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 chara cterize short-term growth. Within a short time, growth proceeds irregu larly 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-exp onential 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 I;lope. The latt er not only refers to the rapidity of each incremental burst, but also alludes to the duration that one incremental burst needs for completi on. In regard to these characteristics, mini growth spurts differ sign ificantly between the sexes in rats. Mean amplitude of mini growth spu rts was 2153 mu m (SD 1034 mu m) in female rats and 2958 mu m (SD 1614 mu m) in male rats. Peak growth velocity of mini growth spurts appear ed lower in male rats than in female rats. Female rats showed mean gam ma of -1.23 (SD 0.72), whereas male rats showed mean gamma of -0.96 (S D 0.72). Partial growth hormone deficiency led to a modification in ra ts that was reversed when exogenous growth hormone was administered. M ean 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 s purt amplitude appeared predictable, strongly suggest chaotic behavior of mini growth spurts. (C) 1998 Elsevier Science Inc.