Rodent

• 1. Introduction

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  Rodents make up about 40 percent of all mammal species, a single order whose name comes from the Latin for gnawing. Every one of them carries the same signature: a pair of incisors in the upper jaw and another in the lower, teeth that never stop growing. They live on every major landmass except Antarctica, from cold tundra under the snow to hot deserts, and human ships carried them to the rest. So how does one body plan, mostly small and stout with short limbs and a long tail, end up dominating almost every habitat on land? Why must an animal grind its own teeth down to avoid piercing its skull? And how did a group this humble cross oceans on floating debris, colonize an isolated Australia, and outlast the dinosaurs? The answers begin with a chisel made of enamel.

• 2. The Endless Tooth

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  Enamel covers only the front of a rodent's incisor, leaving the back bare. The softer dentine behind wears away as the teeth grind together, so the harder enamel edge keeps a self-sharpening shape like the blade of a chisel. Because the incisors never stop growing, the animal must constantly wear them down, or they will reach and pierce the skull.

  A gap called a diastema sits between those incisors and the cheek teeth, with no canines in between. Rodent species carry 12 to 28 teeth in total, usually fewer than 22. The gap lets them suck in or block out inedible material as the incisors chip it off. In chinchillas and guinea pigs, which eat a high-fiber diet, even the molars lack roots and grow continuously like the front teeth.

  The masseter muscle does the heavy work, making up 60 to 80 percent of the masticatory muscle mass. It attaches behind the eyes and produces a strange effect during gnawing called eye-boggling, where rapid contraction makes the eyeballs move up and down. Variations in this zygomasseteric system separate the major chewing types. The eastern grey squirrel, a sciuromorph, has a large deep masseter and bites efficiently with its incisors. The guinea pig, a hystricomorph, has enlarged internal pterygoid muscles that swing the jaw further sideways. The brown rat, a myomorph, has enlarged temporalis and masseter muscles and handles both gnawing and chewing well.

• 3. Built To Move

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  The capybara can reach 66 kilograms, the largest of all rodents, yet most species weigh less than 100 grams. The forefeet usually carry five digits including an opposable thumb, and the elbow gives the forearm great flexibility. Most species are plantigrade, walking on the whole foot, and a nail on the first digit lets them handle hard seeds and nuts, a niche they dominate.

  Locomotion ranges from quadrupedal walking and burrowing to bipedal hopping in kangaroo rats and hopping mice, and even gliding. Flying squirrels stretch parachute-like membranes from fore to hind limbs and glide between trees. The agouti runs fleet-footed and antelope-like, digitigrade with hoof-like nails. Tails come in many forms, prehensile in the Eurasian harvest mouse, bushy or nearly bald, and in some species capable of regenerating if part breaks off.

  Senses sharpen the picture. Smell and hearing are well developed, eyes enlarge in night-active species, and long whiskers called vibrissae handle touch. Many species pack food into cheek pouches; in squirrels and the Muroidea these are extensions of the mouth, while in gophers and the Heteromyidae they are separate, and both kinds reach the shoulders. The digestive system absorbs nearly 80 percent of the energy in their food, helped by coprophagy, the eating of their own fecal pellets so nutrients can be absorbed on a second pass. Many of them may entirely lack the ability to vomit.

• 4. Engineers Of The Land

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  Burrowing rodents eat the fruiting bodies of fungi and spread the spores through their feces, helping fungi form symbiotic relationships with plant roots that often cannot thrive without them. In this quiet way they help keep forests healthy. Some are counted as keystone species and ecosystem engineers.

  Prairie dogs in the Great Plains of North America turn soil through their burrowing, aerating it so it holds more organic material and water. Beavers play a hydrological role across many temperate regions. When they build dams and lodges, they reroute streams and rivers and create wetland habitats where there were none.

  The order's reach owes much to such adaptability. Rodents are the only terrestrial placental mammals to have colonized Australia and New Guinea without human help, and people later carried them to remote oceanic islands. They have settled cold tundra and hot desert alike, climbing trees, digging underground, and swimming, and have thrived in farmland and cities built by humans.

• 5. Diets And Predators

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  The field vole is a typical herbivore, feeding on grasses, herbs, root tubers, moss, and gnawing bark in winter, with the occasional insect larva. The plains pocket gopher stuffs grasses, roots, and tubers into its cheek pouches and caches them in underground larder chambers. The Texas pocket gopher avoids the surface entirely, pulling plants down into its burrow by the roots. The African pouched rat fills its capacious cheek pouches on the surface, then returns underground to sort the haul and eat only the nutritious items.

  Agouti species are among the few animals that can crack open the large capsules of the Brazil nut fruit. With too many seeds to eat at once, the agouti carries some away and caches them, and any it fails to retrieve germinate far from the parent tree. Marmots eat heavily before winter and may be 50 percent heavier in autumn than in spring. Beavers store winter food in rafts, piles of wood sunk in water.

