Australia’s giant kangaroos of yesteryear were likely homebodies — and that inclination ultimately did them in.

The massive marsupial megaherbivores, which, at an estimated 375 pounds, weighed more than twice as much as contemporary kangaroos, really had no reason to wander. Until about 280,000 years ago, they lived in a plant-eater’s Eden — a lush rainforest that, to them, probably seemed like the ultimate salad bar.

But their inability — or unwillingness — likely led to their extinction when the climate started changing, according to a new study in the journal PLOS ONE.

Extinction of Giant Kangaroos

Earlier studies proposed that Protemnodon didn’t have the physical ability to hop very far. Their unwieldy size and ungainly body shape rendered them unsuitable for long-distance foraging. This runs contrary to large mammalian herbivores, in which the bigger the body size, the wider the foraging range. Consider the buffalo. Studies of other ancient kangaroo species suggest they were hunted to extinction, although that view remains controversial.

A new study gives the theory of marsupial stasis more credence. Researchers analyzed Strontium isotopes from Protemnodon teeth found in Mt. Etna Caves in central Queensland. The dental isotopes only matched the local limestone, rather than any rocks or minerals from more distant areas. The researchers concluded the animals had a smaller foraging area than expected.

As the climate turned drier, the increasing aridity dried out Protemnodon’s grazing zone. Their inability to locate greener pastures ultimately led to their extinction. The researchers were surprised at just how geographically limited the giant kangaroo’s dining zones appeared.

“Using data from modern kangaroos, we predicted these giant extinct kangaroos would have much larger home ranges,” Christopher Laurikainen Gaete, a researcher at the University of Wollongong, Australia, and an author of the paper, said in a press release. “We were astounded to find that they didn’t move far at all, with ranges mirroring smaller modern kangaroo species.”

Read More: These 5 Giant Animals Walked Alongside Ancient Humans

Researching Giant Marsupials

The researchers still don’t know if this relatively limited grazing range was true for all Australian giant marsupials of the same time period. They are unsure if Protemnodon’s extinction due to its inability to roam was an isolated incident. More research will be required to determine that those giant marsupials, unlike massive foraging mammals, have home ranges tied more to their habitat than their body size.

The relatively new method of tracking dental strontium remnants may help resolve that question sooner, rather than later.

“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,” Scott Hocknull, a paleontologist at the Queensland Museum and an author of the study, said in the news release. “It’s Paleo Big Brother.”

Read More: 5 Of The Biggest Animals To Ever Live On Earth

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Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.