Richard Musgrove
Dr Richard Musgrove is a zoologist and science journalist. He writes for Cosmos and Double Helix.
Giant kangaroos should have giant home ranges, but researchers were shocked to find that the largest of them all, Protemnodon, was a real homebody. Habitat, not body size was the key says lead researcher, Dr Chris Laurikainen Gaete, from the University of Wollongong.
Protemnodon was a genus of the giant forest kangaroos that lived until about 40,000 years ago in Sahul, the super-continent comprising Papua New Guinea, Australia and Tasmania and the connecting land-bridges.
These ancient marsupials grew up to 170kg, about twice the size of the largest modern male red kangaroo, and were chunkier, with very large arms. Species ranged from hoppers through Australia’s open dry habitats to more quadrupedal forest dwellers.
The largest of the giant kangaroos
was probably more of a quadruped than a hopper, with short feet and more delicate ankle bones than modern kangaroos.
“A study of the limb bones, and the bone proportions to each other, show that Protemnodon, was likely a poor hopper at best,” says Professor Christine Janis of the University of Bristol, “and probably moved mostly quadrupedally, perhaps bounding on all fours like tree-kangaroos do on the ground.” Janis was not involved in the study.
The relatives of the giant kangaroos still hop and shuffle around Australia today, which is why it’s critical to understand their life histories, how they used landscapes, and why they went extinct, says Laurikainen Gaete.
The researchers were expecting Protemnodon to follow the large mammal-large home range trend. Larger mammals generally need to cover more ground to get enough food for growth and reproduction. Australia’s extinct Diprotodon migrated seasonally, with herds moving hundreds of kilometres, and modern kangaroos, African elephants, zebras, wildebeest, and polar bears all have large home ranges.
Laurikainen Gaete led a team to fossil sites in the limestone-rich Mt Etna Caves National Park, which is now a dry rainforest, described as a semi-evergreen vine thicket.
Fossils suggest that 500 to 280,000 years ago it was a lot wetter. A “a biodiverse rainforest ecosystem” says Laurikainen Gaete.
Collected fossil teeth were from 3 Protemnodon species, including the largest of the genus (P. viator).
Aging techniques revealed that these giant
kangaroos were alive between 222,000 and 280,000 years ago.
The big surprise were the small home ranges revealed by the strontium isotope data. Smaller Protemnodon ranged over 3.6km2, the largest species, just 19.8km2.
Coauthor Dr. Scott Hocknull of the Queensland Museum, says, “These new isotopic techniques have blown our field wide open. Imagine ancient GPS trackers—we can use the fossils to track individuals and know where they moved, what they ate, who they lived with and how they died. It’s Palaeo Big Brother.”
Laurikainen Gaete adds: “Using data from modern kangaroos, we predicted these giant extinct kangaroos would have much larger home ranges. We were astounded to find that they didn’t move far at all, with ranges mirroring smaller modern kangaroo species.”
The isotope data also suggested that there were enough resources in that closed canopy rainforest to support animals twice the size of a big red male kangaroo, says Laurikainen Gaete. Such abundance persisted for about 200,000 years in the Mt Etna area, he adds.
Protemnodon’s low-crowned teeth and long forearms suggests a browsing habit, making meals of the local bushes and shrubs. This might also account for the small home range, given such a diet is more nutritious than the grass eaten by modern kangaroos, Laurikainen Gaete says.
Between 280,000 and 205,000 years ago the climate began to dry. Large size and body shape like that of the giant kangaroos meant that long-distance bipedal hopping wasn’t really an option for these giants. Extinction loomed.
Whether habitat or body size determined home range for other extinct Australian marsupials, remains to be seen, Laurikainen Gaete concludes.
The paper was published in PLOS One.
Read more: These giant kangaroos were more resilient to climate change
