The function of haypiles of pikas (Ochotona princeps)

The pika, Ochotona princeps, spends a considerable amount of time during summer caching vegetation. These caches (haypiles) have been suggested to function as the mainstay of diet in winter, as a source of food for intermittent periods when the animal is unable to forage elsewhere, or in nonfood functions, e.g., nest sites. I investigated the haying behavior of a population of pikas on the West Knell of Niwot Ridge, Colorado. In addition to directly measuring sizes of haypiles of pikas on West Knell, I also estimated, using data from the literature, sizes of haypiles of another population of pikas. Haypiles that were excavated immediately after construction revealed that pikas from West Knell stored 350 days of food, more than that required for winter. Initial contents of haypiles, discounted for decay and spring leftovers, indicated that pikas from West Knell consumed at greater than or equal to 175 days of food from their haypile during winter 1992-1993. Sizes of haypiles based on behavioral data collected on a different population of pikas corroborated the large size of haypiles found on West Knell. Given these data, the primary function of haypiles appears to be to provide the major source of sustenance for pikas during winter.