A few years ago, the professor of ecology and evolutionary biology at Brown University (USA) Christine Janis had an intuition when she contemplated in a museum the skeleton of a Stenurine ( Sthenurinea ), an extinct subfamily of giant kangaroos. Janis, an expert on other prehistoric Australian species such as the marsupial wolf, thought upon seeing the inflexible features of this animal that it might not move like modern kangaroos. That was the starting point for an investigation in which the scientist from the University of Malaga Borja Figueirido participates and which he has just published in the journal PLoS ONE . Janis’s thesis is that these kangaroos did not jump, but walked on their two legs. “Those animals may have had different behaviors and lifestyles than their modern relatives, and bone anatomy provides great clues,” says Janis.
Stenurines, also called short-faced giant kangaroos – their faces are similar to today’s rabbits – weighed 240 kg – three times more than today – and were most likely bipedal walkers. The experts did a statistical and biomechanical analysis of the bones of a specimen of Sthenurinea and other kangaroos and wallabies (small-sized marsupials) from the past and present to reach this conclusion. From a biomechanical point of view, stenurines of any size would be poor jumpers due to their anatomy, according to the researchers. In addition, for especially giant individuals, a jump could be an added difficulty. That is why Janis believes that in order to be so big they necessarily had to walk.
Today’s kangaroos jump at high speed and move on all fours when they want to move more slowly . Or, as a recent study has confirmed, they propel themselves with their legs plus their tails. This requires a flexible spine and a robust tail, as well as upper limbs that can support its body weight. But the Sthenurinea individuals do not appear to have had any of these attributes. The scientists found multiple evidence that they were much more adapted than today’s kangaroos to support their weight on one foot at a time, a requirement for walking. “The only current kangaroos capable of doing this, anecdotally, are those of arboreal species,” says Janis.
Also, by comparison, the lower end of the tibia in horses and dogs has a lip that wraps over the back of the joint, providing additional stability to support more weight on each ankle for running and jumping. Today’s kangaroos, which distribute their weight on both feet equally, lack this ridge, but stenurines did. In addition, these extinct animals had proportionally larger hip and knee joints . The shape of the pelvis also differs, as it was wide and flared, and its gluteal muscles were proportionally much larger than those of other kangaroos.
Those muscles would have allowed them to balance weight on only one leg at a time, like humans do to walk.
Therefore, “if they could not, in terms of biomechanics, jump at very slow speeds – they were very heavy – and it was not easy for them to move on all fours, then, since they had to move in some way, what was left? “ the researcher wonders. The bottom line, in light of their anatomy, is that these kangaroos were specialized for walking, not jumping. It remains to be clarified why they became extinct in the late Pleistocene, about 30,000 years ago. “It may be that they had trouble eluding human hunters, or that they couldn’t migrate far enough to find food when the climate became more arid,” Janis concludes.
