Water Rat

As the name suggests water rats are semi-aquatic animals that are found on both terrestrial landscapes and in the aquatic environment, they are mainly dependent on their food sources in the aquatic environment. The term water rat loosely encompasses all the rodents that are semi-aquatic in nature, the term is also used to define some of the species of voles (another member of the rodent family) example of one such vole is water voles. The water rats are evolved to adapt to living around the water streams, they hunt fishes and worms that are found in the water, they live by creating burrows near the water stream.

There are approximately 111 different species that are classified as water rats, there are some species that due to particularly large body size are considered as large water rats.  An Example of such a large water rat species includes the golden-bellied water rat. The scientific name of the animal is Hydromys chrysogaster. It is interesting to note that these different species of water rats do not belong to a single taxonomic genus, as will be evident further in the article. There are approximately 16 different genres among which these species are classified. 

Although the term water rat seems quite simplistic in nature, the term is used to define various different rodent animals, which are not very closely related. The one generalization that can be drawn from the collective data of the study of different species is that majority of the water rats are classified under the family called Murids.

The article focuses on the general description and natural history of the water rats in general. The article also discusses specific examples of the big river rats,  like the montane fish-eating rats and Rakali. The article is set out to discuss the details like habitat, reproduction pattern, diet and metabolism, and general behaviour.

General Description of Water Rats

Water rats are considered as a semi aquatic animal group. These animals are commonly classified as rodents. The majority of the life span is spent on the land, they live by making burrows and nests near the river banks or in general the landscape near any water body. The water body mainly acts as a source of food. The water rats are mainly dependent on fish and other insects and small worms that are present in the river. The mating and reproduction of these big river rats are seen in the land. The young ones once reach a certain age are seen to be more adopted towards the aquatic lifestyle.

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Morphological Features of Water Rats

There are certain adaptations that are common to all the water rats that are not seen in other rodents of the order. The adaptations include morphological features like small eyes, and nostrils that can close while diving in the water. The ears are quite close compared to other completely terrestrial rodents. Another modification that is seen in a large water rat is the presence of hairy ears, in some of the species of large water rats the external appearance of the ear is absent. They are known to have sensitive whiskers. Another important feature of a large water rat is the presence of wide hind feet, the hind feet are covered with fringed and stiff hairs. Another very important feature is the presence of bald soles with conspicuous webbing between the digits.

The colour of the body of the water rats is dependent on the colour of the fur. They are generally found in grey or brown. The fur of the water rats are uniquely thick, the hair density of the fur is very thick. The texture of the fur can be defined as woolly and dense. The important feature that is important from the evolutionary perspective is that they are water repellent in nature.

Anatomical Features

Since water rats are members of the order Rodentia, they also have the significant characteristic of continuously growing incisors. There is only a single pair of incisors in the detention of the buccal cavity. A single incisor is present in the upper and lower jaw of the buccal cavity. The teeth are covered with slightly yellow coloured enamel. Enamel is the outermost covering of the teeth, the main purpose is to provide protection. Most species of water rats have up to 22 teeth,  a common feature that is seen is the absence of canines or anterior premolars.

The tail of the rats is also a very important part of their anatomy the tail allows them to perform the following functions, thermoregulation, minor proprioception, and a nocifensive-mediated degloving response. Thermoregulation refers to the control of the temperature of the body, that is the temperature inside the body is constant even when the temperature of the environment changes. A very common example is the temperature of the human body fixed at 98 degrees Fahrenheit. Minor proprioception on the other hand refers to the ability to sense the change in the position of the body. In simpler terms, it is defined as the motor coordination of the body. 

The major organs that are found inside these animals include the following, heat, lungs, kidney, diaphragm, small and large intestine, gall bladder. All these organs together form the organ system and are essential for the sustenance of the life of a large water rat.

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Habitat and Distribution

The water rats as the name suggests is a group of animals that are partially aquatic so it would not be a drag to hypothesize the general habitat range common to all the different species is near some kind of water body. The water rats are considered carnivorous amphibians, they are found near the lands of rivers, lakes and ponds. It is important to note that there is very little evidence of the presence of the water rat species near the coastal regions. The natural habitat of these kinds of animals includes freshwater lakes, estuaries, and rivers and coastal mangrove swamps.

A unique feature about their habitat is that they are resistant to heavy pollution. The water rats are found in water bodies that are highly polluted and seem to complete their life span without any major hurdles. Although there are some exceptions to this rule. The water rats species that are susceptible to the polluted environment include the following,  rakali (Hydromys chrysogaster), muskrat (Ondatra zibethicus). 

The geographical range of distribution of these animals are very wide, they are almost found in every continent, there is also evident documentation that suggests these species are found even in the Indian subcontinent, which includes India, Sri Lanka, Bangladesh, Pakistan. There are records suggesting that some of the species are native and endemic to particular countries. The species Hydromys chrysogaster is native to Australia, they are found near the brackish water systems, another example of native species is Neusticomys monticolus, which are found in the Columbia, specifically in the foothills of Andes mountain in Columbia. 

Other geographical locations where these animals are found include Queensland, New South Wales, Victoria, Tasmania, South Australia, Western Australia (south-west and north), Northern Territory. there are some of the species that are native to America. North America have large species diversity of water rats. African water rats as the name indicates are found in the countries of the African continent.

