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Chapter ten: Post-partum
adaptations that allow them to give birth, suckle, and nurture their young in water. In contrast,
pinnipeds leave the marine environment to reproduce and give birth on land or ice.
Underneath this broad difference, there exists a diversity of parental strategies and
physiological capabilities that shape the movements, breeding systems, foraging strategies, and life-history patterns of marine mammals. In some species (e.g., some mysticetes and many phocids), movement to an appropriate breeding environment necessitates
a capital breeding strategy, where stored body reserves are accumulated between breeding
episodes and then used for reproduction. Other species (e.g., odontocetes and otariids)
use an income breeding strategy, where offspring are provisioned using resources acquired
from foraging during the lactation period (Figure 10.3).
10.2 Knowledge by order
10.2.1 Cetaceans
The ability to give birth and nurse in the ocean was likely a key feature in allowing the
success and wide distribution of cetacean species. Unfortunately for scientists, aquatic lactation has also made the post-partum physiology of cetaceans extremely difficult to study,
and much of what is known about lactation physiology is based on carcasses from whaling
or strandings and from studies of captive animals. For many species, birth has never been
observed and knowledge of reproductive and parental behavior is fragmentary or based
on comparisons with other species. Despite these limitations, field studies provide some
important insights into post-partum behavior and physiology. Because cetaceans exhibit
low adult mortality and fecundity (birth rates), but high rates of infant mortality, the period
of parental care is critical to offspring survival and population health. Many of the maternal traits that support calf survival are behavioral and social, including communication
and nursing behavior, but important differences in lactation physiology are evident that
influence the life-history strategies of cetaceans.
10.2.1.1 Lactation strategies
The duration of lactation varies widely both between and within species of cetaceans, with
the greatest difference between suborders. However, for many species the duration of lactation is estimated from just a few samples, and the methods used to determine lactation
duration (i.e., milk in the stomach from harvested individuals or behavioral observations
of known individuals) are prone to error. Cetacean offspring can travel with their mothers,
suckling frequently with only brief periods of maternal absence. Offspring suckle from
two mammary teats located in slits on either side of the genital opening. The calf wraps the
tip of its tongue around the nipple from below and pushes against the roof of the mouth.
The base of the tongue creates suction by moving up and down. The mother uses muscular
contraction of both the myoepithelial cells of the mammary gland and surrounding cutaneous muscles to cause milk ejection.
Although both baleen and toothed whales nurse in the water, they exhibit different
lactation strategies. Lactation duration ranges from around 5 months in mysticetes up to
3–6 years in some odontocetes. The mysticete whales exhibit a capital breeding strategy
resulting in a short lactation period relative to their body size (Figure 10.1). Most species
lactate for 5–8 months, typically weaning their calves during the summer months when
food availability is high. Mysticetes have compartmentalized breeding to a distinct part of
the year, requiring high intensity lactation and offspring growth. In the best-studied mysticetes, individuals forage in rich polar waters during summer months, and then move to
Chapter ten: Post-partum
adaptations that allow them to give birth, suckle, and nurture their young in water. In contrast,
pinnipeds leave the marine environment to reproduce and give birth on land or ice.
Underneath this broad difference, there exists a diversity of parental strategies and
physiological capabilities that shape the movements, breeding systems, foraging strategies, and life-history patterns of marine mammals. In some species (e.g., some mysticetes and many phocids), movement to an appropriate breeding environment necessitates
a capital breeding strategy, where stored body reserves are accumulated between breeding
episodes and then used for reproduction. Other species (e.g., odontocetes and otariids)
use an income breeding strategy, where offspring are provisioned using resources acquired
from foraging during the lactation period (Figure 10.3).
10.2 Knowledge by order
10.2.1 Cetaceans
The ability to give birth and nurse in the ocean was likely a key feature in allowing the
success and wide distribution of cetacean species. Unfortunately for scientists, aquatic lactation has also made the post-partum physiology of cetaceans extremely difficult to study,
and much of what is known about lactation physiology is based on carcasses from whaling
or strandings and from studies of captive animals. For many species, birth has never been
observed and knowledge of reproductive and parental behavior is fragmentary or based
on comparisons with other species. Despite these limitations, field studies provide some
important insights into post-partum behavior and physiology. Because cetaceans exhibit
low adult mortality and fecundity (birth rates), but high rates of infant mortality, the period
of parental care is critical to offspring survival and population health. Many of the maternal traits that support calf survival are behavioral and social, including communication
and nursing behavior, but important differences in lactation physiology are evident that
influence the life-history strategies of cetaceans.
10.2.1.1 Lactation strategies
The duration of lactation varies widely both between and within species of cetaceans, with
the greatest difference between suborders. However, for many species the duration of lactation is estimated from just a few samples, and the methods used to determine lactation
duration (i.e., milk in the stomach from harvested individuals or behavioral observations
of known individuals) are prone to error. Cetacean offspring can travel with their mothers,
suckling frequently with only brief periods of maternal absence. Offspring suckle from
two mammary teats located in slits on either side of the genital opening. The calf wraps the
tip of its tongue around the nipple from below and pushes against the roof of the mouth.
The base of the tongue creates suction by moving up and down. The mother uses muscular
contraction of both the myoepithelial cells of the mammary gland and surrounding cutaneous muscles to cause milk ejection.
Although both baleen and toothed whales nurse in the water, they exhibit different
lactation strategies. Lactation duration ranges from around 5 months in mysticetes up to
3–6 years in some odontocetes. The mysticete whales exhibit a capital breeding strategy
resulting in a short lactation period relative to their body size (Figure 10.1). Most species
lactate for 5–8 months, typically weaning their calves during the summer months when
food availability is high. Mysticetes have compartmentalized breeding to a distinct part of
the year, requiring high intensity lactation and offspring growth. In the best-studied mysticetes, individuals forage in rich polar waters during summer months, and then move to