Wild Rats in Captivity and Domestic Rats in the Wild



What happens if you bring a wild animal into captivity? Because of its "hardy" constitution, is it actually more fit than its "degenerate" domestic cousins? What happens to wild animals that are kept for generations in captivity?

What about the reverse -- releasing a domestic animal in the wild? Is it helpless, a soft urbanite released into the harsh natural world? Does it revert to its wild roots?

These two situations are actually very similar. In both cases, an animal adapted to a particular environment is placed in a drastically different environment. The criteria for success are dramatically changed. In the wild environment, a successful rat is one who is reactive to changes, flees humans, finds rats of the opposite sex to breed with, finds food and shelter in a complex environment and avoids predators. In the domestic environment, the successful rat is one who is passive to changes, tolerates humans, breeds willingly with whatever members of the opposite sex are provided, tolerates confinement, bright lights, poor hiding places, a simple environment, and handling by human "predators."

Change their places, and many rats will have a lot of trouble making the switch.

Natural selection is severe in the early generations after the switch from the wild to captivity or from captivity to the wild. Natural selection acts on populations: individuals that are poorly adapted to their new environment die or fail to breed. Hence, mortality and reproductive failure are high in wild rats moved to captivity, and on domestic rats released in the wild. The survivors are those that happen to have the traits that are compatible with these new circumstances.

Here's an analogy. Consider a national sports team of your choice. The players are extremely good at what they do: playing the sport. Some are better than others, but all the members are very good. If they weren't, they'd be eliminated from the team. Now transport them to a new environment: put them all in a room and tell them that the only way out is for them to demonstrate fluency in four languages. All of a sudden, these sports stars find themselves poorly adapted to their new environment. A small number of them may happen to speak four languages, and they'll survive. The rest will fail.

The reverse situation is true too: place a group of linguists who speak four languages in a room and tell them the only way out is to demonstrate world-class mastery of a sport. A few of these linguists may happen to have superb sporting ability, but most will fail.

It is the same with wild animals in the domestic environment or domestic animals in the wild. The criteria for success have changed, and most of the animals, well-adapted to their former environment, find themselves poorly adapted to their new one. Many fail, some survive.

The survivors form the basis of the next generation of animals in the new environment. The second generation, and all subsequent generations, will pass through the same seive, and only those who survive and breed will pass on their traits to the next generations. After generations, the new population may look and act quite differently from the original population.

Natural selection isn't the only force acting on animals in new environments. Relaxed selection (removal of natural selection on certain traits) also plays a role in both environments, and artificial selection (preferential breeding of animals with certain traits by their human managers) plays a role in captivity.

Lastly, animals are not passive subjects of natural selection. They can act on and respond to their environment, and many of them may learn and adjust individually to their changed circumstances.

Wild rats in captivity

Wild animals, including rats, are more active and reactive than animals that have been domesticated for many generations.

Wild animals can be extremely stressed by the captive environment. Compared to the wild, the captive environment is extremely confined, provides few hiding places, is full of bright lights, and is surrounded by human "predators" who approach and handle the animal. These stressful conditions frequently lead to death or reproductive failure in captive wild animals.

Mortality of wild animals in captivity

Mortality of wild animals in captivity can be severe during those first few generations. For example, Blus (1971) established a breeding colony of short-tailed shrews in captivity and found that only 11% of his wild-caught shrews, and 9% of his captive-born shrews, survived for 12 months. The mean age at death was only 5 months (reviewed in Price 2002).

Reproductive failure in captivity

Reproductive failure includes the failure to mate, failure to produce normal-sized litters, and failure to rear young successfully.

Reproductive failure is common in wild and early generation animals in captivity. Only 49% of first-generation wild Norway rats copulate successfully in captivity (Price 1980). Of rats who do give birth, only 43% successfully raised some offspring to weaning age -- the rest were cannibalized or abandoned (Clark and Price 1981). Trut (1999) found that only 14% of field-trapped Norway rats produced offspring that survived to adulthood (see Price 2002).

Litters are generally smaller (averaging 6 offspring) in these first-generation wild rats (Clark and Price 1981). In contrast, wild rats in the wild, and domestic rats in captivity, produced similarly-sized large litters, averaging about 10 offspring (Davis 1951, Boice 1972). It takes about 20 generations in captivity for rat litter sizes to come back to normal (King 1929, 1939). Of course, other changes are happening (e.g. reduction in brain size, Rohrs 1999, Kruska 1975a and b) that are not found in the wild stock, but I won't go into those here.

