Integument,
CHAPTER 6 Support, and
Movement
Integument Imagine a cheetah (Acinonyx jubatus) slowly stalking a Thomson’s ga-
Structure and Function zelle (Gazella thomsonii) on the African savanna. The cheetah blends
Claws, Nails, and Hooves into the dappled shade u nder acacia trees and warily follows the ga-
Horns and Antlers zelle at a distance. Suddenly, it sprints a fter the gazelle in an all-out
dash to capture its prey (Figure 6.1). We are amazed by the cheetah’s
Basic Skeletal Patterns burst of speed and by the graceful, powerf ul movements of its legs as it
Skull races toward the gazelle. What body structures enable the cheetah to
Vertebrae, Ribs, and Sternum make such a swift attack? What body processes are at work to produce
Appendicular Skeleton the smooth, forceful run of this predator?
The cheetah’s body, like that of other mammals, is adapted for its
Muscles way of life. Its fur protects and camouflages it. As it runs or walks, each
stride involves intricately coordinated movements of muscles and bones
Modes of Locomotion throughout its body. To understand the dynamics of locomotion, we
Walking and Running must first examine the arrangement of skin, bones, and muscles in a
Jumping and Ricocheting mammal’s body. This chapter starts with the outside of a mammal—
Climbing the integument—and then moves u nder the skin to explore the skele-
Digging and Burrowing ton and the muscles that power movement. In the final section, we
Gliding and Flying show how the integument and musculoskeletal system relate to modes
Swimming of locomotion.
Integument
The skin, or integument, is the interface between a mammal and the
external environment. Its primary role is to separate the internal, ho-
meostatically regulated milieu of tissues and cells from the vagaries of
outside conditions. The integument of mammals also has two more spe-
cific functions: water conservation and insulation. An impervious outer
layer of skin is a trait shared by all amniotes and an adaptation for pre-
venting evaporative loss of body water in the dry air of terrestrial habi-
tats. As endotherms, mammals must also conserve body heat that is en-
ergetically expensive to produce. The outermost layer of their integument
has evolved a relatively simple structure—hair—that traps a layer of air
108
, Chapter 6 Integument, Support, and Movement 109
next to the skin and prevents convective heat loss. Feathers color or patterns, sensory capabilities for obtaining infor-
perform the same function in birds, the other group of en- mation about the environment, and production of diverse
dothermic vertebrates. The integument serves other criti- secretions from specialized glands. Moreover, the skin has
cal functions as well, including the ability to dissipate ex- produced some uniquely mammalian structures such as the
cess heat by evaporative cooling, communication via pelage surfaces of horns, antlers, nails, and hoofs.
STRUCTURE AND FUNCTION
Skin
Vertebrate skin consists of an outer epidermis and inner
dermis, below which is a hypodermis or subcutaneous re-
gion that overlies muscle (Figure 6.2; Kardong 2012). The
epidermis of mammals consists of several layers: an inner
stratum basale (or germinativum), intermediate strata show-
ing stages of keratinization, and an outer stratum corneum.
Stem cells in the stratum basale divide continually and some
of the daughter cells migrate into the overlying strata where
they manufacture large amounts of the protein keratin (i.e.,
they become keratinized or cornified), a process that causes
them to die by the time they reach the stratum corneum.
Figure 6.1 Cheetah and gazelle. This scene of a cheetah Keratin is water-insoluble and thus the keratinized stratum
attempting to run down a gazelle shows the general body form
of each animal, as well as their integuments and color patterns. corneum prevents desiccation of the underlying skin. Cells
Both species are cursorial, or running, mammals, but the of the stratum corneum are shed and replaced throughout
structures of their limb bones are extremely different. the life of a mammal. The thickness of the stratum corneum
Eccrine sweat pore Hair shaft
Pore Dermal papilla
EPIDERMIS
Meissner’s
Stratum
corpuscle
corneum
Blood capillary Stratum
basale
Hair follicle Meissner’s
corpuscle
DERMIS
Sebaceous (cross section)
gland
Sebaceous
Erector muscle gland
of hair (cross section)
Apocrine
Secreting part
sweat duct
of eccrine
sweat gland
SUBCUTANEOUS LAYER
Nerve fiber (cross section)
(ADIPOSE TISSUE)
Vater-Pacini
Vein
corpuscle
(cross section)
Artery
Secreting part of Hair root
apocrine sweat gland
Secreting part of
Vater-Pacini corpuscle eccrine sweat gland
Figure 6.2 Mammalian skin. Three-dimensional section through the skin and subcutaneous region of a mammal. Glands and
hair are epidermal structures that grow into the dermis during development.
