Allometric Growth: Definition & Example

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we will talk about allometry. We will use several specific examples to discuss the differing rates at which organisms grow and develop.

Neoteny

How has Mickey Mouse evolved since his introduction in 1928? The original Mickey Mouse was ratlike, competent, and a bit mean. The Mickey of today is pudgy, helpless, infantile, and a metaphor for insipidity. In his famous essay ''A Biological Homage to Mickey Mouse,'' biologist Stephen Jay Gould tracks Mickey Mouse's changes and how Mickey's artists have come to design a mouse that is, by his looks alone, better able to evoke an emotional response from his human fans.

In short, Mickey has become neotenous; that is, Mickey now retains the features of an infant. Like an infant, modern Mickey's hands, head, and particularly his eyes are huge compared with the rest of his body. These types of features are generally regarded as cute by humans, and they tend to garner an emotional response. It's not hard to imagine that it benefits us from an evolutionary perspective to find babies cute. If we didn't want to help and protect those helpless babies, our next generation would quickly die off. Mickey has become more neotenous and therefore more lovable, like a baby. Mickey has undergone allometric growth in reverse.

Baby animals have distinct body ratios such as a large head-to-body ratio.
Baby animals

Allometry

Broadly speaking, allometry is how an organism's characteristics change according to its size. In this broadest sense, allometry may include abstract things like how an organism's metabolism changes, how well it runs or flies, and so on. But more narrowly, allometry is concerned with how physical features of an organism grow relative to each other.

Julian Huxley was one of the first people to notice these trends, in 1936. Huxley was interested in the exaggerated claw of the male fiddler crab. Fiddler crabs, which may be any of up to 100 species in the genus Uca, are best known for their giant claws. Female fiddler crabs have two normally-sized claws for feeding. Male crabs have one normal claw for feeding and one giant claw, which can be up to one-third of the crab's total body weight. This giant claw is used for attracting female fiddler crabs and for fighting with other males.

Male fiddler crab
Male fiddler crab

Huxley noticed that the claw grew disproportionately relative to the rest of the crab's body. That is, a young, small male fiddler crab would have a fairly small display claw, while a larger, older fiddler crab would have a huge display claw. Huxley realized that the claw was growing at a faster rate than the body of the crab.

Allometry doesn't only occur in crabs. It occurs quite plainly in humans - we're just so used to it that we don't notice. When we are growing, our arms and legs grow much faster than our heads. Our eyes hardly grow at all. Thus, the proportions between our limbs, body, head, and eyes change as we grow from infant to adult. The big heads, big eyes, and fat limbs that babies have seem to play into why we identify babies as cute. These same proportional ratios seem to be responsible for why we find many baby animals cute, as well.

Evolutionary Allometry

Though the original use of allometry was intended to describe organ size, later authors have used allometry to describe everything from metabolism to running efficiency. Sometimes, allometry isn't even used to describe an individual's growth over its lifetime. It can be used to describe how a trait increases or decreases over many generations. When a trait is compared over different species, this is called evolutionary allometry. Evolutionary allometry can be used to determine phylogenetic relationships between organisms, or where they fit on the family tree. For instance, scientists have used allometry to determine the relationships between different species of South American catfish, genus Corydoras.

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