Domestication: adapting to captivity

As a general rule, when a wild species is brought into captivity it has an enormous adjustment to make to become successful in the captive environment. This adjustment means the death or reproductive failure of many, many individuals who just can't make the switch to captivity.

Sometimes, the animals who die or fail to breed are the very individuals who would be most successful in the wild: the flighty, active individuals with low tolerance for humans. It is the passive, less reactive, less fearful, calm individuals who survive and breed best in captivity. These individuals form the foundation of the domestic stock and their offspring carry on those traits (see Price 2002 for more on this).

This process is called natural selection in captivity, and it is a huge watershed during those first few generations, with large numbers of animals dying or failing to breed, regardless of the actions and wishes of the human breeder. The resulting population of animals, descended from the survivors, may be quite different from the original wild stock, as these animals become adapted to their new, captive environment. (Natural selection in captivity is a problem for programs that breed endangered animals for release in the wild, but that's another topic).

Natural selection in captivity is combined with artificial selection, in which the human preferentially breeds some animals over others in order to produce more animals with a desired trait in the next generation.


Tamability is an important factor in domestication. Animals that can tolerate the presence of and handling by humans are more likely to survive and breed, due to both natural and artificial selection. Natural selection plays a role because animals who cannot tolerate human presence and handling may die or fail to breed. Artificial selection also plays a role because humans prefer to work with animals that are easy to capture and handle, and preferentially breed animals with these traits.

Genetic and experiential factors influence tameness: individuals inherit a greater or lesser capacity to be tamed (tamability), and the experiences each animal has with humans determines the extent to which that potential tameness is reached (Price 2002).

Tamability has a heritable component that responds well to artificial selection. In rats, some coat colors are associated with tameness: non-agouti (black) rats and hooded rats of wild stock are more docile than their solid-colored agouti counterparts (more on coat color and how it affects behavior) (Keeler 1942, Cottle and Price 1987). Today, about 80% of domestic laboratory rat strains are homozygous for the non-agouti allele (Price 2002). This association between tameness and coat color has been found in other species as well (e.g. deermice, Hayssen 1997; foxes, Trut 1999).

Early handling by humans has a big effect on tameness as well. Galef (1970) reared second and third generation wild rats (the direct descendants of wild rats captured on Philadelphia wharfes) under several conditions: with their wild mothers or with domestic mothers, with wild or domestic litter mates, with minimal or maximal exposure to humans, and with or without regular handling by humans (2 minutes per day from age 10 days to 23 days). Only direct handling experience was associated with ease of capture and handling. However, handling did not affect the wild rats' timidity toward novel objects or aggression toward other rats or mice. Handled wild rats were just as timid as unhandled wild rats. Therefore, handling is quite context specific: handling reduces aggression toward humans, but has little effect on other behaviors typical of wild rats.

In a similar experiment, Hughes (1975) raised wild rats under three conditions: (1) he let domestic mother rats rear one group of wild babies, (2) he provided an enriched environment for a group of post-weaning babies, and (3) he regularly handled a third group of wild babies before they were weaned. He found that early handling had a far greater effect on tameness than the other two conditions. Handled rats showed less emotionality and were more like domestic rats in their behavior. In contrast, being reared by a domestic mother or playing in an enriched environment had only minimal effects on tameness.

Evidence suggests that such early handling leads to major changes in the neuroendorcine system (Denenberg et al. 1967). One possible mechanism involves the responsivity of the adrenal glands. Levine et al. (1967) handled 1-20 day old rats each day by picking them up, placing them individually in a can partly filled with shavings for three minutes, then replacing them in their home cage. At age 80 days, the adrenal glands of the handled rats were less responsive to stress when placed in an open arena for 3 minutes (see also Levine 1968).

The biochemistry of taming: The brain biochemistry underlying tameness involves the serotonergic system. Rats selected for tameness had more of the neurotransmitter serotonin and serotonin receptors (Naumenko et al. 1989, Popova et al. 1991, Hammer et al. 1992). Serotonin is involved in inhibiting fear-induced aggression. Serotonin is also implicated with gonadal hormone regulation and stress responses. Interestingly, tameness can be induced pharmacologically with the injection of serotonergic agonists (Blanchard et al. 1988).

