149
Chapter seven: Water balance
some degree of dehydration (Nordøy et al. 1992). In bottlenose dolphin, fasting (days) did
not significantly increase plasma osmolality with levels remaining comparable to fed dolphins suggesting that acute periods of food deprivation are inconsequential (Ortiz et al. 2010;
Ridgway and Venn-Watson 2010; Venn-Watson et al. 2011). Other than plasma osmolality, the
total protein and electrolyte concentrations, hematocrit (Hct: the volume percentage of red
blood cells in blood) is another clinical indicator of hydration state (Shirreffs 2000). However,
its use in marine mammals is not as predictable and indicative. In postweaning northern
elephant seal pups, Hct increased 35% over the course of 8 weeks of fasting (Ortiz et al. 2000);
however, the other indices of dehydration (plasma osmolality, [Na + ], and total protein) were
not altered suggesting that the increase in Hct was likely the result of increased red blood
cell volume and not loss of water from the blood (classical dehydration). This is further supported by the fact that the variability in Hct among marine mammals may be attributed to
an increase in red blood cell volume associated with their deep-diving ability (Kohin 1998).
Therefore, changes in Hct may vary independent of hydration state in deep-diving mammals and may not be a reliable indicator of dehydration (Castellini et al. 1996).
Fasting marine mammals conserve body water by a combination of mechanisms. Fasting
also alters renal hemodynamics (blood flow), which in turn may impart an effect on the reabsorption of filtered electrolytes and free water to maintain fluid and electrolyte homeostasis.
In fasting pinnipeds, urine osmolality is transiently increased before returning to baseline
suggesting that renal mechanisms are dynamically changing in response to fasting duration
(Skog and Folkow 1994; Ortiz et al. 1996). Prolonged fasting (2.5 months) in northern elephant
seal pups decreases protein catabolism, which reduces the nitrogen (primarily urea) load on
the kidneys, GFR, and consequently urine volume (Pernia et al. 1989; Adams and Costa 1993).
Lactating elephant seals can prevent urinary water loss during fasting by increasing fractional reabsorption of urea, despite an increase in GFR in the presence of elevated protein
catabolism necessary for supporting lactation (Crocker et al. 1998). Furthermore, respiratory
evaporative water loss is reduced, resulting in water conservation in fasting northern elephant seals (Lester and Costa 2006). Collectively, these mechanisms comprise a suite of physiological alterations that contribute to the conservation of body water.
Contrary to concentrating urine in fasting pinnipeds, cetaceans excrete diluted urine
in response to fasting as an acute phase response (Bentley 1963; Telfer et al. 1970; Hui 1981;
Ridgway and Venn-Watson 2010). The oxidation of fat can produce more metabolic water
than the catabolism of the other substrates, and fasting marine mammals rely primarily on
the metabolic water from the oxidation of their vast fat stores (Worthy and Lavigne 1987).
Acute food deprivation in the bottlenose dolphin quickly increased plasma non-e sterified
fatty acids (NEFAs) suggesting that lipid metabolism is increased to help alleviate the
potential for nutritional and osmotic stress (Ortiz et al. 2010). Furthermore, food deprivation also induced metabolic water production in the West Indian manatee as in pinnipeds
and cetaceans (Ortiz et al. 1999) suggesting that metabolic water is a significant source
of free water across all marine mammals. In addition to the conservation of body water,
electrolyte homeostasis is also maintained during fasting. Remarkably, the nearly 3-month
long fasts of the northern elephant seal are characterized by the maintenance of plasma
electrolyte concentrations, which is likely achieved by increased renal reabsorption of Na +
and K + at the expense of H + (Ortiz et al. 2000).
