Short-term growth: evidence for chaotic series of mini growth spurts in rat growth.

Five thousand and eighteen quadruplet daily measurements of lower-leg length of 62 female and 81 male rats, were performed in order to characterize short-term growth. Within a short time, growth proceeds irregularly and consists of multiple incremental bursts (mini growth spurts) with no evidence for strict periodic behavior. Mini growth spurts are S-shaped incremental patterns that can be characterized by double-exponential functions (Gompertz's functions). Gompertz's functions are S-shaped, and can be defined by three parameters that identify amplitude, inflection point (age at peak growth velocity), and slope. The latter not only refers to the rapidity of each incremental burst, but also alludes to the duration that one incremental burst needs for completion. In regard to these characteristics, mini growth spurts differ significantly between the sexes in rats. Mean amplitude of mini growth spurts was 2153 microm (SD 1034 microm) in female rats and 2958 microm (SD 1614 microm) in male rats. Peak growth velocity of mini growth spurts appeared lower in male rats than in female rats. Female rats showed mean gamma of -1.23 (SD 0.72), whereas male rats showed mean y of -0.96 (SD 0.72). Partial growth hormone deficiency led to a modification in rats that was reversed when exogenous growth hormone was administered. Mean intervals between subsequent mini growth spurts ranged between 4.2 and 4.6 days, but the large variation of these intervals (SD between 1.6 and 2.3 days) and the fact that neither spurt-spurt interval nor spurt amplitude appeared predictable, strongly suggest chaotic behavior of mini growth spurts.