Introduction
New York City is a vast metropolitan city with a rich history, diverse culture, abundant wildlife, and a thriving population of rats. Although rats are everywhere in urban environments worldwide, New York City’s rats are notorious for their abundance and audacity. Researchers have documented significant differences in the behavior, genetics, and evolution of NYC rats in recent years compared to other rat populations.
My initial interest in this topic stemmed from the amusement I received from looking at pictures and videos online of the rats in New York. As I gathered resources and information, my interest quickly changed into an immeasurable curiosity about why the rats in NYC are so different. This research paper aims to investigate and inform those contributing to the strange evolution of the New York rat by living in these urbanized areas and to identify the factors contributing to these rats’ unique characteristics. Through multiple articles that combine fieldwork, molecular biology, and behavioral analysis, this study aims to shed new light and inform those on the ecology and evolution of urban New York City rats and provide insights into the oversight of this persistent phenomenon in one of the world’s iconic cities.
History, Biology, and Current Population Patterns of New York City Rats
In New York City, rats are widespread, primarily found in densely populated areas. In the city, rats are considered a cultural symbol, and their number is up to five times the number of people. A study conducted in 2014 showed an estimated two million rays in New York City, which is close to a quarter of the human population in the region.
The dominant population of rats is the brown rat, also called the Norway rat. The average body weight of an adult rat is 350 grams in males and 250 grams in females (Harpak et al.). The adults can squeeze through gaps and holes 25mm wide, jump a horizontal distance of up to 1.2 m, and vertically from a flat surface to .91m (Puckett et al.). Similarly, they may survive a fall from a height of approximately 12m and tread water for three days. Most rats thrive in sewers, streets, and unabandoned and abandoned buildings, subways, parks, restaurants, and shoe stores.
There is a vast history of rats in New York City, and their evolution has been a long one. Studies show that brown rats have haunted the parks, basements, and alleys in New York City since the 1700s. They are paradoxically believed to originate in Asia and spread through the Middle East to Europe and Africa. The rat populations from France or England went to New York between 1750 and 1770 (McKie).
Even after the United States began to trade with Africa and Asia, other rodent species could not gain a foothold. Brown rats are also believed to have arrived in America on boats, mainly transported in boxes of grains by German troops. Since then, Norway rats have spread far and wide across the globe.
These rats carry pathogens that have the potential to cause vomiting, diarrhea, and fever in human beings, mostly children (Katz). Additionally, the rats may carry bacteria, including Leptospira and Salmonella, which are spread when an individual comes into contact with the saliva, feces, or urine of the rats. In addition, they may carry viruses and fleas that are disease vectors, including bubonic plague and cerebrospinal fever.
Norway rats thrive in urban areas of New York City by navigating the human-centric environment and using human food and by-products. In general, rats are among the most damaging and troublesome rodents in the United States (Combs et al.). They consume food, damage infrastructure, and structures, and spread diseases and viruses to other animals and humans. They inhabit and thrive in various conditions and climates, including buildings, gardens, farms, and agricultural fields. Individuals do not see rats in daylight, but it is easy to detect their presence using indicators such as their droppings.
The common types of rats are roof (black) rats and Norway or brown rats in the United States. The Norway rats are sturdy burrowing rats that are large in body size compared to the black rats. Their warrens are found at lower levels of woodpiles, along foundations of buildings, and in damp environments. While they invade buildings, they usually stay in the basement and live throughout the forty-eight contiguous U.S. states.
On the other hand, roof rats have a smaller body size than Norway rats. Their tails are longer than the combination of their body and head. Additionally, they are agile climbers and thrive. They nest on the ground in trees, woody plants, and thick vegetation. Roof rats are seen in raised grounds in buildings, including cupboards, walls, roof spaces, and drop ceilings (Combs et al.). Another distinguishing feature between the two species of rats is that the black rat has a more limited geographical coverage than the brown rat, preferring warmer and ocean-influenced environments.
