147
Chapter seven: Water balance
evaporative cooling of the body in aquatic environments (Irving et al. 1935; Ridgway 1972).
Water loss through cutaneous evaporation is considered negligible in phocids due to the
lack of sweat glands (Lester and Costa 2006). Exceptionally, active sweating from the bare
skin of the flippers was observed in the California sea lion (Zalophus californianus) under
hot condition (Matsuura and Whittow 1974). Regardless, the characteristics of the skin such
as low moisture permeability and high lipid content may contribute to the conservation of
body water. Indeed, Kjeld (2003) calculated that in the sei whale (Balaenoptera borealis) and fin
whale that they have no water loss from skin. On the other hand, the skin may serve as an
avenue of water flux in Cetacea, although Telfer et al. (1970) concluded the idea was unlikely.
Hui (1981) calculated that water flux across skin may account for as much as 70% of total
flux in fasting dolphins. Furthermore, delphinids in freshwater may obtain water from the
water flux across the skin dependent on the osmotic gradient of the environment (Andersen
and Nielsen 1983). However, further studies of a more comprehensive nature are needed to
confirm that water flux across the skin is viable in marine mammals, especially in cetaceans.
Respiratory evaporation is another avenue for water loss. In phocids, body water
is conserved by cooling the expired air by nasal a countercurrent heat exchanger (parallel pipes of flowing fluids at different temperatures in opposite directions to exchange
their heat content) resulting in the reduction of evaporative water loss (Huntley et al.
1984; Folkow and Blix 1987). Respiratory water loss through evaporation was calculated
to be lower in dolphins than in terrestrial mammals of similar body mass (Coulombe
et al. 1965). This reduction in respiratory water loss was likely the result of controlling
temperature and pressure within the respiratory system and lowering ventilation rate
(Coulombe et al. 1965). Aside from conserving water, the respiratory system may also
serve as a site of water gain as the air passing through the nasal passage across the nasal
mucosa condenses because of the nasal heat exchange mechanism (Depocas 1971). While
metabolic water and mariposia accounted for 93% of total daily water influx in experimentally dehydrated hooded seals, respiratory water influx was calculated to contribute
as much as 7% of this influx (Alvira-Iraizoz 2014). Skalstad and Nordøy (2000) calculated
that the respiratory water intake accounted for approximately 5% of total water intake in
the hooded seal (Cysophora cristata) and harp seal (Pagophilus groenlandicus); however, they
noted that the net water flux through respiration is negative, so more water is lost than
gained. Nonetheless, the nasal countercurrent heat exchanger abates the conservation of
water loss during respiration in marine mammals.
7.2.3 Source of water
The issue of freshwater availability that confronts marine mammals is often likened to
that of stranded drifters at sea because they cannot access freshwater. However, unlike
drifters at sea, the availability of freshwater for marine mammals is much less of a life
crisis. Their ability to obtain the requisite amount of freshwater to maintain their life at sea
remains a fascinating question.
7.2.3.1 Diet is the main source of free water
In general, the available data, with few exceptions, suggest that the vast majority of marine
mammals that live in hypertonic habitats do not actively consume water. For the few studies of mammals in freshwater environments, the data would suggest that active drinking
is common but not exceptional (Ortiz 2001), and most likely because these species must
guard against the potential for overconsumption inducing hyponatremia or hemodilution.
Regardless, pre-formed water in the diet of marine mammals is a principal source of free water.
Chapter seven: Water balance
evaporative cooling of the body in aquatic environments (Irving et al. 1935; Ridgway 1972).
Water loss through cutaneous evaporation is considered negligible in phocids due to the
lack of sweat glands (Lester and Costa 2006). Exceptionally, active sweating from the bare
skin of the flippers was observed in the California sea lion (Zalophus californianus) under
hot condition (Matsuura and Whittow 1974). Regardless, the characteristics of the skin such
as low moisture permeability and high lipid content may contribute to the conservation of
body water. Indeed, Kjeld (2003) calculated that in the sei whale (Balaenoptera borealis) and fin
whale that they have no water loss from skin. On the other hand, the skin may serve as an
avenue of water flux in Cetacea, although Telfer et al. (1970) concluded the idea was unlikely.
Hui (1981) calculated that water flux across skin may account for as much as 70% of total
flux in fasting dolphins. Furthermore, delphinids in freshwater may obtain water from the
water flux across the skin dependent on the osmotic gradient of the environment (Andersen
and Nielsen 1983). However, further studies of a more comprehensive nature are needed to
confirm that water flux across the skin is viable in marine mammals, especially in cetaceans.
Respiratory evaporation is another avenue for water loss. In phocids, body water
is conserved by cooling the expired air by nasal a countercurrent heat exchanger (parallel pipes of flowing fluids at different temperatures in opposite directions to exchange
their heat content) resulting in the reduction of evaporative water loss (Huntley et al.
1984; Folkow and Blix 1987). Respiratory water loss through evaporation was calculated
to be lower in dolphins than in terrestrial mammals of similar body mass (Coulombe
et al. 1965). This reduction in respiratory water loss was likely the result of controlling
temperature and pressure within the respiratory system and lowering ventilation rate
(Coulombe et al. 1965). Aside from conserving water, the respiratory system may also
serve as a site of water gain as the air passing through the nasal passage across the nasal
mucosa condenses because of the nasal heat exchange mechanism (Depocas 1971). While
metabolic water and mariposia accounted for 93% of total daily water influx in experimentally dehydrated hooded seals, respiratory water influx was calculated to contribute
as much as 7% of this influx (Alvira-Iraizoz 2014). Skalstad and Nordøy (2000) calculated
that the respiratory water intake accounted for approximately 5% of total water intake in
the hooded seal (Cysophora cristata) and harp seal (Pagophilus groenlandicus); however, they
noted that the net water flux through respiration is negative, so more water is lost than
gained. Nonetheless, the nasal countercurrent heat exchanger abates the conservation of
water loss during respiration in marine mammals.
7.2.3 Source of water
The issue of freshwater availability that confronts marine mammals is often likened to
that of stranded drifters at sea because they cannot access freshwater. However, unlike
drifters at sea, the availability of freshwater for marine mammals is much less of a life
crisis. Their ability to obtain the requisite amount of freshwater to maintain their life at sea
remains a fascinating question.
7.2.3.1 Diet is the main source of free water
In general, the available data, with few exceptions, suggest that the vast majority of marine
mammals that live in hypertonic habitats do not actively consume water. For the few studies of mammals in freshwater environments, the data would suggest that active drinking
is common but not exceptional (Ortiz 2001), and most likely because these species must
guard against the potential for overconsumption inducing hyponatremia or hemodilution.
Regardless, pre-formed water in the diet of marine mammals is a principal source of free water.