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What's the Difference Between a Rat and a Mouse?

Updated on August 27, 2017
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Since graduating from university, Paul has worked as a librarian, bookseller, and freelance writer. Born in the UK, he now lives in Florida.

A wood mouse.  This type of mouse is very common across Europe and northwestern Africa.  It also known as a field mouse, common field mouse, long-tailed field mouse, and European wood mouse.
A wood mouse. This type of mouse is very common across Europe and northwestern Africa. It also known as a field mouse, common field mouse, long-tailed field mouse, and European wood mouse. | Source

Mice and rats are both small furry rodents with long tails and beady eyes, which may be confused with each other by an amateur observer.

They are completely different species with separate characteristics, however, and once you know the characteristics of each, you won't mix them up.

So what exactly is the difference between a rat and a mouse? Well, there are many, here is a summary of the seven main ones.

Main Differences Between a Rat and a Mouse

  1. Size and weight - rats are bigger.
  2. Tails - the rat tail is longer and thicker.
  3. Head shape - the mouse head is more triangular.
  4. Droppings - different size and shape.
  5. Biology - young mice develop more quickly.
  6. Nipples - rats have an extra pair.
  7. Rats kill mice, but mice don't kill rats.

I will explore these differences below in more detail.

There are many different types of rat and mouse.  For the sake of argument, I am going to use the most common types encountered, namely the Norwegian (or brown) rat and the house mouse.
There are many different types of rat and mouse. For the sake of argument, I am going to use the most common types encountered, namely the Norwegian (or brown) rat and the house mouse. | Source

Terminology

Note that the terms ‘rats’ and ‘mice’ aren’t actually scientific terms and there are many different species that are labeled ‘rats’ and ‘mice’ by people. For example, there are kangaroo rats and cotton rats, and dormice and field mice.

However, for the sake of argument, I am going to assume that a ‘rat’ in this case refers to the most common type of wild rat that you get around humans in towns and cities, namely the Norway rat (sometimes called the brown rat, Latin name: Rattus norvegicus); and that ‘mouse’ means the most common type of wild mouse that you are likely to see around humans, namely the ‘house mouse’ (Mus musculus).

Although it’s a generalization, it’s fair to say that these are the types of wild rats and mice that ordinary people most often encounter and are talking about when they refer to mice and rats.

1. Size and Weight

Adult rats are much larger in size than mice. In bird terms, they are about the size of a pigeon whereas mice are about the size of a sparrow.

The rat’s body length is longer, around 9 to 11 inches long whereas a mouse's body is just 3 or 4 inches long.

Rats are also over ten times heavier, with an adult male weighing between a half and one pound while mice weigh in around a measly half ounce.

2. Tail Length

Rats also have much longer tails. An adult rat's tail will typically measure around 7 to 9 inches, whereas a mouse tail is more likely to be between 3 and 4 inches.

I had mice that I kept as pets when I was very young, and I've always liked the way they look. Even rats. I'm not scared of them."

— Catherine Deneuve

3. Shape of Head

The other strong visual clue that is useful for telling these rodents apart is their slightly different head shapes.

Essentially, mice have small, triangular heads, whereas the rat head is chunkier and less pointed.

Rats feeding at Rajastan's Karni Mata Temple in India.  The impressive, isolated Hindu temple was built in the early 1900s as a tribute to the rat goddess, Karni Mata, by Maharaja Ganga Singh.  Over 20,000 rats are estimated to live there.
Rats feeding at Rajastan's Karni Mata Temple in India. The impressive, isolated Hindu temple was built in the early 1900s as a tribute to the rat goddess, Karni Mata, by Maharaja Ganga Singh. Over 20,000 rats are estimated to live there. | Source

4. Droppings are Different

If you have a wild rodent living in your house and you are not sure what type it is, one way to identify the rodent without even actually seeing it is to look out for droppings.

Mice droppings are much smaller, measuring around 1/8 inch long and they are pointed at both ends. Rat droppings measure around 5/8 inch long and are curvy.

"If you build a better mousetrap, you will catch better mice."

— George Gobel

5. Biological Differences

There are also some important biological differences in terms each one's development.

For example, young mice develop much more quickly, opening their eyes after around 3 days and having fur after about 10 days, whereas rats open their eyes after 6 days and have fur after about 15 days.

A one day old house mouse.  Mice breed all year round and can reproduce when they reach around fifty days old.  They normally mate at night and the average gestation period is 20 days.  The average litter contains 10–12 young, known as "pups".
A one day old house mouse. Mice breed all year round and can reproduce when they reach around fifty days old. They normally mate at night and the average gestation period is 20 days. The average litter contains 10–12 young, known as "pups". | Source

6. Numbers of Nipples

Rats also have an extra pair of nipples when compared with a mouse. A rat has 6 pairs, whereas a mouse only has 5.

A mouse eating.  The diets of mice and rats have a lot of similarities, although their dietary requirements are not quite the same.  Rats, for instance, need more fiber and less fat than mice. Mice are also slightly pickier eaters than rats.
A mouse eating. The diets of mice and rats have a lot of similarities, although their dietary requirements are not quite the same. Rats, for instance, need more fiber and less fat than mice. Mice are also slightly pickier eaters than rats. | Source

7. Rats Kill Mice but Mice do not Kill Rats

My seventh and final main difference is the fact that mice don’t kill rats, but rats can and do kill mice.

It may surprise some people to know that rats will actively hunt, kill and eat mice – this behavior is known as: “muricide.”

The problem with rats is they have no fear of human beings, they're loaded with foul diseases, they would run the place given half the chance, and I've had them leap out of a lavatory while I've been sitting on it.

— David Attenborough

© 2014 Paul Goodman

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