(Resonance, May, 1997)
Which mammalian order has the largest number of species? The answer, astonishingly, is Rodents, derisively known as the feeble folk, because of their insignificant size and strength. However, rats and mice have, as commensals of man or by natural means, invaded every continent except Antarctica. They are found upto altitudes of 5500 metres and occupy almost every type of habitat. Despite their undisputed success, their extensive adaptability, and their historical use by Science as laboratory animals, Rodents in general and rats in particular have consistently been given bad press. As destroyers of crops, as vectors for disease, and as pests in house and field alike, they have been shunned.
Remember Robert Browning:
They fought the dogs and killed the cats
And bit the babies in the cradles
And ate the cheese out of the vats
And licked the soup from the cooks own ladles
Split open the kegs of salted sprats
Made nests inside the men's Sunday hats
And even spoiled the women's chats
By drowning the speaking
With shrieking and squeaking
In fifty different sharps and flats.
From `The Pied Piper of Hamlin'
However, one could be of the opinion that rodents and other small mammals give credence to the `small is beautiful' thesis. Of the 4200 odd mammal species, 90 % weigh less than 5 kg. 10 of 16 mamma- lian orders contain mostly small species. In fact, both in birds and mammals, the below 1 kg class embraces the most successful order; of some 8600 species of birds, 5100 belong to the order Passeriformes and of the 4200 mammal species, about 1700 are rodents. Rodents are largely herbivorous and their great abundance in most of the world's ecosystems creates a broad layer of primary consumers in the pyramid of numbers. Rodents may affect vegetation communities directly by foraging and indirectly by digging, seed predation and seed dispersal. They can dislodge large quantities of soil which alters its drainage characteristics and modifies its micro-topography. They also play an important role as secondary consumers. And, of course, they form the prey base for a very wide variety of consumers, including snakes, raptors, and small carnivores. The taxonomic and numerous dominance of small mammals shows quite clearly that the pros clearly outweigh the cons of being small. On an evolutionary timescale, small mammals have 33 % higher rate of formation of new genera than large mammals.
The Black Rat and the Brown Rat
Rodents encompass a very wide range of animals such as beavers, squirrels, springhares, porcupines, marmots and capybaras. The suborder including rats and mice, however, is the most widely spread and has the largest number of species. Compared with other rodents, members of the family Muridae (true mice) may have evolved the most recently. One hypothesis suggests that murids evolved in southern Asia, as the earliest fossils of murids have been found in Pakistan. This would have been followed by adaptive radiation to the other continents. The most common and well known of murid rodents is the common rat, Rattus rattus, also known as the black, roof, house or ship rat. The black rat's arrival in Europe is attributed to the increase in trade routes with the East between 400 and 1100 A.D. The brown rat, Rattus norvegicus, also called the field or sewer rat, seems to have invaded Europe a lot later, and was first recorded in Europe and the United States in the eighteenth century. While the brown rat has successfully colonised temperate countries, the black rat has invaded and rules over most tropical countries, including India. The common albino laboratory rats are a variety of the brown rat These `Wistar rats' were first established at the Wistar Institute in Philadelphia, and more than 20 million rats are used per year for medical and physiological studies.
Teeth maketh the Rodent
Rodents are basically herbivorous, their distinctive teeth being specially designed for vegetarianism. They break up their food by nibbling or gnawing with their incisors, which function like chisels; in their incisor, the body of the tooth is soft dentine which, while gnawing, wears more quickly than the resistant enamel, thus maintaining a superb cutting edge. The incisors grow through their life of two to three years.
The social life of rodents
Many species are solitary for most of their lives, such as many sciurids, kangaroo rats, many murids, and jerboas. In the beaver, the family forms the social group. The house mouse occurs in family groups, usually with two to three males and four or five females and their young. Some rodents like the North American black tail prairie dog have extended family social structure; their burrows are juxtaposed to form a colony of upto a thousand individuals.
The populations of many species of rodents are known to fluctuate greatly. The numbers of the Indian Desert Gerbil (Meriones hurrianae) were found to vary between 5 and 800 per hectare depending on the season. Obviously, a number of biological factors such as predation and food availability affect this fluctuation in numbers. Other environmental factors may also affect numbers; eg. seasonal floods may cause high mortality of young in nests, .
Pests and Vectors, Beware !
While rodents do play a very important role in natural ecosystems, they have also become a part of human systems, making a significant economic impact. The damage done by rodents to crops in India alone runs into crores of rupees each year. They also cause considerable damage to fruit, vegetable, poultry and other domestic stock. They act as vectors for many diseases and while plague is the watchword, they also spread other diseases such asrabies, leptospirosis, and trichinosis.
Rodents in India
There are a number of interesting rodents in India. The Common Giant Flying Squirrel (Petaurista petaurista) occurs in forests throughout the country, while many other species are found in the Himalayas. The Giant Squirrel (Ratufa indica) is common in the deciduous and evergreen forests of south India, while the Grizzled Giant Squirrel is more rare. There are also several species of smaller squirrels, the fivestriped palm squirrel (Funambulus pennanti) predominating in North India and the threestriped palm squirrel (Funambulus palmarum) in the South. The porcupine (Hystrix indica) is another highly adaptable rodent. There are several species of gerbils, which are distinguished by their long hind feet and the tassel of hair at the end of their tail. Other rats found in cultivations include the Indian Mole Rat (Bandicota bengalensis) and the Short Tailed Mole Rat (Nesokia indica). Another common rat of the fields is the Metad or the Soft Furred Field Rat (Millardia meltada), though this has also adapted to the high altitude grasslands of the Nilgiris in the Western Ghats. The Indian Field mouse (Mus booduga) and the spiny Field Mouse (Mus platythrix) are found in most habitats. The Indian Bush Rat (Golunda ellioti) and the White Tailed Wood Rat (Rattus blanfordi) inhabit dry and moist deciduous forests throughout India. The Long Tailed Tree Mouse (Vandeleuria oleracea) is a an attractive little forest species found ubiquitously. The Malabar Spiny Dormouse (Platacanthomys lasiurus) inhabits the forested hills of South India at elevations of 600 metres and above.
Many rodent species show interesting colour variations; the ubiquitous Common Rat is an excellent example. In the wild, the animal (Rattus rattus wroughtoni) has a lovely rufous coat, with a pure white underbelly. In agricultural fields and plantations, it has a rufous coat and a yellow underbelly (Rattus rattus rufescens). The house rat (Rattus rattus rattus) is almost uniformly black. Squirrels living in the same area may show marked differences in colour, varying with season and locality. In cities, one comes across the common rat, the brown rat, the house mouse (Mus musculus; found the world over) and the much feared Bandicoot (Bandicota indica), which measure 30-40 inches from nose to the base of the tail.
A last word
Despite occurring in such large numbers, rodents have not been studied in great detail in the wild. Though there have been a number of studies on the population dynamics and community ecology of rodents in Europe, North and South America, it has not been possible to study their behaviour and habits since they are largely nocturnal and may live in inaccessible burrows. In India, very little is known about rodent communities in the different forest types throughout the country. Over the last few decades, Dr. Ishwar Prakash (Zoological Survey of India, Jodhpur) and his colleagues have studied rodents in Indian agricultural fields extensively. More recently, the Centre for Ecological Sciences has been working on small mammals in the Nilgiris, while others from the Wildlife Institute of India, Dehradun and the Salim Ali Centre for Ornithology and Natural History, Coimbatore have also initiated small mammal projects in the Western Ghats. It is, however, time that much more attention is paid to these `feeble folk' since their impact on natural ecosystems and on human systems and economics is not in the least feeble.