Dive Performance and Aquatic Thermoregulation of the World’s Smallest Mammalian Diver, the American Water Shrew (Sorex palustris)

Abstract

Allometry predicts that the 12-17 g American water shrew (Sorex palustris)—the world's smallest mammalian diver—will have the highest diving metabolic rate (DMR) coupled with the lowest total body oxygen storage capacity, skeletal muscle buffering capacity, and glycolytic potential of any endothermic diver. Despite these constraints, the maximum dive time (23.7 sec) and calculated aerobic dive limit (cADL; 10.8-14.4 sec) of wild-caught water shrews match or exceed values predicted by allometry based on studies of larger-bodied divers. The mean voluntary dive time of water shrews in 3, 10, 20, and 30°C water was 5.1±0.1 sec (N=25, n=1584), with a mean maximum dive time of 10.3±0.4 sec. Only 2.3-3.9% of dives in 30 and 10°C water, respectively, exceeded the cADL. Mean dive duration, duration of the longest dive, and total time in water all decreased significantly as temperature declined, suggesting that shrews employed behavioral thermoregulation to defend against immersion hypothermia. As expected from their low thermal inertia, diving shrews had a significantly higher DMR in 10°C (8.77 mL O2 g-1 hr-1) compared to 30°C water (6.57 mL O2 g-1 hr-1). Diving behavior of radio-telemetered shrews exclusively foraging in a simulated riparian environment (3°C water) for 12- to 28-hours suggest that mean (but not maximum) dive times of water shrews in the wild may be longer than predicted from our voluntary dive trials, as the average dive duration (6.9±0.2 sec, n=257) was ~1.75x longer than during 20-min trials with no access to food at the same water temperature. Notably, free-diving shrews in the 24-hr trials consistently elevated core body temperature by ~1.0-1.5°C immediately prior to initiating aquatic foraging bouts, and ended these bouts when body temperature was still at or above normal resting levels (~37.8°C). We suggest this observed pre-dive hyperthermia aids to heighten the impressive somatosensory physiology, and hence foraging efficiency, of this diminutive predator while submerged.