Kangaroo rats revisited: re-evaluating a classic case of desert survival

Kangaroo rats are the archetypical organisms for mammalian survival in North American deserts, yet there are contradictions in the data surrounding their physiology and ecology. The traditional view has been that these nocturnal rodents have little tolerance to high temperatures (e.g., >30°C), reside in cool, humid burrows to escape the heat of the day, and nearly exclusively rely on a dry, carbohydrate-rich diet from which they metabolically derive most of their water supply. To test this view, we measured the microclimates, activity, and diet of Merriam's kangaroo rats (Dipodomys merriami) from a xeric location within the center of the Sonoran Desert. We arrive at the following conclusions: 1. Burrows are much hotter during the summer than previously appreciated. For over 100 days of the year, soil temperatures exceed 30°C at depths to 2 m. For over 50 days, temperatures exceed 35°C at depths to 1.5 m. These high temperatures at such depths preclude kangaroo rats from locating to cool temperatures (e.g., <30°C) by burrowing. 2. Kangaroo rats remain in shallow burrows (<1 m) at relatively high ambient temperatures (>35°C) throughout the daytime during the summer instead of residing deep within the soil. This finding supports recent laboratory experiments that show kangaroo rats have much higher thermal tolerances than previously realized. 3. Kangaroo rats do not restrict their activity to the coolest periods of the night, but are active immediately following sundown, during the hottest time of the night. 4. Burrows are not persistently humid, but can be quite dry. 5. Insects and succulent vegetation constitute a significant portion of a kangaroo rat's diet and may be key to their survival in the hot desert environment.