Sexual Maturation in Underfed Weight‐matched Rats A Test of the “Critical Body Weight” Theory of Pubertal Timing in Males

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Abstract
A popular current theory proposes that the timing of puberty is related to attainment of a critical level of body weight or body fatness. These critical body weight and critical body fat theories have been studied almost exclusively in females. To explore these theories in males, we tested a corollary of these hypotheses: are male rats of the same weight all at the same level of sexual maturation irrespective of prior growth rate? Male rats growing in body weight at five different rates due to various degrees of underfeeding (beginning at weaning) were sacrificed at body weight milestones of 123 and 279 grams. At the first weight milestone, significant (P < 0.01) inverse correlations were observed among these weight‐matched rats between the preceding rate of body weight growth and prostate weight, seminal vesicle weight, testis weight, serum testosterone, and daily sperm production rate, indicating that the underfed animals were more sexually mature. Testis histology also showed that spermatogenic development increased progressively as the prior rate of body weight growth was reduced. These parameters of sexual maturation tended to correlate inversely with body fatness (i.e., leaner animals were more sexually mature) and directly with body length (i.e., longer animals were more sexually mature). By the second body weight milestone, however, the degree of prior underfeeding exerted little effect on those indices of sexual development. We conclude that the degree of sexual maturation in weight‐matched animals with varying previous patterns of body weight growth correlates inversely with body fatness and the rate of body weight growth but correlates directly with body length. These findings would not support either the critical body weight or critical body fatness theories of pubertal timing in underfed male rats and suggest that body length or chronologic age may be related more closely to sexual development than either body weight or fatness.