, 110 Part 2 Structure and Function
varies among regions of the body and among species. It is neum, the hairs of mammals occupy the same structural
very thick in the foot pads of most species. These surfaces position and perform some of the same functions as the
experience regular abrasion against rough substrates dur- epidermal scales of nonavian reptiles and the feathers of
ing locomotion, and thickening of the stratum corneum pro- birds. As noted previously, hair provides insulation for
vides protection against excess wear. Epidermal thicken- mammals, so perhaps its evolution was tied to the origin
ing may even result in a distinct cell layer—t he stratum of endothermy. However, t here is an inconsistency in this
lucidum—just below the stratum corneum. Cells in the stra- scenario: insulatory fur would be maladaptive for an ances-
tum lucidum are translucent due to large amounts of kera- tral ectotherm that relied on efficient heat exchange with
tohyaline, a precursor of keratin. the environment for thermoregulation, but some form of
At the boundary between the epidermis and dermis are insulation appears to be necessary for endothermy to be en-
melanophores, cells that contain melanin. This brown pig- ergetically cost-effective (Pough et al. 2013). Indeed, evi-
ment absorbs ultraviolet radiation from the sun, which dence for endothermy (respiratory turbinate bones in the
might otherwise damage the underlying dermal tissue. Mel- nasal cavity) can be found in Permian therapsids long be-
anin in the dark tongues of giraffes (Giraffa camelopardalis) fore the earliest mammal (Ruben and Jones 2000; but see
protects the tissue from sunburn as the animals lift their Kemp 2006). The Permian therocephalian Estemmenosu-
heads into the branches of trees to forage for leaves. In chus has an exceptionally well-preserved skin impression
humans, synthesis of melanin increases upon exposure to that lacks any trace of hair (Kardong 2012). In contrast, the
sunlight and produces tanning. Extensions of melanophores oldest synapsid known to have had fur, the docodont Cas-
reach into the epidermis where they inject their pigments torocauda, did not occur u ntil the M iddle Jurassic (Ji et al.
into developing hair cells (see next section). In a few cases, 2006), though indirect evidence suggests that hair may
such as the bright ischial callosities of baboons, pigmented have been present in the Late Triassic stem mammal Mor-
patches of skin are used as visual signals. The blue and red ganucodon (Pough et al. 2013). Given this long lapse between
coloration on the scrotum and perineal region of male ver- the apparent origin of endothermy and the origin of hair,
vets (Chlorocebus aethiops) is used in dominance displays. it seems unlikely that hair evolved in response to selection
The thick mammalian dermis contains connective tis- for insulation. What, then, explains its origin?
sue, blood vessels, nerves, and slips of integumentary mus- Maderson (1972) suggested that hairs originated as tac-
cle. The epidermis has no blood supply of its own, so meta- tile receptors between the scales of early synapsids and later
bolic needs of cells in the stratum basale are met by diffusive were coopted as insulation a fter the evolution of endo-
exchange of nutrients and waste products with the highly thermy. Stenn and colleagues (2008) and Dhouailly (2009)
vascularized dermis. Constriction and dilation of dermal ar- proposed that hairs evolved through modification of the
terioles helps regulate heat loss by directing blood toward or developmental processes that produced skin glands in an-
away from the surface of the skin. Encapsulated sensory cestral synapsids, a view supported by some experimental
nerve endings in the dermis terminate at tactile receptors evidence (e.g., Alibardi 2012). These authors suggest that
such as Pacinian (Vater-Pacini) corpuscles or the Meissner’s lepidosaur scales, avian feathers, and mammalian hairs—
corpuscles of primates (Figure 6.2). Receptor stimulation all derivatives of the stratum corneum—represent divergent
results in firing of the associated nerve and transmission of evolutionary trends in development of the amniote epider-
an impulse to the central nervous system. A tiny smooth mis. Each had distinct adaptive values, but all owe their
muscle, the arrector pili, inserts at the base of each hair origin to changes in a common set of morphogenet ic sig-
within the dermis. When they contract, arrectores pilorum nals between the epidermis and underlying dermis.
