This episode is sponsored by Manta Sleep.

Manta Sleep makes sleep masks  and sleep accessories that help   you get the energy to create your best life. Head to the link in our  description to get 10% off.

Christmas Island is not huge– it’s only 135 square kilometers – and it was uninhabited by humans for  millennia, making it a biodiversity hotspot. It also has no large native predators,  which can mean only one thing: millions and millions of crabs. And not just in the surrounding waters, either, the island itself is home to 20  different species of land crabs.

But one in particular outnumbers  and outshines the rest,   due to its astonishing annual mating spectacle. The Christmas Island red crab’s  yearly migration has captured   nature enthusiasts’ attention for decades. But… could we be partly responsible  for this world-famous crab-valanche? [♪♪ INTRO ♪♪] The Bizarre Beasts pin club is open  for subscriptions for this whole month!

You can sign up by December 20th and the first pin  you get will be the Christmas Island red crab. Christmas Island, named by a British  East India Company captain when he   spotted it on Christmas Day, 1643,  can be found just south of Sumatra. And the Christmas Island red  crab can be found in forests   all over the island for the majority of the year… not just Christmas Day.

They hide in crevices and shady  spots to avoid dehydration,   plus they dig lots and lots of little burrows  in the ground that they close off with leaves. And those slightly rotten leaves are most of their diet,

though they will happily scavenge  whatever they can get their pincers on. As a result, the red crabs do a  pretty effective job of keeping   the forest floors tidy, often dramatically  controlling plant growth in the process.

And their cleaning efficiency is helped by the  sheer mass of crab in play at any given time. Adults max out at around 11.6 centimeters  across their shell, and their population is   somewhere over 100 million, so scientifically  speaking, this island is made of crabs. We’re talking an island-wide average of about  three red crabs for every four square meters, and that’s not including the 19 other land crab  species that also call Christmas Island home.

But the red crab’s sheer abundance is  not the most impressive thing about them. That honor instead goes to what they  get up to at the end of every year. Most months, the crabs lay low in the forest,  eating leaves and trying not to dry out.

Then, as the rainy season approaches,  they get ready to make their move – wetter conditions allow them to  travel without needing shelter,   so the start of the rains is their cue. But the timing isn’t just based on the weather, the crabs also follow the lunar cycle in  order to optimize their timing with the tides. So if the first rainfall of the wet  season doesn’t start during the right   phase of the moon, the crabs will wait  another full month before venturing out.

Once those conditions are met, though, usually between October and  December, the journey begins! Over the course of about a week or so, all of the  island’s adult red crabs march to the coastline. When they arrive, the males dig dens for the  females, and mate when the females get there.

Then the males make their way  back inland to the forests,   while the females stay in their dens to  brood their eggs for about a week and a half. This is when timing the tides comes into play. During the first or last  quarter of the lunar cycle,   the difference in height between high tide and  low tide is the smallest it’ll be all month.

This has a name, it's called a neap tide, and it’s when the  female crabs release their eggs into the ocean. At night, just as the high tide starts to turn,  each female crab releases up to 100,000 eggs,   which hatch immediately, and the larvae are swept  into to sea, where they develop for about a month. The next step is usually that  most of the larvae get eaten.

But every few years, when the conditions are  just right, they will survive in huge numbers   and return to shore and replenish  the island’s red crab population. The baby crabs start the slow march from the  beaches to the forest on their tiny little legs,   creating a moving sea of red that  blankets the island for over a week. The new crabs that successfully reach  their long-term home will continue to   grow for the next three years before they’re  big enough to repeat the process themselves.

And those who make it to adulthood will have  plenty of chances to experience the migration again:  red crabs are estimated  to live for a decade or longer. But if surviving the whole  ordeal wasn’t enough on its own,   all of this yearly crab legwork now  involves a series of human-made obstacles: houses, roads, also cars on the roads. Christmas Island can be a scary  place to be a crab pedestrian!

Which is too bad, because there are a lot of crab pedestrians! But we have put some safeguards into  place to make the migration smoother. These include selectively closing roads and  redirecting traffic as much as possible,   putting up barriers to guide  crabs away from dangerous areas,   and building a climbable bridge to  help the crabs avoid getting run over.

One wildlife photographer even found a way  to make his car’s tires mostly crab-proof. Even so, thousands of crabs die every year  from human causes like vehicle collisions,   so there’s still work to be done in  order to keep crab numbers strong. But we have to ask: was the migration always like this?

Because, there’s a case to be made that the red  crab’s population explosion is actually our fault. The first recorded human visit  to Christmas Island was in 1688,   and the crew immediately took  note of all the crabs they saw, the robber crabs, that is. They’re also known as coconut crabs,  and these guys are absolutely gigantic, growing up to a meter across, legs included, and they still cover the  island in huge numbers today.

But they aren’t the crabs we’re looking for. Almost two full centuries later,  in December of 1886,   the H. M.

S. Flying Fish arrived at Christmas  Island to perform a proper survey. Despite coming ashore in potentially  the middle of migration time,   the crew made no report of the red crab at all. Instead, they once again found  an abundance of robber crabs… as well as a species of rat  that was all over the place.

They named the swarming rodents after  the ship’s captain, John Maclear,   and Maclear’s rat became a fixture  of future excursions to the island. For example, a much more thorough  expedition the following year notably   featured Maclear’s rats crawling all over  the sailors in their hammocks as they slept. But no mention on that trip was made of  the red crabs, either.

