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Chapter ten: Post-partum
has been identified as an important factor causing apoptosis and involution in mice and
human mammary epithelial cells. Mutations in this gene identified in several otariids lead
to lack of production of functional α-lactalbumin and may protect the otariid mammary
gland from involution while foraging. In contrast, phocids lack this mutation and have
shown involution of the mammary gland within 24 hours of weaning (Reich and Arnould
2007). The mutation of the LALBA gene in otariids and walruses may have been an important factor in allowing the divergence of pinniped life-history strategies between capital
and income breeding strategies.
10.2.3 Sea otters
Although they are less specialized for the marine environment when compared to other
marine mammals, sea otters (Enhydra lutris) spend most of their time at sea, including
the period of maternal investment. Sea otters give birth, lactate, and rear their young in
shallow coastal waters. Females give birth to a single pup and lactate for an average of
6 months. Along the income/capital breeding continuum, sea otters likely represent the
extreme level of the income strategy found among marine mammals. The high surfaceto-volume ratios of sea otters cause elevated heat loss and contribute to them having the
highest metabolic rates for their body size among marine mammals (Costa and Williams
1999). Because of this, they must consume 20%–25% of their body mass in food each day
(Costa and Kooyman 1982) and spend as much as 50% of their time foraging, depending
on reproductive status and prey availability (Tinker et al. 2008). Sea otters lack the typical
blubber layer of marine mammals and are dependent on increased food intake to supply
the nutrients for lactation. The increased energy cost of lactation imposes a significant
energetic burden on sea otter mothers with energy demands nearly 100% higher than prepregnancy levels by weaning (Thometz et al. 2014). Despite a lack of fat reserves, females
often lose mass during lactation (Monson et al. 2000).
Female sea otters breed annually, irrespective of body condition or environmental
factors. This “bet-hedging” strategy allows an otter to abandon pups early in development if she is not able to meet the energy cost of lactation through increased foraging (Monson et al. 2000). Similarly, females make decisions about when to wean
their pup that are influenced by resource availability. Female sea otters must balance
the benefits of extending lactation to the survival to their current offspring against
potential loss of future reproduction from the health impacts of nutritional deficiencies. In areas of high otter population density, reductions in prey resource availability
may make it difficult for mothers to complete lactation. These energetic constraints
have been implicated in poorer body condition and higher mortality rates among sea
otters nearing the end of lactation on the central California Coast, termed end-lactation
syndrome (Thometz et al. 2014).
10.2.4 Polar bears
Polar bears (Ursus maritimus) are considered marine mammals because they move over a
wide range of sea ice, enter the water to move between ice floes and land and rely on the
ocean for food. This less marine-adapted lifestyle is reflected in their reproductive and
lactation strategy. The post-partum physiology of polar bears has many similarities to that
of brown bears. Polar bear offspring are unique among marine mammals in that they are
altricial, being initially dependent on their mother for thermoregulation.
Chapter ten: Post-partum
has been identified as an important factor causing apoptosis and involution in mice and
human mammary epithelial cells. Mutations in this gene identified in several otariids lead
to lack of production of functional α-lactalbumin and may protect the otariid mammary
gland from involution while foraging. In contrast, phocids lack this mutation and have
shown involution of the mammary gland within 24 hours of weaning (Reich and Arnould
2007). The mutation of the LALBA gene in otariids and walruses may have been an important factor in allowing the divergence of pinniped life-history strategies between capital
and income breeding strategies.
10.2.3 Sea otters
Although they are less specialized for the marine environment when compared to other
marine mammals, sea otters (Enhydra lutris) spend most of their time at sea, including
the period of maternal investment. Sea otters give birth, lactate, and rear their young in
shallow coastal waters. Females give birth to a single pup and lactate for an average of
6 months. Along the income/capital breeding continuum, sea otters likely represent the
extreme level of the income strategy found among marine mammals. The high surfaceto-volume ratios of sea otters cause elevated heat loss and contribute to them having the
highest metabolic rates for their body size among marine mammals (Costa and Williams
1999). Because of this, they must consume 20%–25% of their body mass in food each day
(Costa and Kooyman 1982) and spend as much as 50% of their time foraging, depending
on reproductive status and prey availability (Tinker et al. 2008). Sea otters lack the typical
blubber layer of marine mammals and are dependent on increased food intake to supply
the nutrients for lactation. The increased energy cost of lactation imposes a significant
energetic burden on sea otter mothers with energy demands nearly 100% higher than prepregnancy levels by weaning (Thometz et al. 2014). Despite a lack of fat reserves, females
often lose mass during lactation (Monson et al. 2000).
Female sea otters breed annually, irrespective of body condition or environmental
factors. This “bet-hedging” strategy allows an otter to abandon pups early in development if she is not able to meet the energy cost of lactation through increased foraging (Monson et al. 2000). Similarly, females make decisions about when to wean
their pup that are influenced by resource availability. Female sea otters must balance
the benefits of extending lactation to the survival to their current offspring against
potential loss of future reproduction from the health impacts of nutritional deficiencies. In areas of high otter population density, reductions in prey resource availability
may make it difficult for mothers to complete lactation. These energetic constraints
have been implicated in poorer body condition and higher mortality rates among sea
otters nearing the end of lactation on the central California Coast, termed end-lactation
syndrome (Thometz et al. 2014).
10.2.4 Polar bears
Polar bears (Ursus maritimus) are considered marine mammals because they move over a
wide range of sea ice, enter the water to move between ice floes and land and rely on the
ocean for food. This less marine-adapted lifestyle is reflected in their reproductive and
lactation strategy. The post-partum physiology of polar bears has many similarities to that
of brown bears. Polar bear offspring are unique among marine mammals in that they are
altricial, being initially dependent on their mother for thermoregulation.