cause hairs to stand erect, a response that may conserve heat A hair follicle begins its development in the stratum ba-
by thickening the dead-air layer above the skin, or act as a sale (Butcher 1951) and grows down into the dermis, induc-
visual signal (e.g., when “hairs stand on end” on the neck of ing the formation of a dermal papilla (Figure 6.3). The
a snarling dog). Although hairs and integumentary glands papilla becomes vascularized and serves as a conduit for
are derived from epidermal cells, their bases grow down into nutrients and waste products with the developing hair.
the dermal layer as they develop (Figure 6.2). Where it reaches the base of the dermis, the follicle swells
Below the skin lies a hypodermis consisting of loose to form a bulb around the dermal papilla. Continual mito-
connective and adipose (fat) tissues. Connective tissue sis occurs within the bulb, where root cells synthesize ker-
causes the skin to adhere to underlying muscle, whereas atin and grow outward to form a shaft of dead cells, which
subcutaneous fat serves as insulation and an energy reserve. eventually emerges from the surface of the skin. As the hair
Some sensory receptors (e.g., Pacinian corpuscles) also oc- is differentiating, so too are dermal cells that w ill form the
cur in the hypodermis. arrector pili muscle and follicle cells that w ill form a seba-
ceous gland. The root of each hair becomes surrounded by
sensory nerve endings that transmit tactile signals to the
Hair brain whenever the shaft is displaced (Figure 6.2).
A typical hair has three structural layers. The medulla
Hair is a unique characteristic of mammals, but its evolu- occupies the center of the shaft and consists of sparse, ir-
tionary origin is obscure. Derived from the stratum cor- regular cells connected by keratin strands and surrounded
CHAPTER 6 Support, and
Movement
Integument Imagine a cheetah (Acinonyx jubatus) slowly stalking a Thomson’s ga-
Structure and Function zelle (Gazella thomsonii) on the African savanna. The cheetah blends
Claws, Nails, and Hooves into the dappled shade u nder acacia trees and warily follows the ga-
Horns and Antlers zelle at a distance. Suddenly, it sprints a fter the gazelle in an all-out
dash to capture its prey (Figure 6.1). We are amazed by the cheetah’s
Basic Skeletal Patterns burst of speed and by the graceful, powerf ul movements of its legs as it
Skull races toward the gazelle. What body structures enable the cheetah to
Vertebrae, Ribs, and Sternum make such a swift attack? What body processes are at work to produce
Appendicular Skeleton the smooth, forceful run of this predator?
The cheetah’s body, like that of other mammals, is adapted for its
Muscles way of life. Its fur protects and camouflages it. As it runs or walks, each
stride involves intricately coordinated movements of muscles and bones
Modes of Locomotion throughout its body. To understand the dynamics of locomotion, we
Walking and Running must first examine the arrangement of skin, bones, and muscles in a
Jumping and Ricocheting mammal’s body. This chapter starts with the outside of a mammal—
Climbing the integument—and then moves u nder the skin to explore the skele-
Digging and Burrowing ton and the muscles that power movement. In the final section, we
Gliding and Flying show how the integument and musculoskeletal system relate to modes
Swimming of locomotion.
Integument
The skin, or integument, is the interface between a mammal and the
external environment. Its primary role is to separate the internal, ho-
meostatically regulated milieu of tissues and cells from the vagaries of
outside conditions. The integument of mammals also has two more spe-
cific functions: water conservation and insulation. An impervious outer
layer of skin is a trait shared by all amniotes and an adaptation for pre-
venting evaporative loss of body water in the dry air of terrestrial habi-
tats. As endotherms, mammals must also conserve body heat that is en-
ergetically expensive to produce. The outermost layer of their integument
has evolved a relatively simple structure—hair—that traps a layer of air
108
, Chapter 6 Integument, Support, and Movement 109
next to the skin and prevents convective heat loss. Feathers color or patterns, sensory capabilities for obtaining infor-
perform the same function in birds, the other group of en- mation about the environment, and production of diverse
dothermic vertebrates. The integument serves other criti- secretions from specialized glands. Moreover, the skin has
cal functions as well, including the ability to dissipate ex- produced some uniquely mammalian structures such as the
cess heat by evaporative cooling, communication via pelage surfaces of horns, antlers, nails, and hoofs.
