234
Marine Mammal Physiology: Requisites for Ocean Living
and cetaceans, but the accuracy of the mass measurements were often greatly impacted
by camera angle and animal position. Three-dimensional photogrammetry, which uses
several photographs to create a 3D model of the animal, has improved the accuracy and
precision of photogrammetric mass estimates (Waite et al. 2007). Estimates of whale size
and mass have been obtained using aerial stereophotogrammetry. For large animals like
whales, these types of tools are currently the only way to obtain mass and condition information on wild animals.
10.3.2 Field metabolic rate
Although changes in mass and condition can be used to investigate energy transfer in fasting animals, these techniques are not appropriate for animals that feed during lactation
like otariids and odontocetes. In non-fasting species, it is possible to measure field metabolic rate over short periods of time (~7–10 days), and this, combined with milk energy
output (method discussed below), can be used to investigate lactation and post-partum
reproductive effort.
Field metabolic rate can be measured using the doubly labeled water (DLW) method
(Costa 1987; Speakman 1997). The basis of the DLW method is to follow the decline in
enrichment of the isotopes of oxygen ( 18 O) and hydrogen ( 2 H or 3 H) in the body water.
CO 2 production is measured by injecting known amounts of 2 H 2 O (or 3 H 2 O) and H 2
18 O
into the animal. An initial blood sample is taken after the isotopes equilibrate with the
animal’s body water (as described above) and is followed by a final blood sample at the
end of the study period, 7–10 days later depending on animal size and metabolic rate.
The decline in the hydrogen isotope is a measure of total water influx (TWI), which
is composed of metabolic water production (MWP) and water consumed in the food
(i.e., milk or fish). The 18 O isotope declines as a function of both water flux and CO 2
production. The difference between the rates of decline of these two isotopes is proportional to the animal’s CO 2 production. Energy expenditure can then be calculated from
CO 2 production using an appropriate conversion factor depending on the diet of the
female or pup (Costa 1987).
10.3.3 Milk composition and intake
Knowledge on both the proximate composition of milk and rate of milk production is key
to understanding the intra- and interspecific variation in the patterns of energy transfer
observed in nature and described in this chapter. However, obtaining milk samples from
large and/or difficult to access species can be challenging, and accurate estimates of milk
production may be impractical. Despite this challenge, milk composition has been determined for many marine mammals. From these studies, we know that milk composition
can change substantially over the course of lactation and attendance bouts, so it is important to consider the timing of sampling when studying milk composition and intake. In
pinnipeds, an intramuscular injection of oxytocin is usually administered to help initiate
milk let down before sampling. The lipid, protein, water, and ash components of the milk
are measured independently and in duplicate following standard protocols (reviewed in
Oftedal and Iverson 1995). Lipids are typically extracted using organic solvents and protein
content is usually measured based on the nitrogen content of milk. The water content of
milk is easily measured using oven drying. Carbohydrates are often not analyzed because
of their established minor contribution to marine mammal milk. If measured accurately,
the sum of the individual milk components should total ~100% of the initial sample mass.
Marine Mammal Physiology: Requisites for Ocean Living
and cetaceans, but the accuracy of the mass measurements were often greatly impacted
by camera angle and animal position. Three-dimensional photogrammetry, which uses
several photographs to create a 3D model of the animal, has improved the accuracy and
precision of photogrammetric mass estimates (Waite et al. 2007). Estimates of whale size
and mass have been obtained using aerial stereophotogrammetry. For large animals like
whales, these types of tools are currently the only way to obtain mass and condition information on wild animals.
10.3.2 Field metabolic rate
Although changes in mass and condition can be used to investigate energy transfer in fasting animals, these techniques are not appropriate for animals that feed during lactation
like otariids and odontocetes. In non-fasting species, it is possible to measure field metabolic rate over short periods of time (~7–10 days), and this, combined with milk energy
output (method discussed below), can be used to investigate lactation and post-partum
reproductive effort.
Field metabolic rate can be measured using the doubly labeled water (DLW) method
(Costa 1987; Speakman 1997). The basis of the DLW method is to follow the decline in
enrichment of the isotopes of oxygen ( 18 O) and hydrogen ( 2 H or 3 H) in the body water.
CO 2 production is measured by injecting known amounts of 2 H 2 O (or 3 H 2 O) and H 2
18 O
into the animal. An initial blood sample is taken after the isotopes equilibrate with the
animal’s body water (as described above) and is followed by a final blood sample at the
end of the study period, 7–10 days later depending on animal size and metabolic rate.
The decline in the hydrogen isotope is a measure of total water influx (TWI), which
is composed of metabolic water production (MWP) and water consumed in the food
(i.e., milk or fish). The 18 O isotope declines as a function of both water flux and CO 2
production. The difference between the rates of decline of these two isotopes is proportional to the animal’s CO 2 production. Energy expenditure can then be calculated from
CO 2 production using an appropriate conversion factor depending on the diet of the
female or pup (Costa 1987).
10.3.3 Milk composition and intake
Knowledge on both the proximate composition of milk and rate of milk production is key
to understanding the intra- and interspecific variation in the patterns of energy transfer
observed in nature and described in this chapter. However, obtaining milk samples from
large and/or difficult to access species can be challenging, and accurate estimates of milk
production may be impractical. Despite this challenge, milk composition has been determined for many marine mammals. From these studies, we know that milk composition
can change substantially over the course of lactation and attendance bouts, so it is important to consider the timing of sampling when studying milk composition and intake. In
pinnipeds, an intramuscular injection of oxytocin is usually administered to help initiate
milk let down before sampling. The lipid, protein, water, and ash components of the milk
are measured independently and in duplicate following standard protocols (reviewed in
Oftedal and Iverson 1995). Lipids are typically extracted using organic solvents and protein
content is usually measured based on the nitrogen content of milk. The water content of
milk is easily measured using oven drying. Carbohydrates are often not analyzed because
of their established minor contribution to marine mammal milk. If measured accurately,
the sum of the individual milk components should total ~100% of the initial sample mass.