Zoology is the subdivision of biology (the scientific study of living systems) that deals specifically with animals. Animals belong to the Kingdom Animalia (see Evolutionary Biodiversity). Animals are multicellular eukaryotic organisms—meaning that animal bodies are composed of many cells that have most of their genetic information located in a distinct, double-walled nucleus within the cell.

Animals are heterotrophs—meaning that they derive the energy they need to develop, grow, and stay alive through the consumption of other animals, plants, or fungi. Most animals are capable of movement (locomotion) during at least some phases of their life cycle; for example, though barnacles (crustaceans) are rooted to the sea bottom, they develop from free-swimming larvae.

Animals have developed a wide range of feeding and locomotion mechanisms that determine in large measure the role that each species of animal plays in its ecosystem.

Although the variety of feeding and locomotion types among mammals, birds, reptiles, amphibians, and fishes are familiar, marine invertebrates are less so. Feeding mechanisms among marine invertebrates include filter feeding—mostly in groups that are attached to the seafloor, such as corals and sea anemones (coelenterates), bryozoans, tunicates (sea squirts), and sedentary polychaete worms. Filter feeding involves the use of a straining device (tentacles of various sorts, sometimes lined with cilia) to remove very small particles of biological material (bacteria, eukaryotic microbes, as well as particles of decaying tissue) from the water (along with chemical nutrients and oxygen). The term deposit feeding refers to the actions of some clams and snails that derive their nourishment from removing small food particles that are trapped in muds or stuck to the sides of sand grains.

The original subdivisions of zoology reflected the development of laboratory instrumentation. For example, Antoni van Leeuwenhoek was the first to reveal the existence of the microbial world, using a microscope in the seventeenth century. Microscopic examination of animal tissues became the subject of the science of cytology. Similarly, when the science of genetics was founded at around 1900, the famous "fly room" of Thomas Hunt Morgan's Zoology Department of Columbia University became the locus where the very notion of genes and chromosomes was first developed, through careful experimentation and observations on fruit flies.

Physiology, study of the way animal bodies function, is another early and important branch of zoology. Anatomy, too, was an early subfield of zoology; anatomy continues to be an important field especially in medicine, for which all entering graduate students are required to complete a rigorous course specifically in human anatomy. "Comparative anatomy" involves the study of how the same parts of an animal's body takes different forms: for example, the forelegs of some animals are modified (through the course of evolution) into wings for flying, or fins or paddles for swimming. Embryology is the study of how animals develop from a fertilized egg.

Modern biology tends to be divided up differently from these older subdivisions of zoology (and botany and microbiology). Separate zoology and botany departments for the most part no longer exist. For example, the advent of the serious study of evolution in the mid-nineteenth century, and ecology somewhat later, has led, in modern times, to university departments of ecology and evolution that are separate from departments of molecular biology. These divisions maintain the older interests in the anatomy, physiology and embryology of animals (and plants), but do so as unified subject areas utilizing similar analytic techniques. Today's embryology, for example, is usually called developmental biology, and it is associated most closely with molecular biology because the central goal has become the explanation of the development of the adult form from the underlying genetic instructions—which can now be read in molecular terms. A laboratory studying mouse development is thus likely to be next door to one in which the development of ferns from spores is being analyzed, all with the techniques of molecular biology.

Although there has been a natural tendency in the history of biology to delve deeper and deeper into the smaller anatomical and, ultimately, chemical workings of the bodies of organisms (to the point where molecular biology now receives the majority of the attention and funding in modern biological science), none of the prior interest in larger-scale phenomena—such as anatomy, evolution, and ecology—has been lost. That zoology is usually no longer taught as a subject distinct from botany or the other traditional fields of biology reflects a growing realization that there is a unity to all life—from the molecular processes of the genes, the biochemical workings of energy production and development, on up through the behavior of organisms and the organization of such larger-scale biological systems as ecosystems and species.

—Niles Eldredge

See also: Ecology; Ecosystems; Evolutionary Biodiversity; Food Webs and Food Pyramids; Systematics


Eldredge, Niles. 1998. Life in the Balance. Princeton: Princeton University Press; Margulis, Lynn, and Karen V. Young. 1998. The Five Kingdoms, 3d ed. New York: W. H. Freeman; Wilson, Edward O. 1993. The Diversity of Life. Cambridge: Harvard University Press.

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