Rodentia is an order of mammals also known as rodents, characterized by two continuously growing incisors (front teeth), two on the upper and lower jaws respectively, which must be kept short by gnawing. This is the origin of the name, from the Latin word rodere, which means to gnaw. These teeth are used for cutting wood, biting through the skin of fruit, or for defense. The teeth have enamel on the outside and exposed dentine on the inside, so they self-sharpen during gnawing. Rodents lack canines, and have a space between their incisors and premolars.
Forty percent of mammal species world-wide are rodents (around 2,277 species). They are found in vast numbers present nearly on all continents and islands, and in all habitats except oceans and Antarctica. Their success is probably due to their small size, short breeding cycle, and ability to gnaw and feed on a wide variety of foods.
Rodents are important in many ecosystems because they reproduce rapidly, and can function as food sources for predators, mechanisms for seed dispersal, and as disease vectors. Humans use rodents as a source of fur, as pets, as model organisms in animal testing, for food, and even for detecting landmines.
In the Maltese Islands four species of rodents are known to occur. These are divided in 2 species of rats and 2 species of mice. Rats are typically distinguished from mice by their size; rats are generally large rodents, while mice are smaller. The best-known rat species (and these are what we have in our islands) are the Black Rat (Rattus rattus) and the Brown Rat (Rattus norvegicus). The group is generally known as the Old World rats or true rats, and originated in Asia. Rats are bigger than most Old World mice, which are their relatives, but seldom weigh over 500 grams in the wild. Male rats are generally called bucks, unmated females are called does, pregnant or parent females are called dams, and infants are called kittens or pups. A group of rats is either referred to as a pack or a mischief.
These common species are opportunistic survivors and often live with and near humans, therefore they are known as commensals. They may cause substantial food losses, especially in developing countries. Wild rats and mice can carry many different “zoonotic” pathogens, such as Leptospira, Toxoplasma gondii and Campylobacter, and may transfer them to other species, for example to humans. The Black Death is traditionally believed to have been caused by the micro-organism Yersinia pestis, carried by the Tropical Rat Flea (Xenopsylla cheopis) which parasitized on the Black Rat living in European cities during the epidemic outbreaks of the Middle Ages; these rats were used as transport hosts. Today, this cycle still exists in many countries of the world and plague outbreaks still occur every year. Besides transmitting zoonotic pathogens, rats are also linked to the spread of contagious animal pathogens that may result in livestock diseases such as Classical Swine Fever and Foot-and-mouth disease. The normal lifespan of rats ranges from two to five years, and is typically three years.
Regarding mice, we have two species: the Wood Mouse (Apodemus sylvaticus) and the House Mouse (Mus musculus). The latter thrives under a variety of conditions: they are found in and around homes and commercial outlets as well as in open fields and agricultural lands. House mice consume and contaminate food meant for humans, pets, livestock, or other animals. In addition, they often cause considerable damage to structures and property. They can transmit pathogens that cause diseases such as salmonellosis, a form of food poisoning.
Mice are afraid of rats, which often kill and (partially) eat them. This rat behaviour is known as muricide. Despite this behaviour, free-living populations of rats and mice do exist together locally. House mice are generally poor competitors and cannot survive away from human settlements in areas where other small mammals, such as wood mice, are present.