The marsupial mole, an elusive creature that swims through the sands of remote Australian deserts, seems to have suffered an abrupt population crash about 70,000 years ago, possibly due to climate change.
That's according to a new analysis, described in the journal Science Advances, that looked at DNA from one of these secretive critters.
Northern and southern marsupial moles are so rare that a sighting in the wild can make headlines, as people marvel over its lush golden fur and flipper-like front feet.
Scientists don't know much about how a marsupial mole makes its way in life, or how these moles have evolved over time.
"It's endangered, so you're not allowed to go looking for it. It's extremely rare," notes Nathan Clark of the University of Pittsburgh. "It's in Australia, in the desert. How are you going to find this thing? It doesn't have permanent tunnels."
He's studied various moles from around the world, to understand how a mammal can adapt to living underground, and says a subterranean life does have advantages like protection from predators and extreme temperatures.
On the other hand, moles have to live in constant darkness and breathe oxygen-poor air. "That's clearly a very extreme change," he says.
Among mammals, the fully underground lifestyle has evolved independently at least five times, says Clark, who was curious to understand how it happened in Australia, where the moles have been isolated for millions of years.
"Their body plan and limbs and stuff look just like the European mole or the star-nosed mole," he says, "but they are completely unrelated."
Like kangaroos, marsupial moles have a pouch, although it's upside down, presumably so that sand doesn't collect in the pouch as the mole swims forward through sand.
Clark and some other colleagues recently teamed up with biologists in Australia to analyze DNA from a single specimen: a female southern marsupial mole.
"Marsupial mole specimens are very rare and no one has ever managed to keep one alive for very long in captivity," Stephen Frankenberg of the University of Melbourne told NPR in an email.
Decades ago, tissues from this particular mole were collected after its death and kept frozen at the South Australian Museum, he says, adding that samples were made available to the team for analysis thanks to museum researcher Stephen Donnellan.
Frozen tissues are well suited to DNA sequencing, says Frankenberg, calling the availability of this tissue "an opportunity too good to resist."
According to the study's results, the mole's closest living relatives are two small nocturnal marsupials, the bandicoot and the bilby,
What's more, genetic evidence suggests that the population of marsupial moles had been stable before declining sharply about 70,000 years ago, roughly coinciding with a time of cooler temperatures and dropping sea levels.
"I think that's definitely one of the major findings of this paper," says Sarah Lucas, a biologist currently with the University of Münster in Germany and a member of the research team.
Biologists really know nothing about the population dynamics of these moles, she says, noting that "again, it's really hard to find a living one."
The researchers say their analysis suggests that the mole's declining numbers can't be blamed on the spread of people to this continent.
"It's very difficult to say, but our results suggest that it was due to climatic changes rather than the arrival of humans," says Frankenberg, noting that in the region inhabited by marsupial moles, humans showed up ten to twenty thousand years after the population crash.
Another part of their study looked at how the marsupial mole lost its vision, using comparisons with other species to find approximate dates for when various vision-related genes started to degrade.
The genes lost first appear to be involved in forming the eye's lens. This suggests that for a time the moles were still able to see light and dark, but could not see very clear images.
Then the mole lost functional genes for cone photoreceptors, which are involved in perceiving bright light and color vision. At that point, the moles could see in dim light, and perhaps still came above ground at night.
Eventually, though, even the eye's rod photoreceptors used to perceive dim light went away, and the mole was left completely blind.
"You can basically create a mental picture, over millions of years, of this thing slowly going more and more underground and then never coming back out," says Clark. "I thought that was just really cool."
"It nicely maps out how the evolution of marsupial moles and changes in their ecological niches would have progressed, with burrowing in the floor of a rainforest being a precursor to eventually becoming fully subterranean," agrees Frankenberg.
To see if other mammals that have lost vision follow a similar sequence of genetic events, says Lucas, "we're trying to actually study how other mammals lose their eyesight to see how unique the marsupial mole is."
Bats, for example, also evolved to rely less on vision, even though they live in the darkness of the night air instead of the blackness underground.
And as this study of marsupial moles shows, says Lucas, there's plenty of information still out there about mammals that remains a mystery.
"We like to think most of the mammals have been well discovered and well-characterized," she says. "But that's not true."
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