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  • In 1873, a geologist named Robert Stearns reported a rather odd find on the Channel

  • Islands off the coast of southern California.

  • He had discovered a single tooth from an animal that wasn’t known to have lived on the islands:

  • a mammoth.

  • The tooth was was a bit on the small side, but it was obviously from a mammoth.

  • And it was enough for Stearns to conclude that the Channel Islands must have once been

  • connected to the mainland.

  • Otherwise, he thought, how could the mammoth have gotten there?

  • Well, Stearnshypothesis turned out to be wrong; the islands weren’t ever part

  • of the mainland.

  • But that tooth would eventually be recognized as evidence for something even more unexpected.

  • By 1928, enough mammoth fossils had been found on the Channel Islands that paleontologists

  • realized that many of them were unusually small, around 2 meters tall at the shoulder.

  • The mammoths were so small that they came to be seen as their own species, commonly

  • known as the Channel Islands Pygmy Mammoth.

  • These animals aren’t known anywhere else in the world.

  • Which means that they must have evolved in place on the Channel Islands.

  • But theyre also extremely similar to the mammoths that lived just a few dozen kilometers

  • away, on the California mainland: the 4 meter tall Columbian Mammoth, one of the largest

  • mammals to ever walk North America.

  • So if the islands were never connected to the mainland, then how did these little mammoths

  • get there?

  • Were they related to the giant Columbian mammoths?

  • And how -- and why -- were they so small?

  • The answers can be found in the way species respond to isolated environments with limited

  • resources.

  • It’s a phenomenon that’s been observed all over the world.

  • And the evolutionary forces that drive it are slow and gradual, but powerful.

  • Powerful enough to shrink a mammoth.

  • The oldest mammoth fossil ever found on the Channel Islands is a tusk found on Santa Rosa

  • Island in 2014.

  • Uranium-series dating of nearby corals showed it to be about 80,000 years old, from the

  • late Pleistocene Epoch.

  • And even though the tusk was discovered more than 140 years after Stearns found that first

  • tooth, both fossils raise the same question:

  • How did mammoths manage to get to these islands, some of which are more than 40 kilometers

  • from the mainland?

  • To solve that puzzle, researchers began by studying what southern California was like

  • in the Pleistocene.

  • Back then, coastal California was often cooler and drier than it is today.

  • And southern California was covered with grasses and conifer trees, which fed herds of enormous

  • Columbian mammoths, as well as horses, and giant ground sloths.

  • And in turn, these herbivores were stalked by skilled predators like saber-tooth cats

  • and dire wolves.

  • But the Pleistocene was a time of tumultuous change.

  • The planet cooled, then warmed, then cooled again, over and over.

  • And those changes dramatically altered the coastline of California, as well as the size

  • of the Channel Islands themselves.

  • During warmer spells, as nearby glaciers retreated, portions of the Earth’s crust underneath

  • slowly rose back up.

  • But every time the climate turned colder, sea levels dropped, as more water became locked

  • up in glaciers and ice caps.

  • So as the seas dropped, more of the islandsland was exposed, making them bigger and bigger

  • until some of them eventually became connected.

  • The four northernmost islands were joined together into a single island that geologists

  • call Santarosae.

  • And during these times, the surface of Santarosae was much closer to the mainland than than

  • the islands had been.

  • Sometimes, when the sea level was really low, it was just over 7 kilometers away.

  • So, that’s a shorter distance; but it still doesn’t explain how mammoths crossed the

  • water.

  • Well, the thinking among paleontologists is: They swam!

  • Believe it or not, swimming comes easily to elephants.

  • Theyre strong, buoyant, and their trunks can act like snorkels.

  • In rare instances, African elephants have been known to swim as much as 48 kilometers,

  • so it seems at least plausible that a Columbian mammoth could paddle less than a quarter of

  • that distance to Santarosae when sea levels were low.