STRUCTURE AND FUNCTION
Skin
Vertebrate skin consists of an outer epidermis and inner
dermis, below which is a hypodermis or subcutaneous re-
gion that overlies muscle (Figure 6.2; Kardong 2012). The
epidermis of mammals consists of several layers: an inner
stratum basale (or germinativum), intermediate strata show-
ing stages of keratinization, and an outer stratum corneum.
Stem cells in the stratum basale divide continually and some
of the daughter cells migrate into the overlying strata where
they manufacture large amounts of the protein keratin (i.e.,
they become keratinized or cornified), a process that causes
them to die by the time they reach the stratum corneum.
Figure 6.1 Cheetah and gazelle. This scene of a cheetah Keratin is water-insoluble and thus the keratinized stratum
attempting to run down a gazelle shows the general body form
of each animal, as well as their integuments and color patterns. corneum prevents desiccation of the underlying skin. Cells
Both species are cursorial, or running, mammals, but the of the stratum corneum are shed and replaced throughout
structures of their limb bones are extremely different. the life of a mammal. The thickness of the stratum corneum
Eccrine sweat pore Hair shaft
Pore Dermal papilla
EPIDERMIS
Meissner’s
Stratum
corpuscle
corneum
Blood capillary Stratum
basale
Hair follicle Meissner’s
corpuscle
DERMIS
Sebaceous (cross section)
gland
Sebaceous
Erector muscle gland
of hair (cross section)
Apocrine
Secreting part
sweat duct
of eccrine
sweat gland
SUBCUTANEOUS LAYER
Nerve fiber (cross section)
(ADIPOSE TISSUE)
Vater-Pacini
Vein
corpuscle
(cross section)
Artery
Secreting part of Hair root
apocrine sweat gland
Secreting part of
Vater-Pacini corpuscle eccrine sweat gland
Figure 6.2 Mammalian skin. Three-dimensional section through the skin and subcutaneous region of a mammal. Glands and
hair are epidermal structures that grow into the dermis during development.
, 110 Part 2 Structure and Function
varies among regions of the body and among species. It is neum, the hairs of mammals occupy the same structural
very thick in the foot pads of most species. These surfaces position and perform some of the same functions as the
experience regular abrasion against rough substrates dur- epidermal scales of nonavian reptiles and the feathers of
ing locomotion, and thickening of the stratum corneum pro- birds. As noted previously, hair provides insulation for
vides protection against excess wear. Epidermal thicken- mammals, so perhaps its evolution was tied to the origin
ing may even result in a distinct cell layer—t he stratum of endothermy. However, t here is an inconsistency in this
lucidum—just below the stratum corneum. Cells in the stra- scenario: insulatory fur would be maladaptive for an ances-
tum lucidum are translucent due to large amounts of kera- tral ectotherm that relied on efficient heat exchange with
tohyaline, a precursor of keratin. the environment for thermoregulation, but some form of
At the boundary between the epidermis and dermis are insulation appears to be necessary for endothermy to be en-
melanophores, cells that contain melanin. This brown pig- ergetically cost-effective (Pough et al. 2013). Indeed, evi-
ment absorbs ultraviolet radiation from the sun, which dence for endothermy (respiratory turbinate bones in the
might otherwise damage the underlying dermal tissue. Mel- nasal cavity) can be found in Permian therapsids long be-
anin in the dark tongues of giraffes (Giraffa camelopardalis) fore the earliest mammal (Ruben and Jones 2000; but see
protects the tissue from sunburn as the animals lift their Kemp 2006). The Permian therocephalian Estemmenosu-
heads into the branches of trees to forage for leaves. In chus has an exceptionally well-preserved skin impression
humans, synthesis of melanin increases upon exposure to that lacks any trace of hair (Kardong 2012). In contrast, the
sunlight and produces tanning. Extensions of melanophores oldest synapsid known to have had fur, the docodont Cas-
reach into the epidermis where they inject their pigments torocauda, did not occur u ntil the M iddle Jurassic (Ji et al.
