In biology, heterokairy is used to define the variability or plasticity in the timing of the onset of developmental events 'at the level of an individual or population during development'. Physiological heterokairy, for example, would then refer specifically to plasticity in the timing of the onset of physiological regulatory systems or their components. Heterokairy derives from the Greek words Hetero meaning "different" and Kairois meaning "at the right time."
The term “heterokairy” was first coined in 2003 by John Spicer and Warren Burggren from the University of Plymouth and the University of North Texas, respectively. It was introduced to differentiate this intra-specific developmental phenomenon from the more widely known heterochrony, which refers to differences in developmental timing between rather than within species. While heterochrony is genetically based, heterokairy can result from epigenetic, environmental factors that modify the relative development of physiological processes.
Relatively few experimental studies have dealt explicitly with the issue of heterokairy, although it may potentially be a confounding factor in many studies of individual and population level developmentwithin a species. Nevertheless, several studies provide a reasonable data base to illustrate and to begin an investigation of the phenomenon of heterokairy.