Generally, rats are active at night and have bad eyesight but good taste, hearing, touch, and smell senses (Strand). Their embryology shows that rats have the potential to learn and explore new things, remembering the locations of food, shelter, routes, and features of their environment. Thus, the rats can detect and avoid novel food products and strange objects by escaping traps and baits for multiple days (Combs et al.). Both rat species can access structures by chewing, diving, climbing, or jumping through sewage systems, and they can gain entry through broken pumps and toilets.
Furthermore, Norway rats have larger body sizes and are considered more dominant than black rats. When a roof and a brown rat live in the same building, the latter can occupy the ground level, with roof rats occupying the top floors. In addition, both species can share some food resources but cannot interbreed. The brown rat consumes various foods, including meals, fruits, cereal grains, and nuts. When looking for water and food, they can travel between 100 to 150 feet in diameter (Harpak et al.).
An adult female brown rat typically has between 4-6 liters per year and may successfully wean more than 20 offspring yearly. The natural populations of Norway rats show resistance to second-generation anticoagulant rodenticide (SGAR). A study by Arbel Harpak et al. showed that vitamin K epoxide reductase complex subunit 1 (VKORC1) plays an integral part in SGAR resistance (Harpak et al.). Laboratory rats’ genetics, for instance, of resisting, indicated low death rates in response to some bromadiolone and difenacoum.
Additionally, the rat’s resistance to the second-generation anticoagulant rodenticide bromadiolone exhibited high expression of a suite of cytochrome P450 genes (Harpak et al.). Being quite cautious and shy creatures, it is virtually impossible to determine the exact number of rats in New York City. Norway rats have significantly less genetic diversity than other rodents, potentially because of a population bottleneck. The brown rats have faced large expansive ranges, apparently because of their connection with agricultural and urban human culture.
Since approximately 9 million people live in New York City, there are about a 36million rats in the same area (Harpak et al.). After reaching Europe, Norway rats have become rapidly urban pests and have spread to America, Africa, and Australia as a result of European colonization. The introductions to coastal towns, accompanied by urbanization and industrial development, exerted a strong survival of the fittest on rat populations.
Urban settings witnessed radical change between the 19th and 20th centuries (Harpak et al.). Natural selection has influenced several traits in the expanding rat population. For instance, rat populations resist (FGAR) first-generation anticoagulant rodenticides, including Warfarin, which arelinked with nonsynonymous substitutions in the vitamin K epoxide reductase complex subunit one gene.
Factors Contributing to the Strange Evolution of NYC Rats
New York City is known for having the tallest buildings and theaters in the world. In addition, it is the leading populous metropolitan region in the United States and has the most extensive subway system. The city has ample food sources, which offer diversity and different varieties as compared with other cities around the world (Combs et al.).
While many people in the city eat a large amount of food daily, a significant portion ends up as waste. That waste ends up in city streets, trash cans, and dustbins, which is why NYC rats are getting larger and advancing fast. The rats obtain pretty much any food they want at any given time. New York is also leading in terms of the garbage produced in the U.S., meaning there is plenty of food for the rats.
Furthermore, the city’s vast sewer system and massive high-rises provide an excellent opportunity for the rats to find a home. This means the rats can stay together in colonies in large numbers, not just for one or two rats at a time (Combs et al. 89). The rats are well-fed and have abundant places to live, making them healthier and in better physical conditions than ever before. These conditions allow the rats to get bigger as they eat better and live longer.
To examine how rats are genetically adapted to living in New York City, Katz isolated genomes of twenty-nine Norway rats in the city and contrasted them to deoxyribonucleic acid samples from Norway rats in the countryside of northeastern China. The study exposed thousands of genes that indicated manifestations of genetic hitchhiking among the brown rats in New York City, some linked with diet, behavior, and movement. Those sweeps were present mutations that appeared after the rodents separated from their ancestry, accompanied by the migration of the rodents from Asia to Europe and then to the United States. Some of the genes were found to be linked with resistance to rodenticides. Other genes were found to be associated with psychiatric disorders in human beings.
