ALLOMETRIC RELATIONSHIPS IN LIONS VS. DOMESTIC CATS

Gene-controlled allometric growth rates are capable of producing profound and complex evolutionary changes in the proportions of organisms. Their role in vertebrate evolution has recently been emphasized by Rensch (1948, 1960). Allometric rates may reflect the results of natural selection in two ways: (1) selection for increased (or decreased) organism size may result in exponential changes in the relative sizes of structures, and such changes may or may not be adaptive, and (2) natural selection may operate directly on the allometric rates themselves to produce changes that are by definition adaptive. Finally, (3) many organs and tissues have the capacity of responding directly to the physiological needs of the organism, and vary allometrically without any change in the genetic background, although the capacity for such physiological response is of course inherited. Rensch (1960) has summarized pertinent data from the literature, and Frick (1957b) has critically discussed procedural and mathematical methods. The major problem in evaluating these data is eliminating other variables. The most important of these other variables are differences in genetic background, differences in physiological requirements that are unrelated to size, and differences in the procedures used by the several investigators. Others are sex, age, and physical condition. Inadequate size differences between organisms being compared may be a limiting factor unless samples are very large. The ideal situation would be comparisons between closely related organisms with similar habits and behavior, but differing substantially in size, for then minor individual variations and differences in technique are minimized. This ideal at least approached in the Felidae, a compact group of very similar mammals among which the largest species are more than 60 times as heavy as the smallest. Studies of allometric weight relationships between species have been limited almost entirely to a few compact organs: brain, heart, liver, kidneys, etc. No attempt has been made to evaluate more diffuse structures, such as the wet skeleton or the musculature, perhaps because preparing these for weighing presents formidable practical problems. Yet the locomotor system may account for nearly two-thirds of total body weight in a mammal, and is no less subject to allometric growth rates than are the organs of metabolism. The following data are from two captive-bred lions that were destroyed for reasons other than ill health. Both were received within a few hours after death, and all weights and measurements of soft parts (except brain and spinal musculature) were made within seven hours after the animal was received. The male was destroyed at the Chicago Zoological Park on Feb. 2, 1962. He was 3-4 years old, and was below normal size. The skeleton showed that he was somewhat rachitic, but otherwise he seemed to be in good condition although there was very little body fat. The skull of this individual is preserved in the Chicago Natural History Museum under the catalog number 0-1526. The female was received from the Lincoln Park Zoo on May 28, 1946. She was mature, but not aged, and in excellent physical condition, with considerable body fat. Her skeleton is preserved under the catalog number 54639. Both of these specimens were below the average size of wild lions. The gross weight of the male was 109.3 kg, of the female 138.8 kg. The average and ex-