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Great Migrations: March of the Crabs

Princeton doctoral student Allison Shaw discusses the Christmas Island red crab's improbable migration to the sea, and the forces that prompt and guide all animal migrations. Great Migrations premieres tonight in the U.S. on the National Geographic Channel.

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By Ford Cochran

The largest programming event in the ten-year history of the National Geographic Channel, Great Migrations premieres in the U.S. beginning at 8 p.m. EST/PST tonight, with related coverage in National Geographic magazine and an official companion book.

Tens of millions of red crabs inhabit Christmas Island in the Indian Ocean. The first hour of Great Migrations, "Born to Move," relates the crabs' surprising and perilous migration from the island's interior to the sea. Fourth-year Princeton doctoral student Allison K. Shaw traveled to Christmas Island to study the crabs with a National Geographic Society/Waitt research grant. I spoke to Shaw about her work.

Tell me about your research.

It started with the red crabs. My first and second years of graduate school, I was looking for a pilot project to test some ideas, and I went to Christmas Island and worked with the crabs. Based on what I learned about them and what I saw, I got interested in animal migration in general.

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My background is actually in applied math and biology, so a lot of my interests are in modeling as well. I talked to a bunch of people who worked on migration, and I discovered that most work on migration is done at a very specific level. People who study a particular species of bird study its migratory pattern because they're interested in that bird. Any summaries on migration that are done are done at a taxa level, so there are summaries of bird migration, summaries of fish migration, mammal migration, insect migration. But there's not much work that's been done to span different groups and to try to come up with general patterns of migration.

What my thesis research tries to do is describe migration at a level that spans taxa--sort of, to come up with general ideas about under what conditions should any species should migrate, and then after that, try to link it to specific examples.

So are you trying to create mathematical classifications of different types of migration into which different migratory species could be classified?

Yes, in part. For example, one of the models that I just finished working on looks at species where individuals have to migrate in order to reproduce. You'll see examples where if they don't migrate, they don't reproduce. That was inspired directly by the crabs: All the crabs live in the middle of the island, they're fully terrestrial, but their eggs have to develop in the ocean. So in order to reproduce, in order to actually have eggs that develop into offspring, they have to migrate to the beaches, release their eggs in the water, and reproduce like that.

Any crabs that don't migrate won't reproduce that year. Those conducting past studies noticed that there were a bunch of adults in the middle of the island during migration that weren't participating. I saw that myself while I was there, and the question came up: If you're a species that has to migrate in order to reproduce, why would you ever skip, why would you ever postpone?

This sort of migratory pattern is also seen in sea turtles. Adult sea turtles have the opposite pattern: They live in the open ocean, but then when they want to reproduce, they come back to nesting beaches and lay their eggs in the sand. You'll sea cases where in most sea turtles, they don't reproduce every year, although they could. They will have different remigration intervals--two years, three years, four years, five years between migration events.

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And not all the individuals are synchronized, so it's happening in different years for different turtles of the same species?

Exactly. So there will always be some sea turtles that are migrating, but it won't be the same ones every year. Same thing with amphibians: a lot of amphibians will develop in local pools and ponds and then spend their adult lives terrestrially, but then when they reproduce they have to go back to those pools. Atlantic salmon live in the open ocean, and then when they want to reproduce, they have to migrate back into freshwater, so into streams, and reproduce there.

I built a model that said, okay, if you're a salmon or a sea turtle or a red crab, a species that has to migrate in order to reproduce, under what conditions should you ever skip reproduction.

Reproduction is one motivator for migration, but there are others. What are some of the other motives animals have to migrate?

At the broadest level, animals are migrating because it pays for them to spend part of their lives in one area and the rest of their lives in a different area. That seems to be related to different food distributions, different levels of predation, different needs in their life cycles (so whether they need to be focusing on accumulating resources versus spending their times reproducing). It's generally understood that all those factors are important, but how they interact is less well put together.

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Are there just a few mechanisms that seem to explain how migratory animals know which way to go, or does it appear that there are many different mechanisms animals use to follow their routes?