So where were they? Well, from 1897 to 1898, a paleontologist  attempted to document as many Christmas   Island species as he could before  phosphate mining began there,   and he finally seemed to notice the red crabs. But despite commenting on the number of red crabs,  and even describing their annual migration, his statement kind of lacks the scale we would   expect if there were nearly as  many crabs back then as there are today.

And sure, a lackluster account of an old red crab  migration isn’t exactly proof that things have   changed drastically, but you’d still expect some  enthusiasm if there were tens of millions, right? He also noted during his stay that the  population of the island’s other native   rodent, the bulldog rat, seemed to be declining. And we’ll come back to that in a minute.

If we jump forward in time a decade,   that’s when it seems like the  crab ball really started rolling. In a 1908 letter home, a medical  doctor living on Christmas Island   wrote much more animatedly of the  migration spectacle, concluding, “...they are so thick they are  piled up one on top of another,   and on the seashore they are sometimes six deep. “I know it sounds like a fairy tale and  is hard to believe unless actually seen”. So this guy saw it, what changed?

Well, the same year as that doctor’s letter,  the paleontologist paid a return visit to   gauge the effects of human colonization  on the island’s wildlife over time. He reported that conditions looked more or  less the same as when he was there… except for the rats. Despite being ubiquitous across the island  and repeatedly described as moving in “swarms”   only a few years prior, Maclear’s rat had gone  entirely extinct, along with the bulldog rat.

Both of these rodents were most likely wiped out by a   parasitic disease introduced  to the island by ship rats. It took less than nine years. So how does this rat extinction  fit into our crab story?

Well it’s impossible to truly confirm now,   but Maclear’s rat may have been one of  the red crab’s only natural predators. With the rats gone, there was nothing to stop  the crab-stravaganza that continues to this day. It might even explain why the 1908 migration was   so much more attention-grabbing than  those of just a few years earlier.

The Time of the Rat was at an end, the Time of  the Crab had arrived, and we had ushered it in. The human effect on Christmas Island was acute and   fast, and it’s an important example of just  how temperamental island ecosystems are. The cascading effect of the rat-pocalypse  doesn’t stop with the crab boom, either: Maclear’s rat was the host for a  species of flea that’s now extinct,   as well as a species of tick  that may also be gone for good.

Meanwhile, the spectacular nature of the red  crab migration has led to a bunch of conservation   efforts on their behalf, to maintain the numbers  that we, ironically, inflated in the first place. It’s easy to forget that  living in an ecosystem means   only ever seeing a snapshot of it – a single moment in its long history. And while ecosystems certainly change  over time, the Christmas Island red   crab forces us to ask what we should do  when we’re the ones bringing the changes.

It’s an ongoing story, and one that  certainly involves a lot of bizarre beasts, including us. Sign up for the pin club at BizarreBeastsShow.com  and help keep the channel going. If you want a crab to be your first  pin, sign up by December 20th.

And now, for some bonus facts… [♪♪ BONUS FACTS ♪♪] In addition to black rats and their deadly  protozoan payloads, humans have introduced   wolf snakes, feral cats, and even a big  snail to Christmas Island over the years. Though that last one has proven more of a tasty  snack for the red crabs than an invasive threat. The yellow crazy ant was introduced likely  from west Africa sometime in the first   third of the 20th century, but it didn’t  start wreaking crab havoc until the 1990s.

These ants form massive, multi-queen supercolonies  that can cover 25 square kilometers or more,   which really started taking off in the late 1980s. Strengthened by such overwhelming numbers,   the ants crawl all over the red crabs  and spit acid into their eyes and mouths. The ants eat the crabs, steal their  burrows, and impede their migration.  The result?

Yellow crazy ants killed between a  quarter and a third of the island’s   total red crab population in the early 2000s. The lack of forest floor cleanup, in turn, can  cause a major feedback effect on forest plants,   which helps scale insects,  who further feed the ants – it’s a vicious cycle. This has understandably caused concern, but  is all ecological change automatically bad?

The crab bounty is only because  of other big human disruptions,   so maybe the yellow crazy  ants can restore balance! And like, do two wrongs make a right? One research team doesn’t think so, which is  why they’ve started deliberately introducing   a Malaysian parasitoid wasp to kill  the scale insects that feed the ants.

The initial test successfully restored ant-invaded   parts of the rainforest to  crab strongholds once more. And crab numbers are certainly back up in general! Still, with all the alterations that humans  have already brought to Christmas Island,   it’s hard not to feel like  we’re approaching a full-blown “Old Lady Who Swallowed a Fly” situation here.

All this is just more reason to stop and appreciate  the delicate balance that is island biogeography. If you have trouble getting to sleep and mentally  counting endless crabs doesn’t do the trick,   you need to hear about today’s  episode’s sponsor, Manta Sleep. They make the world’s best sleep masks  and functional sleep accessories,   designed to enable better lives  through better sleep and regular naps.

Take this one for example, the Manta Sleep Mask Pro. With its premium feel  and infinite adjustability,   it’s engineered to ensure a true 100% blackout  for deeper sleep with zero eye pressure. I could be surrounded by crabs right  now and I would not know it.

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And then, take a nap! [♪♪ OUTRO ♪♪]