How do people walk on two legs?

Life on two legs

Their life is upside down: Slowly but surely, all six species of sloth living today are hanging upside down through the treetops of the tropical rainforests. Megatherium americanum and other giant sloths could not do this: They were much too heavy to stay in trees for long periods of time.

Instead, over the course of evolution, they had adapted surprisingly well to life on the ground. Megatherium americanum, for example, did not move forward in a quadruped gait, but often stood up on its hind legs and was apparently able to walk longer stretches upright. Scientists found evidence of this type of locomotion at least in 1987 near the small town of Pehuen-Có on the Atlantic coast of Argentina.

Prehistoric footprints in Pehuen-Có © MoMiA07 / public domain

Telltale footprints

In addition to many other fossil footprints, they also discovered prints there that most likely come from Megatherium americanum. “Each footprint is almost a meter long,” says the scientist Teresa Manera from the Universidad Nacional del Sur in Bahía Blanca, describing the enormous dimensions of the finds in the magazine “Geo”. However, almost only relics of the hind legs have been preserved in the rock, and according to the researchers, the hands were only used sporadically for support.

The valuable 12,000-year-old fossils confirm a theory about the two-legged gait of the giant sloth, which the Austrian paleontologist Othenio Abel had already put forward in 1912, which was considered absurd for a long time.

Slower than a jogger

The giant sloth did not advance particularly quickly in this way. This was the result of calculations by scientists based on the available data on the anatomy of prehistoric beings in 2003. A brisk human hiker or jogger would have easily overtaken Megatherium - the sloth reached a maximum speed of four kilometers per hour - and left behind.

Carnivores or Vegetarians?

The diet of the giant sloth was also controversial among scientists for a long time. Most thought it was a pure herbivore that stood on its hind legs and fished for branches with its claws in order to graze them afterwards.

However, some researchers vehemently contradicted this theory and took a completely opposite view. Richard Fariña and Ernesto Blanco from the Universidad de la República in Montevideo assumed that Megatherium did not use its claws to collect vegetarian food or as a weapon of defense, but to actively capture prey. In her opinion, potential victims were so-called glyptodons, giant relatives of today's armadillos, long extinct.

The scientists based their theory on an investigation of the so-called olecranon. This is the part of the ulna where the tendon of the upper arm muscle - musculus triceps brachii - attaches. The measurement of the olecranon showed that it was astonishingly small at Megatherium. However, the shorter this part of the bone, the faster the arm can move - an ability typically attributed to predators.

Fariña and Blanco therefore suspected that the giant sloth initially knocked over the glyptodon, which weighed up to 1,400 kilograms, with its physical strength. It then thrust the claws into the soft sub-habit of its victims to kill them.

Teeth refute the predator hypothesis

Today, however, this predator hypothesis has been disproved. A detailed analysis of fossil Megatherium teeth provided important evidence for this. “The edges are […] not sharp enough to cut meat. It is therefore unlikely that Megatherium was specialized in meat nutrition, ”Wighart von Koenigswald from the Steinmann Institute for Geology, Mineralogy and Paleontology at the University of Bonn and Simone Hoffmann from Stony Brook University in New York summarize the results.

According to scientists, the giant sloths themselves hardly had to fear predators due to their size and dangerous claws. They only had to be careful of saber-toothed cats or primeval dogs.

Dieter Lohmann
Status: 02/10/2012

February 10, 2012