7.2.3.3 Drinking of seawater and freshwater
Mariposia, or the voluntary and deliberate consumption of seawater, is not common in marine
mammals, despite the fact that the vast majority of these animals can produce more concentrated urine than seawater (Table 7.2). The consumption of seawater in cetaceans is thought
Chapter seven: Water balance
some degree of dehydration (Nordøy et al. 1992). In bottlenose dolphin, fasting (days) did
not significantly increase plasma osmolality with levels remaining comparable to fed dolphins suggesting that acute periods of food deprivation are inconsequential (Ortiz et al. 2010;
Ridgway and Venn-Watson 2010; Venn-Watson et al. 2011). Other than plasma osmolality, the
total protein and electrolyte concentrations, hematocrit (Hct: the volume percentage of red
blood cells in blood) is another clinical indicator of hydration state (Shirreffs 2000). However,
its use in marine mammals is not as predictable and indicative. In postweaning northern
elephant seal pups, Hct increased 35% over the course of 8 weeks of fasting (Ortiz et al. 2000);
however, the other indices of dehydration (plasma osmolality, [Na + ], and total protein) were
not altered suggesting that the increase in Hct was likely the result of increased red blood
cell volume and not loss of water from the blood (classical dehydration). This is further supported by the fact that the variability in Hct among marine mammals may be attributed to
an increase in red blood cell volume associated with their deep-diving ability (Kohin 1998).
Therefore, changes in Hct may vary independent of hydration state in deep-diving mammals and may not be a reliable indicator of dehydration (Castellini et al. 1996).
Fasting marine mammals conserve body water by a combination of mechanisms. Fasting
also alters renal hemodynamics (blood flow), which in turn may impart an effect on the reabsorption of filtered electrolytes and free water to maintain fluid and electrolyte homeostasis.
In fasting pinnipeds, urine osmolality is transiently increased before returning to baseline
suggesting that renal mechanisms are dynamically changing in response to fasting duration
(Skog and Folkow 1994; Ortiz et al. 1996). Prolonged fasting (2.5 months) in northern elephant
seal pups decreases protein catabolism, which reduces the nitrogen (primarily urea) load on
the kidneys, GFR, and consequently urine volume (Pernia et al. 1989; Adams and Costa 1993).
Lactating elephant seals can prevent urinary water loss during fasting by increasing fractional reabsorption of urea, despite an increase in GFR in the presence of elevated protein
catabolism necessary for supporting lactation (Crocker et al. 1998). Furthermore, respiratory
evaporative water loss is reduced, resulting in water conservation in fasting northern elephant seals (Lester and Costa 2006). Collectively, these mechanisms comprise a suite of physiological alterations that contribute to the conservation of body water.
Contrary to concentrating urine in fasting pinnipeds, cetaceans excrete diluted urine
in response to fasting as an acute phase response (Bentley 1963; Telfer et al. 1970; Hui 1981;
Ridgway and Venn-Watson 2010). The oxidation of fat can produce more metabolic water
than the catabolism of the other substrates, and fasting marine mammals rely primarily on
the metabolic water from the oxidation of their vast fat stores (Worthy and Lavigne 1987).
Acute food deprivation in the bottlenose dolphin quickly increased plasma non-e sterified
fatty acids (NEFAs) suggesting that lipid metabolism is increased to help alleviate the
potential for nutritional and osmotic stress (Ortiz et al. 2010). Furthermore, food deprivation also induced metabolic water production in the West Indian manatee as in pinnipeds
and cetaceans (Ortiz et al. 1999) suggesting that metabolic water is a significant source
of free water across all marine mammals. In addition to the conservation of body water,
electrolyte homeostasis is also maintained during fasting. Remarkably, the nearly 3-month
long fasts of the northern elephant seal are characterized by the maintenance of plasma
electrolyte concentrations, which is likely achieved by increased renal reabsorption of Na +
and K + at the expense of H + (Ortiz et al. 2000).
7.2.3.3 Drinking of seawater and freshwater
Mariposia, or the voluntary and deliberate consumption of seawater, is not common in marine
mammals, despite the fact that the vast majority of these animals can produce more concentrated urine than seawater (Table 7.2). The consumption of seawater in cetaceans is thought