  • And even though the oldest definitive mammoth fossil on the island is 80,000 years old,

  • some experts think that the first mammoths made their historic journey as much 150,000

  • or even either 250,000 years ago, when sea levels were especially low.

  • But the next question you might be wondering is: Why would these animals even want to swim

  • out there to begin with?

  • I mean, southern California is nice, right?

  • Well, for much of the Pleistocene, mainland California was an ideal habitat for Columbian

  • mammoths, but in some ways, the islands were even better.

  • Pollen samples from the Late Pleistocene show us that Santarosae was covered with vegetation,

  • like pines, firs, grasses, and sage.

  • And although they mostly ate grasses and sedges, mammoths also sometimes ate woody plants.

  • Additionally some researchers think that Columbian mammoths knew that food was there, because

  • not only could they see this nice, lush island, they may have even been able to smell it.

  • Research in 2014 found that modern elephants have twice as many genes dedicated to smell

  • as dogs do, and they can smell food and water from considerable distances.

  • No matter what drew them to the island, once they got there, the mammoths found another

  • benefit to their new home: there were no big predators.

  • There’s no fossil evidence of saber-toothed cats, dire wolves or any other large carnivores

  • on the island.

  • It’s not clear why; maybe they just didn’t want to get their feet wet.

  • But even in the absence of predators, those pioneering Columbian mammoths eventually ran

  • into trouble in their new home.

  • Because, the same climate changes that made the glaciers grow also sometimes made them

  • shrink, dumping fresh water into the ocean and rising sea levels worldwide.

  • This happened a lot in the Pleistocene; the most recent period of rising sea levels began

  • around 20,000 years ago.

  • And when the seas rose, much of the terrain flooded, including some of the grasslands

  • that grew the mammothspreferred food.

  • When seas were at their highest, only Santarosae’s highest points were above water, and in fact

  • those high points form the four northern islands that we see today.

  • This new island environment was totally different from the one the mammoths experienced when

  • sea levels were lower.

  • It was also very different from the mainland habitats that their mammoth ancestors had

  • adapted to originally.

  • And one of the most important adaptations that the ancestral Columbian mammoths had

  • acquired was their enormous size.

  • Back on the mainland, the mammothssize gave them a big advantage: helping to deter

  • the saber-toothed cats, American lions, and dire wolves that shared their ranges.

  • And big animals typically need more food, and there was plenty of that on the mainland.

  • But on Santarosae, there were no large predators, but there was a limited food supply.

  • So, suddenly, being big wasn’t helpful; it was detrimental.

  • And this is when a fascinating phenomenon took place.

  • In this new island environment, natural selection began to favor mammoths with reduced body

  • sizes, mostly because they needed less food.

  • These smaller mammoths not only required less food, they also began eating different things.

  • A 2015 analysis of scratches and pits found in their teeth showed that Pygmy mammoths

  • ate a lot more twigs and leaves than Columbian mammoths did.

  • This may be because they were better able to access the steep slopes near the peaks

  • of the islands, where these foods grew.

  • One study even compared things like the limb proportions and centers of gravity of both

  • kinds of mammoths, and concluded that the pygmys could have climbed steeper inclines

  • to reach parts of the islands that the Columbians couldn’t.

  • Either way, the newer, smaller mammoths were able to take fuller advantage of the forests

  • that were prevalent on the islands, and probably gave them an edge when sea levels rose and

  • flooded the grasslands that the columbian mammoths preferred.

  • Pygmy mammoths eventually grew so specialized to their unique, isolated environment, that

  • by at least 80,000 years ago, they became what’s considered to be a new species.

  • This is a classic example of Insular Dwarfism, the tendency for large organisms living in

  • isolated environments to become smaller over evolutionary time.

  • But there’s an interesting wrinkle in this tale: The two species -- Columbian mammoths

  • and pygmy mammoths -- apparently coexisted on the island.

