A COMPARISON OF GROWTH AND DWARFING TRENDS OBSERVED IN PLEISTOCENE MAMMOTHS AND MASTODONS: HOW DOES ONTOGENETIC SCALING OF LIMB BONES COMPARE TO CHANGES ASSOCIATED WITH INSULAR DWARFING?

Elephants, much like other terrestrial megafauna, are supported by thick robust limbs, which are necessary to accommodate their larger body mass. How do these limbs grow between different species? Do baby mammoth and mastodon limbs get more robust as they grow, or do their proportions stay largely the same, as observed in most other mammals, including other proboscidean species? How do the trends of growth from small juvenile to large adult compare to the reduction in size seen in Pleistocene insular dwarf mammoths? Thanks to the large sample of juvenile limb bones at Rancho La Brea and many other museums, we were able to investigate the growth curves of three different species of proboscideans (the extinct American mastodon, Mammut americanum, the Channel Islands pygmy mammoth Mammuthus exilis, and the Columbian Mammoth, Mammuthus columbi) to determine their ontogenetic patterns, and compared their growth to data from extant African elephants. The limbs were measured for total length, length between the diaphyseal growth plates, and circumference at the midshaft, and plotted on bivariate graphs. Reduced major axis (RMA) slopes and their confidence intervals were calculated. Unsurprisingly, neither the pygmy mammoths nor the living African elephants show growth that were negatively allometric. Instead, the humerus, femur, and tibia change isometrically as they grow, which is consistent with trends observed in most mammalian taxa, and only the ulna tends to grow more robust as the mass increases rapidly. What was surprising was the growth trend of the American mastodon which were quite negatively allometric. Previous studies have shown that dwarfed hippos and rhinos tended to develop more robust limbs as they shrank in size. In addition, previous studies regarding dwarfed proboscideans, such as the various species of Mediterranean mammoths have shown that their proportions shrank isometrically. However, the Channel Island pygmy mammoths only became more robust in the shrinking proportions of the humerus and ulna, but the femur and tibia became reduced in size isometrically.