Earth Almanac: July/August 2008

Ancient Beavers

As summer wanes, mountain beavers start weaning their young. If you live in the Northwest, you may glimpse them at dusk as they venture from their elaborate burrows for the night’s foraging. But you don’t have to restrict your search to mountains, and don’t look for anything remotely resembling those paddle-tailed dam builders that abound in river systems across the continent. The mountain beaver, with no living family relative, is about the size, shape, and color of a muskrat. And like so many other burrowing rodents, it is richly bewhiskered and has small ears, eyes, and tail. The most primitive of all rodents, it traces its ancestry to the early Tertiary Period, some 40 million years ago, when mammals had just started to assert themselves in global ecosystems. That early form may have even given rise to the entire line of chipmunks and squirrels. Mountain beavers construct elaborate burrows with bathrooms, bedrooms, pantries (stocked with vegetation that they have allowed to dry on the surface), and as many as 30 exit/entrance holes. Droppings are stored and re-ingested once, making for more efficient metabolism of woody forage.

Doing the Wave

In high summer, robin-size shorebirds called sanderlings sweep down from their high Arctic breeding grounds, fanning out over the world’s beaches. Few, if any, shorebirds have greater ranges or migrate farther. The round-trip route for sanderlings trading between the Arctic and southern Argentina, for instance, is 20,000 miles. From a distance it can be easy to mistake sanderlings for dunlins or red knots. But in flight they show more white on dark wings than any other sandpiper, and their foraging behavior is distinctive. Watch them as they chase the retracting tongues of waves, frantically probe for invertebrates with their black, broad-based bills, then dash back to high ground just ahead of the next breaker. On many beaches mole crabs account for the major part of their diet. Horseshoe crab eggs are also important, particularly when migrants require a fast energy boost. The collecting of horseshoe crabs for eel-pot bait has been hurtful to sanderlings and other shorebirds, but it has recently come under tighter control, and last March New Jersey imposed a  moratorium.