Rattus Biologicus:
The Domestication of the Rat

This is the first installment of a regular column that will deal with the biology of rats. It is the goal of the author to present scientific information in a clear, non-threatening and interesting format. In this way, rat owners will gain an appreciation and understanding of their pets' behaviors and biological functions in order to maintain the best possible care for them. Future topics include: reproductions, aggression, and circadian rhythms. If you have any questions or suggestions for future columns, Caryl welcomes email at branta1@aol.com.

The concept of domestication can be traced to the Latin words domus and domesticus, meaning home or house. A domestic animal is a tame creature that lives in or near a human domicile. Pets are domesticated animals that often occupy a special place in their owner's heart and home, and are treated with the utmost fondness and affection. Some (okay, most) humans would balk at the idea of having a RAT as a pet --that nasty, aggressive, disease-ridden, sewer-inhabiting beast! However, over the last century, the rat has transcended its reputation as a feral pest, due in most part to the selective breeding efforts of biological researchers.

Domestication of the rat is quite recent, and parallels that of human civilization. Pet rats are descended from the Norway rat, Rattus norvegicus. R. norvegicus was originally documented in the temperate regions of Asia. Its niche was soon established as that of "economic pest," allowing its population to increase greatly and spread over the world in close association with man. Eventually, the rat entered western Europe by way of the Norwegian peninsula in the first half of the eighteenth century. Hence, its scientific designation reflect sits route of arrival into the industrialized world. The Norway rat is noted for being extremely aggressive and fierce, and it ultimately replaced the less virile Black or House rat (Rattus rattus) in Europe.House rats are the second-most numerous wild rats in the world (behind the Norway), and are reproductively isolated from the Norway rat; that is, the two species cannot interbreed. Incidentally, it was the House rat that was responsible for spreading the bubonic plague in London in 1664-65; the Norway rat did not even reach England until around 1730.

Soon after the introduction of the Norway rat into Western Europe, albino mutants made their appearance in wild populations, which is a common occurrence in the case of wild mammals. Due to their attractive coats and distinctive appearance, these white rats were captured and tamed by humans. Thus, albinos were most likely the first domesticated rats to be kept as pets. In fact, throughout the eighteenth and nineteenth centuries in France and England, lots of rats were being caught by humans, due to the despicable yet popular sport of "rat baiting." Rat baiting involved placing a terrier dog in a pit with 100 or so rats. A keeper measured the time until the last rat was killed, and men would place bets on how long it took the terrier to kill all the rats. Hundreds of rats were captured and placed in pounds prior to these contests. Here, too, the albinos were removed and kept for show purposes and breeding. Rat baiting was finally ended by decree, but this sport flourished for nearly 70 years.

The wild Norway rat made its way into the United States by way of ship around 1775, replacing the already established House rat, as it had in Europe. It is speculated that tame albino rats of European origin were brought to America by the biologist Dr. H.H. Donaldson, and that these rats comprised the foundation of the Wistar Institute race of albinos. The Wistar Institute in Philadelphia had originally served as a museum of scientific artifacts during the 1800s, but under the direction of Dr. M.J. Greenman in 1907, it became the first independent research institute in the U.S. It was here that the initial investigations of nutrition, growth and domestication in the rat were undertaken.

Domestication is a form of evolution -- a shift in the gene allele frequency that creates a change in a species. Changes occur in both form (i.e., coloration, size) and behavior. The pet rat is similar in form to its wild ancestors, but its behavior is clearly different. Most notably, the trait of docility was selected for by biological researchers to ease handling of their experimental subjects. Comparative studies of the tame albino and the wild Norway rat were undertaken by Dr. Helen D. King of the Wistar Institute in 1908. It was her goal to re-enact the domestication of the rat in the lab and thus determine the changes that took place during the transformation of the "excitable and ... savage" wild Norway to the gentle and trusting rat we share our homes with today. Examination of 26 generations of captive-bred rats led to the conclusions that domesticated rats exhibit (1) increased body size, (2) decreased savageness, and (3) increased fertility. Furthermore, it was noted that domesticated rats lost the very distinctive "high-pitched squeak" that is characteristic of the frightened wild rat. These were the very traits that made the rat such an ideal experimental subject for biological researchers.

Selection has played the most important part in the process of domestication of rats. In their natural habitat, only the fiercest, wildest and strongest - the fittest for that type of environment -survive. Selection in an artificial environment (e.g., a laboratory) is different. In this environment, the fittest rats are those that are most gentle and fertile. Wild rats do not mate well in captivity, with very few pregnancies resulting when males and females are placed together. However, it just so happens that the most fertile rats incaptivity are those that are docile. Therefore, each succeeding generation of gentle rats will propagate its own characteristics, and in time, the entire breeding colony will exhibit the desired trait of docility. It is not clear how the traits for gentleness and fertility in captivity are linked, but they clearly have been propagated together.

So, the next time someone makes a gagging sound when you mention your pet rat, you can give them an informed response -- domesticated pet rats are far removed from their wild counterpart, just as the pet dog is removed from the wolf. You may also remind them that humans were once wild creatures, as well, and like the rat have adapted to a controlled and secure environment much different from that in which our ancestors survived! We are all products of both our genes AND our environment.