Diet of Water Rats

Water rats are carnivorous amphibians that hunt their prey by swimming under it. An important point to note about these primarily carnivorous groups of animals is that they are opportunistic feeders. They can eat the prey hunted by some other predators and have the habit of foraging the food supply. 

The primary animals that are included in the diet of big river rats include small fish, snails, semi-aquatic insects. A large water rat hunts both invertebrate animals and vertebrate animals. The invertebrates population that serves as their food source includes large aquatic insects, snails, mussels, crabs, and crayfish. The vertebrate animals that are hunted by the water rats include animals like fish, frogs, turtles, young and adult birds, bird eggs, bats, and mice. Some species of water rats that prefer living in the clean unpolluted water also seen predating over crustacean species and occasionally small fish.

Some of the species of water rats like Neusticomys monticolus are insectivorous they are seen predating over insect that primarily includes the population of beetles, flies, mayflies, stoneflies, caddisflies, moths and butterflies, and spiders. 

Behavioural Adaptations

The rodents are generally animals that are found both living a solitary and social life. The preferred way for the rodents is to have a social organisation and live within a group. But nonetheless, sometimes rodents are also observed, this is due to forced domestication, or some other reasons. It is proven that rodents can be clinically diagnosed with mental illnesses like depression and anxiety if forced to live a solitary life. But there is not enough research evidence that can conclude the social behaviour of all the rodents including the water rats. It can be said that water rats prefer living in pairs or in groups when compared with solitary lifestyles, although keeping in mind that there can be some exceptions to this generalisation.

There are some feeding adaptations that are unique to the water rats, an example of such a habit is foraging the food, they are opportunistic feeders and have a tendency to store the food in their burrows. Another unique behaviour observed in water rats is that they can hunt the food from the water body, and will move the dead prey over their burrow to eat. The exact reason for this behaviour is not very clear. One of the feeding adaptations seen in the species that can not efficiently tolerate the colder temperature is that they switch to terrestrial prey from the aquatic prey during the winter. The sole reason for this is to protect their body from the excessive cold temperature of the water.

Water rats including the big river rats are considered to be nocturnal animals. That is they hunt during the night, the most active hour for these animals is the time after the sunset, during this time most of the activities are recorded. 

Water rats are considered territorial animals, in an area with dense population water rats tend to turn aggressive, the aggressive nature is more prominent in the male members of the groups. One of the major signs of this behaviour is the presence of the injured tail. The tail of the water rats is an important part of their defence system. The injured tail or tail with various marks indicates that they are part of intraspecies or sometimes interspecies competition.

Mating and Reproduction

Water rats similar to all rodents perform sexual reproduction. The male gamete fuses with the female gamete inside the body of the female, indicating that the fertilization is an internal process. The females give birth to progeny and provide parental care, since they are classified under the category of Mammalia, they are considered placental mammals. That is the placenta is the tube-shaped structure produced during the gestation period that connects the fetus with the mother. The primary function of the placenta is to exchange nutrients between the mother and the fetus.

The sexual maturity of water rats is attained at about a year old. The females can produce litter from that age. Litter is described as a group of progeny produced by a single female during a single pregnancy. The average gestation period of these organisms is about 34 to 38 days. The average number of progeny in the litter is about 2 to 3. 

The parental care in water rats is mostly provided by the females. Since they belong to the Mammalia category, they have the significant feature of mammary glands. These glands are used for lactation, to provide nutrition to the young progeny.

The average life span of water rats ranges from 3 to 5 years and they are reproductively about 4 years.

Conservation Status

There are several species of water rats that are available in the wild. Although there is high species diversity of the animal the major threat that these animals face is the alteration of the habitat due to human activity. Urbanisation, salinisation, waterway degradation and pollution have all attributed to the threat of water rats. Some other potential threats to these animals include habitat alteration as a result of flood mitigation and swamp drainage. Some species like Hydromys chrysogaster were hunted in the past for their fur. The majority of species that are classified as water rats are categorized as “least concern” species. The only potential threat to these animals is the activities mentioned above.

Rakali

It is one of the most common species classified as the water rat. The scientific name of the species is Hydromys chrysogaster, they are native to the Australian continent. They are among the big river rat. They are generally found near permanent water bodies.

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Taxonomical Classification

The Taxonomical Classification of the Species is Mentioned Below:

• Kingdom -------> Animalia

• Phylum -------> Chordata

• Subphylum -------> Vertebrata

• Class -------> Mammalia

• Subclass -------> Eutheria

• Order -------> Rodentia

• Family ------->  Muridae

• Genus -------> Chrysogaster 

• Species -------> hydromys

Morphological Features

They have thick and dense fur. A pair of incisors. The head is long and flattened in shape, the eyes and ears are small. The distinctive morphological feature is the webbed feet of the hind feet. They are generally brown, black or grey in colour.

Distribution

They are native to Australia, found near permanent water bodies and wetlands. Specific geographical locations include Queensland, New South Wales, Victoria, Tasmania, South Australia.

Diet 

They are carnivorous in nature. The primary diet includes large insects, crustaceans, mussels and fishes, and even frogs, lizards, small mammals and water birds.

In conclusion to the article, it can be said that most of the water rats belong to the Rodentia order. It is an Umbrella term that consists of various species and genus some of which are not closely related to each other. Some of the examples of water rats include montane fish-eating water rats, African water rats, and rakali. These diverse groups of animals are classified as carnivores and are found in various landscapes. These animals are not under the threatened list of IUCN.