Learning Objectives
In this section, you will explore the following questions:
- What are the various types of body plans that occur in animals?
- What are the limits on animal size and shape?
- How do bioenergetics relate to body size, levels of activity, and the environment?
Connection for AP® Courses
As you have learned, specialized cells in the animal body are organized into tissues, organs, and organ systems, which efficiently localize functions, such as the digestion of food and the elimination of wastes. As we explore the information in this section, our primary focus is homeostasis—the ability to maintain dynamic equilibrium around a set point. Animals need to maintain their “normal” internal environments while also responding to external environmental changes.
In our study of biology thus far, we have seen numerous examples of structure-function relationships, and the design of the animal body is no exception. Specialization in multicellular animals contributes to efficiency in cell processes. For example, animals must be able to procure nutrients and eliminate wastes, and cells that line the small intestine allow for diffusion. Furthermore, the relationship between metabolic rate and body mass is typically an inverse one: The smaller the animal, the higher its metabolism, with mice having a higher metabolic rate than, for example, elephants. Because mice have a greater surface area-to-volume ratio for their mass than larger animals, they lose heat at a faster rate and, consequently, require more energy to maintain constant body temperature.
Speaking of temperature, we learned that the body temperature of ectothermic animals varies according to environmental temperatures. When snakes need to warm up, they bask in the sun; when they need to cool down, they go into the shade. Other animals, including mice, kangaroos and humans, are endothermic because they are able to maintain a fairly constant internal body temperature despite environmental temperatures; for example, shivering generates heat, whereas sweating returns our body temperature to its normal set point of 37◦C. We will explore the control of these responses in more detail in the Homeostasis section.
The information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven Science Practices.
| Big Idea 2 | Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. |
| Enduring Understanding 2.A | Growth, reproduction and maintenance of living systems require free energy and matter. |
| Essential Knowledge | 2.A.1 All living systems require constant input of free energy. |
| Science Practice | 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. |
| Learning Objective | 2.1 The student is able to explain how biological systems use free energy based on empirical data that all organisms require constant energy input to maintain organization, to grow and to reproduce. |
| Essential Knowledge | 2.A.1 All living systems require constant input of free energy. |
| Science Practice | 6.1: The student can justify claims with evidence. |
| Learning Objective | 2.2 The student is able to justify a scientific claim that free energy is required for living systems to maintain organization, to grow or to reproduce, but that multiple strategies exist in different living systems. |
| Essential Knowledge | 2.A.1 All living systems require constant input of free energy. |
| Science Practice | 4.2 The student can design a plan for collecting data to answer a particular scientific question. |
| Learning Objective | 2.35 The student is able to design a plan for collecting data to support the scientific claim that timing and coordination of physiological events involve regulation |
| Essential Knowledge | 2.A.1 All living systems require constant input of free energy. |
| Science Practice | 6.1 The student can justify claims with evidence. |
| Learning Objective | 2.36 The student is able to justify scientific claims with evidence to show how timing and coordination of physiological events involve regulation. |
| Essential Knowledge | 2.A.1 All living systems require constant input of free energy. |
| Science Practice | 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas. |
| Learning Objective | 2.37 The student is able to connect concepts that describe mechanisms that regulate the timing and coordination of physiological events. |
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. Animals’ bodies are also designed to interact with their environments, whether in the deep sea, a rainforest canopy, or the desert. 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.
Body Plans
Animal body plans follow set patterns related to symmetry. They are asymmetrical, radial, or bilateral in form as illustrated in Figure 24.2. Asymmetrical animals are animals with no pattern or symmetry; an example of an asymmetrical animal is a sponge. Radial symmetry, as illustrated in Figure 24.2, describes when an animal has 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, like a rock or a boat, and extract their food from the surrounding water as it flows around the organism. Bilateral symmetry is illustrated in the same figure by 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. Additional terms used when describing positions in the body are anterior (front), posterior (rear), dorsal (toward the back), and ventral (toward the stomach). Bilateral symmetry is found in both land-based and aquatic animals; it enables a high level of mobility.
Limits on Animal Size and Shape
Animals with bilateral symmetry that live in water tend to have a fusiform shape: this is 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. Table 24.1 lists the maximum speed of various animals. Certain types of sharks can swim at fifty kilometers an hour and some dolphins at 32 to 40 kilometers per hour. Land animals frequently travel faster, although the tortoise and snail are significantly slower than cheetahs. 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. On the other hand, land-dwelling organisms are constrained mainly by gravity, and drag is relatively unimportant. For example, most adaptations in birds are for gravity not for drag.
| Animal | Speed (kmh) | Speed (mph) |
|---|---|---|
| Cheetah | 113 | 70 |
| Quarter horse | 77 | 48 |
| Fox | 68 | 42 |
| Shortfin mako shark | 50 | 31 |
| Domestic house cat | 48 | 30 |
| Human | 45 | 28 |
| Dolphin | 32–40 | 20–25 |
| Mouse | 13 | 8 |
| Snail | 0.05 | 0.03 |
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 (Figure 24.3). 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, and 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. 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
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 and the center of the cell does not receive adequate nutrients nor is it able to effectively dispel its waste.