  • Fossil evidence shows that Columbian Mammoths lived on Santarosae as recently as 15,000

  • years ago.

  • But, based on the amounts of fossils from both animals, scientists estimate that pygmy

  • mammoths may have outnumbered the Columbian mammoths by more than three to one.

  • So, it’s possible that these big Columbian mammoths were from a population that -- for

  • whatever reason -- never evolved a smaller body size.

  • But others suggest that they might represent repeated waves of new visitors from the mainland.

  • In which case, full-sized Columbian mammoths may have swum out to islands populated by

  • what were essentially miniature versions of themselves, like a Pleistocene version of

  • Gulliver’s Travels.

  • Now, we understand pretty well what happened to the Channel Islandsshrinking mammoths,

  • because ... it happened more than once.

  • For example, during the Late Pleistocene, Europe was home to an enormous elephant, Palaeoloxodon

  • antiquus, also known as the straight tusked elephant.

  • And at some point during an Ice Age cold snap, some of these elephants seem to have made

  • their way to the island of Cyprus, where they too found themselves stranded when the sea

  • levels rose again.

  • By about 11,000 years ago, a very similar, but much smaller elephant was living on Cyprus,

  • Palaeoloxodon cypriotes.

  • While Palaeoloxodon antiquus probably tipped the scales at 10 metric tons, the new dwarf

  • species weighed only 200 kilograms, a reduction in size of about 98 percent!

  • And the same thing happened with a species of hippos that lived on Cyprus until about

  • 9,000 years ago.

  • This phenomenon is so common that it’s got its own Rule, proposed by biologist J. Bristol

  • Foster in 1964.

  • Foster’s Rule says that, in isolated environments, animals can acquire either smaller or larger

  • body sizes, depending on the availability of resources like food.

  • The rule was inspired by Foster’s study of modern island animals, like the pygmy raccoon

  • on Cozumel in Mexico.

  • Or the pygmy tree sloths that live off the coast of Panama.

  • Both mammals are about half the size of their mainland counterparts.

  • And Foster’s rule isn’t a set-in-stone scientific law.

  • It’s more like a trend, to which there are plenty of exceptions.

  • But in both the fossil record and modern ecosystems, we can find many examples of insular dwarfism,

  • which enables organisms to handle the limited resources that come with living on an island.

  • But, even miniaturization can't always save an animal from extinction.

  • The Channel Island pygmy mammoths disappear from the fossil record shortly after a global

  • cooling episode known as the Younger Dryas came to an end about 11,600 years ago.

  • With the climate getting warmer and drier, the pygmy mammothsforests were replaced

  • with coastal scrub and grassland.

  • And while these animals had survived lots of warm periods before, this time, they also

  • had to share their island with a new neighbor: us.

  • The first humans appeared on the channel islands around 13,000 years ago.

  • And while we don’t know for sure if people hunted the pygmies, there are butcher sites

  • elsewhere in North America that show humans did hunt and eat mammoths.

  • So, as the Channel Islandsshrinking mammoths can attest, islands can offer safe refuge

  • from predators, and harbor unique habitats that some animals can exploit.

  • But when the climate changes, or when new predators arrive, there’s nowhere to run,

  • and a mammoth’s paradise can become a trap.

  • But did you notice was I said earlier?

  • About Foster’s Rule?

  • Even though mammoths may have shrunk after they arrived on islands, there are plenty

  • of other ancient animals that became giants in island habitats.

  • So join us again in a few weeks, when well explore how an island grew massive waterbirds

  • and huge hedgehogs, and how Foster’s Rule applies to them, too.

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  • You can find activity ideas and additional information on their website at g.co/ScienceJournal

  • or check out the link in the description below.

  • And extra big thanks to our current Eontologists, Jake Hart, Jon Ivy, John Davison Ng and STEVE!

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猛Ma象岛(The Island of Shrinking Mammoths)

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    joey joey 發佈於 2021 年 05 月 02 日
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