Definition of Terms
Throughout this article, when I say rat, I will be referring to Norway rats since they are the type of rodent-rat that normally infest houses through the sewers and other moist places. It is quite interesting to note though that not all rodents which fall under the classification of being a rat are actually related (the same is true with mice). In fact, the label rat is not based on a specific rodent specie because many rodent species have been classified as rats like the cotton rats, black rats, kangaroo rats, wood rats, Polynesian rats, mole rats and of course, Norway rats. As a general guide, a rodent that is medium-sized and has a long, thin tail is given the label "rat".
On the other hand, the term mouse is reserved for rodents with tiny bodies, having long thin tails. And since many rodents fall under this category such as the field mice, deer mice, spiny mice, dormice, house mice, along with many others, throughout this article when I say "mouse", I am referring to the common house mouse that usually infests houses.
Norway Rats versus House Mice
It might come as a surprise to you but mice and rats are actually not related to each other, although they have many similarities. These rodents do not belong to the same specie so they are not capable of interbreeding as well. In fact, rats can be considered as predators to mice because of their tendency to kill mice known as matricide. Mice also scurry away from the smell of rats.
So as a clear heads-up, mice and rats are not friends by nature.
However, despite their natural difference, these rodents also have very important similarities. From the evolution theory of Charles Darwin to the theory of natural selection, it was deduced that about eight to forty million years ago, an ancestor that is common to both mice and rats once "crawled" on this planet. So, technically, although these rodents are now from different species, they have still offspring's of the same prehistoric rodent in the past.
Difference between a Rat and a Mouse
The most obvious difference between the two rodents is their adult size. Rats are much larger than mice though the ears of rats are relatively small for their head as compared to the ears of the mice. Moreover, the head of a rat looks chunky and heavy unlike that of a house mouse that is small and triangular. If ever you get the chance to weigh both rodents, you will see that a Norway rat's weight ranges from 350 to 650 grams while a house mouse's weight is barely about 30 to 90 grams.
If you look beyond the exteriors of both rodents, you will see their genetic differences too. It was known that a house mouse only has twenty chromosome pairs which are two pair less than that of rats. If compared to humans having about 2.9 million base pairs of chromosomes, rats are only short by 0.15 of a million while mice are short by 0.30 million. Moreover, rats take a longer time to grow as compared to mice. Norway rats are gestated for 21 to 24 days while mice only need to be gestated for 19 to 20 days. Rats also need to be nourished with rat milk for three long weeks while mice only need two. Which makes me think that this has something to do with the fact that rats have six pairs of nipples while mice only have five?
Well, of course, after looking inwardly, mice and rats also show a big difference in the traces that they left. Because of their disparity in size, rat's feces are generally much larger than the droppings left by mice. As a graphical representation, a single rat dropping is equivalent in amount to twenty to thirty mouse droppings.
Baby Rat and Adult Mice: They are of the same size, how to tell which is which?
Of course, since rats also pass the stage of being a baby rat, there is a certain period in their life cycle when they are of the same size with mice. But by close examination or at least, by keen observation, you will still be able to delineate a mouse from a baby rat. As a simple clue how, just like humans, baby rodents will have "child-like" body proportions. Baby rats will have proportions that are relatively large for the size of their bodies. Its feet and head will be big and its face will be stubby, having a wide and blunt nose. Adult mice of the same size, on the other hand, will have adult body proportions. An adult mouse's tail will be of the same length (or probably longer) of its body while a baby rat will have a tail that is shorter that the length of its body. An adult mouse's tail is also thin while a baby rat's tail is somewhat fatter. Moreover, a baby rat has hind legs with a very pronounced sized in contrast with that of an adult mouse that looks quite small for its body.
More about Muricide
Like what has been revealed earlier in this article, rats can actually kill mice, in fact, rats have the tendency to look at mice as preys. Wild rats even hunt down mice for food. Scientists who have studied this rat behavior found out that about 70 percent of wild rats have this tendency while laboratory-grown rats show 12% possibility. Muricide is not exclusive to one rat gender alone, as male and female rats manifest the same behavior at the same extent.
Rats kill mice by biting their necks with their sharp incisor; rats also target the head and the upper back of mice. In most cases, the rat's first bite to the mouse is fatal killing the mouse in a few seconds. Sometimes, mice are able to fight back and prolong the killing by laying on its back or by "boxing" using their front paws, but in the end, rats usually (if not all the time) overpower them and then kill them.
Wild rats usually eat the mouse they kill on the spot starting with its neck then down to its thorax and then consume the liver. Domesticated rats however, have the appetite for the brain and proceed to opening the skill of the mouse and leave the mouse's carcass after consuming the brain.
Because of this rat tendency, mice have also developed a unique response to rats. Rat odor would cause stress to mice, disrupting their behavior and their reproduction. It was found out through a research that mice will lose appetite for up to two days after being exposed to excessive rat odor (like urine or feces). When a mouse catches the scent of a rat, it will definitely flee for safety and when it is prevented from running away, it will exhibit wild defensive behavior (just like humans all other animals in the face of their predators).