Rats and mice: what's the difference?

What do the terms rat and mouse mean?

Rat and mouse are actually not scientific classifications. These words are common names for rodents that look alike to the casual eye.

Rat is used to describe medium-sized rodents with long thin tails. There are many species of rodent that are called rats -- kangaroo rats, cotton rats, Norway rats, black rats, African pouched rats, naked mole rats, wood rats, pack rats, Polynesian rats, and many others. These different rodent species may not be closely related to each other at all!

Mouse is used to describe tiny, sparrow-sized rodents with long thin tails. As with rats, there are many species of rodents called mice which may or may not be closely related to each other: house mice, field mice, deer mice, smoky mice, spiny mice, and dormice are all called mice.

So, which rats and mice are you talking about? Generally, people are referring to the domestic or pest rats and mice, which means Norway rats (Rattus norvegicus), black rats (Rattus rattus), and house mice (Mus musculus).

Differences between Norway rats and house mice

Norway rats and house mice belong to different species. A species is a group of related individuals or populations which are potentially capable of interbreeding and producing fertile offspring. So Norway rats and house mice belong to different species and cannot interbreed. Humans and orangutangs, chipmunks and red squirrels, bottlenosed dolphins and killer whales, all belong to different species.

Norway rats and house mice are related, however. They descend from a common ancestor that lived millions of years ago -- how long ago is currently under debate, with estimates ranging from 8 to 41 million years ago. That estimate will probably become more precise over time. The descendants of that common ancestor diverged into different species, among which are Norway rats and house mice.

Norway rats and house mice now have many genetic, reproductive, developmental, morphological and anatomical differences. The list below is not exhaustive, but for those with a casual interest it should get you started:

 Genetic differnces: Norway rats have 22 chromosome pairs, house mice have 20 (see Levan 1991). Norway rats have 2.75 billion base pairs while mice have 2.6 billion (humans have 2.9). About 90% of rat genes have counterparts in the mouse and human genomes (Rat Genome Sequencing Consortium 2004). See Burt et al. 1999, Grutzner et al. 1999, and Watanabe et al. 1999 for more.

 Growth differences: In general, Norway rats develop more slowly than house mice. For example, Norway rat gestation is slightly longer (21-24 days) than house mouse gestation (19-20 days). Norway rats lactate for about 3 weeks, house mice for 2 weeks. Both species are born naked and blind, but Norway rats open their eyes at 6 days, they are fully furred at 15 days. House mice open their eyes at 3 days, have fur at 10 days (etc.).

 Anatomical differences: Norway rats have 6 pairs of nipples, house mice have 5 pairs.

 Morphological differences: Norway rats are larger, heavier and longer than house mice (Norway rat: 350-650 grams, 9-11 inch bodies and 7-9 inch tails; house mice: 30-90 grams, 3-4 inch bodies and 3-4 inch tails). Correlated with this larger size, Norway rat body parts are larger than those of the house mouse -- rats have larger ears, feet etc. The heads of Norway rats are heavy, blunt and chunky, house mouse heads are small and sharply triangular with pointed muzzles. Note, however, that Norway rats have smaller ears relative to their heads than house mice.

 Sign differences: Due to their larger body size, rat feces are larger than mouse feces (also see differences in rat and mouse sign from a pest management perspective).

Life-size drawing of mouse and rat feces.

How can I tell Norway rats and house mice apart?

Adult rats and mice

Adult mice are much smaller than adult rats (Fig. 1). Adult mice weigh about 30 grams, and fancy mice tip the scales at about 50 grams. Adult mice have bodies that are 3-4 inches long with 3-4 inch tails.

Adult rats are far heavier and longer: they can weigh ten times as much, averaging 350-450 grams for females and 450-650 for males (with an overall range of 200-800 grams). They have 9-11 inch long bodies and 7-9 inch tails (ref).

Figure 1. Drawing showing the relative size of rats and mice

Young rats vs. adult mice

Young, weaned rats are still larger than adult mice, weighing around 100 grams at six weeks. However, to the casual observer, very young rats and adult mice can be difficult to tell apart.