  The herbivore label hides a wider truth. Most small rodents opportunistically take insects, worms, snails, mussels, and even vertebrates, and a few specialize in animal matter, like the shrewlike rats, the rakali, and the grasshopper mouse. A study of the rodent tooth system supports the idea that primitive rodents were omnivores rather than herbivores. The stomach contents of the North American white-footed mouse showed 34 percent animal matter, and the grasshopper mouse, which hunts insects, scorpions, and other small mice, can kill prey as large as itself.

• 6. Colonies And Kings

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  Brown rats live in small colonies of up to six females sharing a burrow, with one male defending the territory around it. At high density that system breaks down into a dominance hierarchy with overlapping ranges, female offspring staying while males disperse. Prairie dog towns can stretch for kilometers and number in the thousands, divided into family groups called coteries, each often an adult male, three or four females, and their young, friendly within and hostile to outsiders.

  The extreme case is eusociality, found in the naked mole rat and the Damaraland mole rat, which live like colonial insects. Their underground colonies, numbering in the hundreds for the naked mole rat, contain a single breeding female and a few males, while the rest stay non-reproductive with their fertility suppressed. The non-breeders dig, maintain, and seal tunnels, gather food, and tend the young.

  Communication binds these societies. Rodents read an individual's species, sex, reproductive status, health, and social rank from urine, and recognize close kin by smell to favor relatives and avoid inbreeding. Major urinary proteins act as pheromone transporters, and each male house mouse carries uniquely encoded ones. Sound matters too. Prairie dogs have different alarm calls for aerial and ground predators, each carrying details about the threat. At least fifteen call types have been recorded in adult Kataba mole rats, and the common degu also has fifteen vocalizations. Laboratory rats, which are brown rats, emit ultrasonic chirps during play, mating, and tickling, a sound likened to laughter, though the chirping declines as they age.

• 7. Deep Time

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  The earliest fossils with rodent dentition come from the Paleocene of Laurasia, shortly after the extinction of the non-avian dinosaurs some 66 million years ago. Glires, the clade of rodents and lagomorphs, split from other placental mammals within a few million years of the Cretaceous-Paleogene boundary. Some molecular clock data suggest modern rodents had appeared by the late Cretaceous, and rodents may have outcompeted and replaced multituberculates, though that is debated.

  The Castoridae, which includes modern beavers, first appeared in North America in the late Eocene and crossed into Eurasia by the Bering Land Bridge in the early Oligocene. Late in the Eocene, hystricognaths invaded Africa, most likely originating in Asia at least 39.5 million years ago. From Africa, caviomorphs reached an isolated South America by 41 million years ago, evidently riding ocean currents across the Atlantic on floating debris, and arrived in the Greater Antilles by the early Oligocene. Nesomyid rodents rafted from Africa to Madagascar 20 to 24 million years ago.

  Some of these animals grew enormous. The largest known rodent, Josephoartigasia monesi, was a pacarana that may have reached 3 meters in length and 1000 kilograms in weight, living 4 to 2 million years ago. In Australia, rodents all belonging to Muridae appear in the fossil record as early as 4.5 million years ago, the Hydromyini old endemics arriving first, the true rats of the genus Rattus following as new endemics. Rodents also joined the Great American Interchange around 5 million years ago, when the Isthmus of Panama formed and New World porcupines headed north while murids migrated south.

• 8. Living With Humans

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  Humans have eaten at least 89 species of rodent, and in 1985 rats were consumed by at least 42 different societies. Guinea pigs were first raised for food around 2500 B.C., and by 1500 B.C. they were the main source of meat for the Inca Empire. In Peru today, around 20 million domestic guinea pigs yield roughly 64 million pieces of meat a year. Dormice were farmed in ancient Rome in special pots called gliraria, fattened on walnuts, chestnuts, and acorns. Beaver pelts drove the North American fur trade, prized by Europeans for felting into beaver hats.

  In the laboratory, albino mutant rats were first used for research in 1828 and became the first animal domesticated purely for science. The house mouse is now the most common laboratory rodent; in 1979 an estimated fifty million were used worldwide each year. Guinea pigs helped identify Mycobacterium tuberculosis as the cause of tuberculosis in 1882 and were the site of the discovery of vitamin C in 1907, which they cannot make on their own. The USSR launched guinea pigs into orbit on the Sputnik 9 biosatellite of the 9th of March 1961, with a successful recovery. The Gambian pouched rat can detect tuberculosis bacilli with a sensitivity up to 86.6 percent and has been trained to find land mines.

  The same intelligence makes rodents formidable pests. In 2003, crops lost to rats in Asia were estimated to be enough to feed 200 million people, and in South America losses have reached ninety percent. The black rat spreads Yersinia pestis, the bacterium behind bubonic plague, through its fleas. Carried on sailing ships in the Age of Exploration, the black rat caused the extinction of over 40 percent of the terrestrial bird species on Lord Howe Island just ten years after arriving in 1918. On the island of Lundy in the United Kingdom, the killing of an estimated 40,000 brown rats is giving Manx shearwater and Atlantic puffin a chance to recover, a reminder that the Bramble Cay melomys of the Great Barrier Reef became the first mammal declared extinct from human-caused climate change.