The mouse that roars - grasshopper mouse

National Wildlife, Oct-Nov, 1995 by Chris Madson

The grasshopper mouse, with its strange calls and appetite for meat, is no ordinary rodent

A deck of clouds slides over the moon, and the desert floor subsides into perfect blackness. Somewhere in the far distance a lone coyote serenades the night. Close by, a different hunter speaks out in a cry unique to western prairies and desert basins. One biologist described it as "a fine shrill whistle or prolonged squeak, insectlike in attenuated quality but as smooth and prolonged as the hunting call of a wolf."

The sound carries nearly 200 yards in the midnight silence, signaling that a grasshopper mouse is on the prowl. This tiny rodent scarcely projects the look of a voracious hunter. It weighs about half an ounce, a little on the small side even by the standards of its close relatives, the white-footed mouse and prairie deer mouse. It has the same big-eyed appeal as the rest of its tribe, but, somewhere along the evolutionary line, it acquired a taste for meat. Research on the grasshopper mouse shows that the predatory rodent has many of the behavioral patterns found in large predators, suggesting that the dynamics of hunting are invariable regardless of a predator's size.

Other small rodents eat insects and carrion to fill out vegetarian diets, but the three species of grasshopper mouse, all natives of the American West, subsist almost entirely on meat. The staple of the grasshopper mouse's diet is, appropriately enough, grasshoppers, to which the mouse may add beetles, caterpillars, spiders, ants, mites and other small invertebrates, even scorpions, to whose venom the mouse seems immune.

The mouse may occasionally stalk bigger game. Mammalogist Lester Flake found vertebrate tissue, primarily from other mice, in the stomachs of northern grasshopper mice from southeastern Colorado, as well as feathers and lizard skin. Some of these meals may have come from carrion, since small rodents sometimes scavenge dead carcasses. However, captive grasshopper mice have revealed a pronounced predatory potential, killing and eating deer mice, pocket mice and mountain voles, the last weighing perhaps half again as much as its attacker. A captive male mouse in Oklahoma slaughtered a male cotton rat three times his size.

In the wild, grasshopper mice probably never attack prey as large as a cotton rat and seldom take on even small mammals. Denise Frank, a biologist who has studied grasshopper mice since 1981, has never seen her subjects kill a mammal and objects to the lurid focus on the species as a killer. "It's such a small part of their ecology," she says, "and there are so many other interesting things about them."

Frank began studying grasshopper mice as a graduate student at Cornell University because she was interested in their breeding habits. For decades, biologists thought grasshopper mice were monogamous. Technicians trapping small rodents often captured a single male and a single female near each other with no sign of neighboring grasshopper mice. Researchers had also reported that captive males sometimes watch nestlings while females feed, occasionally even washing the young or pulling nesting material over them. Such evidence suggested the mice mated for life, and Frank wanted to see how monogamy worked for them.

She found that it did not. Her radio telemetry studies of the southern grasshopper mouse show that during the mating season, territories of breeding males are much larger than those of breeding females. When the season ends, male territory size drops to about that typical of females. The increase in male territory during mating season reflects the male's need to corral as many females as possible for his exclusive use. According to Frank, when a male finds a receptive female, he mates with her, then settles down to defend her from other males until she goes out of heat, at which time he moves on to other liaisons. When the breeding season ends in early fall, the male no longer needs a large territory that encompasses those of several females and so restricts winter movements to the much smaller space he needs to find food.

By the standards of other desert mice, the grasshopper mouse's territory can be huge - as much as 9 acres for breeding males and more than 4 acres for adult females. A typical territory for a deer mouse is less than an acre. The size of the territory reflects the grasshopper mouse's predatory habits. To ensure that they have an adequate prey base for meeting nutritional needs, predators always require larger foraging areas than do the herbivores upon which they prey.

Keeping a territory that may be more than 200 yards across is a handful for an animal that measures barely 6 inches from nose to tail. A grasshopper mouse asserts its claim in several ways. Scent posts are important territorial markers among these mice, as they are among many large predators. Captive grasshopper mice mark territories with a series of dust baths, apparently impregnating the ground with scent from anal oil glands. Frank says the mice have "an enormous number of scent glands," many of which have not been carefully studied.