CHAPTER 1 The Science of
Mammalogy
Mammals Mammals
Why Study Mammals?
Mammalogy is the study of mammals, organisms belonging to the
History of Mammalogy taxonomic group Mammalia. One way of defining Mammalia is “the
Antiquity most recent common ancestor of monotremes (platypus and echidnas)
Discovering Biological Diversity in and therians (marsupials and placental mammals) and all its descen-
the 17th and 18th Centuries dants” (Rowe 1988; Figure 1.1). As in this example, modern biologists
Explaining Biological Diversity in define all taxonomic groups by their evolutionary ancestry, not by
the 19th Century their traits. Taxonomic groups such as Mammalia do indeed have di-
Integrating Knowledge of Biological agnostic, distinguishing, characteristic, or common traits, but it is
Diversity in the 20th Century ancestry, not traits, that makes an organism a mammal. Thus, for
instance, members of all living mammal species have hair and mam-
Modern Mammalogy as an mary glands, but a completely hairless mutant mouse would still be a
Interdisciplinary Science mammal. This way of understanding taxonomic diversity is relatively
new and confusing to many students, but it is one of the major con-
Resources for Mammalogists ceptual advances of 20th-century biology. (See Chapter 4 for more in-
Information formation on the taxa shown in Figure 1.1.)
Experience Still, what is a mammal? How would you know one when you saw it,
given that organisms do not have their ancestry pinned to their sleeves?
We can trace that ancestry by characterizing the familiar groups to
which mammals belong in terms of key adaptations—traits that en-
hance an organism’s evolutionary fitness and exert a major influence on
the biology of a major taxonomic group. In order from more to less
inclusive, mammals are the following: eukaryotes (organisms with cells
containing a nucleus, membrane-bound organelles, and a cytoskele-
ton); animals (eukaryotes that are mobile, multicellular, and heterotro-
phic); chordates (animals with a notochord, dorsal hollow nerve chord,
postanal tail, and pharyngeal slits); vertebrates (chordates with verte-
brae and a cranium); tetrapods (vertebrates with four limbs); amniotes
(tetrapods with three unique extraembryonic membranes—amnion,
chorion, and allantois); and synapsids (members of Synapsida, amniotes
with a single temporal opening on each side of the skull). Among living
vertebrates, tetrapods are ancestrally terrestrial animals derived from
one group of fishes; they include amphibians (frogs, salamanders, and
2
, Chapter 1 The Science of Mammalogy 3
thermal habitats, but also requires them to obtain much
more energy from food than do comparably sized ecto-
therms. Indeed, one apt characterization of a mammal rela-
tive to other vertebrates is “an eating machine.” Many of
the traits we associate with mammals ultimately serve this
purpose and form a correlated suite of adaptations that vary
tremendously among major groups. Complex teeth and
Theria
chewing actions, foraging strategies, locomotor specializa-
tions, and a wide range of body sizes—all have something
to do with the voracious appetites of these high-energy ani-
Mammalia (Crown Group Mammalia) mals. Mammals have big brains (themselves a major energy
sink), which evolved from an early emphasis on olfaction to
Mammaliaformes include the elaboration of neocortical layers that process
diverse sensory information and coordinate more or less
Figure 1.1 What is a mammal? The phylogenetic defini- “intelligent” responses (Kas 2013), mostly for the purpose
tion of Mammalia is all descendants from the last common of eating. Hair (or fur), a trait mammals inherited from
ancestor of living mammals. This is the “Crown Group Mam
malia” of many authors (Benton 2015), who consider Mammalia their more recent synapsid ancestors, is also correlated with
equivalent to Mammaliaformes as shown here (synapsids with endothermy: Acting as insulation, hair (or fur) traps body
a dentary-squamosal jaw joint; see below and Chapter 4). heat produced at great cost and keeps it from dissipating
There is still debate about the branching order for some of with every gentle breeze that passes over the skin.
these lineages. Daggers indicate extinct groups. Based on A second adaptation that exerts heavy influence on
Pough et al. (2013).
mammalian biology is the energy invested in offspring (see
Chapter 9). Mammals are either oviparous (i.e., lay eggs, as
caecilians) and amniotes. Amniotes comprise sauropsids in the case of monotremes) or viviparous (i.e., give birth to
(including reptiles and birds) and synapsids. Mammals are live young, as marsupials and placentals do), but all provide
the only living synapsids, but there were many nonmam- their young with nutritious milk produced in the mammary
malian synapsids (please don’t call them “mammal-like glands of mothers, the trait for which mammals are named.