Domestic rats in the wild

Not surprisingly, there is less information on domestic animals adapting to the wild (feralization) than on wild animals adapting to captivity (domestication). We humans aren't often around to observe feralization, but we are definitely around for domestication.

Domestic animals released in the wild may have a number of handicaps compared to their wild counterparts. They may lack some of the structural, physiological and behavioral responses to environmental stimuli that are normally acquired by their free-living counterparts, usually early in life. These deficits may lead to increased mortality of domestic rats released in the wild (see Price 2002 for more).

Physical condition: Living in the wild may require greater physical condition than in captivity. The large size of some captive animals may be due to better nutrition and lack of exercise. Such large size may be a handicap in the wild if it impairs mobility and agility (Price 2002).

Structural attributes: Many domestic rats are white or have white patches on their fur. These patches may be conspicuous to predators, leading to higher predation on animals with white fur. In addition, albino animals have poor vision, which may impair their chances of survival even more.

Behavioral responses: Released animals must find food and shelter, develop anti-predator skills, interact appropriately with conspecifics if they encounter them, and orient (disperse, navigate etc.) in a complex environment (Price 2002). Deficits in behavior may result in slower growth and higher mortality.

Success of domestic rats in the wild

Domestic rats are therefore at a disadvantage as compared to their free-living counterparts. Reports of domestic rats establishing feral populations in the wild are rare. Donaldson (1916) made four attempts to feralize albino rats but was unsuccessful. King and Donaldson (1929) tried five times to establish populations of feral albino rats, at sites ranging from Massachusetts to an island group in the Gulf of Mexico. All attempts failed.

However, in some cases, domestic rats may survive and establish populations under natural or semi-natural conditions.

Domestic rats in semi-natural conditions

Albino rats in an outdoor pen in Missouri, USA: Boice (1977) released ten domestic rats (5 males, 5 females) in a large, fully-enclosed outdoor pen and studied them for two years. He found that the rats constructed and lived in burrows that were indistinguishable from wild rat burrows. They followed regular pathways above ground like those seen in wild rats. The rats were hardy throughout climactic extremes, surviving cold temperatures as low as -30º C. The rats reproduced successfully, producing litters mainly in the spring and fall. They established a stable population that stayed around 50 rats, for a total of five generations.

Boice's experiment demonstrates that domestic rats can survive harsh temperatures, construct shelter for themselves and breed successfully under such conditions. They have not lost these abilities even after hundreds of generations in captivity. Note, however, that Boice's experiment was not a true release into the wild: these rats were given food and water daily and were protected from predators.

Albino and hooded rats in a farmyard in Oxfordshire, UK: In another report, Manuel Berdoy released 75 albino and hooded laboratory rats into an enclosure in a farmyard in Oxfordshire. The rats found water, food, and shelter. They established paths through their environment and dug burrows. They built social hierarchies and bred successfully. Due to the enclosure, they were not confronted with natural predators or with wild rats (documentary film Berdoy 2003, reported in Peplow 2004).

Beige dumbo rats under a chicken coop in New Orleans, US: In the spring of 2004, a litter of six 5-6 week old beige dumbo rex and standard-haired rats escaped from their cage, which was kept outside on a porch in an urban neighborhood. The rats discovered a chicken coop in the backyard and began living in the space underneath the coop's floor, with a few deep tunnels. Food was provided to the chickens every day so the rats had an easy food source. Attempts to catch them were unsuccessful. As they did not harm the chickens they were allowed to stay. When the floorboards to the coop were removed, the rats moved to tunnels deeper underground. At least one litter was produced under the coop, which was discovered when the coop floor was taken up. The new litter born under the coop was brought back into captivity, but the original escapees were not captured. No other litters were known.

The rats had been extensively handled before their escape, and were quite tame. After their escape, however, they became shy, though not as shy as a wild rat. Most of the rats rarely emerged from the coop, though a single bolder one made forays back onto the porch. The original boldest rat, a female, probably died from an infected tail injury that occurred during a failed capture attempt. She was succeeded by a male. This male was quite unafraid and allowed a hand to get almost, but not quite, within grabbing distance before fleeing. The entire human family, including the dog, could stand on the porch and watch him and he remained unperturbed, as long as nobody moved quickly. If an attempt was made to catch him, he fled, but was usually back the next night [L. J. pers. comm. 2005].