An important concept in understanding how efficient diffusion is as a means of transport is the surface area 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 area 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.
The solution to producing larger organisms is for them to become multicellular. Specialization occurs in complex organisms, allowing cells to become 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. Moreover, surface area-to-volume ratio applies to other areas of animal development, such as the relationship between muscle mass and cross-sectional surface area in supporting skeletons, and in the relationship between muscle mass and the generation and dissipation of heat.
Link to Learning
Visit this interactive site to see an entire animal (a zebrafish embryo) at the cellular and sub-cellular level. Use the zoom and navigation functions for a virtual nanoscopy exploration.
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It shows bilateral symmetry.
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It shows dorsal symmetry.
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It shows radial symmetry.
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it does not show any symmetry.
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, and 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, and 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 and called 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 variously in joules, calories, or kilocalories (1000 calories). Carbohydrates and proteins contain about 4.5 to 5 kcal/g, and 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 to 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 (Figure 24.4). 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.
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. For example: the type of grasses, leaves, or shrubs that an herbivore eats affects the number of calories that it takes in. The relative caloric content of herbivore foods, in descending order, is tall grasses > legumes > short grasses > forbs (any broad-leaved plant, not a grass) > subshrubs > annuals/biennials.
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 and allows animals to survive adverse conditions. Torpor can be used by animals for long periods, such as entering a state of hibernation during the winter months, in which case it 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 use this 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.
Science Practice Connection for AP® Courses
Activity
Read about how scientists developed a method using today’s technology to collect data on heart rates in hibernating bears at this website. Design an experiment that would allow you to collect body temperature and heart rate at the same time. Discuss how combining data on body temperature with heart rate can give you information on the animal’s overall metabolism.
Think About It
- Small mammals, such as squirrels need to eat at least once a week during hibernation. Why is it impossible for them to go through the entire winter without eating, as bears do? Also, why must smaller mammals, like squirrels, store food for the winter while larger mammals, like bears, do not?
- Hummingbirds lower their metabolic rate and body temperature at night, an example of torpor. What advantage does torpor provide hummingbirds on a nightly basis? Think about the high metabolic rates of hummingbirds.
Teacher Support
- This activity is an application of Learning Objective 2.35 and Science Practice 4.2 because students are designing a plan for collecting data to investigate the timing and coordination of physiological events with metabolism. Students are asked to come up with a hypothesis and design an experiment that would allow them to test it.
- The first Think About It question is an application of AP® Learning Objective 2.1 and Science Practice 6.2 because students are explaining how the size of an animal affects energy input and metabolic processes. This second Think About It is an application of Learning Objective 2.36 and Science Practice 6.1 because students are justifying a claim about the timing and coordination of physiological events with evidence.
- Smaller mammals have higher metabolic rates and also lose heat more readily due to their smaller body sizes. This requires smaller mammals to take in additional food during winter, preventing them from being able to hibernate the entire winter. Instead, they periodically emerge to access stored food caches throughout the winter.
- The second Think About It question is an application of Learning Objective 2.37 and Science Practice 7.2 because students can connect concepts relating to hibernation and torpor with the timing of physiological events.
- A hummingbird’s high metabolic rate allows it to move very quickly and hover while flying. However, this high metabolic rate is not needed to support the bird while sleeping, which requires considerably less energy.
Animal Body Planes and Cavities
A standing vertebrate animal can be divided by several planes. 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 belly (ventral) or stomach from the back (dorsal). A transverse plane (or, horizontal plane) is perpendicular to the sagittal planes and the long axis of the body. This is sometimes called a cross section, and, if the transverse cut is at an angle, it is called an oblique plane. Figure 24.5 illustrates these planes on a goat (a four-legged animal) and a human being.
Vertebrate animals have a number of defined body cavities, as illustrated in Figure 24.6. Two of these are major cavities that contain smaller cavities within them. The dorsal cavity contains the cranial and the vertebral (or spinal) cavities. The ventral cavity contains the thoracic cavity, which in turn contains the pleural cavity around the lungs and the pericardial cavity, which surrounds the heart. The ventral cavity also contains the abdominopelvic cavity, which can be separated into the abdominal and the pelvic cavities.
Career Connection
Physical Anthropologist
Physical anthropologists study the adaptation, variability, and evolution of human beings, plus their living and fossil relatives. They can work in a variety of settings, although most will have an academic appointment at a university, usually in an anthropology department or a biology, genetics, or zoology department.
Non-academic positions are available in the automotive and aerospace industries where the focus is on human size, shape, and anatomy. Research by these professionals might range from studies of how the human body reacts to car crashes to exploring how to make seats more comfortable. Other non-academic positions can be obtained in museums of natural history, anthropology, archaeology, or science and technology. These positions involve educating students from grade school through graduate school. Physical anthropologists serve as education coordinators, collection managers, writers for museum publications, and as administrators. Zoos employ these professionals, especially if they have an expertise in primate biology; they work in collection management and captive breeding programs for endangered species. Forensic science utilizes physical anthropology expertise in identifying human and animal remains, assisting in determining the cause of death, and for expert testimony in trials.