Effects of surface and sub-surface temperature on the activity budget of a microhabitat specialist

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Small mammals that make use of sub-surface microclimates might be able to adapt quickly to a warming climate by altering the amount and timing of certain surface activities. For example, energy-intensive territorial defense or foraging activities might be shifted to cooler times of day. We explored this hypothesis using data on the American pika (Ochotona princeps), a small, diurnal lagomorph that uses talus and other rocky microhabitats for shedding heat between bouts of surface activity. Over three consecutive summers (mid-June to mid-August, 2012-2014), we conducted 128 observations (45 minutes each) involving N = 69 unique pikas, using a standardized protocol to record behaviors during continuous focal-animal sampling. During each observation, data loggers were used to record shade temperatures in surface and sub-surface microhabitats available to the focal pika. Most observations were conducted at the Niwot Ridge Long Term Ecological Research site in Boulder County, Colorado (USA), with the goal of comparing historical data from this site with contemporary and future data. Here, we report a contemporary, temperature-indexed activity budget for pikas in this area. Surface temperatures averaged 5-10°C higher than sub-surface temperatures during diurnal observations, and pikas spent about two-thirds of their time below the surface. Modeling surface activity as a linear function of microclimate and microhabitat variables, sub-surface temperature was a significant, inverse predictor of surface activity. The difference between surface and sub-surface temperature also predicted surface activity, but this relationship was nonlinear and suggested that a strong temperature gradient was more important for pika thermoregulation when the surface temperature was greater than 15°C. This study provides a current baseline for studying climate-related shifts in pika behavior, by providing data on behavior and temperature variation within currently available microhabitats.