INTRODUCTION

Rats belong to the family Muridae, specifically to the suborder Myomorpha; this group of rodents is the largest and includes some 650 species of mammals. Originally widespread in Eurasia, Africa and Australia, it is now a cosmopolitan species (except at the poles). The Muridae also includes mice and mice like them. 

THE FIVE SENSES IN RATS

• VISION IN RATS: The vision of rats and humans is similar, although it differs in some respects.  Humans have two types of light receptors: cones and rods; the former are responsible for sensitivity to light and colour, the latter only to light.  Rats in particular are sensitive to two colours (green and blue) as opposed to humans who are able to recognise three main colours, all the others moving in different shades of grey, the most important thing for them being the brightness of the substances or objects around them. The visual acuity of rats is quite poor: 1 CPG in pigmented rats and 0.5 CPG in albino rats; but what is a ‘CPG’? A ‘CPG’ is a ‘cycle per degree’ which is the formula for measuring an individual's visual acuity. An average human being has 30 CPG.
  On the other hand, his field of vision is very wide due to the situation of his eyes (on both sides of his head), calculating depth through head movements.
  As we have already mentioned, albino rats have extremely poor vision, as they have no pigmentation in their eyes (the reddish colour is not a colour, but the capillaries of the eye), they are saturated with light and, over time, the retina weakens. This type of animal has no problems living day to day as they make up for their vision by using all their other senses, especially their sense of smell and whiskers.  In the case of albinos, special care must be taken with direct sunlight or excessively strong lights, as even if they are ‘almost blind’, they can feel a strong discomfort or pain when exposed to this element. 

• RATS' HEARING: Unlike larger animals in which the distance between the auditory pavilions is wide, in rats and small animals this is much smaller and, although they are able to hear the sounds around them much more quickly, they are not so good at locating where they are coming from.
  In the case of fancy rats, which have a variety of pinna shapes, the shape of the pinna does not influence hearing.
  Deafness in these animals is not common but can occur, especially genetically (passed on from ancestors to descendants); like ‘blind’ or ‘nearly blind’ rats, deaf rats have a great capacity to adapt to the environment and can live with the help of their other senses. From my point of view in this case (as with the blind) the way of approaching or touching the animal is important to prevent them from becoming frightened and acting defensively.

• SMELL IN RATS: Smell works in the following way: Air enters the body through the nostrils and, after encountering the olfactory receptors, travels to the brain, where it forms an image (or series of images) to which it relates the smell. In the case of rats, they have a second way of ‘smelling’, and that is through a thin ‘tunnel’ located next to the nasal cane, through which, either by licking or sniffing, the rats allow the ‘smell’ to stick to this ‘tunnel’ and reach the ‘vomeronasal organ’. The main function of this organ is to detect hormones and it is essential for living and interacting properly. These hormones are found in faeces, urine or any gland of the skin; the most common way is through urine, both males and females in fertile age mark a lot, being more common in the case of females that it happens before coming into heat. These chemical signals are very important and give a lot of information about other rats: What sex are you? What is your reproductive status? Do I know you? Who are you? 

• TASTE IN RATS: Rats are ‘picky’ and the sense of taste is essential to perform several tasks, but one of them is to check that what they eat is of good quality. Rats are omnivorous animals (they eat everything) and this, in the wild, can be dangerous, as they have to try everything they think is ‘edible’; in captivity this does not happen as a conscientious owner will always give them food that is beneficial to them. Like humans, rats are able to distinguish the same tastes: salty, sweet, bitter, sour and ‘umami’ (such as monosodium glutamate). In the case of newborns, they tend to acquire their mother's ‘tastes’ (through breastfeeding) and it is common that, once they reach adulthood, they have tastes similar to their mother.
  Likewise, the sense of taste warns them of possible ‘dangers’, since if a food has previously harmed them they will not eat it again, as they associate it with pain or illness. 

• TOUCH IN RATS: Whiskers in rats grow on both sides of the nose, on the eyebrows, cheeks and around the mouth (a little shorter) and are important for various functions, forming in the embryo around eleven or twelve days of gestation but it is not until twelve days after birth that the rat can move them. Every time one of these whiskers bends it sends a message to the barrel cortex (a part of the brain) and this creates a map that it decodes to understand what is happening, so they are important to them, but what happens if they lose one? In that case the area of the brain that processes the messages sent by that moustache adapts, the others (still present) become larger to ‘work for that moustache’ and, once it grows back, the brain adapts again. In the case of rats with curly whiskers, as it is ‘natural’ for these rats (they are born with it) their brains are adapted to this singularity, although their worlds will be a little more limited than those rats with normal whiskers. 

THE RAT'S TAIL

The rat's tail is made up of bone (from the spinal column), tendons, blood vessels and skin. But what is the function of the tail in this animal? 

In rats, the tail has two main functions, the first is to regulate body temperature (17-19% of body heat) and the second is to aid balance when climbing ropes, walls... It maintains the function of being the rat's counterweight, preventing it from falling easily, and helping it to slightly change its centre of gravity.

It may happen that we find rats that do not have a tail (either from birth or due to some traumatic event), in this case the rat's power to regulate its temperature decreases and not only is its body temperature higher than usual, but it will take longer to lower it. 

THE RATS' TEETH

Rats have only two types of teeth: Incisors and molars. These teeth are unique throughout their lives and are really strong. 

The incisors are located at the front of the face, they are four, narrow and long, but the upper ones are shorter and yellowish than the lower ones. They emerge 8-10 days after birth and grow steadily throughout the animal's life (which is why they gnaw and why it is so important that they can do so), renewing themselves completely after 40-50 days, but how long they take to renew depends very much on their diet and the objects they gnaw, as much faster wear (gnawing on hard things) will cause them to grow much faster. 

But why are rats' teeth yellow in colour? This is due to the enamel, which is also very hard; throughout the rat's life it changes colour, starting with the upper incisors at 21 days of age while the lower incisors start at 25 days; the upper incisors will always be a more intense shade than the lower ones, reaching an orange-yellow shade in adulthood.