Pest Profiles: Norway/Brown Rat

The common brown or "Norway" rat.The common “brown” or “sewer” rat (Rattus norvegicus) is arguably the most despised and successful vertebrate pest of all time.

While the species hails from northern China (the origin of the misnomer “Norway” rat is still unclear) the brown rat is literally everywhere today – most likely within a hundred meters of where you are right now in fact…that is unless you happen to live in Antarctica.

The remarkable intelligence and complex social behavior of the brown rat, coupled with its amazing reproduction rates and unusual adaptability to varying climates and food sources certainly poised it for world dominance.

However, it is this rat’s uncanny ability to thrive in and around human infrastructure which propelled it out of Asia, throughout Europe and eventually into the Americas within a short few hundred years.

Yet, as interesting as this species’ biology may be, the brown rat has left of legacy of destruction in its wake. It is not only an unparalleled scourge on native flora, fauna and vegetation communities, but also poses a threat to human welfare worldwide by consuming/tainting our food and carrying numerous diseases and pathogens.

One way to control the brown rat around the home is through responsible airgun use. But before we get to that, you should learn a little more about what makes these animals “tick” – as it will likely make you more successful at defeating such a clever and adaptable foe.

Identification & Superficial Characteristics

Common physical characteristics of the brown rat.

Although it’s covered in a nondescript dark brown/grey pelt with a slightly paler underside, the hairless/slightly scaly tail and relatively large proportions (up to ¾ of a pound and 10 inches in length, without the tail) readily distinguishes the brown rat from squirrels and native mice. Moreover, this species is considerably larger and more robust (up to 2 times more massive) than the next most common member of the genus, the black rat (Rattus rattus), which is a more arboreal species that has a proportionally longer tail, more pointed snout and overall slimmer build among other distinguishing features (see figure below).

Distinguishing the black rat vs. brown rat

General Biology

Social Behavior

Despite being portrayed as the German cockroach of the rodent family, the brown rat is a highly intelligent, social mammal that operates within complex social hierarchies that closely resemble that of wild dogs. There are clear levels of dominance within groups, which are established/reinforced through play fighting, boxing, grooming and huddling behaviors. Individual rats tend to be territorial and are prone to violence when resources become limited. Those rats ranking lowest on the social ladder are typically among the first to die off in such situations. Nevertheless, family groups are strong and members have a rich repertoire of audible and ultrasonic vocalizations and chirps they use to communicate moods and warn each other of danger.

Reproductive Biology

Brown rats are incredibly prolific animals.As you can guess from their pest status, the brown rat is incredibly fecund.

Unlike many other mammals, female rats have no breeding “season” per se, and can essentially breed year-round if food/shelter is not limiting. Moreover, the brown rat has a very short gestation period (~21 days) and can produce up to six litters per year consisting of up to 14 pups at a time. Add to that a very short five-week period before the young reach sexual maturity and can start reproducing on their own, and it becomes clear why the brown rat is the very definition of an “invasive” species.

Diet & Foraging Behavior

To match the brown rat’s remarkable fertility is a willingness to eat just about anything.

Brown rat foraging

Brown rat foraging among kelp beds during low tide.

Indeed, while the species is most known for consuming various types of grains/cereals, populations of rats can adjust their behavior to exploit local food sources. For example, one West Virginia population was reported to learn how to catch and eat small fish from a nearby fish hatchery (brown rats are excellent swimmers); while other reports confirm that they can dive for mollusks, hunt and eat songbirds/waterfowl and, perhaps most disturbing, even grow so bold as to attack human infants.

However, despite such a decidedly eclectic palate, one well- known researcher and the founder of the Animal Behavior society concluded after a 1964 study that the brown rat ultimately preferred more mundane, familiar fare, such as mac & cheese, scrambled eggs and cooked corn! It is no wonder why they have taken so well to our garbage.

Preferred Habitats & Home Range

Brown rat habitat preferences

The brown rat will happily utilize drainage pipes.

The brown rat is a nocturnal/crepuscular burrowing species that, soil permitting, is prone to constructing complicated networks of tunnels, often with multiple levels and entrances. These tunnels and chambers, often situated along/adjacent/beneath a man-made or natural structures, are believed to provide a more stable thermal environment, easy food storage and a convenient refuge from predators and other above-ground threats.

