Rodent mammals popularly known as the rats and mice; possibly the most numerous in species and abundance, worldwide in distribution, and are responsible for causing colossal damage to the life and property of human beings. They are found almost everywhere on land, excepting the two poles and other permanent ice caps. They come under the order Rodentia with nearly 2,000 species and 30 families. The order includes Agouti, Beaver, Capybara, Chinchilla, Chipmunk, Dormice, Flying Squirrel, Hamster, Gerbil, Gopher, Guineapig, Jard, Jerboa, Lemming, Mara, Mole-Rat, Paca, Porcupine, Prairie Dog, Spring Hare, Squirrel, Woodchuck, and Vole.
Rodents have three distinct groups based on their arrangements of chewing muscles, orbits and teeth. These are the Squirrel Forms or Sciuromorphs, Porcupine Forms or Hystricomorphs and Rat Forms or Myomorphs. All the groups are well-represented in Bangladesh.
All rodents, small or big, have four highly developed, long and prominent chisel-shaped incisor teeth, two in the upper jaw and two in the lower jaw. To keep the incisors in perfect shape and form all rodents must keep nibbling at something throughout their whole life. Otherwise the teeth will grow out of proportion and curl or become twisted making the rodents unfit for survival. Also they have an area on the jaw called diastema that has been formed due to the absence of canine teeth. With these special arms they are capable of gnawing any plant matter that they reach and use them as food. Their usual food is the seeds but will consume stems, roots, leaves and flowers. In addition they eat a variety of insects and other invertebrates. A few are even known to kill and eat small vertebrates such as reptiles, birds, their eggs and baby mammals.
The insectivorous species have a slender muzzle and their molars have sharp cusp. The herbivorous species are armed with capacious incisors, milling molar teeth and a stocky skull. The omnivorous species stands between these two extremes. Hard enamel cover of the front surface of an incisor tooth helps it getting sharpened continuously. Some have cheek pouches and they have a tendency to store food for future use. Because of their burrowing, bounding, climbing and gliding modes of life, several groups of rodents have specialized body forms. By and large all have well-developed tail, ears and eyes and have keen sense of hearing, smell and vision.
Many of them are nocturnal and hence dependent on sound and scent. Scent is also used in regulation of social behaviour. Rodents live in underground, on land, in water, over the trees and in the dwellings of humans and their domestic companions. Several species, the house mouse and house rat in particular, have been transported by human beings from one continent to the other mostly fortuitously; the house mouse has a worldwide distribution.
Largest living rodent is the Capybara Hydrochaeris hydrochaeris (head and body measuring 130 cm, tail 52 cm and weight 50 kg) from South America and the smallest is the European Harvest Mouse Micromys minutus weighing 5-7 g, measuring 5.5-7.5 cm and tail 7 cm.
Rodents are possibly the most prolific breeders among the terrestrial vertebrates. The mice females start breeding at the age of 21 days, have a gestation period of 19-21 days and can produce 6-8 litters of 2-6 babies per litter. It can breed all year round under suitable conditions. Larger species, squirrels and porcupines, breed once a year while rats and others can breed several times in a year. Litter size varies from one to several babies. Many species build nests in the trees, in hollows and crevices when others make an elaborate tunnelling system either for living or for nest building and residing. Tree nests of rats and squirrels are very big with a specialized chamber for rearing the babies. Their nests are called dray. Most rodent babies are born naked and blind. However, the babies of porcupines and flying squirrel are born with eyes open and fully furred.
