Rodent Retreat is found in most terrestrial habitats. Their ability to adapt to different environments has led to a number of convergent traits.
They share several anatomic features, such as having a single upper and lower pair of ever-growing incisors. They are also known for having a well-developed pterygoid fossa, elongated glen o id fossa (allowing the lower jaw to move forward and backward), a constricted area between the orbits, cingulate gyrus reduced or absent, primitively 5-toed feet, and a reduced temporalis muscle.
Rodents first appear in the fossil record toward the end of the Paleocene epoch. They share some of their archaic features with a group of small fossil mammals called the multituberculates, and are closely related to lagomorphs. The most basal rodents are the eurymylids (springhares and scaly-tailed flying squirrels). Early taxonomists, prior to recognition of convergent evolution in masseteric architectures, divided the order Rodentia into the monophyletic suborders Ctenohystrica, Sciuromorpha (dormice, sewellel or mountain beavers and squirrels), Anomaluromorpha (eurymylids and leptictids), Castorimorpha (beavers and pocket gophers), Hystricomorpha (porcupines, agoutis, guinea pig-like cavies, chinese crested porcupines and plains vizcachas), and Myomorpha (mouse-related).
The unifying feature of all rodents is a pair of continuously growing incisor teeth in each jaw adapted for gnawing. The incisors are separated from the cheek teeth by a sizable gap called a diastema. The chewing action of the incisors grinds edible matter into crumbs and throws away non-edible material. In most species of rodents, the incisor teeth are brachydont and the molars are elondont. A few of the most common rodents, such as rats and mice, are serious pests and vectors of disease, including plague, murine typhus, hantavirus, scrub typhus, tularemia, and rat-bite fever. The remarkably high rates of speciation and extinction observed in the rodents are attributable to a combination of rather unspecialised anatomy and physiology, ability to cope with a wide range of foods, extraordinary fecundity, and an unexplained power of niche filling.
Habitat
Rodents are abundant in many parts of the world, filling a diverse niche in nature. Some, like kangaroo rats and beavers, build dams that purify water systems or moderate floods and droughts. Others, such as anteaters, dusky toads and tarantulas, forage for insects, destroying crop pests or even preying on them themselves. The rest of the species, including mice, shrew-rats, true rats and hamsters, are more elusive, inhabiting quiet corners in rainforests, mountains, deserts and rivers. They are an essential part of the ecosystem, spreading seeds and providing food for predators such as birds of prey and snakes.
Rodents also play an important role in human societies, serving as pets, agricultural pests and the subjects of scientific experiments and laboratory studies. However, some species, such as invasive house mice, are considered pests that spread pathogens and spoil food stored by humans.
A new rodent habitat designed at NASA’s Ames Research Center will help scientists better understand what changes occur in the rodents during spaceflight. This information is vital for future human spaceflight, as it will help scientists develop countermeasures to reduce the risks of disease in astronauts. The experiment utilizes a rodent habitat designed to comply with ISS Express Rack standards and an Animal Access Unit (AAU) to facilitate delivery of animals to the habitat and retrieval after the experiment. A video camera is used to capture the rodents’ behavior in the arena, and a sensor measures oxygen, temperature, pressure and relative humidity inside the habitat.
Diet
Rodents are small mammals that are omnivores with a wide range of food sources. Their sharp incisors allow them to gnaw seeds, excavate burrows and defend themselves from predators. They are largely herbivorous, but some species also eat insects and other animals. Rodents are social animals and may be monogamous, polygynous or promiscuous. They reproduce quickly, and their young are born precocial, which allows them to scavenge food soon after birth. Some species live in colonies with complex social structures, while others are solitary. They are often carriers of human diseases, including bubonic plague and typhus.
The dietary composition of rodent diets influences gut microbiome structure and function, but the exact ingredients in commercial diets are largely unknown. This lack of information has been a source of significant concern, with calls for details of diet composition to be included as part of animal research guidelines.
