233
Chapter ten: Post-partum
the temporal separation of foraging and parental investment. This makes it possible to
look at reproductive effort without the confounding variable of food intake. The earliest studies investigating reproductive effort used changes in mass across lactation as an
estimate of energy investment in pups. Although this seems like a simple approach, when
working with large wild animals it is important to develop techniques to safely chemically or physically immobilize the animal allowing for a basic measurement, like mass.
Even when safely immobilized, the large body size of some phocids provided distinct
challenges. For example, in order to obtain mass measurements from large female phocids
(250–600 kg), researchers must lift the animal using a winch attached to a scale hanging
from a 3 m tall tripod. For large male pinnipeds, researchers have even gotten animals to
move across truck scales.
Although these studies were important to the emerging field of life-history studies,
it was recognized that mass just gives an approximation of energy investment, as not all
mass is created equal (i.e., fat has twice the energy density of protein). Researchers started
to use techniques that allowed them to calculate energy expenditure (mothers) or gain
( offspring) from changes in body composition and mass by converting the mass of tissue
lost (or gained) to an energy equivalent based on the energy content of lipid and protein.
This allowed for more precise estimates of total post-partum reproductive effort (milk output and metabolic overhead) and to calculate transfer efficiency to offspring (energy gained
by pup/energy lost by mother).
Obtaining precise body composition measurements in live animals is difficult. The
most widely used techniques in field studies measure fat content and then partition the
animal’s mass into two compartments—fat and fat-free mass (often referred to as lean
body mass). A number of approaches are used to estimate the body composition of marine
mammals, with the size and accessibility of the animal determining what method is used.
One technique, often considered the gold standard for determining body composition of
live animals, is the isotope dilution method (Bowen and Iverson 1998). Total body water is
determined by injecting a known quantity of labeled water (usually 2 H 2 O or 3 H 2 O) into
the animal, allowing the isotope to equilibrate with the animal’s body water, and taking a
blood or urine sample to determine the isotope dilution space. The estimate of total body
water is combined with estimates of tissue hydration state to derive total lipid mass. This
is similar to how body composition bathroom scales calculate your percent fat from estimates of total body water based on electrical conductivity.
Morphometric measurements including length, girth, and blubber depth can also be
used to estimate body composition. One such approach is the truncated cones method, where
body composition is estimated by combining morphometrics with ultrasonic measures of
blubber thickness at various sites on the body. These measurements are used to model the
seal as a series of truncated cones, allowing calculation of blubber and non-blubber compartments of each cone, which can then be summed to estimate total body composition
(Gales and Burton 1987). This technique has been shown to be an accurate estimate of lipid
content in a wide variety of phocid seals. Since this method is based on estimating the
volume of the subdermal blubber layer, it works best in species, like some phocids, whose
fat reserves are largely subcutaneous. However, it may be less accurate in species that store
significant lipid inside the body cavity or within muscle.
The size of some marine mammals, especially the larger odontocetes and mysticete whales, does not allow researchers to use the methods described above, so they
rely on photogrammetric techniques for estimating size and condition. Photogrammetry
is the technique of making measurements based on photographs. Two-dimensional photogrammetry has been used to estimate size and mass in several species of pinnipeds
Chapter ten: Post-partum
the temporal separation of foraging and parental investment. This makes it possible to
look at reproductive effort without the confounding variable of food intake. The earliest studies investigating reproductive effort used changes in mass across lactation as an
estimate of energy investment in pups. Although this seems like a simple approach, when
working with large wild animals it is important to develop techniques to safely chemically or physically immobilize the animal allowing for a basic measurement, like mass.
Even when safely immobilized, the large body size of some phocids provided distinct
challenges. For example, in order to obtain mass measurements from large female phocids
(250–600 kg), researchers must lift the animal using a winch attached to a scale hanging
from a 3 m tall tripod. For large male pinnipeds, researchers have even gotten animals to
move across truck scales.
Although these studies were important to the emerging field of life-history studies,
it was recognized that mass just gives an approximation of energy investment, as not all
mass is created equal (i.e., fat has twice the energy density of protein). Researchers started
to use techniques that allowed them to calculate energy expenditure (mothers) or gain
( offspring) from changes in body composition and mass by converting the mass of tissue
lost (or gained) to an energy equivalent based on the energy content of lipid and protein.
This allowed for more precise estimates of total post-partum reproductive effort (milk output and metabolic overhead) and to calculate transfer efficiency to offspring (energy gained
by pup/energy lost by mother).
Obtaining precise body composition measurements in live animals is difficult. The
most widely used techniques in field studies measure fat content and then partition the
animal’s mass into two compartments—fat and fat-free mass (often referred to as lean
body mass). A number of approaches are used to estimate the body composition of marine
mammals, with the size and accessibility of the animal determining what method is used.
One technique, often considered the gold standard for determining body composition of
live animals, is the isotope dilution method (Bowen and Iverson 1998). Total body water is
determined by injecting a known quantity of labeled water (usually 2 H 2 O or 3 H 2 O) into
the animal, allowing the isotope to equilibrate with the animal’s body water, and taking a
blood or urine sample to determine the isotope dilution space. The estimate of total body
water is combined with estimates of tissue hydration state to derive total lipid mass. This
is similar to how body composition bathroom scales calculate your percent fat from estimates of total body water based on electrical conductivity.
Morphometric measurements including length, girth, and blubber depth can also be
used to estimate body composition. One such approach is the truncated cones method, where
body composition is estimated by combining morphometrics with ultrasonic measures of
blubber thickness at various sites on the body. These measurements are used to model the
seal as a series of truncated cones, allowing calculation of blubber and non-blubber compartments of each cone, which can then be summed to estimate total body composition
(Gales and Burton 1987). This technique has been shown to be an accurate estimate of lipid
content in a wide variety of phocid seals. Since this method is based on estimating the
volume of the subdermal blubber layer, it works best in species, like some phocids, whose
fat reserves are largely subcutaneous. However, it may be less accurate in species that store
significant lipid inside the body cavity or within muscle.
The size of some marine mammals, especially the larger odontocetes and mysticete whales, does not allow researchers to use the methods described above, so they
rely on photogrammetric techniques for estimating size and condition. Photogrammetry
is the technique of making measurements based on photographs. Two-dimensional photogrammetry has been used to estimate size and mass in several species of pinnipeds