Isolated from other red squirrels since the end of the Pleistocene glacial period about 10 thousand years ago, and previously thought to have gone extinct in the 1960s, the endangered Mount Graham red squirrel has worked its tail off to survive. Nestled in the upper-elevation conifer forests of southeastern Arizona's Pinaleño Mountains, this sub-species of red squirrel is territorially landlocked and highly susceptible to environmental dynamics, including wildfire and drought. 

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To survive in times of scarcity, red squirrels hoard their food in larders, or middens – piles of debris and scales from pine, fir and spruce cones that can grow to considerable depths. The animals also sometimes scatter their food around in different locations. To understand why a Mount Graham red squirrel would choose one food hoarding strategy over another, and to understand the caching strategies of mammals at large, Sean Mahoney and Bret Pasch of the Mt. Graham Biology Programs in the University of Arizona College of Agriculture, Life and Environmental Sciences, conducted a study that revealed temperature and population density –  which can lead to competition for food and the potential for petty theft – are factors that can impact hoarding behavior. 

Storing solutions

The study, published in the journal Animal Ecology, explored the evolution of food caching through a comprehensive analysis of a 25-year dataset on the hoarding behavior of Mount Graham red squirrels. It also integrated behavioral data for nearly 40% of all mammal species.

"We incorporated data on nearly all mammals that cache food, whether they feed on seeds, mushrooms or other animals, all of which have important implications for nutrient cycling," said Mahoney, a research scientist in the School of Natural Resources and the Environment. "Tree squirrels are particularly interesting because scattered cones once forgotten can influence forest regeneration."

The study found that food caching strategies are very flexible, with many species switching between one strategy and the other numerous times. Temperature and population density were identified as factors that influence hoarding behavior, with colder temperatures and higher population densities being strongly associated with larder hoarding.

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When it came to the Mount Graham red squirrel, variation in mean annual temperature over the past 24 years in the area did not correlate with changes in food hoarding behavior. However, researchers found that squirrels invested less in larder hoards during years of low population numbers, which suggests that competition for food and animals stealing from one another may explain the variation in hoarding decisions.

"Squirrels appear to be sensitive to their social environment and upon seeing or hearing other squirrels decide, 'OK, I need to bring cones to my midden before others take them,'" explained Pasch, an associate professor in the School of Natural Resources and the Environment and director of the Mt. Graham Biology Programs. "In addition, increased rates of petty theft can occur at higher densities, and squirrels may compensate for what's been stolen."

Beyond contributing to the general body of scientific knowledge, the study provides practical data for conservation efforts. Biologists have historically used middens as a one-to-one index of squirrel population size. However, changing environmental conditions might promote scatter hoarding, necessitating alternative methods for monitoring squirrel populations. By observing mammalian food hoarding behavior, wildlife managers can also better predict the factors that could influence species' caching behaviors in the future.

"The data suggests that we need to be mindful of population density and temperature, and so we know that it's getting warmer, and cooler temperatures are associated with storing food in one place," Mahoney said. "This gives us some insight into the factors that might be pushing animals toward certain behaviors in the future."

As temperatures change and population numbers shift, critical habitat designations may need to be refined, and Mahoney and Pasch's work highlights how the study of animal behavior can be a powerful tool in the development of adaptive conservation management strategies.