reptiles”—reptiles are sauropsids) that lived during the past As with endothermy, making milk requires energy from
300 million years. Mammals (or the Mammaliaformes, as food. For monotremes and marsupials, milk production
some authors term them) are distinguished from their ex- (lactation) constitutes the major maternal investment in
tinct synapsid relatives by the structure of their jaw joint, offspring survival; but for placental mammals, suckling is
which forms between the dentary bone of the mandible preceded by a lengthy period of gestation (development in
(in extant mammals, the dentary and mandible are syn- utero) during which the mother is supplying all offspring
onymous; this was not the case in cynodonts) and the needs. In no species of mammal do mothers simply leave
squamosal bone of the skull. Thus, the classification of their offspring to fend for themselves (a common strategy in
Mammalia looks like this: other vertebrates such as turtles). In many species a pro-
longed postnatal association between offspring and mothers
Eukarya
(and sometimes fathers or more distant relatives) lies at the
Animalia
heart of complex social systems so familiar to humans. So,
Chordata
what are mammals? They are smart, mobile eating ma-
Vertebrata
chines that exhibit great maternal care (Figure 1.2).
Tetrapoda
Amniota
Synapsida
Mammalia
Wh y Study Mammals?
But still, what is a mammal? From the information
above, you could (in principle) identify one if you could ex-
amine its entire anatomy, both macroscopically and micro- Why are mammals important (“to us” is implied)? The
scopically. But ancestry and diagnostic traits are not the first answer modern biologists give is that mammals are
whole story of mammalian evolution. That story is very significant components of functioning ecosystems, and
long indeed (see Chapter 4 and many of the chapters that functioning ecosystems are what give Earth a biosphere
follow), but a few aspects are worth mentioning at the rather than a “dead-o-sphere.” The anthropocentric cor-
outset. Again these are key adaptations. For example, mam- ollary is that humans depend (more completely than most
mals are like birds in that they are endotherms—that is, of us realize) on a biosphere that will support our swelling
they maintain a relatively high body temperature by main- population (Crist et al. 2017). Mammals form critical links
taining a correspondingly high metabolic rate. Endo- in terrestrial and aquatic food webs, transferring energy
thermy has allowed mammals to thrive in a wide range of and nutrients that power biotic processes for maintaining
Mammalogy
Mammals Mammals
Why Study Mammals?
Mammalogy is the study of mammals, organisms belonging to the
History of Mammalogy taxonomic group Mammalia. One way of defining Mammalia is “the
Antiquity most recent common ancestor of monotremes (platypus and echidnas)
Discovering Biological Diversity in and therians (marsupials and placental mammals) and all its descen-
the 17th and 18th Centuries dants” (Rowe 1988; Figure 1.1). As in this example, modern biologists
Explaining Biological Diversity in define all taxonomic groups by their evolutionary ancestry, not by
the 19th Century their traits. Taxonomic groups such as Mammalia do indeed have di-
Integrating Knowledge of Biological agnostic, distinguishing, characteristic, or common traits, but it is
Diversity in the 20th Century ancestry, not traits, that makes an organism a mammal. Thus, for
instance, members of all living mammal species have hair and mam-
Modern Mammalogy as an mary glands, but a completely hairless mutant mouse would still be a
Interdisciplinary Science mammal. This way of understanding taxonomic diversity is relatively
new and confusing to many students, but it is one of the major con-
Resources for Mammalogists ceptual advances of 20th-century biology. (See Chapter 4 for more in-
Information formation on the taxa shown in Figure 1.1.)
Experience Still, what is a mammal? How would you know one when you saw it,
given that organisms do not have their ancestry pinned to their sleeves?
