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Tuesday, June 2, 2020
Their View / Opinion

Just now, crabs are the thing for Christmas … Island

Q.  Christmas Island, a remote 35-square-mile island in the Indian Ocean, is home to about 1,500 people and some 35,000,000 red crabs, reports Dan Lewis on his “Now I Know” website.  Though the crabs usually live in the forest, once every November or December, they migrate to the beaches to breed, then return home.  What did the residents do to deal with this annual migration?

A.  They built a crab bridge.  As Lewis says, “When crabs cross the road, bad things happen.”  The tiny crustaceans might be run over by cars and trucks zipping by, plus, per Wikipedia, the crabs’ tough exoskeletons can puncture tires, sometimes causing accidents.  But since the crabs predictably take the most direct route from the forest to the coast, the residents decided to erect 20 kilometers (12 miles) of plastic barriers, redirecting the crabs to 30 underpasses and to a specially designed bridge over the road.

The annual crab migration is a major event for the island and a point of local pride, Lewis adds.  It has become a big tourist draw, and “the constant stream of hundreds of crabs flowing over the bridge only enhances the experience.”

Q. “When it comes to the nuts and bolts of tool crafting, only chimps and orangutans match or exceed the New Caledonian crow’s sophistication,” says Jennifer Ackerman in her book “The Genius of Birds.”  Can you name its hunting tool that is unique among (non-human) animals?

A.  Hooks!  Not to catch fish but for prying larvae from holes and crevices in wood.  “A crow fishing for larvae looks a lot like the termite-fishing Jane Goodall observed in the chimpanzees of Gombe.”  Humans attempting such larvae fishing report that it requires “remarkable levels of sensorimotor control” and is “surprisingly difficult to master.”

The New Caledonian crows actually make two very different types of hooks.  One is formed from a branched twig, pruned until the branching point remains as the hook and then sharpened.  The other, carved from a leaf of the pandanus tree, “works as a tool only after the crow makes a final cut to separate it from the leaf.”  Of the three different types of leaf tool, the prevalent style varies from region to region.  There is evidence that the styles are faithfully transmitted within local groups.  “If it’s true, that fairly well defines the term culture.”

 Q.  An asteroid impact some 66 million years ago killed off the dinosaurs.  And, as “Science” magazine reports, it also wiped out 75% of living species, including any mammals much larger than a rat, plus half the plant species.  What happened next, and how do we know?

A.  An archaeological site at Corral Bluffs, Colorado, contained a treasure trove of thousands of plant and vertebrate animal fossils, offering a detailed chronology of how life recovered, says the magazine’s Elizabeth Pennisi. From a post-impact ferny world where seed-and-fruit-bearing plants were scarce, palm forests emerged, along with larger and more numerous mammals.

As Corral Bluffs researcher Ian Miller describes it, over a period of some 200,000 years, this “palm period” gave way to the “pecan pie period,” when walnutlike plants arose, with new mammals evolving to take advantage of the nutritious seeds.  Mammal diversity increased threefold, and the biggest of the new species reached beaver size.  After about 700,000 years came a “protein bar period,” with pea and bean species providing protein-rich meals that further boosted mammalian size and diversity, says co-researcher Tyler Lyson.

But a sobering message underlies this comeback, Pennisi concludes: “Even a recovery that geologists call ‘fast’ took hundreds of thousands of years, and the world was never the same.”

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