Allometry

Allometry is the study of changes in the characteristics of organisms relative to changes in the body size of these organisms. Originally, allometry specifically referred to the size of a particular body part in relation to the size of the entire body as both grow over time. In contemporary study, however, the concept of allometry has been expanded to include a variety of biological scaling relationships, such as the relationship between brain size and body size (morphological), the relationship between body size and metabolic rate (physiological), and the relationship between body size and home-range size (ecological). Whatever the specifics of study, the common theme of allometry is the question of how different biological traits or physiological processes scale with one another in a given organism. As a result, the study of allometry focuses on the causes of these scaling relationships, how they affect ecology, and how they are tied to evolution.

Brief History

The concept of allometry first began to emerge in the late nineteenth and early twentieth centuries as scientists attempted to understand the intricacies of Charles Darwin's theory of evolution. Among the scientists working in this realm was Julian Huxley, the grandson of famed Darwin supporter Thomas Henry Huxley and the brother of novelist Aldous Huxley. In 1924, Huxley turned his attention to studying the relative size of different organs. During his studies, Huxley observed that the relative size of one part of an organism compared to that of another part depended on the absolute size of the organism as a whole. In his first paper on the subject, he described a study of the fiddler crab's unusually large major claw. At that point, Huxley called the phenomenon heterogonic development. Although Huxley wrote Problems of Relative Growth, his landmark paper on the subject, in 1932, it was not until 1936 that he first introduced the term allometry. He conceived of an allometry as being the opposite of an isometry, which is when the relative size of an organism's individual body parts is independent from the organism's absolute size. Over the time that Huxley studied allometry, he proved the concept was a widespread phenomenon that applied to both the dimensions of a single individual at different stages of its development and the dimensions of different individuals belonging to the same species at any point in their development. Huxley also created an equation called the simple allometry formula that could be used to explain the mathematics behind why living organisms have the physical dimensions that they do. This mathematical approach made it possible to calculate how the physical forms of various living organisms developed over time. In the years since Huxley first outlined the concept, numerous scientists around the world have continued to study allometry and the relationship between mathematics and biology. Their research has further refined humankind's understanding of this complicated concept and how it works. Moreover, allometry has come to be seen as the most important principle involved in the determination of differences in form among closely related plants and animals.

Overview

Allometry is often defined as a study of the biological consequences of changes in size. While accurate, this definition reflects only the broadest possible understanding of the concept. More specifically, the term also refers to changes in the size or shape of a specific body part in relation to the size of the organism as a whole. When considered in this sense, there are two different types of allometry: negative allometry and positive allometry. Negative allometry occurs when one part of an organism's body grows at a slower rate than that of the body as a whole. Positive allometry, on the other hand, occurs when one part of an organism's body grows at a faster rate than that of the body as a whole. In either case, changes in size automatically lead to changes in shape. These characteristics are what separate allometry from isometry, which is when an organism's individual body parts grow at the same rate as the body as a whole.

Few organisms actually experience isometric growth. Rather, most living things, including humans, grow allometrically. This can easily be seen in the way humans develop as they grow from fetuses into full-sized adults. Instead of simply scaling up as they grow, humans have different proportions at different stages of their development. In the early weeks of its development, a fetus's head accounts for around half of its body length. As development continues, the body begins to develop at a faster rate. By thirty-eight weeks, the head accounts for only about a quarter of the body's length. If humans grew isometrically, their heads would be so large by adulthood that a person's chin would reach down to around his or her belly button.

Head size also plays a role in another type of allometry that is seen in other types of animals, such as chimpanzees. Chimpanzees experience allometric growth when it comes to the development of their skulls versus that of their brains. At birth, chimpanzees have a relatively human-shaped skull with an equivalently sized brain. While the brain stops growing soon after, the skull continues to develop as the chimpanzee gets older. By the time a chimpanzee reaches adulthood, it has a significantly larger skull marked by a protruding jaw and a heavily projecting brow. However, despite the skull's dramatic growth, the size of the chimpanzee's brain remains unchanged.

One of the best-known examples of allometric growth in animals is seen in the claws of the fiddler crab. Adult male fiddler crabs have an unusual appearance in that one of its claws is much larger than the other. Known as the major claw, this larger claw can account for as much as two-fifths of the crab's body weight. When fiddler crabs are born, however, both claws are the same size. It is only as the male fiddler crab develops over time that the major claw begins to develop at a faster rate than the rest of the body. Female fiddler crabs have claws that grow isometrically, meaning that both grow at the same rate and in proportion to the rest of their bodies.

Bibliography

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