Currently, seven commercial rodent diets are used in research laboratories. They fall into two categories: grain-based diets and purified ingredient diets. The following table summarizes the main ingredients of these diets, excluding supplements and additives. In addition, it shows how each diet varies from its GB counterpart in terms of nutritional content and other variables (e.g., protein, fat and contaminant levels). The tables include mill dates and sample sizes for each diet. The data reveal that GB diets differ significantly in composition, with differences in grain and cereal components and, to some extent, in supplement content.
Breeding
Millions of mice and rats are bred for use in biomedical research each year. Many large commercial breeding facilities exist, but many laboratories also choose to breed in-house to control genetics and to time pregnancies for particular projects. In-house breeding is also advantageous because it allows researchers to work with embryos and neonates.
Rodents are polyestrous, meaning they come into heat frequently throughout the year. The duration of the heat cycle varies with the weather and individual females. After parturition, the female enters a postpartum estrus. During this period she may conceive again. Most commonly, the rat breeding system is monogamous; one male to one female.
A rat is ready to mate when her vulva becomes swollen and red, usually at the start of her reproductive cycle (known as proestrus). The male and female are placed together in a tub and observed for mating. If a copulatory plug is present in the morning, mating has occurred.
When a female begins a litter, she will often adopt other babies of the same size as her own to ensure that all the pups survive. She is also very protective of her offspring. If she is stressed during delivery or if her litter becomes too large, she may kill and eat healthy pups, even if they are not her own. It is important to keep the number of litters under control and separate breeding animals promptly if overcrowding occurs in order to prevent this stress.
Behavior
Rodents are a very diverse group of animals, found in most terrestrial habitats. They have short limbs, long tails, and a single upper and lower pair of continuously growing incisors that they use to gnaw food, excavate burrows and defend themselves. Species vary in lifestyles, from arboreal to fossorial, and mating systems can range from monogamy to promiscuity.
In the laboratory, rodents can be used for a wide range of behavioral tests, from studies of basic locomotor function to analyses of cognition and emotionality. Tests can include the Morris water maze to measure spatial navigation skills, the delayed nonmatching to place (DNMP) task to assess pattern separation and the Y-maze spontaneous alternation task to measure short-term spatial memory.
Rodents are a valuable part of the natural environment and are a keystone species in many ecosystems. However, rodents are also a significant pest in human-made environments, and many species are in decline or facing extinction. Despite the importance of these animals, rodents receive little scientific attention and are often misunderstood by the general public. Despite this, a large number of simple preventive measures can be taken to minimize rodent infestation. These include reducing the availability of food and water by sealing garbage cans, promptly removing pet foods and cleaning up crumbs, and regularly repairing and emptying outdoor trash containers. In addition, reducing overgrowth and weeds around buildings can reduce rodent harborage and eliminate the potential of providing a pathway for entry.
Evolution
The extraordinary morphological diversity of rodents and the richness of their fossil record make them an ideal model for understanding the link between convergent evolution and taxonomic diversification. Rodents have adapted to many different environments, sometimes even crossing oceans. Water rats can drink saltwater, field voles digest poisonous bark, and kangaroo rats and jerboas leap on their toes. The body forms of these specialized species also resemble those of nonrodent orders, such as shrews, moles, hares, pikas and guinea pigs.
In addition, the evolutionary histories of some families are very close, leading to a phenomenon called homology. This can occur when distantly related groups evolve similar traits that improve their fitness, such as reduced digits in kangaroo rats or jerboas (see figure 4).
One of the most important developments for rodents was the evolution of enlarged ever-growing incisors (called protrogomorphy) that allowed them to consume more food and thereby expand their ranges. The emergence of protrogomorphy was probably a key factor in the expansion of Glires, the clade that contains both true rodents and lagomorphs (the basal mammals).
Dr Lucja Fostowicz-Frelik, of the Institute of Paleobiology PAS, has used fossils from the Gobi Desert to study the way the ankle bones of early members of this incredibly successful clade evolved. She found that different lifestyles required different heel structures – fast running requires a different shape to slow clambering or jumping.