MEET MONSTER MOUSE

focus on nature These Tiny Critters Weigh Only an Ounce, but They're Monsters Just the Same

The live-traps have been out for seven nights, but I haven't caught any of the killers yet. Because of their nocturnal nature and the fact that predator populations encompass fewer members than those of prey species capturing one will not be easy. But I know they're out there, stalking the desert grasslands, dispatching their victims with ferocity, defending their territory from intruders, and occasionally piercing the night air with their unique cry.

These tiny but fearsome creatures are known by the misleadingly benign name of grasshopper mice, Onychomys spp. They range throughout much of the western United States with two species, the northern and the southern, found in Arizona.

Stocky, grayor cinnamoncolored rodents with stubby white-tipped tails, white feet, and white bellies, they average four inches in length and about an ounce in weight. Their beady black eyes and house-mouse visages belie their true character. Unlike most of their cousins, who are content surviving on vegetable diets, grasshopper mice assume the role of aggressive, even cannibalistic, hunters after more worthy game. They are often compared to weasles and lynxes.

Imagine a harvest mouse, a potential meal, foraging through the gramma grass and scrub under a starlit sky. Unknowingly crossing an invisible line, it enters the seven-acre territory of a grasshopper mouse. The hunter tracks the trespasser by scent, then sight. The grasshopper mouse rushes alongside its prey from behind, throws strong forepaws over the neck of its victim, and bites into the base of the skull.

The harvest mouse expires quickly, and the killer chews an access hole into the side of its quarry's head. During the night, the grasshopper mouse will methodically turn back the skin from this opening, eating everything within until there is nothing left but the discard-ed hide and tail. When it is fin-ished, the victor may sit up on its hind legsor simply throw back its head point its muz-zle at the sky like a howling wolf, and split the darkness with its cry.

The unusual name "grass-hopper mouse" came from the 19th-century naturalists who discovered the rodent's appetite for such insects. But they did-n't realize the mouse was an opportunistic hunter, taking whatever prey was available. It considers reptiles, amphibians, small mammals, and all insects fair game. At the Arizona-Sonora Desert Museum, I watched one grass-hopper mouse eat crickets and mealworms. The mice also have survived in captivity on a diet of horsemeat. Others have en-joyed toads and earthworms. In the wild, it is no less than amazing how these hungry lit-tle mice kill potentially harmful prey. They will jump upon a tarantula, for example, so it can't use its heavy black fangs in defense. Scorpions, for which these rodents have a real liking, are first incapacitated by quick bites to the tail before they are devoured alive. The stinger may or may not be eaten. Horned lizards with their array of spiny defensive scales are vanquished by continuous biting around the eyes. The mice learn to assassinate darling bee-tles by jamming the insects' hind ends into the sand, preventing them from squirting an irritat-ing secretion. Whipscorpions, which spray a concentrated acid discharge, also are easily overcome. Nature designed grasshopper mice with not only a pugnacious temperment but also a uniquely adapted digestive system for their specialized diet, which includes habitually ingesting sand to aid in the breakdown of bony or chitinous material from their prey. Like many predators, they may go days without eating, gorging themselves when the next opportunity arises. When animal sources in their range are scarce, though, they will eat seeds and other plant parts to survive.

As their individual territories are so large, they are never abundant in any one locale. A male and a female, called a consort pair, will bond in a monogamous manner similar to wolves' behavior. Highly adaptable and somewhat nomadic, they will either take over another rodent's abandoned den or dig their own.

The voracious mice dig four types of burrows in their domain: for caching food, nesting, providing a refuge from larger predators, and marking their boundaries with scat and pungent scent from glands near the base of the tail. They also maintain "dust baths" to keep their fur from becoming oily and matted.

Litters averaging four in number are born between March and September. When the young are old enough to forage with the adults, the group ranges much as a wolf pack does.

An enduring reference to the rodents' "howls" is a bit erroneous. Although grasshopper mice do assume a wolflike position when making these vocalizations, the sounds are less like howls or wails and more like a shrill whistle, some of it too high to register in human ears. No one has confirmed the purpose or meaning of the cries, but as with other territorial hunters, the howls may be a warning to other grasshopper mice not to enter their killing zone.

Maybe the next time I check my live-traps, I'll hear their strange call of the wild.