heterometry

Heterometry is a term used in morphology and related fields to describe variation in the absolute size or length of a trait that is not strictly tied to the overall body size. The word combines the Greek roots hetero- “different” and metron “measure,” underscoring its focus on differences in measurement rather than proportional growth alone.

In practice, heterometry is used to discuss differences in trait magnitude across individuals, populations, or species that cannot be accounted for by isometric scaling (where all parts grow proportionally with body size) or by standard allometric relationships (where trait size scales with body size according to a predictable nonlinear pattern). It highlights variation in the measured size of a feature beyond simple, expected size changes tied to overall body size.

Applications of heterometry appear in comparative anatomy, paleontology, and developmental biology. For example, measurements of limb bones, cranial elements, or dental dimensions may vary independently of body size, reflecting genetic, functional, or ecological factors. Researchers may attempt to separate size effects using allometric corrections or regression analyses to examine residual heterometry, which represents size-independent variation in a trait.

Terminological note: heterometry is not universally defined with a single, agreed-upon definition, and some authors describe it as overlapping with allometry or as a framework for discussing variation in trait magnitude beyond straightforward size scaling. Consequently, when encountered, it is important to review the specific context and definitions used in that study.