The Desert Shrew
(Notiosorex crawfordi crawfordi)
by Jay Sharp
The desert, or Crawford's gray, shrew belongs to the broadly distributed taxonomic family called "Soricidae," which includes some 250 species of shrews worldwide and more than 30 species in the United States and Canada.
Collectively, the desert shrew and its kindred species rank among the smallest and most biologically primitive of the placental mammals. (The placenta is the organ that joins mother and fetus during pregnancy.) The shrew burns energy at an extraordinary rate. Even when resting a typical shrew's heart rate may equal 1000 beats per minute, and its respiratory rate, 800 breaths per minute—parameters that approach the physiological limits for vertebrate systems. A shrew, which feeds primarily on insects, is a ravenous predator, sometimes consuming more than its own weight in food within 24 hours. Deprived of food, it may starve to death within a matter of hours.
Characteristics of the Desert Shrew
The desert shrew, specially adapted to an arid environment, is the smallest of the mammals of our Southwestern deserts. It is also one of the smallest homeotherms in the world. (A homeotherm is an animal, like a human, for instance, that maintains its body temperature within a narrow range, independent of the surrounding environment. By comparison, an endotherm is an animal, like a snake, that maintains its body temperature by absorbing heat from the surrounding environment.)
- Appearance: Although the desert shrew looks something like a house mouse, it is only about a fourth the size. It has a more narrow and elongated snout. The desert shrew feeds primarily on insects, while the mouse consumes primarily plant material. The shrew has 28 teeth with sharp pincerlike incisors and single cusp molars that are suitable for catching and chewing its prey. The mouse has 16 teeth with incisors designed for gnawing on plant and other material. The shrew has five clawed digits on its feet, the mouse, four.
- Adult Size and Weight: An adult desert shrew measures an inch or two in length, with its tail accounting for half its length. It weighs about 0.1 to 0.2 ounces, approximately as much as a copper penny.
- Coat and Color: The shrew, with a short dense coat, has a grayish brown back, sides and head, and it has a light gray belly, throat, legs and paws. Its tail is darker on top than on the bottom.
- Head: In addition to its pointed snout, the desert shrew has tiny eyes and conspicuous rounded ears. It has long tactile bristles on its muzzle, giving it a means for feeling its way through passageways in total darkness.
- Senses: Equipped only with poor vision, the animal relies, like a bat, on echolocation, emitting high pitches squeaks and interpreting the echoes to determine the direction and distance of objects such as potential prey. It also uses its highly developed senses of hearing and smell to locate and track down prey.
- Communication: The desert shrew often leads a solitary, nocturnal existence, and its processes of communication with potential mates or adversaries are little understood. If threatened the animal, like other shrew species, emits a foul odor from musk glands on its hindquarters, presumably warning would-be predators that it is not a desirable meal. It may also use the glands to mark territory.
- Desert Adaptations: As a species, the desert shrew has developed specialized behavioral and biological adaptations to life in an arid environment, oftentimes without a free source of water for prolonged periods. Their primary source of moisture is their food. They are idle during the heat of the day, saving energy and water. Biologically, the desert shrew has a lower metabolic rate than other shrews, reducing its water requirements, and it has highly efficient kidneys and a specially adapted respiratory system, which minimize its water losses.
Distribution and Habitat
The desert shrew ranges across much of the southwest quarter of the United States and the northernmost states of Mexico. Somewhat misnamed, it occupies a range of habitats well beyond desert basin scrubs and sand dunes, including various shrublands, prairie grasslands, streamside wetlands and mountain flank forestlands. In lower elevations, they may habitate in relatively humid locations such as in burrows or under rocks.
The desert shrew, say Davis and Schmidly in The Mammals of Texas, "is thought to feed largely on both larval and adult insects; captive specimens have eaten a wide variety of food including mealworms, cutworms, crickets, cockroaches, houseflies, grasshoppers, moths, beetles, earwigs, centipedes, the carcasses of skinned small mammals and birds, and dead lizards." Because of its lower metabolic rate, the desert shrew consumes relatively less food –only about 75 percent of its body weight per day – than many other shrews.
Behavior and Life Cycle
A rapacious, quick and aggressive nighttime predator, the desert shrew not only attacks its favorites, the arthropods (invertebrate animals – for example, insects, scorpions and millipedes – that have external skeletons, segmented bodies and jointed appendages), it may also attack prey larger than itself, sometimes including pets. It may even resort to cannibalism to satisfy its driving hunger.
The desert shrew may eat prey at once or cache it for later. If it attacks a grasshopper, for instance, it may bite off the legs and bite into the head, leaving the segmented and now immobilized body alive and fresh for later consumption. If it attacks a scorpion, the shrew will nip off the stinger before disabling and eating the creature.
The shrew's courtship and mating practices are largely unknown. The breeding season may range from spring to fall, or all year. The male and female build a nest of shredded plant materials in a secluded spot such as a packrat midden or a dead agave, and about three weeks after mating, the femaie gives birth to three to five naked and pink offspring, each about as big as a pinto bean. For the first two or three weeks, she nurses her charges, able to produce milk although she has not had access to water. She then changes them over to regurgitated prey, then to live prey. At about five to six weeks the juveniles will reach adult size, leave the nest and parental care and begin an independent life. The mother may produce another litter in the course of the year. Typically, the desert shrew will live for about a year or, possibly, two in the wild.
The desert shrew's major predator is the owl, a nocturnal hunter with a weak sense of smell (hence it is presumably not bothered by the shrew's foul musk gland odor). Although the shrew has lost habitat in some areas and is regarded as a threatened species in Mexico, it is listed as a species of "least concern" by the International Union for Conservation of Nature and Natural Resources: "There are no major threats to the widespread species."
The Zuni people, said Schmidt, regard the shrew as a "beast god," which provides "protection for stored grains from raids by rats and mice."
By comparison, the Egyptians believed the shrew to be the spirit of darkness, likely because of its quick and aggressive predation, voracious appetite and pugnacious nature.
Like shrews, the hummingbird – the Lilliputian of the bird world – also has an extremely high metabolic rate, with a resting heart rate of perhaps 500 beats per minute. It has been suggested that if human beings burned energy at the same rate, relatively, as a shrew or a hummingbird, our body temperatures would rise so high we could erupt in flame.
In 16th century England, the word "shrew" often referred to an evil-tempered person, typically a woman, which led Shakespeare to write "The Taming of the Shrew."
Robert H. Schmidt, Assistant Professor, Department of Fisheries and Wildlife Utah State University, Internet Center for Wildlife Damage Management.
Kerry R. Foresman, The Wild Mammals of Montana, SPECIAL PUBLICATION NO. 12, American Society of Mammalogists, Published 8 June 2001 .
San Diego Natural History Museum.
William B. Davis and David J. Schmidly, The Mammals of Texas - Online Edition, 1997 Texas Tech University.
Randall D. Babb, Arizona Wildlife Views, "A Penny's Worth of Terror,"March–April 2009.
U. S. Army Center for Health Promotion and Preventative Medicine.
David Allen, University of Michigan Museum of Zoology Animal Diversity Web, National Science Digital Library.
Tiffany Duffield, National Park Service.
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