Here's what to look for: baby rats will have more juvenile proportions than adult rodents. Their heads and feet will be large relative to their bodies, their faces will be stubby and blunt with wide noses. Adult mice, on the other hand, will have adult proportions: a small, triangular head with a small nose and little delicate feet as compared to the body. In addition, mouse ears are very large relative to their heads, rat ears are smaller relative to their heads. Rats also have thicker tails than mice.

Feature Baby Rat Adult Mouse Head short, stubby, broad, large relative to body small, triangular, small relative to body Muzzle large and blunt with wide muzzle narrow with sharp muzzle Ears ears are small relative to the head ears are large relative to the head Tail thick thin Tail/body ratio Tail shorter than body Tail same length/longer than body Feet Large relative to body, especially the hind feet Small relative to body Weight around 100 grams at 6 weeks, 200 grams at 8 weeks 30-50 grams 6 week old rat Adult mouse

Quiz: Rat or Mouse?

Take this quiz to test your ability to tell photos of rats and mice apart.

History and evolution of the Norway rat and the house mouse

Common ancestry of the Norway rat and house mouse

True rodents first appear in the fossil record at the end of the Paleocene and earliest Eocene in Asia and North America, about 54 million years ago. They are widely considered to have originated in Asia (Meng et al. 1994). These original rodents were themselves descended from rodent-like ancestors called anagalids, which also gave rise to the Lagomorpha, or rabbit group.

Murids (Muridae), the family that gave rise to present-day Norway rats, house mice, hamsters, voles, and gerbils, first appeared during the late Eocene (around 34 million years ago). Modern murids had evolved by the Miocene (23.8-5 mya) and radiated during the Pliocene (5.3-1.8 mya) (for more, see Introduction to the Rodentia).

The Norway rat and the house mouse had a common murine rodent ancestor. How long ago that common ancestor lived is a matter of debate, however. The fossil record indicates that the most recent common ancestor of Norway rats and house mice lived about 8-14 million years ago (Jacobs and Pilbeam, 1980). Geneticists, however, estimated that their most recent common ancestor lived about 41 million years ago (Kumar and Hedges, 1998).

Brief history of house mice

The ancestors of the house mouse (Mus musculus) lived in the steppes of present-day Pakistan. Ten thousand years ago, at the end of the last ice age, neolithic farmers moved from the Fertile Crescent into the steppes of Pakistan, and these small rodents found a delightful new source of food and shelter.

When humans migrated away from the steppes to colonize other areas, mice went along as stowaways in the humans' carts and later, their ships. House mice arrived in the new world in the 16th century, arriving with explorers and colonists. Mice went everywhere with humans, living in and around their houses, a human-dependent association called commensalism. Today, commensal house mice live in and around human dwellings on every continent, in every climate.

Today, commensal house mice are divided into four subspecies: M. musculus bactrianus are the descendants of the original, ancestral house mice first encountered by our neolithic ancestors. They live in India, Pakistan and Afghanistan. M. musculus castaneus lives in Southeast Asia. M. musculus musculus lives in Russia and western China, and M. musculus domesticus lives in Europe, from whence it traveled to the Americas, Australia, New Zealand, and Africa with the colonists.

Domestication: Domestic mice originated from stocks captured in China, Japan and Europe and developed into fancy mice. These fancy mice were found in pet shops in the 20th century, and were developed into laboratory mouse strains. Fancy mice are primarily descended from M. musculus domesticus, with a little admixture of the other three subspecies. As such, domestic mice do not represent one of the single subspecies, but are a mixture of all four. (Silver, 1995).

Brief history of Norway and black rats

See separate article, History of the Norway rat.

Are rat-mouse hybrids possible?

See separate articles, are rat-mouse hybrids possible? and the hybridization page.

Mouse-killing behavior in rats: muricide

Rats can, and do, kill mice, a behavior known as muricide. Muricide is a form of predatory behavior: rats hunt, kill and eat mice.

