Animal Form and Function

Characteristics of the Animal Body

Every animal has a distinct body plan, adapted in response to environmental pressures, that limits its size and shape.

Learning Objectives

Describe how form and function are related in an organism

Key Takeaways

Key Points

  • A body plan encompasses symmetry, segmentation, and limb disposition.
  • Almost all animals have bodies made of differentiated tissues, which in turn form organs and organ systems.
  • Animal bodies have evolved to interact with their environments in ways that enhance survival and reproduction.

Key Terms

  • physiology: a branch of biology that deals with the functions and activities of life or of living matter (as organs, tissues, or cells) and of the physical and chemical phenomena involved
  • body plan: an assemblage of morphological features shared among many members of a phylum-level group
  • anatomy: the art of studying the different parts of any organized body, to discover their situation, structure, and economy; dissection

Animal Form and Function

Animals vary in form and function. From a sponge to a worm to a goat, an organism has a distinct body plan that limits its size and shape. The term body plan is the “blueprint” encompassing aspects such as symmetry, segmentation, and limb disposition. Body plans have been considered to have evolved in a geologically-sudden flash during the Cambrian Explosion (roughly 542 million years ago). However, there is also evidence of a more gradual development of body plans. With a few exceptions, most notably the sponges and Placozoa, animals have bodies differentiated into separate tissues, which in turn make up more complex organs and organ systems. These include tissues such as muscles, which are able to contract and control locomotion, and nerves, which send and process signals. Typically, there is also an internal digestive chamber with one or two openings. Animals’ bodies are also designed to interact with their environments, whether in the deep sea, a rainforest canopy, or the desert. In addition, animal body plans have evolved in response to environmental pressures, as observed in fossil records, in order to enhance survival and reproductive success. Therefore, a large amount of information about the structure of an organism’s body (anatomy) and the function of its cells, tissues, and organs (physiology) can be learned by studying that organism’s environment. The arctic fox is an example of a complex animal that has adapted to its environment and illustrates the relationships between an animal’s form and function.

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Arctic fox: An arctic fox is a complex animal, well adapted to its environment. It changes coat color with the seasons and has longer fur in winter to trap heat.

Body Plans

Animal body plans can have varying degrees of symmetry and can be described as asymmetrical, bilateral, or radial.

Learning Objectives

Describe the body plan of an animal

Key Takeaways

Key Points

  • Some animals have a body with no pattern or symmetry, making them asymmetrical.
  • Animals (mostly aquatic) with an up-and-down orientation have a radial symmetry in which there is no definite right or left side, but any longitudinal plane cut produces equal halves.
  • Animals, either aquatic or terrestrial, that have a high level of mobility usually have a body plan that is bilaterally symmetric.
  • Terms such as anterior (front), posterior (rear), dorsal (toward the back), and ventral (toward the stomach) are used to describe the position of parts of the body in relation to other parts.

Key Terms

  • asymmetrical: having disproportionate arrangement of parts; exhibiting no pattern
  • bilateral symmetry: having equal arrangement of parts (symmetry) about a vertical plane running from head to tail
  • radial symmetry: a form of symmetry wherein identical parts are arranged in a circular fashion around a central axis

Body Plans

Animal body plans follow set patterns related to symmetry. They can be asymmetrical, radial, or bilateral in form. Asymmetrical animals are those with no pattern or symmetry, such as a sponge. Radial symmetry describes an animal with an up-and-down orientation: any plane cut along its longitudinal axis through the organism produces equal halves, but not a definite right or left side. This plan is found mostly in aquatic animals, especially organisms that attach themselves to a base, such as a rock or a boat, and extract their food from the surrounding water as it flows around the organism. Bilateral symmetry is found in both land-based and aquatic animals; it enables a high level of mobility. Bilateral symmetry is illustrated in a goat. The goat also has an upper and lower component to it, but a plane cut from front to back separates the animal into definite right and left sides.

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Body symmetry: Animals exhibit different types of body symmetry. The sponge is asymmetrical, the sea anemone has radial symmetry, and the goat has bilateral symmetry.

In order to describe structures in the body of an animal it is necessary to have a system for describing the position of parts of the body in relation to other parts. For example, it may be necessary to describe the position of the liver in relation to the diaphragm or the heart in relation to the lungs. The most common terms used when describing positions in the body are anterior (front), posterior (rear), dorsal (toward the back), and ventral (toward the stomach). Note that the terms superior and inferior are usually not used to describe animals. They are only used to describe the position of structures in the human body (and possibly apes) where the upright posture means some structures are above or superior to others.

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Directional terms: The table illustrates common directional terms that are used to describe the position of body parts in relation to other body parts.

Limits on Animal Size and Shape

Animal shape and body size are influenced by environmental factors as well as the presence of an exoskeleton or an endoskeleton.