We can trace that ancestry by characterizing the familiar groups to
which mammals belong in terms of key adaptations—traits that en-
hance an organism’s evolutionary fitness and exert a major influence on
the biology of a major taxonomic group. In order from more to less
inclusive, mammals are the following: eukaryotes (organisms with cells
containing a nucleus, membrane-bound organelles, and a cytoskele-
ton); animals (eukaryotes that are mobile, multicellular, and heterotro-
phic); chordates (animals with a notochord, dorsal hollow nerve chord,
postanal tail, and pharyngeal slits); vertebrates (chordates with verte-
brae and a cranium); tetrapods (vertebrates with four limbs); amniotes
(tetrapods with three unique extraembryonic membranes—amnion,
chorion, and allantois); and synapsids (members of Synapsida, amniotes
with a single temporal opening on each side of the skull). Among living
vertebrates, tetrapods are ancestrally terrestrial animals derived from
one group of fishes; they include amphibians (frogs, salamanders, and
2
, Chapter 1 The Science of Mammalogy 3
thermal habitats, but also requires them to obtain much
more energy from food than do comparably sized ecto-
therms. Indeed, one apt characterization of a mammal rela-
tive to other vertebrates is “an eating machine.” Many of
the traits we associate with mammals ultimately serve this
purpose and form a correlated suite of adaptations that vary
tremendously among major groups. Complex teeth and
Theria
chewing actions, foraging strategies, locomotor specializa-
tions, and a wide range of body sizes—all have something
to do with the voracious appetites of these high-energy ani-
Mammalia (Crown Group Mammalia) mals. Mammals have big brains (themselves a major energy
sink), which evolved from an early emphasis on olfaction to
Mammaliaformes include the elaboration of neocortical layers that process
diverse sensory information and coordinate more or less
Figure 1.1 What is a mammal? The phylogenetic defini- “intelligent” responses (Kas 2013), mostly for the purpose
tion of Mammalia is all descendants from the last common of eating. Hair (or fur), a trait mammals inherited from
ancestor of living mammals. This is the “Crown Group Mam
malia” of many authors (Benton 2015), who consider Mammalia their more recent synapsid ancestors, is also correlated with
equivalent to Mammaliaformes as shown here (synapsids with endothermy: Acting as insulation, hair (or fur) traps body
a dentary-squamosal jaw joint; see below and Chapter 4). heat produced at great cost and keeps it from dissipating
There is still debate about the branching order for some of with every gentle breeze that passes over the skin.
these lineages. Daggers indicate extinct groups. Based on A second adaptation that exerts heavy influence on
Pough et al. (2013).
mammalian biology is the energy invested in offspring (see
Chapter 9). Mammals are either oviparous (i.e., lay eggs, as
caecilians) and amniotes. Amniotes comprise sauropsids in the case of monotremes) or viviparous (i.e., give birth to
(including reptiles and birds) and synapsids. Mammals are live young, as marsupials and placentals do), but all provide
the only living synapsids, but there were many nonmam- their young with nutritious milk produced in the mammary
malian synapsids (please don’t call them “mammal-like glands of mothers, the trait for which mammals are named.
reptiles”—reptiles are sauropsids) that lived during the past As with endothermy, making milk requires energy from
300 million years. Mammals (or the Mammaliaformes, as food. For monotremes and marsupials, milk production
some authors term them) are distinguished from their ex- (lactation) constitutes the major maternal investment in
tinct synapsid relatives by the structure of their jaw joint, offspring survival; but for placental mammals, suckling is
which forms between the dentary bone of the mandible preceded by a lengthy period of gestation (development in
(in extant mammals, the dentary and mandible are syn- utero) during which the mother is supplying all offspring
onymous; this was not the case in cynodonts) and the needs. In no species of mammal do mothers simply leave
squamosal bone of the skull. Thus, the classification of their offspring to fend for themselves (a common strategy in
Mammalia looks like this: other vertebrates such as turtles). In many species a pro-
longed postnatal association between offspring and mothers
Eukarya
(and sometimes fathers or more distant relatives) lies at the
Animalia
heart of complex social systems so familiar to humans. So,
Chordata
what are mammals? They are smart, mobile eating ma-
Vertebrata
chines that exhibit great maternal care (Figure 1.2).
Tetrapoda
Amniota
Synapsida
Mammalia
Wh y Study Mammals?
But still, what is a mammal? From the information
above, you could (in principle) identify one if you could ex-
amine its entire anatomy, both macroscopically and micro- Why are mammals important (“to us” is implied)? The
scopically. But ancestry and diagnostic traits are not the first answer modern biologists give is that mammals are
whole story of mammalian evolution. That story is very significant components of functioning ecosystems, and
long indeed (see Chapter 4 and many of the chapters that functioning ecosystems are what give Earth a biosphere
follow), but a few aspects are worth mentioning at the rather than a “dead-o-sphere.” The anthropocentric cor-
outset. Again these are key adaptations. For example, mam- ollary is that humans depend (more completely than most
mals are like birds in that they are endotherms—that is, of us realize) on a biosphere that will support our swelling
they maintain a relatively high body temperature by main- population (Crist et al. 2017). Mammals form critical links
taining a correspondingly high metabolic rate. Endo- in terrestrial and aquatic food webs, transferring energy
thermy has allowed mammals to thrive in a wide range of and nutrients that power biotic processes for maintaining