That's actually a complicated question. In general, there are a few different types of information you can use. You can use some sort of perception that you as an individual have of your environment. So either you're looking around and seeing what's there or you're perceiving the local magnetic fields and following magnetic pathways, or the local olfactory ... anything that you smell, or local visual stimuli. Or you can look at what other individuals of your species are doing. A lot of organisms such as wildebeests will migrate socially, where they are actually paying attention to what other animals are doing Or maybe you have a genetic predisposition to head in a certain direction. Or memory, if you've migrated somewhere before in a previous year.

So it's sort of personal perception of the environment, genetic or memory cues, and social cues. My interest is at that level: Should you use your own cues, should you talk to the other individuals, or should you use some sort of memory. A lot of people work in great detail on problems such as quantifying whether it's olfactory cues or magnetic cues, and there's evidence for almost every type of cue I think. To try to sort out which ones are the most important or what's their order of importance is pretty tricky.

What are some of the species you're studying most closely right now, and are you looking mostly at data collected by others and trying to pull it together in order to extract principles about animal migration?

A lot of it is just reading through the literature. As I mentioned, lots of summaries have been done, say, of bird migration or bat migration. So most of what I do is read through summaries by taxa to see, okay, what do people who study this group of birds think are the important factors, and what do people who study bats think are the important factors. And then I try to generalize those in a model.

With the breeding model that I spoke about, once I had specific predictions, I then tracked down species that had that type of migration pattern just in the literature, not collecting my own data. I looked to see what was published and what I could map to my model predictions.

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One of the many species featured in Great Migrations is the monarch butterfly, which makes an astonishing journey from Mexico to the northern United States and Canada over several generations, then returns south thousands of miles in a single generation. Is that sort of migration relatively common, with different generations responsible for different facets of a migration, or is a full migratory cycle typically traced by a single individual?

That's common for insects. As far as I know, there's not a single species of insect where the same individual will complete an entire cycle of migration--move from point A to point B and then back to point A again. But I think for most other species that's usually the case: They'll complete a full migration cycle or multiple migration cycles within their lifetimes.

I also think of an example that's familiar from March of the Pengiuns: The males and the females migrate, but they migrate at different times. The same is true of the Christmas Island crabs. Will males and females often participate at different times in a migratory cycle?

It depends on which factors are driving the migration. Sometimes you actually see cases where only one sex will migrate and the other one won't. In some bats, there tends to be a female bias, where females will migrate longer distances or be more likely to migrate than males will.

Any other amazing facts about migration you'd like to share?

Before I learned about the Christmas Island red crabs, I had never heard of a land crab. The idea that there were these crabs that live in the rainforest and have to migrate to the ocean, but the adults can't survive in water, was a crazy concept for me. There are actually quite a few species of land crabs where the adults are terrestrial. Some crabs are fully aquatic, some hang out on the beach and need to spend their time in the water, and some are fully terrestrial. All land crabs have eggs that have to develop in seawater, so all land crabs have to make some sort of trip back to the water.

The thing that's spectacular about the Christmas Island crabs is that they're going such long distances, and they're mostly active during the day. There are lots of land crabs in the Caribbean that are mostly active at night. If you want to study their migration, you have to go out and watch them at night, which is a lot harder to do.

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  • Watch Great Migrations on the National Geographic Channel beginning at 8 p.m. EST/PST tonight in the U.S. (Check local listings for air dates and times outside the U.S.)

  • Help conserve the landscapes of migratory species with National Geographic's Global Action Atlas.

Photos by Allison K. Shaw; book cover courtesy National Geographic Books

Ford Cochran directs Mission Programs online for National Geographic. He has written for National Geographic magazine and NG Books, and edits BlogWild--a digest of Society exploration, research, and events--and the Ocean Now blog. Ford studied English literature at the College of William and Mary and biogeochemistry at Harvard and Yale, with a focus on volcanoes, forests, and long-term controls on atmospheric CO2. He was an assistant professor of geology and environmental science at the University of Kentucky before joining the National Geographic staff.

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