Biology:Muridae
There are roughly 250 more species in two more subfamilies (Arvicolinae and Gerbillinae), with the remaining 14 subfamilies housing various other genera, some of which have only one species. Not all experts believe in the number of subfamilies or if they should all be classified as Muridae. Some assemblages, such as blind mole rats and bamboo rats, are quite different and have been considered as different families in the past.
Murids may be found in a wide range of habitats, from tropical jungles to tundras. Although most murid species are terrestrial, there are several that are fossorial, arboreal, and semiaquatic. Murids occupy a wide range of niches, which helps to explain their relative abundance. Not all specialists agree on the number of subfamilies or that all of these should be included within Muridae.
As a result of the large number of species, there is much variation in the physical characteristics of murids. Murids breed frequently, often producing large litters several times per year. They typically give birth between 20 and 40 days after mating, although this varies greatly between species. The young are typically born blind, hairless, and helpless, although exceptions occur, such as in spiny mice. The evolution of murids, like that of many other tiny mammals, is poorly understood due to the shortage of fossils.
Unlike other rodent-borne viruses such as arenaviruses, there is no vertical transmission from the dam to its offspring, and maternal antibodies can protect offspring from infection for several months (Kallio et al., 2006; Taruishi et al., 2008). This view has been recently disputed by Calisher et al. (2009), and several scenarios have been presented for the transmission of hantaviruses from infected to uninfected hosts (Calisher et al., 2009). As with many other small mammals, the evolution of the murids is not well known, as few fossils survive. They probably evolved from hamster-like animals in tropical Asia some time in the early Miocene, and have only subsequently produced species capable of surviving in cooler climates. They have become especially common worldwide during the Holocene, as a result of hitching a ride commensally with human migrations. Rats, mice, and relatives, sometimes called murids (MYOO-rids; members of the family Muridae), are divided into seventeen subfamilies, including voles and lemmings, hamsters, Old World rats and mice, South American rats and mice, and many others.
Mice that are free from murine infectious diseases and parasites are produced by many vendors. The acquisition costs for healthy and infected mice approach parity, while differences in experimental outcome and reliability of data between the two groups are likely to be significant. With use of appropriate housing procedures and equipment, disease-free mice may maintain their health status despite the presence of endemic rodent infectious diseases within the animal facility. The most commonly used type of mouse housing unit is the polycarbonate shoebox cage. The cage bottom is covered with 1–2 cm of appropriate contact bedding. A wire bar lid is placed on top of the shoebox to hold a water bottle and commercial rodent chow.
In general, phylogenetic trees constructed using L, M or S RNA genomic sequences are the same, suggesting a similar evolution for all three RNA segments (Nemirov et al., 1999). Hantavirus evolution resembles that of an extrachromosal genetic element of murines rather more than it does of that of an autonomous, horizontally transmitted agent (Hjelle and Yates, 2001). Dr Ho Wang Lee and his colleagues at the Seoul National University were the first to isolate the causative agent in 1978 from the lungs of a captured A. This common feral rodent is the major rodent host of the virus now referred to as Hantaan virus after the Hantaan River which flows through the Korean peninsula close to the demilitarised zone.
Others may be essential (“keystone”) species in maintaining the health of our forests, through their role in spreading mycorrhizal fungi or dispersing seeds. And a few species play an essential role as “domestic animals” used in medical research that has been enormously beneficial to mankind. Switching to a new rodent species can have a profound effect on the evolution of hantaviruses. Adaptation to the new host can stimulate modifications of virus phenotype and expansion into new ecological niches, eventually giving rise to “new” hantaviruses sufficiently distinct that are able to exploit new geographical areas. At least three instances of this happening have now been recorded.
Several inbred, specific pathogen-free strains currently exist in various laboratories throughout the world (Yamamoto et al., 1999). Although Mastomys is currently considered the most appropriate genus designation, these animals are Climate change and animal diets also referred to by the genus name Praomys in some literature (Yamamoto et al., 1999). The taxonomy is gradually being refined based on genetic analysis (Hisatomi et al., 1994; Fieldhouse et al., 1997; Volobouev et al., 2001).