Learning Objectives

Explain how the environment and skeletal structure can put limits on the size and shape of animals

Key Takeaways

Key Points

  • Aquatic animals tend to have tubular shaped bodies ( fusiform shape) that decrease drag, enabling them to swim at high speeds.
  • Terrestrial animals tend to have body shapes that are adapted to deal with gravity.
  • Exoskeletons are hard protective coverings or shells that also provide attachments for muscles.
  • Before shedding or molting the existing exoskeleton, an animal must first produce a new one.
  • The exoskeleton must increase thickness as the animal becomes larger, which limits body size.
  • The size of an animal with an endoskeleton is determined by the amount of skeletal system required to support the body and the muscles it needs to move.

Key Terms

  • fusiform: shaped like a spindle; tapering at each end
  • exoskeleton: a hard outer structure that provides both structure and protection to creatures such as insects, Crustacea, and Nematoda
  • apodeme: an ingrowth of the arthropod exoskeleton, serving as an attachment site for muscles
  • endoskeleton: the internal skeleton of an animal, which in vertebrates is comprised of bone and cartilage

Limits on Animal Size and Shape

Animals with bilateral symmetry that live in water tend to have a fusiform shape: a tubular shaped body that is tapered at both ends. This shape decreases the drag on the body as it moves through water and allows the animal to swim at high speeds. Certain types of sharks can swim at fifty kilometers an hour, while some dolphins can swim at 32-40 kilometers per hour. Land animals frequently travel faster (although the tortoise and snail are significantly slower than sharks or dolphins). Another difference in the adaptations of aquatic and land-dwelling organisms is that aquatic organisms are constrained in shape by the forces of drag in the water since water has higher viscosity than air. However, land-dwelling organisms are constrained mainly by gravity; drag is relatively unimportant. For example, most adaptations in birds are for gravity, not for drag.

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Animal speeds: Land and marine animals travel at varying speeds. Land animals usually travel at higher speeds, but marine animals such as dolphins and sharks travel relatively fast.

Most animals have an exoskeleton, including insects, spiders, scorpions, horseshoe crabs, centipedes, and crustaceans. Scientists estimate that, of insects alone, there are over 30 million species on our planet. The exoskeleton is a hard covering or shell that provides benefits to the animal, such as protection against damage from predators and from water loss (for land animals); it also provides for the attachments of muscles. As the tough and resistant outer cover of an arthropod, the exoskeleton may be constructed of a tough polymer, such as chitin, and is often biomineralized with materials, such as calcium carbonate. This is fused to the animal’s epidermis. Ingrowths of the exoskeleton called apodemes function as attachment sites for muscles, similar to tendons in more advanced animals. In order to grow, the animal must first synthesize a new exoskeleton underneath the old one and then shed or molt the original covering. This limits the animal’s ability to grow continually. It may limit the individual’s ability to mature if molting does not occur at the proper time. The thickness of the exoskeleton must be increased significantly to accommodate any increase in weight. It is estimated that a doubling of body size increases body weight by a factor of eight. The increasing thickness of the chitin necessary to support this weight limits most animals with an exoskeleton to a relatively-small size.

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Apodemes: Apodemes are ingrowths on arthropod exoskeletons to which muscles attach. The apodemes on this crab leg are located above and below the fulcrum of the claw. Contraction of muscles attached to the apodemes pulls the claw closed.

The same principles apply to endoskeletons, but they are more efficient because muscles are attached on the outside, making it easier to compensate for increased mass. An animal with an endoskeleton has its size determined by the amount of skeletal system it needs in order to support the other tissues and the amount of muscle it needs for movement. As the body size increases, both bone and muscle mass increase. The speed achievable by the animal is a balance between its overall size and the bone and muscle that provide support and movement.

Limiting Effects of Diffusion on Size and Development

Less efficient diffusion in larger cells led to multicellular organisms with specialized tissues that supply nutrients and remove waste.

Learning Objectives

Describe how diffusion limits cell size and development

Key Takeaways

Key Points

  • Diffusion is effective over a specific distance, so it’s more efficient in small, single-celled microorganisms.
  • Diffusion becomes less efficient as the surface-to-volume ratio decreases, so diffusion is less effective in larger animals.
  • To overcome the limitations of diffusion, multicellular organisms have developed specialized tissues and systems that are responsible for completing a limited number of nutrient and waste tasks.

Key Terms

  • surface-to-volume ratio: the amount of surface area per unit volume of an object or collection of objects; decreases as volume increases

Limiting Effects of Diffusion on Size and Development

The exchange of nutrients and wastes between a cell and its watery environment occurs through the process of diffusion. All living cells are bathed in liquid, whether they are in a single-celled organism or a multicellular one. Diffusion is effective over a specific distance and limits the size that an individual cell can attain. If a cell is a single-celled microorganism, such as an amoeba, it can satisfy all of its nutrient and waste needs through diffusion. If the cell is too large, then diffusion is ineffective at completing all of these tasks. The center of the cell does not receive adequate nutrients nor is it able to effectively dispel its waste.

