Meeting Abstract

73.4  Tuesday, Jan. 6  Could Giant Kangaroos Hop? Scaling of tendon geometry and skeletal features. MCGOWAN, C.P.; Univ. of Texas at Austin cpmcgowan@mail.utexas.edu

Large species of Macropodoidea, the superfamily containing kangaroos and wallabies, have the amazing ability to decouple oxygen consumption from speed. This is largely due to the capacity to store and return increasing amounts of elastic energy in the ankle extensor tendons. However, elastic energy storage in tendons is proportional to tendon strain and as elastic energy storage increases, tendon safety factor decreases. Recent scaling studies of macropodoids have shown that the capacity for elastic energy return increases with body size, while tendon safety factor decreases and may limit maximal body size ( ~140 kg). Yet fossil evidence suggests that several species of extinct macropodoids likely reached 150 kg and the largest, Procoptodon goliah, may have been 250 kg or more. Clearly, if these animals followed the same scaling relationship as extant species, they would have had safety factors well below one, and thus would have been very limited in their ability to hop. In this study, we examined the scaling of morphological features on the calcaneus and tendon cross-sectional area in extant macropodoids (n=15, size range: 0.8 to 27 kg). The results of this analysis showed that the area of the tendon attachment site on the calcaneus is highly correlated with gastrocnemius tendon area (r²=0.91), plantaris tendon area (r²=0.91) and combined tendon area (r²=0.93). We then measured the same morphological features from fossilized calcanei of extinct giant kangaroo species (n=4). The relationship between tendon attachment area and tendon cross-sectional area was used to estimate the tendon cross-sectional area for these extinct species. Our results suggest that extinct giant kangaroos did not follow the same scaling patterns as smaller extant species, but rather had much larger ankle extensor tendons for their size and therefor could likely hop in a similar manner to modern kangaroos.