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, Stearns' hypothesis 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 they're 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 islands' land 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.

They're 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 mammoths' preferred 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 mammoths' size 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 Islands' shrinking 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 mammoths' forests 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 Islands' shrinking 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 we'll explore how an island grew massive waterbirds

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

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