An important concept in understanding the efficiency of diffusion as a transportation mechanism is the surface-to-volume ratio. Recall that any three-dimensional object has a surface area and volume; the ratio of these two quantities is the surface-to-volume ratio. Consider a cell shaped like a perfect sphere: it has a surface area of 4πr2, and a volume of (4/3)πr3. The surface-to-volume ratio of a sphere is 3/r; as the cell gets bigger, its surface-to-volume ratio decreases, making diffusion less efficient. The larger the size of the sphere, or animal, the less surface area for diffusion it possesses.

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Surface-to-volume ratio: The image illustrates the comparison of spheres of one to one thousand volume units. The surface-to-volume ratio of a sphere decreases as the sphere gets bigger. The surface area of a sphere is 4πr2 and it has a volume of (4/3)πr3 which makes the surface-to-volume ratio 3/r. This has an effect on diffusion because it relies on the surface area of a cell: as a cell gets bigger, diffusion becomes less efficient.

The solution to producing larger organisms is for them to become multicellular. Specialization occurs in complex organisms, allowing cells to become less efficient at completing all tasks since they are now more efficient at doing fewer tasks. For example, circulatory systems bring nutrients and remove waste, while respiratory systems provide oxygen for the cells and remove carbon dioxide from them. Other organ systems have developed further specialization of cells and tissues and efficiently control body functions. Surface-to-volume ratio also applies to other areas of animal development, such as the relationship between muscle mass and cross-sectional surface area in supporting skeletons or in the relationship between muscle mass and the generation of dissipation of heat.

Animal Bioenergetics

An animal’s body size, activity level, and environment impacts the ways it uses and obtains energy.

Learning Objectives

Differentiate among the ways in which an animal’s energy requirements are affected by their environment and level of activity

Key Takeaways

Key Points

  • An animal is endothermic (warm-blooded) if it maintains a relatively-constant body temperature by conserving heat with the help of insulation.
  • An animal is ectothermic if it does not have insulation to conserve heat and must rely on its environment for body heat.
  • Metabolic rate is the amount of energy expended by an animal over a specific time; in endotherms, it is described as the basal metabolic rate (BMR), while in ectotherms, as the standard metabolic rate (SMR).
  • Smaller endothermic animals have a higher BMR than larger endothermic animals because they lose heat at a faster rate and require more energy to maintain a constant internal temperature.
  • More active animals have higher BMRs or SMRs and require more energy to maintain their activity.
  • A long period of inactivity and decreased metabolism ( torpor ) that occurs in the winter months is hibernation; estivation is torpor that occurs in the summer months.

Key Terms

  • endotherm: a warm-blooded animal that maintains a constant body temperature
  • ectotherm: a cold-blooded animal that regulates its body temperature by exchanging heat with its surroundings
  • hibernation: a state of inactivity and metabolic depression in animals during winter
  • estivation: to go into a state of inactivity during the summer months

Animal Bioenergetics

All animals must obtain their energy from food they ingest or absorb. These nutrients are converted to adenosine triphosphate (ATP) for short-term storage and use by all cells. Some animals store energy for slightly longer times as glycogen, while others store energy for much longer times in the form of triglycerides housed in specialized adipose tissues. No energy system is one hundred percent efficient as an animal’s metabolism produces waste energy in the form of heat. If an animal can conserve that heat and maintain a relatively-constant body temperature, it is classified as a warm-blooded animal: an endotherm. The insulation used to conserve the body heat comes in the forms of fur, fat, or feathers. The absence of insulation in ectothermic animals increases their dependence on the environment for body heat.

The amount of energy expended by an animal over a specific time is called its metabolic rate. The rate is measured in joules, calories, or kilocalories (1000 calories). Carbohydrates and proteins contain about 4.5-5 kcal/g, while fat contains about 9 kcal/g. Metabolic rate is estimated as the basal metabolic rate (BMR) in endothermic animals at rest and as the standard metabolic rate (SMR) in ectotherms. Human males have a BMR of 1600-1800 kcal/day, and human females have a BMR of 1300 to 1500 kcal/day. Even with insulation, endothermal animals require extensive amounts of energy to maintain a constant body temperature. An ectotherm such as an alligator has an SMR of 60 kcal/day.

Energy Requirements Related to Body Size

Smaller endothermic animals have a greater surface area for their mass than larger ones. Therefore, smaller animals lose heat at a faster rate than larger animals and require more energy to maintain a constant internal temperature. This results in a smaller endothermic animal having a higher BMR, per body weight, than a larger endothermic animal.

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Body size and metabolic rate: The mouse has a much higher metabolic rate than the elephant since it has greater surface area relative to mass.