How common is muricide?

Karli (1956) found that about 70% of wild rats and 12% of domestic laboratory rats kill mice. Male and female rats are equally likely to kill mice. Similarly, Galef (1970) found that 67% to 77% of captive born wild rats kill mice.

Description of muricide

Muricide is a stereotyped behavior, performed in much the same way each time: the rat chases the mouse around the cage and bites it using its sharp front incisors, usually aiming for the mouse's head, neck, or upper back. The first bite is frequently fatal, but the mouse may delay the rat's attacks by defending itself (by rearing up and boxing with its front paws or laying on its back). Eventually, however, the rat delivers a fatal bite. Mouse-killing behavior is very rapid, lasting only a few seconds (Hsuchou et al. 2002).

The preferred area to bite is the back: out of 671 mice killed by rats, 89% were bitten on the spinal cord (specifically: 65% neck, 13% thoracic, 11% lumbar). Only 7% were bitten on the belly and 4% were bitten on the head (Karli 1956).

Do rats eat the mice they kill?

Karli (1956) found that all mouse-killing rats (wild or domestic) consumed part of the mice they killed. Specifically, out of 683 mice killed by rats, after 7 hours 25% of the mice had been entirely eviscerated (brain, thoracic and abdominal viscera), 67% had been partially eviscerated, and only 8% had not been eaten.

Wild rats tended to start eating at the spot where they had bitten the mouse, which is usually the neck. They gradually opened the thorax and consumed the thoracic viscera, then proceeded to the liver. In contrast, domestic rats went right to the brain, opening the skull and consuming all or part of the brain, no matter what killing method had been used (Karli 1956).

Exogenous and endogenous influences

Mouse-killing is a complex behavior involving several neurotransmitter systems (Miachon et al. 1997, Onodera et al. 1981, Tadano et al. 1997, Ueda et al. 1999, Vergnes and Kempf 1982, Yamamoto et al. 1982), neural systems (Hull and Homan 1975, Spector et al. 1972) and hormonal systems (Miachon et al. 1995; Rastegar et al. 1993).

Mouse-killing is also affected by by rearing (Garbanati et al. 1983), environmental conditions (Garbanti et al. 1983, Giammanco et al. 1990), social conditions (Eisenstein and Terwilliger 1984), diet (Bac et al. 2002, Onodera et al. 1981), learning (Tingstrom and Thorne 1978). Rats are more likely to kill mice at night than during the day (Russel and Singer 1983).

Mouse-killing is affected by hunger, too: rats kill mice more when they are hungry (Malik 1975) and at times when they are normally inclined to eat food (Russel et al. 1985). Rats may start killing mice when they are starving, but stop when they are given plenty of food (Karli 1956).

Familiarity with mice also plays a role: rats reared with mice tend not to kill mice as adults. Specifically, Galef (1970) found that 67-77% of captive born wild rats kill mice. However, if captive born wild rats are raised with a mouse from weaning to age 3 months, none of them kill the mice they're familiar with. When presented with an unfamiliar mouse, only 7% of the mouse-reared rats killed it.

Response of mice to rats

Rat odor is stressful to mice and has an effect on their behavior and reproduction. In fact, rat odor is sometimes used as a predator odor to study anxiety and antipredator behavior in mice.

Specifically, domestic and wild-stock mice who are exposed to a conscious or anesthetized rat tend to flee, and if prevented from fleeing, they show defensive or attack behavior (Griebel et al. 1995, Blanchard et al. 1998).

Mice housed in the same room as rats tend to be more stressed than mice housed without rats (Calvo-Torrent et al. 1999). Mice who can smell rat urine take ten times longer to start eating a treat than mice who cannot (Merali et al 2003). Mice who were exposed to rat urine for just a few minutes startle more afterwards, even up to two days after the rat urine exposure (Hebb et al. 2003). Pregnant mice exposed to rat urine produce fewer litters than mice who were not exposed (de Catanzaro 1988).