Bangladesh has about 25 species of rodents. Of these, house-mouse Mus musculus (Nenggti Indur), is the smallest (head and body 5-8 cm plus 5-8 cm tail, weight 18 g), and Indian Porcupine (Shjaru), Hystrix indica (head and body 90 cm plus 10 cm tail) is the largest weighing about 20 kg. Porcupine's body is covered with modified hairs called quills that are very long and spiny. Common species in Bangladesh are the Porcupine, House Mouse; Common House (Indur), Rattus rattus; Bandicoot Rat (Dhadi Indur), Bandicoota indica; Lesser Bandicoot Rat (Metho Indur), Bandicoota bengalensis; Malayan Giant Squirrel (Kalchey Kathbidali), Ratufa bicolor; Flying Squirrel (Uranta Kathbidali), Petaurista petaurista; Five-striped Palm Squirrel (Dora Kathbidali), Funumbalus penanti, and Irrawady Squirrel (Badami Kathbirali), Callosciurus pygerythrus.
Shjaru used to be present all over the country with woodlands and other vegetation. Now they are found only in the forested areas. Kalchey Kathbidali has disappeared from the sal forests and is present in small numbers in the mixed evergreen forests of Sylhet and Chittagong divisions; it is the same with the Irrawady Squirrel. The Palm Squirrel never occurred east of the River Padma and is still found in certain pockets of Khulna and Rajshahi divisions. It has been introduced to the east of Padma through escapees from the Dhaka Zoo. The number of rats and mice is increasing at an alarming rate.
With the exception of a few species, rodents are usually detrimental to the cultivated crops, vegetables, fruits and sugarcane. They often spread diseases to human beings and other animals, the notable among these being the Bubonic Plague that has cost millions of lives during the past century. [Ali Reza Khan]
Rodent control Methods or devices to control rodent pests. The various control techniques used by Bangladeshi farmers include poison baits, flooding, and excavation of burrows, trapping, and applications of smoke or phosphine gas. Most farmers use poison bait (74%) during a certain time of the year in their houses, fields, or both.
In Bangladesh, seasonal floods often control or limit the distribution and abundance of field rodents during the monsoon. Heavy rains may flood the burrows of field rodents, resulting in the death of young rodents. Several predators, such as, jungle cat, civet, jackal, owl, hawk, snake, monitor lizard, and cat are reported to feed on rodents.
Farmers use a large variety of locally made traps, including snap traps, wooden traps, bamboo traps, earthen traps, and cage traps. The most common type of snap trap used by farmers in houses is Kachi kall.
Glue boards are very useful to control rats and mice in houses, laboratories, and storages. A pieces of card board (30 x 30 cm) or wood or galvanised iron sheet coated with rodent glue is placed in rodent runways in an infested area. Rodents become stuck while walking or running across the glue board.
In Bangladesh, digging is very popular because of the reward offered of getting cached rice from the burrow, besides killing the rats. Digging out is more effective and sure policy. In fields or farmhouse, rats can be driven out by pouring water on burrows. In rural areas, rats are often driven out of burrows by blowing smoke (with chillies) into active burrows.
Chemical control includes the use of zinc phosphide, brodifacoum, bromadiolone, flocoumafen, and Yusidion. These are the main registered rodenticides available in Bangladesh. Zinc phosphide is generally used at 2 to 3%. All anticoagulant rodenticides are available in the form of ready-made baits.
Rodenticides are generally organic or inorganic compounds, and are classified arbitrarily (based on their mode of action) into four groups. These are acute single dose or non-anticoagulants, chronic or multiple dose or anticoagulants, fumigants, and chemosterilants.
There is no registered chemosterilant in Bangladesh for rodent control. Some fumigants are however, used to kill rodents within their burrows, in ships, godowns, and other inaccessible places. Common fumigants used against rodents are: calcium cyanide, aluminium phosphide, and carbon monoxide. Aluminium phosphide is available as 0.6g pellets and 3.0g tablets that release phosphine when comes in contact with atmospheric moisture or damp soil. Cyanides are available as grey 'gassing' powders that similarly release hydrogen cyanide gas. Use of 1-2 aluminium phosphide tablets per burrow entrance is commonly recommended. It is registered and sold under the following trade names: Agriphos 57%, Gastroxine 57%, Selphos 57%, Quickphos 57%, Alumphos 57%, and Quickphum 57%. [Santosh Kumar Sarker]