Interspecies variation in mammary gland growth rate: relationship to gestation length.

Growth of the mammary gland is measured by several indices including total wet weight, dry fat-free tissue, and deoxyribonucleic acid. The latter is a superior measure of true growth because it represents changes of cell numbers. Sufficient data have been generated to determine the relationship among species of mammals between gestation length and differences in rates of mammary growth. Exponential growth equations were estimated for eight mammalian species with gestation lengths from 16.5 d for the hamster to 280 d for the cow. The form of the most appropriate equation was Y = AeBx, where Y is mammary deoxyribonucleic acid or dry fat-free tissue, x is day of gestation, e is the base of natural logs, and A and B are constants. The A term was related to body weight (W) and the B-term to gestation length (G). Resulting equations were deoxyribonucleic acid (mg) = .0547W.803 e1.98 G-.98x and dry fat-free tissue (mg) = 2.35W.779 e.719 G-.77x. First-order rate constants of mammary growth ranged in a reverse order from a high of .141 d-1 in hamsters to a low of .008 d-1 in cows; in other words, mammary deoxyribonucleic acid in hamsters doubled in 4.9 d but in the bovine it took 87 d to double.