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Two new studies examining gray wolves in California paint a complex picture of life on California’s ranching landscapes: wolves eat cattle more than anything else and the presence of the predators causes significant stress among livestock.

In the , University of California, Davis, researchers found wolves from the Lassen and Harvey packs in northeastern California were primarily eating cattle. They collected scat samples during the summer months of 2022 and 2023 and found 72% of wolf scat contained cattle DNA. The research was published in PLOS One.

“Whether it's through scavenging or whether it's through depredation, it’s a huge component of the wolves’ diet,” said lead author , a professor of Cooperative Extension in the �鶹��ý Agricultural and Resource Economics Department. “Their conservation success is because of livestock producers in the state.”

Cattle appeared as the most frequently occurring food item during both summers of the study, present in 86% of samples in 2022 and 55% in 2023. While all 2022 samples were from the Lassen Pack, the 2023 samples included eight from the newly established Harvey Pack. 

Wolves are a state and federally recognized endangered species. The first confirmed wolf entered California in 2011, following a nearly 100-year absence. The California Department of Fish and Wildlife estimates the state is now home to nine wolf packs. 

Few natural prey

Wolves in California live in landscapes shaped by people. They have fewer wild ungulate prey options than wolves in other parts of North America. Mule deer are their only significant wild food source, but populations have dropped sharply since the 1970s. 

Saitone said the scarcity of wild prey may be one reason wolves are eating cattle instead. Researchers found mule deer in just 45% of the scat samples, significantly less common than cattle. 

Keeping wolves away from cattle is difficult on many of these working landscapes. 

“Conservatively, we’re talking about a million acres in our study area and 10,000 cow-calf pairs,” said Saitone. “It’s not as simple as putting up an electric fence on a two-acre pasture or putting cattle in the barn at night.”

Cattle are stressed

Researchers also examined how living among wolves affects cattle. Their findings suggest the costs of wolf reintroduction extend well beyond animals lost to predation.

A published in Ecology and Evolution, measured cortisol levels in tail hair samples collected from beef cattle grazing rangelands in northeastern California — some herds sharing territory with wolf packs, others in areas without wolves. Cortisol, often called the stress hormone, accumulates in hair over time, making it a reliable indicator of chronic stress rather than a momentary response to fright.

Cattle herds living among wolves had cortisol levels 58% higher than those in control herds — a significant physiological difference. This marks one of the first times hair cortisol analysis has been used to study how the reintroduction of predators reshapes livestock physiology in the field.

"What this really confirms is that death or depredation is not the only impact here," said Saitone. "Living among wolves for cattle is a chronically stressful experience, and that could ultimately have production-related impacts in both the short and the long term."

Could it affect reproduction?

Saitone said the findings are just a starting point. Researchers next want to understand whether elevated stress levels translate into lower conception rates. It’s a concern long raised by ranchers that has not yet been established through direct biological evidence. When breeding cows fail to conceive, it can directly affect a rancher’s bottom line.  

"That's their whole purpose,” said Saitone. “These are mother cows that are supposed to be generating calves as a marketable product.”

Other authors of the PLOS One study include Kenneth W. Tate and Benjamin J. Sacks of �鶹��ý. Funding came from the Russell L. Rustici Rangeland and Cattle Research Endowment and California Cattle Council. 

Christina Nord was corresponding author of the Ecology and Evolution study. Other authors include Alexander Pritchard, Rosemary A. Blersch, Brenda McCowan, Jessica J. Vandeleest and Kenneth W. Tate. Funding came from Western Sustainable Agriculture Research and Education and the Russell L. Rustici Rangeland and Cattle Research Endowment.