Mice can be picked up by mid-tail and held either manually or within a restraining device. Manual restraint is achieved by grasping the loose skin over the mouse’s neck and shoulders. Intraperitoneal or subcutaneous injections are easily accomplished in manually restrained mice. A restraint device that permits access to the tail veins is recommended for intravenous injections. Intramuscular injection in the mouse is restricted to the small volume of material that can be placed within the caudal thigh muscles. Small amounts of urine and fecal material are usually expelled when a mouse is manually restrained.
This procedure does not require anesthesia and is most effective when the mouse has been briefly exposed to a heat lamp to promote vasodilation of the tail vessels. The site must be digitally compressed immediately after sampling is completed to prevent hemorrhage. Terminal blood collection requires that the mouse be under general anesthesia. For cardiac puncture, a 1–3-ml syringe and 21- or 23-gauge needle are used to draw blood directly from the heart or large vessels.
Mice are members of the family Muridae in the order Rodentia. Numerous outbred stocks and inbred strains of the laboratory mouse (Mus domesticus) may be purchased from research animal vendors. Coat colors and other characteristics vary considerably among mouse strains. Several other species and genera of mice are also available commercially.
Muridae is home to two-thirds of all rodent species and genera. Murids, or muroid rodents, are the collective name for members of this family. Members of the family can be found on all continents except Antarctica and on many oceanic islands.
Many African rats and mice are sometimes treated as members of the family Nesomyidae (and its various subfamilies) while others are still referred to the Old World Muridae along with all of the Asian species. In the Afrotropics, an herbivore-frugivore semi-aquatic rodent niche can be defined, including species in the genera Otomys, Pelomys, Dendromus, Dasymys as well as Delanymys brooksi, Thryonomys swinderianus and Mus bufo (Kingdon, 1974). Although most of them do not show marked anatomical adaptations to aquatic life, they occur only in close proximity to permanent water and swim readily. The dendromurine genus Dendromus (four semiaquatic species) and Delanymys brooksi (Delanymyinae) have prehensile tails adapting them to climb and nest in riparian forest.
This can be prevented by using a dental drill, bur, small hacksaw blade, or embryotomy wire under sedation or general anesthesia to bring the teeth into occlusion (Emily, 1991; Donnelly, 1997). A speculum, otoscope, or small tongue depressor may assist in visualizing the site (Harkness and Wagner, 1995). Rats with severe malocclusion will likely require re-trimming every 4 to 6 weeks (Emily, 1991).
Muridae (rat family) is the biggest extant rodent family, and indeed the biggest of all animal families, with over 1,383 species of “real” mice and rats. The use of nuclear and mitochondrial DNA, as well as DNA-DNA hybridization, has provided strong statistical support for these claims. Due to the absence of physical traits that support this group, the subfamily as it is now classified will likely expand.
The murids are tiny mammals that range in size from 4.5 to 8 cm in length (without the tail) in the African pygmy mouse to 48 cm (19 in) in the southern giant slender-tailed cloud rat. They have thin bodies, scaled tails that are longer than the body, and pointed snouts with prominent whiskers, however, these characteristics vary greatly. Some murids have lengthened legs and feet to allow them to hop around, while others have broad feet and prehensile tails to help them climb, and yet others have neither adaptation. This would be OK if each group’s common progenitor could be proven.
Studies on normative biology of Mastomys are rather limited, yet still under active investigation. Yamamoto et al. (1999) documented normal serum biochemistry values for two inbred strains, and Schares and Zahner (1994) characterized the immunoglobulin (Ig) IgG system. This species (along with other desert rodents) has been used to investigate mechanisms of body temperature regulation (Shukla et al., 1997). Mammary gland growth and hormonal response have been compared between Mastomys and C3H/He mice (Nagasawa et al., 1989).
