Rates of O(2) consumption and CO(2) production, telemetered body temperature (T(b)) and activity level were recorded from adult and subadult water shrews (Sorex palustris) over an air temperature (T(a)) range of 3-32 degrees C. Digesta passage rate trials were conducted before metabolic testing to estimate the minimum fasting time required for water shrews to achieve a postabsorptive state. Of the 228 metabolic trials conducted on 15 water shrews, 146 (64%) were discarded because the criteria for inactivity were not met. Abdominal T(b) of S. palustris was independent of T(a) and averaged 38.64 +/- 0.07 degrees C. The thermoneutral zone extended from 21.2 degrees C to at least 32 degrees C. Our estimate of the basal metabolic rate for resting, postabsorptive water shrews (96.88 +/- 2.93 J g(-1) h(-1) or 4.84 +/- 0.14 ml O(2) g(-1) h(-1)) was three times the mass-predicted value, while their minimum thermal conductance in air (0.282 +/- 0.013 ml O(2) g(-1) h(-1)) concurred with allometric predictions. The mean digesta throughput time of water shrews fed mealworms (Tenebrio molitor) or ground meat was 50-55 min. The digestibility coefficients for metabolizable energy (ME) of water shrews fed stickleback minnows (Culaea inconstans) and dragonfly nymphs (Anax spp. and Libellula spp.) were 85.4 +/- 1.3% and 82.8 +/- 1.1%, respectively. The average metabolic rate (AMR) calculated from the gas exchange of six water shrews at 19-22 degrees C (208.0 +/- 17.0 J g(-1) h(-1)) was nearly identical to the estimate of energy intake (202.9 +/- 12.9 J g(-1) h(-1)) measured for these same animals during digestibility trials (20 degrees C). Based on 24-h activity trials and our derived ME coefficients, the minimum daily energy requirement of an adult (14.4 g) water shrew at T(a) = 20 degrees C is 54.0 kJ, or the energetic equivalent of 14.7 stickleback minnows.

Abstract. This paper documents a cost-effective method for the long-term housing and maintenance of water shrews. Wild-caught American water shrews (Sorex palustris) were successfully maintained in this set up for up to 2 years, suggesting a maximum lifespan of 28 months for this species in captivity. In addition, we describe the postnatal growth and development of S. palustris based on data collected from two litters born in captivity, together with corresponding changes in maternal mass over the gestational and postpartum periods. Minimum gestation for this species is 21 days with neonatal mass at birth averaging 0.42 g (n = 3). This study recorded one of the fastest growth rates to date for any Sorex shrew (0.51 g/day), with body mass beginning to plateau by 23 days of age. During the last two weeks of pregnancy, body mass of one pregnant shrew increased from 16.0 to 22.8 g (0.45 g/day). Maternal mass quickly dropped by 5 to 7 g following birth. However, maternal body mass then gradually increased by ca. 20% during the 24 to 27 day lactation period, despite a >30 fold increase in litter mass over this same interval. Exposure of young to water was initiated on day 25 with only surface swimming occurring on this day. All shrews were observed diving the following day, and appeared to be fully proficient divers by 40 to 45 days of age.

Jones (1997) formulated the adaptive hypothermia hypothesis to explain why some species routinely exceed their oADL. By intentionally decreasing core T6, divers can theoretically decrease DMR, due to the Q¡6 effect, and thereby extend dive duration while maintaining an aerobic metabolism (Geiser 1988).However, the benefit of increased aerobic dive endurance comes with the mandatory energetic cost of re-warming cooled tissues after immersion. Studies examining the "adaptive hypothermia" argument have yielded mixed results (Hall et al. 1987, Ponganis et a|.2003, Hindle et a|.2006). The water shrew offers an ideal model for testing this hypothesis. This is because it has the largest surface-area-to-volume ratio, smallest onboard Oz stores and the highest mass-specific diving metabolic rate of any diver, all factors that predict rapid cooling and a limited scope for dive endurance.