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Chapter ten: Post-partum
may be to avoid predation on young calves. An exception to this general pattern is the
bowhead whale (Balaena mysticetus) that calves at high latitudes in the summer and has an
extended lactation period (12 months) compared to other mysticetes (Oftedal 1997). The
life-history pattern of most mysticetes suggests that prey ingestion only overlaps slightly
with nursing in offspring, indicating a relatively abrupt weaning compared to odontocetes.
Odontocetes exhibit an income breeding strategy, with mothers foraging during lactation. They nurse their calves for 6 months to several years, with lactation duration tending
to increase with body size. Within species, there are records of extraordinarily long lactation periods for individuals, exceeding a decade, though these records may reflect artifacts
of the observation methods employed. Long periods of parental investment may facilitate
training of young in migration, foraging, and social behavior. Weaning in odontocetes is
more gradual than mysticetes, and offspring may remain with the mother in social groups
after weaning. In many species, females become pregnant while still lactating and may
provision their young for a significant portion of gestation. This may allow females to
“bet-hedge” against loss of the developing fetus, with the option of continuing investment
in the current offspring.
The duration of lactation is influenced by several factors in cetaceans (Whitehead and
Mann 2000). Cetacean young do not fast after weaning. Odontocetes show a gradual transition from nursing to foraging, while mysticetes have only a brief period where ingestion of
solid food overlaps with nursing. The period of maternal investment may reflect (1) foraging
strategies and the time required for the offspring to develop sufficiently to find and catch
food; (2) the fitness consequences of continued parental investment; (3) the impact of continued lactation on the next breeding attempt; and (4) occurrence of weaning conflict between
the mother and calf. Weaning conflict arises from the fact that the mother and offspring
may be selected to desire different levels of investment in current offspring (Trivers 1974).
10.2.1.2 Investment
Though data from individual species are sparse, the milk of cetaceans has the highest fat and
energy content of any mammalian taxa except for the pinnipeds (Oftedal and Iverson 1995)
(Table 10.1). Fat content varies across lactation but is inversely related to duration of parental
investment, being ~30%–50% at mid-lactation in mysticetes and ~10%–30% at mid-lactation in
odontocetes (Oftedal 1997). While data are sparse, studies suggest that milk fat content peaks
near mid-lactation. The high energy cost of lactation is reflected in thinner blubber layers in
lactating females when compared to non-lactating individuals. The high milk energy outputs
of mysticete whales represent the most extreme examples of parental investment in nature.
A pregnant blue whale (Balaenoptera musculus) may weigh 119,000 kg of which as much as
45,000 kg is blubber. During lactation the mother converts most of this blubber to milk, allowing the calf to gain 17,000 kg by weaning (Lockyer 1981). Most odontocetes have smaller body
reserves than mysticetes and their food resources are more varied in time and space. This
may limit their ability to maintain the high milk energy outputs of large baleen whales. The
duration of lactation in cetaceans can be thought to ultimately reflect the spatial and temporal characteristics of the food resources used by the population and whether sufficient body
reserves are accrued to permit rapid energy transfer to the offspring in milk.
10.2.2 Pinnipeds
In contrast to the cetaceans, pinnipeds give birth on land, despite spending most of their
lives in the water. Most neonates require some period of time on land to develop before
being able to cope with life in the sea. This requires use of an appropriate breeding site,
Chapter ten: Post-partum
may be to avoid predation on young calves. An exception to this general pattern is the
bowhead whale (Balaena mysticetus) that calves at high latitudes in the summer and has an
extended lactation period (12 months) compared to other mysticetes (Oftedal 1997). The
life-history pattern of most mysticetes suggests that prey ingestion only overlaps slightly
with nursing in offspring, indicating a relatively abrupt weaning compared to odontocetes.
Odontocetes exhibit an income breeding strategy, with mothers foraging during lactation. They nurse their calves for 6 months to several years, with lactation duration tending
to increase with body size. Within species, there are records of extraordinarily long lactation periods for individuals, exceeding a decade, though these records may reflect artifacts
of the observation methods employed. Long periods of parental investment may facilitate
training of young in migration, foraging, and social behavior. Weaning in odontocetes is
more gradual than mysticetes, and offspring may remain with the mother in social groups
after weaning. In many species, females become pregnant while still lactating and may
provision their young for a significant portion of gestation. This may allow females to
“bet-hedge” against loss of the developing fetus, with the option of continuing investment
in the current offspring.
The duration of lactation is influenced by several factors in cetaceans (Whitehead and
Mann 2000). Cetacean young do not fast after weaning. Odontocetes show a gradual transition from nursing to foraging, while mysticetes have only a brief period where ingestion of
solid food overlaps with nursing. The period of maternal investment may reflect (1) foraging
strategies and the time required for the offspring to develop sufficiently to find and catch
food; (2) the fitness consequences of continued parental investment; (3) the impact of continued lactation on the next breeding attempt; and (4) occurrence of weaning conflict between
the mother and calf. Weaning conflict arises from the fact that the mother and offspring
may be selected to desire different levels of investment in current offspring (Trivers 1974).
10.2.1.2 Investment
Though data from individual species are sparse, the milk of cetaceans has the highest fat and
energy content of any mammalian taxa except for the pinnipeds (Oftedal and Iverson 1995)
(Table 10.1). Fat content varies across lactation but is inversely related to duration of parental
investment, being ~30%–50% at mid-lactation in mysticetes and ~10%–30% at mid-lactation in
odontocetes (Oftedal 1997). While data are sparse, studies suggest that milk fat content peaks
near mid-lactation. The high energy cost of lactation is reflected in thinner blubber layers in
lactating females when compared to non-lactating individuals. The high milk energy outputs
of mysticete whales represent the most extreme examples of parental investment in nature.
A pregnant blue whale (Balaenoptera musculus) may weigh 119,000 kg of which as much as
45,000 kg is blubber. During lactation the mother converts most of this blubber to milk, allowing the calf to gain 17,000 kg by weaning (Lockyer 1981). Most odontocetes have smaller body
reserves than mysticetes and their food resources are more varied in time and space. This
may limit their ability to maintain the high milk energy outputs of large baleen whales. The
duration of lactation in cetaceans can be thought to ultimately reflect the spatial and temporal characteristics of the food resources used by the population and whether sufficient body
reserves are accrued to permit rapid energy transfer to the offspring in milk.
10.2.2 Pinnipeds
In contrast to the cetaceans, pinnipeds give birth on land, despite spending most of their
lives in the water. Most neonates require some period of time on land to develop before
being able to cope with life in the sea. This requires use of an appropriate breeding site,