Energy Requirements Related to Levels of Activity

The more active an animal is, the more energy is needed to maintain that activity and the higher its BMR or SMR. The average daily rate of energy consumption is about two to four times an animal’s BMR or SMR. Humans are more sedentary than most animals and have an average daily rate of only 1.5 times the BMR. The diet of an endothermic animal is determined by its BMR.

Energy Requirements Related to Environment

Animals adapt to extremes of temperature or food availability through torpor. Torpor is a process that leads to a decrease in activity and metabolism, which allows animals to survive adverse conditions. Torpor can be used by animals for long periods. For example, animals can enter a state of hibernation during the winter months, which enables them to maintain a reduced body temperature. During hibernation, ground squirrels can achieve an abdominal temperature of 0° C (32° F), while a bear’s internal temperature is maintained higher at about 37° C (99° F).

If torpor occurs during the summer months with high temperatures and little water, it is called estivation. Some desert animals estivate to survive the harshest months of the year. Torpor can occur on a daily basis; this is seen in bats and hummingbirds. While endothermy is limited in smaller animals by surface-to-volume ratio, some organisms can be smaller and still be endotherms because they employ daily torpor during the part of the day that is coldest. This allows them to conserve energy during the colder parts of the day when they consume more energy to maintain their body temperature.

Animal Body Planes and Cavities

Vertebrates can be divided along different planes in order to reference the locations of defined cavities.

Learning Objectives

Describe the major body planes and cavities of animals

Key Takeaways

Key Points

  • A sagittal plane divides the body into right and left portions; a midsagittal plane divides the body exactly in the middle.
  • A frontal or coronal plane separates the front from the back.
  • A transverse or horizontal plane divides the animal into upper and lower portions; it is called an oblique plane if it is cut at an angle.
  • The posterior (dorsal) cavity is a continuous cavity that includes the cranial cavity (brain) and the spinal cavity (spinal cord).
  • The anterior (ventral) cavity includes the thoracic cavity and the abdominopelvic cavity.
  • The thoracic cavity is divided into the pleural cavity (lungs) and pericardial cavity (heart); the abdominopelvic cavity includes the abdominal cavity (digestive organs) and the pelvic cavity (reproductive organs).

Key Terms

  • transverse plane: divides a body into upper and lower portions
  • frontal plane: divides a body into dorsal (back) and ventral (front) parts
  • sagittal plane: divides the body into right and left halves

Animal Body Planes and Cavities

A standing vertebrate animal can be divided by several planes that can be used to as references to describe locations of body parts or organs. A sagittal plane divides the body into right and left portions. A midsagittal plane divides the body exactly in the middle, making two equal right and left halves. A frontal plane (also called a coronal plane) separates the front (ventral) from the back (dorsal). A transverse plane (or, horizontal plane) divides the animal into upper and lower portions. This is sometimes called a cross section; if the transverse cut is at an angle, it is called an oblique plane.

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Body planes: Shown are the planes of a quadruped goat and a bipedal human. The midsagittal plane divides the body exactly in half into right and left portions. The frontal plane divides the front and back, while the transverse plane divides the body into upper and lower portions.

Vertebrate animals have a number of defined body cavities. The posterior (dorsal) and anterior (ventral) cavities are each subdivided into smaller cavities. In the posterior cavity, the cranial cavity houses the brain and the spinal cavity (or vertebral cavity) encloses the spinal cord. Just as the brain and spinal cord make up a continuous, uninterrupted structure, the cranial and spinal cavities that house them are also continuous. The brain and spinal cord are protected by the bones of the skull and vertebral column and by cerebrospinal fluid, a colorless fluid produced by the brain, which cushions the brain and spinal cord within the posterior (dorsal) cavity.

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Body cavities: Vertebrate animals have two major body cavities. The dorsal cavity, indicated in green, contains the cranial and the spinal cavity. The ventral cavity, indicated in yellow, contains the thoracic cavity and the abdominopelvic cavity. The thoracic cavity is separated from the abdominopelvic cavity by the diaphragm. The abdominopelvic cavity is separated into the abdominal cavity and the pelvic cavity by an imaginary line parallel to the pelvis bones.

The anterior cavity has two main subdivisions: the thoracic cavity and the abdominopelvic cavity. The thoracic cavity is the more superior subdivision of the anterior cavity and is enclosed by the rib cage. The thoracic cavity contains the pleural cavity around the lungs and the pericardial cavity, which surrounds the heart. The diaphragm forms the floor of the thoracic cavity, separating it from the more inferior abdominopelvic cavity. The abdominopelvic cavity is the largest cavity in the body. Although no membrane physically divides the abdominopelvic cavity, it can be useful to distinguish between the abdominal cavity, the division that houses the digestive organs from the pelvic cavity, the division that houses the organs of reproduction.