Salt Marsh Harvest Mouse

It may never be depicted on the cover of a major environmental groups complimentary calendar, but the Salt Marsh Harvest Mouse is nevertheless a very cute little rodent. And it’s all ours: the Salt Marsh Harvest Mouse exists only in the San Francisco Bay, living among the dense “pickleweed” stands in tidal and diked coastal salt marshes.

The Salt Marsh Harvest Mouse is no more than three-inches long—not counting their tails, which are at least as long as their bodies. They weigh less than half an ounce—about the same as a nickel. Like the much-more common Western Harvest Mouse, they have brown backs and grayish undersides, except that some individuals instead have a reddish-colored belly (hence their alternate moniker, the “red-bellied harvest mouse”). They are truly unique among the ranks of harvest mice, however, for two important reasons: They can swim really well, and they can drink salty water.

Those two characteristics make for a very interesting lifestyle. The Salt Marsh Harvest Mouse is largely nocturnal. It holds pieces of pickleweed in its two front paws and nibbles, just like we would with an ear of corn. When the tide comes in, the mice float, swim, or scurry up the stalks of taller vegetation growing on the periphery of the marsh. When breeding, they will sometimes use old bird nests they find on the ground.

Sadly, the odds of survival run heavily against the Salt Marsh Harvest Mouse individually, and as a species. Its life span is brief—less than a year, and the females only bear one litter in a lifetime. It is liable to become food for hawks, owls, herons, and other predators any time it is forced out into the open. But most of all, the Salt Marsh Harvest Mouse is at the mercy of development. Since 1850, more than 80 percent of the tidal marshes bordering San Francisco Bay—or roughly 164,000 acres—have been diked, drained, filled, or back hoed—often with the assistance of the U.S. Army Corps of Engineers. The Salt Marsh Harvest Mouse has been protected by the Endangered Species Act since the Act’s inception in 1973.

Although the rate of destruction of the Salt Marsh Harvest Mouse’s habitat has been slowed by Endangered Species Act protection, much of what’s left is too small, fragmented, degraded, or otherwise altered to do the struggling Salt Marsh Harvest Mouse much good. Isolated pockets of wildlife, unable to migrate when conditions change, are easily wiped out. And every population has a critical threshold below which it cannot survive.

In the GGNP the Salt Marsh Harvest Mouse is very difficult to find, and may be extirpated. Your best bet is to look in the Marin lands for appropriate habitat, and be particularly observant on a full moon, watching for the mouse to move.

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