Interestingly, virtually all of the digging required for these tunnels is done by females and young males; older males rarely contribute. This drive to excavate of course doesn’t mean that the brown rat is above using man-made tunnels, such as sewers, drainage ditches and other human excavations.

In addition to their affinity for natural/artificial burrows and tunnels, the brown rat appears to gravitate towards moisture. And in areas away from human development, the species is often found along the banks of river systems. This is presumably why they have taken so readily to municipal sewer/waste water systems in metropolitan areas.

Studies suggest that rats will roam in search of food if necessary but are true “homebodies” at heart. In fact, where a stable supply of food is close by, rats may never venture further than 20 meters from the nest.

Origin and Current Distribution

Brown rat distribution

Red shading represents current distribution of brown rat (Rattus norvegicus).

Believed to originate from the high plains of northern China, the brown rat is now common to every continent save Antarctica. Reportedly, this movement from its native lands was launched during the Middle Ages, where human migratory routes presumably facilitated their spread by providing both food and shelter. Yet, regardless what promoted their movement, by around 1750 it is clear that the brown rat was already a regular feature throughout much of industrial Europe and was beginning its “hostile take over” of North America.

Such conquests were not without interspecific drama, however. The black rat that was native to many of these areas may have resisted the advance initially, but likely quickly gave way to its much larger, more aggressive congener. This displacement was reportedly hastened also due to the brown rat’s comparatively greater adaptability to various foods and weather extremes, as well as its ability to burrow in/around human dwellings.

There is no consensus as far as how many brown rats presently occur worldwide; however, some estimates put the populations of brown rats in the UK alone at roughly 1.3 rats for each person alive!

Impacts and Threats

Displacement of Native Species

As far as threats to native species go, the brown rat is one of the most destructive invasive vertebrate species of all time, and may singularly responsible for causing or threatening the extinction of more plants and animal species worldwide than any other, next to mankind.

While the extent and nature of such impacts is well beyond the scope of this article, the brown rat threatens native flora and fauna in two general ways.

Island wilife is particularly vulnerable to impact from rats.

Brown rats wiped out the Atlantic puffin on Ailsa Craig Isle (UK).

The first and most obvious manner is by predating on plants/animals directly. Brown rats can and will make a meal of just about any plant and animal they can catch and overpower. Nesting birds, small snakes, lizards and other terrestrial organisms are particularly easy targets for the brown rat, as are their eggs and young. And the threat is not limited to land; their strong swimming/diving ability also makes them a scourge upon intertidal and other shallow water aquatic organisms as well. This extreme opportunism and voracity has decimated countless species worldwide, with particularly obvious impacts to more vulnerable island populations. From devouring rare aloe plants on an island off the coast of Madagascar, to feasting on the eggs of the endangered Hawksbill turtles on the shores of Brazil, the brown rat is a common theme among reports of species declines/extinctions everywhere.

Hawksbill nests are often plundered by brown rats.

Hawksbill nests are often plundered by brown rats.

In addition, the brown rat also contributes to biodiversity loss indirectly by eroding habitats. By foraging on seed material and new tree/shrub starts, the brown rat can prevent forests/vegetation communities from recruiting new plants to replace dead and dying members, thereby permanently transforming/degrading those environments. Forests can give way to grasslands, and rare and specialized shrub communities can cede to near monotypic stands of noxious weeds. Yet, however they are altered, the point is that these communities can be changed to the point where they are no longer suitable for the wide assemblage of plants/animals that had previously evolved to live in these communities and nowhere else. The extent of these impacts is inherently vastly harder to quantify but potentially much more devastating to biodiversity overall.

Agricultural Losses

The United States Fish and Wildlife Service estimates that rats are responsible for destroying roughly 20% of all agricultural products worldwide. This is likely due to consuming food material directly, as well rendering much more food unsanitary as a result of contamination of stores from rat feces.

Public Health Issues & Property Damage

Rats can get in the home between walls, under foundations and in roof spaces.The brown rat is a known vector for many diseases and pathogens ranging from viral hemorrhagic fever and toxoplasmosis to trichinosis and even bubonic plague (which can also be transmitted by a number of other mammals, such cats, dogs, squirrels and native wood rats). Ironically, it is believed that the brown rat’s displacement of the black rat – the rat species technically responsible for the “Black Death” in 14th century Europe – actually led to the decline of the plague there.

