The Effect of Retarded Growth Upon the Length of Life Span and Upon the Ultimate Body Size

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Abstract
Rats were retarded in growth and not allowed to attain maturity until after periods of 766 and 911 days. The rat body still retains the power to grow at these extreme ages. After such periods of retardation the rat cannot attain a body size equal to that of an animal that grows to maturity younger. This conclusion is based upon the smaller size of the entire body, the weight of such organs as the heart, and the size of the bones represented by the femur. Even after these long periods of suppressed growth the male rat retains a growth potential greater than the female although the males of the retarded groups grow no larger than the normal females of this species. The hearts of all these animals dying in old age were larger than normal while the livers were smaller. The kidneys corresponded in weight at the time of death to the maximum weight attained by the body. The femurs of members of the retarded groups were less dense than those that matured normally. In both retarded groups individuals of both sexes attained extreme ages beyond those of either sex that grew normally. The mean age of the males of both retarded groups was greatly increased in comparison with ‘rapid growth’ males while the mean age for the females was about the same in all three groups. The males of the retarded groups exceeded the females in age in contrast to the ‘rapid-growth’ group. At a constant weight level in the course of retarded growth the female requires more calories for maintenance than the male. In the course of retarded growth, the diameter of the hair as well as the growth of the body reflects the retardation.

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