A Rat Is Not “Just a Rat”

By Julie Etra

OK, here is your primer on rats – perhaps more than you would ever like to know. There are a lot more rats than the rat with the deservedly “bad rat” rap, the Norway rat, Rattus norvegicus, hence the common name. It is also known as the brown rat, the sewer rat, the wharf rat, the Hanover rat, and the Norwegian rat. (White lab rats are a product of selective breeding of the Norway rat for the specific purpose of research.)

The other common rat in temperate climates (including deserts) is various species of the woodrat, or pack rat, Neotoma. There are well over a dozen species of this genus, considered a “good” rat. 

That Bad Rat

It is thought that the Norway rat originated in China and Mongolia; it now enjoys worldwide distribution, with the exception of the Arctic, Antarctic, and the province of Alberta, Canada (see article elsewhere in this issue). From northern Asia, they most likely migrated to southeast Asia, Siberia, and Japan. They did not appear in Europe until the 1500s. 

Norway rats are nocturnal and have almost hairless tails that are shorter than their bodies; they have short ears.  Females have 12 nipples and they reproduce year around, ovulating spontaneously. Females can be sexually active immediately after giving birth. Litters range from two to 22 offspring, with gestation between 21-24 days, and the young are weaned in about 28 days. In summary they reproduce like crazy. Adding to their adaptability and reproductive success, they are omnivorous and opportunistic eaters, consuming just about everything. Although they can swim, they cannot climb out of a bucket (see below).  They are smart and have an acute sense of smell, although poor vision. 

The Norway rat is almost exclusively associated with human beings; how it evolved from a wild animal to ubiquitous pest associated with humans has been studied through relatively recent (2016) genetic investigations. In addition to being the notorious vector of the bubonic plague in Europe, they urinate and defecate where they eat, contaminating food supplies. 

Perhaps even more problematic they eat insulation and chew through electrical cables. In fact, a few years ago, I met with several Mexican businessmen in Guadalajara who were working on a project with South Korean investors to grow habaneros for production of a concentrate to coat cables and prevent the sheath’s consumption. Apparently, Norway rats to do not like the super picante habanero. 

From Norway to Mexico

As with many introduced species, the Norway rat most likely arrived in eastern North America (US) aboard ships around 1775, if not earlier, as stowaways, with wharfs and associated supplies being preferred habitat. Norway rats arrived in Mexico with the Spaniards during the beginning of the conquest, most probably with Hernán Cortés when he landed in Veracruz in 1519, bringing supplies from Cuba, although Cortés was preceded by Francisco Hernández de Córdoba and Juan de Grijalva.  

As for whether Christopher Columbus brought rats to the Caribbean – he beached on both Hispaniola and Cuba on both the first and second of his four voyages – we do not know.  Documentation confirms that Columbus and his men went ashore in Bariay Bay on the northeast coast of Cuba when he landed on October 28, 1492.  There were no docks and it is unknown what, i.e., rats, may have accompanied them ashore. The first voyage was relatively small with the three ships (La Niña, La Pinta, and La Santa María) and only 90 sailors.  In contrast, the second voyage included 17 ships and approximately 1500 sailors, making land in what is now Puerto Rico; stowaways may have been more likely that time. We know that Cortés arrived with horses, as his expedition was one of conquest and colonization, which he and his men accomplished with the help of horses. Over the 15th and 16th centuries, the Spaniards brought pigs, chickens, and wheat, among other goods (this trade was known as the Columbian Exchange, since goods went in both directions). 

Urban Rats

In Mexico City, according to exterminators, there are currently between nine and 18 rats for each of the over 21 million people living in greater Mexico City. Even with the low estimate, that is a scary 189 million rats!  New York City has suffered for three centuries from sewer rat populations that occasionally explode – the New York Times recently reported a 38% increase in complaints, measured by 17,353 calls to the rat hotline in the first nine months of 2019. New York has tried rodent birth control, dry ice and even a ban on eating in the subway, with few noticeable results. The city is now trying a new “high tech” rat trap:  a bucket, vinegar, and drowning. The vinegar is supposed to prevent the rats from rotting too fast, and to keep the bucket of dead rodents from stinking too much, according to its maker, Rap Trap Inc. “ICK” is all I have to say, and until humans and their crumbs disappear, the Norway rats will continue to live with us.

But, Yes, There Are “Good” Rats

All rats provide some ecological benefit, such as scavenging dead animal and plant material, and can serve as important prey species for wildlife such as owls, hawks, foxes, bobcats, mountain lions, etc. when they share the same habitat. 

One “good rat” is the pack rat.  Pack rats are nest builders, and uses twigs and other scavenged material for construction. Although most people call the plural of the wood rat a “pack” that is technically called a “mischief.”  

Pack rat nests can be complex with several chambers and are called middens. They resemble a pile of debris. Middens can be very old, and scientists have dated some as old as 50,000 years; their study can provide a window into past site conditions.   Although diversity is greatest in the deserts of the US southwest and northern Mexico, they are also found in a wide range of habitats including rocky terrain and caves, but are unknown in the tropics. They are also opportunistic and known to nest in attics and garages, to which my husband and I can attest, as we discovered one living on our top shelf.  They have bushy tails, feed on twigs, shoots, fruits and nuts.  Babies are born naked and are raised in the middens. Females can have up to five litters per year with up to five babies per litter. Lifespan is usually short, 1-2 years. 

Want to know more? Check this link. 


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