If eating and pooping in our food stores wasn’t enough, the brown rat can also cause property damage due to its proclivity for gnawing on everything; power cables, irrigation lines, and all manner of wooden/plastic objects are all fair game for the restless brown rat. Likewise, the species tendency to burrow around homes can lead to problems where rats infiltrate walls, roofs or other structures.

Controlling Rats With an Air Rifle

Given their incredible reproductive rates and largely nocturnal behavior, an air rifle is definitely not the first or most effective line of defense when dealing with a rat infestation, and it is most critical to ensure that all nearby sources of food (such as granaries, waste receptacles, animal feed, etc.) are removed or secured. However, for dealing with individual rats that have established a pattern of behavior that allows an open shot, a good pellet rifle can provide targeted eradication without the need for indiscriminate poisons that can harm native wildlife or pets.

Choosing a Pellet Rifle for Eradicating Rats

This wadcutter pellet makes a big statement when used at close range.Because rats are typically engaged at close range (within 10 yards typically), all of the hunting air rifles featured on this site, and some of the more powerful pellet guns, should do the job. However, due to the brown rat’s extreme toughness, we strongly encourage you to opt for a .22 over a .177 caliber model. It’s not that a .177 can’t kill rats – they can – but the .22 has greater stopping power (as we’ve discussed before) and can put them down more convincingly. For example, while it can also happen with a .22, it is very common for an adult rat to be shot clean through with a .177 caliber pellet  – and still manage to run back to the safety of a burrow! If you must use a .177 bore gun, do yourself a favor and use soft, hollow-point or wadcutter-type pellets to maximize impact.

Keep in mind, however, that regardless how much of a scourge rats may be to humans, they like any pest species deserves to be put down humanely. This means that if you don’t have a clear shot, or are too far away to reliably hit a rat in the head region for a one-shot kill, wait until a better opportunity presents itself.

How & Where to Hunt Rats

The biggest challenge when hunting rats with a pellet rifle is lighting. As nocturnal animals, they are difficult to target during most of their active period (unless you can afford night vision optics like this guy!). However, brown rats typically begin moving in earnest at dusk (which technically makes them “crepuscular”), when bright optics (or open sight shooting) can provide a good, albeit limited window of shooting opportunity.

brown rat out during the day

The promise of food will get rats out in daytime.

In many cases, rats may emerge during broad daylight too. As usual, this normally revolves around food; bird feeders, left over dog/cat food and other items will often get rats out in the open and used to moving about during daylight hours. I personally recall a rat that came out promptly from underneath a doghouse (where it constructed a tunnel) to steal a bit of my dog’s food, every day and roughly at the same time – in broad daylight. As you can guess, this only went on for so long.

Yet, before the sun begins to set, do a careful recon of the affected area beforehand. Get a sense of where rats are coming from and are likely to be engaged. Obviously, food sources/dump sites/feed lots etc. will be the main destination of the local rat population, so start from there. But don’t forget routes between these food sources and nesting areas, which can be in/around drainage ditches, creek beds/banks, walls, along power lines, fence lines, woodpiles, hedgerows, and virtually any other natural/man-made feature that provides cover.

Rats are creatures of habit, and will typically avoid new objects in their environment and use the same routes to go from point A to B. Intercepting rats along these routes can be highly effective. Likewise, look for signs of burrowing/droppings around things like sheds, doghouses, foundations, etc., as these provide ideal opportunities for rats to build tunnels or temporary refuges between food stores. In many cases, with a bit of patience, you may be able to get clear shots as they emerge from or move around these structures.

As usual, above all, remember to shoot safely. Be certain that your shot will not carry and result in harm to people or unintended pets/wildlife, and remember to check with local rules and ordinances before undertaking any eradication effort, since these local rules often dictate how/if a firearm of any kind may be fired within city limits.

Below is video showing some very good air rifle shooting. Note that only clear head shots are taken.

Please don’t watch if you are uncomfortable with this method of extermination.

 

Image Credits (top down):

Jeans-Jacques Boujot under CC BY-SA 2.0
Jeans-Jacques Boujot under CC BY-SA 2.0
Vergleich Hausratte Wanderratte under CC-BY-SA-3.0
Alexey Krasavin under CC BY-SA 2.0
Ingrid Taylar under CC BY 2.0
jans canon under CC BY 2.0
Jrockley [Public domain]
Richard Bartz under CC BY-SA 3.0
Aquaimages under CC-BY-SA-2.5
Peter Firminger under CC BY 2.0
Robert Pittman under CC BY-ND 2.0

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