The Ecuador fish-eating rat (Anotomys leander) is an endangered rodent (IUCN, 2006) confined to high altitude streams and wetlands. It has well-developed vibrissae, velvety fur, and the ears are sealed during immersion by a muscular membrane. The broad hind feet are not fully webbed but have stiff hairs which aid in swimming (Nowak, 1999). These and other Neotropical mice and rats listed in Table VI are sometimes treated as distinct from the Muridae, and most are placed in the Sigmodontinae within the Cricetidae which contains all endemic South American rodents.
A second example is Puumala virus from Clethrionomys “host-switching” species to Lemmus and then onto Microtus species, thence establishing lineages to the present day Topografov and Khabarovsk viruses (Vapalahti et al., 1999b). Upper molars are hypsodont and strongly lophodont or loxodont; individual teeth appear to be made up of transverse laminae; third molar larger than the first two. Unusually strongly developed zygomatic spine and notch, and zygomatic arch strong and bowed. A number of other unusual characteristics of cranium and mandible. Medium sized rats, primarily terrestrial, and mainly herbivorous.
The members of this family are often collectively called murids, or muroid rodents. Southeastern Europe west to Middle East and south to northeastern Africa along the Mediterranean coast. Medium to large rats with bodies highly modified for fossorial life. Feet small and claws less developed than might be expected; most digging is done with the head and incisors.
This species jump may cause the virus to evolve differently and result in more dangerous forms of the virus, which could be more pathogenic to humans (Zeier et al., 2005). Though the lifespans of most genera are generally less than two years, murids have high reproductive potential and their populations tend to increase rapidly and then drastically decline when food resources have been exhausted. The 300 genera of muroid rodents are classified within 18 subfamilies, but more than 200 of them (and nearly 1,000 species) belong to only two subfamilies—Sigmodontinae (New World rats and mice) and Murinae (Old World rats and mice). Two other subfamilies (Arvicolinae and Gerbillinae) include approximately 250 additional species, with the remaining 14 subfamilies accommodating various other genera, some of which consist of a single species. Rats, mice, and relatives are active during the day, at night, or both night and day (depending on the species).
They may be found in a variety of environments, including forests, grasslands, and mountain ranges. Several animals, particularly gerbils, have evolved to desert environments and can survive for long periods with only a little amount of water. With the help of powerful jaw muscles and gnawing incisors that grow during life, they devour a broad variety of meals, depending on the species. Murids are prolific breeders, having huge litters many times a year. They usually give birth twenty to forty days after mating, however, this varies widely by species. Hamsters are mouse-like Old World rodents with large cheek pouches used to carry food; stout body; short legs; wide, (sometimes) furry feet; and short, furry tails.
Schematic representation of our current understanding of alveolarization (left) from birth to early adulthood and senescence-related changes in tissue structure (right) for the species described in this chapter. The data available are currently not sufficient to draw true trajectories for alveolarization between birth and adulthood. As such, the lines drawn should only be considered as indicative of the pattern that may occur based on the data currently available.
The yellow-rumped leaf-eared mouse (Phyllotis xanthopygus) has the world’s biggest elevation range of any animal, at more than 20,000 feet. The Muridae are classified into five subfamilies, around 150 genera, and about 834 species. Not surprisingly, even the most basic characters are subject to continuing evolutionary change; most of the characters listed as diagnostic in the next paragraph do in fact show some variation within the group. All, however, are believed to have characterized primitive murids. Previously, these animals were used extensively in pesticide research (Gill and Redfern, 1979) because they constitute a significant agricultural pest in Africa.
More extensive collection of these samples can be obtained by housing the mouse in a metabolic cage. Blood sampling may be performed as a survival or non-survival procedure. The easiest method for obtaining a blood sample employs a scalpel blade. The ventral tail artery can be nicked in a restrained mouse to permit blood collection.
The Muridae, or murids, are the biggest rodent and mammal family in the world, with over 700 species. Muridae is derived from the Latin mus (genitive murids), which means “mouse.” Murids are found almost everywhere on the planet, but Antarctica and many maritime islands are devoid of murids. Some species, such as the house mouse and black rat have been transported to other parts of the world. The 300 genera of muroid rodents are divided into 18 subfamilies, although only two have more than 200 genera each (Old World rats and mice).