into developing hair cells (see next section). In a few cases, 2006), though indirect evidence suggests that hair may
such as the bright ischial callosities of baboons, pigmented have been present in the Late Triassic stem mammal Mor-
patches of skin are used as visual signals. The blue and red ganucodon (Pough et al. 2013). Given this long lapse between
coloration on the scrotum and perineal region of male ver- the apparent origin of endothermy and the origin of hair,
vets (Chlorocebus aethiops) is used in dominance displays. it seems unlikely that hair evolved in response to selection
The thick mammalian dermis contains connective tis- for insulation. What, then, explains its origin?
sue, blood vessels, nerves, and slips of integumentary mus- Maderson (1972) suggested that hairs originated as tac-
cle. The epidermis has no blood supply of its own, so meta- tile receptors between the scales of early synapsids and later
bolic needs of cells in the stratum basale are met by diffusive were coopted as insulation a fter the evolution of endo-
exchange of nutrients and waste products with the highly thermy. Stenn and colleagues (2008) and Dhouailly (2009)
vascularized dermis. Constriction and dilation of dermal ar- proposed that hairs evolved through modification of the
terioles helps regulate heat loss by directing blood toward or developmental processes that produced skin glands in an-
away from the surface of the skin. Encapsulated sensory cestral synapsids, a view supported by some experimental
nerve endings in the dermis terminate at tactile receptors evidence (e.g., Alibardi 2012). These authors suggest that
such as Pacinian (Vater-Pacini) corpuscles or the Meissner’s lepidosaur scales, avian feathers, and mammalian hairs—
corpuscles of primates (Figure 6.2). Receptor stimulation all derivatives of the stratum corneum—represent divergent
results in firing of the associated nerve and transmission of evolutionary trends in development of the amniote epider-
an impulse to the central nervous system. A tiny smooth mis. Each had distinct adaptive values, but all owe their
muscle, the arrector pili, inserts at the base of each hair origin to changes in a common set of morphogenet ic sig-
within the dermis. When they contract, arrectores pilorum nals between the epidermis and underlying dermis.
cause hairs to stand erect, a response that may conserve heat A hair follicle begins its development in the stratum ba-
by thickening the dead-air layer above the skin, or act as a sale (Butcher 1951) and grows down into the dermis, induc-
visual signal (e.g., when “hairs stand on end” on the neck of ing the formation of a dermal papilla (Figure 6.3). The
a snarling dog). Although hairs and integumentary glands papilla becomes vascularized and serves as a conduit for
are derived from epidermal cells, their bases grow down into nutrients and waste products with the developing hair.
the dermal layer as they develop (Figure 6.2). Where it reaches the base of the dermis, the follicle swells
Below the skin lies a hypodermis consisting of loose to form a bulb around the dermal papilla. Continual mito-
connective and adipose (fat) tissues. Connective tissue sis occurs within the bulb, where root cells synthesize ker-
causes the skin to adhere to underlying muscle, whereas atin and grow outward to form a shaft of dead cells, which
subcutaneous fat serves as insulation and an energy reserve. eventually emerges from the surface of the skin. As the hair
Some sensory receptors (e.g., Pacinian corpuscles) also oc- is differentiating, so too are dermal cells that w ill form the
cur in the hypodermis. arrector pili muscle and follicle cells that w ill form a seba-
ceous gland. The root of each hair becomes surrounded by
sensory nerve endings that transmit tactile signals to the
Hair brain whenever the shaft is displaced (Figure 6.2).
A typical hair has three structural layers. The medulla
Hair is a unique characteristic of mammals, but its evolu- occupies the center of the shaft and consists of sparse, ir-
tionary origin is obscure. Derived from the stratum cor- regular cells connected by keratin strands and surrounded