Note: Two additional adult rats that found their way out of the cage eventually returned to captivity. One of these was the escapees' mother, who had never been particularly tame. After a month outdoors, she started coming to the porch again, and one day walked up to one of the humans and sat on his shoe to receive a treat. She also came through the open back door and into the house several times. She was taken back into captivity and has since become quite tame. The other adult escapee was an adult agouti female who was not very tame. She escaped but refused to leave the vicinity of the porch and after a day and a half managed to find her own way back into her cage during cage cleaning [L. J. pers. comm. 2005].


Domestic rats in the wild

 Albino rat colony in Montana, USA: Minckler and Pease (1938) mention a colony of albino rats living in a landfill in Montana, which numbered about 2,000 rats in 1937. The exact source of these rats was unknown, but they were presumed to have been released by students from the local university. Abundant food, water, refuge and few predators created a sheltered environment in which an albino colony could survive, even in harsh winters with temperatures as low as -25 F.

Interestingly, these albino rats traveled on paths through the refuse heap, and they never left these paths, even to try a fragrant food source just a few inches to one side of the path. Apparently, these rats had to come directly on the food, almost touching it, before they responded to it.

Diseases and parasites: several of Minckler and Pease's rats had liver cysts, and a few had "crusted eyes" and showed symptoms of the common "mouse pneumonia" [by this, I assume the authors mean the eyes were crusted with porphyrin and the rats had symptoms of mycoplasma pulmonis -- A.]. Many rats had hair loss, possibly due to parasites or a dietary deficiency. A large number, especially among the younger rats, were affected with rickets.

Albino and hooded rats on Lanai, Hawaii, USA: Svihla (1936) reported albino and "spotted" (white belly and sides -- probably hooded) Norway rats living in fields under natural conditions on the island of Lanai, which is an island southeast of Hawaii. This island environment is sheltered: food is abundant, there is no competition for habitat, and there are few predators (no mongooses, few feral cats and native owls). The rats were presumed to be the descendants of escaped pet rats belonging to Filipino plantation hands. The escaped rats interbred and became common in the pineapple fields, houses, and buildings of Lanai City.

Interactions between wild and domestic rats

Very few studies place domestic and wild rats together, so there is little information on how wild and domestic rats might get along.

Note, however, that resident rats usually attack intruders in their colony -- domestic rats attack domestic intruders, and wild rats attack wild intruders. Studies of rat aggression against intruders are very common. To read more about them, visit the aggression page. Therefore, a reasonable prediction about wild and domestic rat interactions is that resident wild rats would attack a domestic rat intruder, and domestic rats would attack a wild rat intruder.

How are domestic rats received in wild colonies?

To my knowledge, no studies have examined how wild rat colonies receive a domestic intruder in the wild.

However, one study has examined how colonies of wild rats in captivity received a domestic intruder. The wild rats attacked the domestic intruder intensely. Most attacks were performed by the dominant male, who lunged and leaped at, sidled, chased, and bit the intruder. The intruder fled, froze, and spent time on his back. The wild dominant rat inflicted about 10 bites on the intruder in 10 minutes, and most of the bites were to the intruder's back. For comparison, wild intruders introducted to wild colonies in captivity receive about 6.9 bites in 10 minutes (Takahashi and Blanchard 1982).

How do domestic rats receive wild rats?

When a wild Norway rat was placed in a colony of domestic rats in captivity, the level of dominant male rat attack on the intruder was low. This may have been because the wild intruder displayed intense defensive behavior, and the wild intruder was also much faster than the domestic rats. Therefore, the dominant male chased him but rarely managed to catch him. In addition to chasing, the domestic rat sidled, and when possible, bit the wild intruder. The intruder displayed defensive behavior: he fled, froze, boxed, and spent time on his back. The dominant rats inflicted about 1 bite on the wild intruders in 10 minutes, and most of the bites were to the intruder's back. For comparison, domestic intruders introduced to domestic colonies receive about 5.6 bites in 10 minutes (Takahashi and Blanchard 1982).


Wild-domestic rat interactions are similar to wild-wild and domestic-domestic interactions. Regardless of whether the rats are wild or domestic, resident rats, especially the dominant males, tend to attack intruders. The two asymmetries uncovered between wild and domestic rat are that wild rats display a lunging or leaping attack rarely seen in domestic males, and wild rats are faster than domestic rats. This speed difference means that wild rats may have an advantage in wild-domestic interactions: when the wild rat is the resident, he can press a more effective attack, and when the wild rat is the intruder, he can elude attack more easily.

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