What is the term for being younger than the body age? I remember that I once attended a seminar in which the speaker talked about the heart rate of different kinds of butterflies. Normally, the heart rate of the adult butterfly will be more complex than the heart rate when it is in the larvae form. However, there are some species that have complex heart rates even when they are in the larvae form, and there are others that have simple heart rates even when they are in butterfly form. So if we are talking about the butterflies' heart, we can say that some species' heart are younger than their real age, while others are older than their real age. What is the name of this phenomenon?

# Neoteny

> Neoteny (/niˈɒtəni/), also called **juvenilization** , is the delaying or slowing of the physiological (or somatic) development of an organism, typically an animal. Neoteny is found in modern humans. In **progenesis** (also called **paedogenesis** ), sexual development is accelerated.
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> Both neoteny and progenesis result in **paedomorphism** (or **paedomorphosis** ), a type of heterochrony. Some authors define paedomorphism as the retention of larval traits, as seen in salamanders.
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> Both neoteny and progenesis cause the retention in adults of traits previously seen only in the young. Such retention is important in evolutionary biology, domestication and evolutionary developmental biology.