Hooded seal (Cystophora cristata) pups ingest snow and seawater during their post-weaning fast

Abstract

The purpose of this study was to evaluate the importance of exogenous water intake (snow/seawater) in hooded seal (Cystophora cristata) pups during their post-weaning fast. In this study, five hooded seal pups had ad lib access to snow and seawater for the first 12 and last 21 days of their post-weaning fast, respectively. Total body water and water flux were determined during both exposure periods by use of the tritiated water method. Blood samples were collected to monitor changes in hematocrit, plasma urea and plasma osmolality. Body mass loss was on average 0.36 kg day−1. Average total body water changed from 15.7 to 11.4 L, while total water influx changed from 15 to 18 mL day−1 kg−1 during snow and seawater exposure, respectively. Of this influx an average of 35% can be attributed to metabolic water, while approximately 8% was due to respiratory water influx. Interestingly, 56 and 58% of the total water influx was due to snow and seawater ingestion, respectively, amounting to 8 mL day−1 kg−1 snow (counted as liquid water) and 10 mL day−1 kg−1 seawater. Based on the results of the plasma parameters it is concluded that fasting hooded seal pups maintain water balance and homeostasis when access to snow or seawater is permitted. It is further concluded that snow and seawater intake, in addition to metabolic and respiratory water, is important for maintenance of water balance and excretion of urea during the post-weaning fast of hooded seal pups.Electronic supplementary materialThe online version of this article (doi:10.1007/s00360-016-1048-3) contains supplementary material, which is available to authorized users.

Seasonal changes in background levels of deuterium and oxygen-18 proves water drinking by harp seals, which affects the use of the DLW method

“…Second, this study may apply to studies on energy expenditure of wild marine mammals, such as the freshwater Baikal seal (Pusa sibirica) or icebreeding phocids of which some have been shown to depend on freshwater in the form of snow and ice during fasting (e.g. Schots et al, 2017). Finally, the findings in this study may also apply to all DLW studies of wild populations of birds and mammals that rely on freshwater drinking for maintenance of water balance homeostasis.…”

“…Total body water was calculated as the relationship between the injected isotope dose and the isotope dose found in the seal body water after equilibration time as explained by Lifson and McClintock (1966) and applied by Schots et al (2017). The fractional turnover rate was calculated based on the difference in isotope concentration at the beginning and end of the 8 day experiment, as also explained by Lifson and McClintock (1966) and applied by Schots et al (2017). Under the assumption that total body water and the turnover rate remain stable during the 1 week experimental period, the total water influx rate (which equals the water efflux rate) could be calculated using eqn 8 from Lifson and McClintock (1966).…”

“…Under the assumption that total body water and the turnover rate remain stable during the 1 week experimental period, the total water influx rate (which equals the water efflux rate) could be calculated using eqn 8 from Lifson and McClintock (1966). The total water influx rate can be divided into the respiratory water influx, free water in the food and metabolic water influx, and water influx due to (freshwater) drinking using similar calculations as explained by Skalstad and Nordøy (2000) and Schots et al (2017).…”

“…reduced influx of metabolic water and free water in the food, the seals probably needed to drink more water to maintain water balance. For instance, young hooded seals do ingest snow and seawater during the respective beginning and end of their post-weaning fast to maintain water balance when water influx through food is absent (Schots et al, 2017).…”

“…Thus, it has often been assumed in studies of seals that all water is obtained from free and metabolic water from the food they eat and that this is sufficient to maintain water balance (Depocas et al, 1971). Even so, a number of studies have shown that seals may on occasion both eat snow and drink freshwater and seawater (Gales and Renouf, 1993;How and Nordøy, 2007;Renouf et al, 1990;Schots et al, 2017;Skalstad and Nordøy, 2000).…”