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Marine Mammal Physiology: Requisites for Ocean Living
The diverse phylogenetic origins of marine mammals also offer scientists an important opportunity to better understand the complex interplay between diet (or “energy in”)
and how marine mammals go about their aquatic lifestyles (or “energy out”). Biology has a
long tradition of employing a comparative approach to understanding general underlying
principles, and the marine mammals with their differing evolutionary histories, but convergent lifestyles are particularly valuable in this regard. By comparing and contrasting
the diets and nutrition of marine mammals, we can learn a lot about how these influence
life-history strategies.
As a group, marine mammals have a wide range of diets, from exclusive herbivory for
the sireniens (manatees and dugongs), through grazers on lower trophic levels (mysticete
whales), those that exploit mesopelagic fish and squid (most of the odontocete whales and
pinnipeds), up to top predators that take other marine mammals (killer whales, Orcinus
orca; polar bears, Ursus maritimus; and leopard seals, Hydrurga leptonyx). It is important to
note that most of these dietary types are used by several orders of marine mammals. For
example, both phocid seals and mysticete whales have evolved filter feeding as a mechanism for collecting small zooplankton. This convergence in diets and feeding techniques
is a good example of how species with diverse phylogenetic origins are able to exploit the
same abundant resources available in the marine environment. With the exception of the
sirenians, all marine mammals are carnivorous, and indeed are likely to have had carnivorous terrestrial antecedents.
In this chapter, we review the diets of all the major families of marine mammals; this
includes the sirenians, mysticete, and odontocete whales, as well as the three families of
pinnipeds (ottarids, phocids, and odobenidae), but not sea otters (Enhydra lutris) or polar
bears as these families are not exclusively marine. The overall scope of the chapter is to
detail the range of diet types and, in so doing, highlight the diversity of taxa using each
diet and their morphological convergence. We begin with an overview of foraging strategies as well as the morphological and behavioral adaptations and implications this has for
their life history and broader ecology. As marine mammals are a particularly challenging
group of organisms for dietary studies (due in large part to the difficulties in observing
and accessing them in the ocean), we then include a brief review of the techniques that
scientists use to quantify (with varying degrees of taxonomic and temporal resolution)
what marine mammals eat.
6.2 Life history and diet
The biggest dichotomy in marine mammal life histories is the aquatic breeding of all the
cetaceans and sirenians compared to the terrestrial (or ice) breeding of the pinnipeds.
Both life-history strategies have implications for diet and nutrition. Many species of pinnipeds are central place foragers, meaning that they must regularly return to a specific
location between foraging trips. This is most pronounced for those species that are income
br eeders. These are those species having too few energy stores to sustain the mother and
her pup throughout the entire lactation period, requiring mothers to make regular trips to
sea to replenish energy stores. A good example of this is the fur seals. This is in contrast to
capital breeders, which have very large energy stores and do not need to feed during the
lactation period. Capital breeders tend to be large and have very short lactation periods,
ranging from less than a week in hooded seals (Cystophora cristata) to a month in Weddell
seals (Leptonychotes weddellii). The requirement to breed and raise young onshore of the
income breeders requires them to be central place foragers, at least during the pup-rearing
period. In extreme cases, such as the Australian sea lion (Neophoca cinerea) where pups take
Marine Mammal Physiology: Requisites for Ocean Living
The diverse phylogenetic origins of marine mammals also offer scientists an important opportunity to better understand the complex interplay between diet (or “energy in”)
and how marine mammals go about their aquatic lifestyles (or “energy out”). Biology has a
long tradition of employing a comparative approach to understanding general underlying
principles, and the marine mammals with their differing evolutionary histories, but convergent lifestyles are particularly valuable in this regard. By comparing and contrasting
the diets and nutrition of marine mammals, we can learn a lot about how these influence
life-history strategies.
As a group, marine mammals have a wide range of diets, from exclusive herbivory for
the sireniens (manatees and dugongs), through grazers on lower trophic levels (mysticete
whales), those that exploit mesopelagic fish and squid (most of the odontocete whales and
pinnipeds), up to top predators that take other marine mammals (killer whales, Orcinus
orca; polar bears, Ursus maritimus; and leopard seals, Hydrurga leptonyx). It is important to
note that most of these dietary types are used by several orders of marine mammals. For
example, both phocid seals and mysticete whales have evolved filter feeding as a mechanism for collecting small zooplankton. This convergence in diets and feeding techniques
is a good example of how species with diverse phylogenetic origins are able to exploit the
same abundant resources available in the marine environment. With the exception of the
sirenians, all marine mammals are carnivorous, and indeed are likely to have had carnivorous terrestrial antecedents.
In this chapter, we review the diets of all the major families of marine mammals; this
includes the sirenians, mysticete, and odontocete whales, as well as the three families of
pinnipeds (ottarids, phocids, and odobenidae), but not sea otters (Enhydra lutris) or polar
bears as these families are not exclusively marine. The overall scope of the chapter is to
detail the range of diet types and, in so doing, highlight the diversity of taxa using each
diet and their morphological convergence. We begin with an overview of foraging strategies as well as the morphological and behavioral adaptations and implications this has for
their life history and broader ecology. As marine mammals are a particularly challenging
group of organisms for dietary studies (due in large part to the difficulties in observing
and accessing them in the ocean), we then include a brief review of the techniques that
scientists use to quantify (with varying degrees of taxonomic and temporal resolution)
what marine mammals eat.
6.2 Life history and diet
The biggest dichotomy in marine mammal life histories is the aquatic breeding of all the
cetaceans and sirenians compared to the terrestrial (or ice) breeding of the pinnipeds.
Both life-history strategies have implications for diet and nutrition. Many species of pinnipeds are central place foragers, meaning that they must regularly return to a specific
location between foraging trips. This is most pronounced for those species that are income
br eeders. These are those species having too few energy stores to sustain the mother and
her pup throughout the entire lactation period, requiring mothers to make regular trips to
sea to replenish energy stores. A good example of this is the fur seals. This is in contrast to
capital breeders, which have very large energy stores and do not need to feed during the
lactation period. Capital breeders tend to be large and have very short lactation periods,
ranging from less than a week in hooded seals (Cystophora cristata) to a month in Weddell
seals (Leptonychotes weddellii). The requirement to breed and raise young onshore of the
income breeders requires them to be central place foragers, at least during the pup-rearing
period. In extreme cases, such as the Australian sea lion (Neophoca cinerea) where pups take