (Bison bison), Garvey shows that, with digesta included, a group of 25 adult foragers could meet the USDA’s average recommendations for proteins and carbs for 3 days without additional supplementation. Such a resource could have been vital in certain contexts– like in areas where plants were scarce or indigestible to humans. It could also have eliminated the need to hunt and gather separately since total nutrition could be obtained from a single resource.

These are skulls excavated from an archaeological site near Roswell, New Mexico.

Bison skulls excavated near Roswell, N.M. UMMAA #83209 a and b, courtesy of the University of Michigan Museum of Anthropological Archaeology


Folsom points

North American folsom points from 10-11,000 years ago, likely used for bison hunting. UMMAA #27673, 27737, 30442, and 39802, courtesy of the University of Michigan Museum of Anthropological Archaeology

In this way, Garvey’s “bison model” raises questions about the assumption that human subsistence labor was always strongly sex-divided and that large game hunting was always dominated by men. She explains: Women’s participation in large game hunting becomes likelier when high energy resources can be acquired with low risk. In times and places where large bodied herbivores were abundant, a group’s overall hunting success could have been improved, and plant – animal scheduling conflicts reduced through women’s hunting.

There is some archaeological evidence to suggest female hunting (and the childcare provisioning that would have entailed)– was more common during prehistory than in the later, ethnographic period, Garvey reports. A study of “grave goods” in burials across North and South America, for example, found that perhaps 30-50% of all large-game hunters in the Americas during the late glacial (late Pleistocene and early Holocene) era– between 8,000 and 13,000 years ago– may have been female. This strategy might have been influenced during that period by both the relative abundance of large-bodied herbivores like bison and by people’s high mobility. Following migratory game, groups increased the reliability of hunting, but created conditions that reduced the accessibility of edible plants. Digesta consumption would have bridged this resource gap.

Social Organization

Digesta consumption could also have contributed to major changes in dietary breadth and human mobility in the Americas, Garvey says. Her “bison model” indicates that while large-bodied herbivores could provide small human groups’ total nutrition for short periods, the ratio of protein to carbohydrates in a single animal makes digesta an unsustainable source of carbohydrates in the long run. That is, a1000-pound bison could provide a group of 25 adult foragers with three days’ worth of protein and carbs, but if they pursued a fresh animal every fourth day, they would leave approximately 6 days’ worth of protein unconsumed each bison kill. As human populations grew in size, it would have become increasingly difficult to address the carb deficit: The level of hunting required to ensure sufficient carbohydrates would become unsustainable. In such cases, Garvey says, the need for reliable access to both protein and carbohydrates, without which humans can’t survive, would necessitate significant social reorganization of settlement and subsistence.

Prehistoric battle scene

An 1890 etching presents a culturally-biased, unlikely picture of prehistoric times. (Getty Images)


Even if herbivores and their stomach contents were not a group’s sole source of key macronutrients—as Garvey notes they likely were not in most times and places—digesta’s positive effect on herbivores’ nutrient profiles could, when available, have affected plant foraging behaviors, relaxing the demand for fresh plant foods and perhaps freeing up time and energy for other activities.

“Many anthropologists are concerned that our evolutionary understanding of humans is disproportionately influenced by contemporary behaviors and cultural expressions that may bear little resemblance to those of the past,” Garvey says. “Allowing for the possible dietary importance of large herbivore digesta and other understudied resources can lead to new interpretations of past behaviors and a healthy reevaluation of assumptions on which our evolutionary hypotheses rest.”

Raven Garvey is the Associate Curator of the UM Museum of Anthropological Archaeology and received an Andrew W. Mellon Foundation fellowship last year to train in engineering and explore the effects of wind on hunter-gatherers’ livelihoods and technologies.

This post was written by RCGD communications specialist Tevah Platt.