Animal models for growth

The translation of nutrients into increased body mass is achieved in different ways by different organs and organisms. The dimensions to which the bodies of vertebrates grow can be classified as determinate or indeterminate. Determinate types include higher vertebrates in which a species specific stature is attained more or less at the time of sexual maturity. Many of the lower vertebrates have indeterminate body sizes. They may continue to grow throughout life, some of them reaching the impressive proportions of certain big game fishes, giant tortoises, or dinosaurs. Even those forms that stop growing seem to retain at least the capacity for further enlargement, something that is not possible in birds and mammals. One may ask why these different modes of growth exist. There are two answers, the proximate and the ultimate. The proximate explanation relates to the developmental and physiological factors responsible for the programmed cessation of growth when body size is predetermined. The ultimate answer seeks to understand why one or the other type of growth evolved in the first place. Proximate answers can sometimes be found in experimental results. Ultimate ones arise from intelligent speculation.