Another exciting finding lies in gene changes linked with the metabolism of sugars and carbohydrates. Scavenging off the remains of their human resources, rats living in urban settings eat increasingly significant parts of refined sugars and fats (“Why Are NYC”). However, like human beings, the analysis noted that the unhealthy diet of the rats might make them susceptible to health issues.
Another reason contributing to these rats’ rapid advancement is that it is difficult to trap them because they are wary of new objects. In addition, the rats adapt quickly as humans change; they have complex social structures in that the females all give birth at one time and raise the offspring in the same nest. Similarly, the New York City rats have prodigious reproduction and the potential to survive on nearly anything.
The Differences Between Rats Based on the Environment
Urbanization can drive drastic evolutionary and ecological consequences for wildlife, including rats. The most frequently explored developmental response to urbanization is alterations in the intensity of genetic drift and gene flow (Zhang). Different studies have investigated the biological evolution in urban places, such as behavioral modification, adaptive life history changes, structural adaptations to urban infrastructure, and temperature tolerance to warming cities.
Recent research has revealed some associations between encroaching urban rat populations that spread worldwide with human beings and the impact of heterogeneous urban climates on gene migration. Rats are widely distributed in urban environments in the United States. New York City is the home of rats; they climb out of toilets, drag slices of food down the underground passages, bite babies in their cribs, and crawl on sleeping commuters.
In Manhattan, the rats are genetically similar to those in Western Europe, mainly France and Great Britain. They are believed to have come on ships when New York was a British colony in the 18th century (Zhang para. 3) because Norway rats’ deoxyribonucleic acid is similar to European rats. Moreover, Manhattan rats are grouped into two genetically distinguishable categories: the uptown and the downtown rats. Similarly, the two groups of rats do not mix since they only move a few blocks in their lifetimes, so it’s easy for them to stay in a specific portion of the city (Zhang para. 4). After a close analysis of the rat’s deoxyribonucleic acid, those who live in uptown are distinct from those who live in downtown.
Even though the rats live in separate areas of Manhattan, they still adapt to the same conditions and have similar diets. They are separated geographically by Midtown, but the scientific definition of a rat species requires its DNA to have changed enough that the two would not be able to reproduce (Yirka para. 5). Midtown is filled with buildings and not many residential buildings, translating to fewer habitats for rats. The midtown break is based on the population density and the layout of the sewer system. The rats in this area have high levels of inbreeding and reduced gene flow. Furthermore, rats found below 14th Street in Manhattan are distinct from those above 59th Street because of the generations of rats breeding relatively close to home (Zhang).
The average lifespan of a Norway rat found in New York is two years, implying that isolated breeding has gone on for hundreds of generations since the rats arrived in Manhattan. The reason for the isolated breeding is that there are far fewer human homes and, thus, less food garbage for the rats to feed on and fewer places to burrow in the Midtown of Manhattan.
Comparisons of European and Asian rats show that most immune-response genes are highly differentiated due to natural selection. The rats in New York City have longer noses, mainly interpreted as adaptations to the cold. Additionally, they have short tooth rows, attributed to adaptations to softer, high-quality diets (Wynne et al.).
Even though no current study has explored the gene expression signatures of selection in urban rodents, research on the wood mouse offers further hypotheses on the features contingent on selection in city rodents. Spermatogenesis, detoxifying exogenous substances, metabolism genes, and immune response are overrepresented among apparently selection areas. These results demonstrate that some genetic modifications in urban rats have arisen regarding high exposure to pollutants, disease pressure in densely populated urban environments, and shifts to novel diets.
The Asian rats are a dominantly adaptable species primarily found in artificial and natural habitats, including urban environments, agricultural lands, and residential and grasslands. Their tails are almost the same length as their bodies, and the fur on their back is brown and olive. The males are larger than their female counterparts (Puckett et al. 4742). Generally, the house rats in Asia are fast runners, are primarily active at night, and tend to live in polygynous groups. The dominant males have higher mating access and mate more regularly, while females are more aggressive and less mobile.
European rats are generally black with a colored ventral belly. They are found in large numbers in coastal regions because the species is spread through human seafaring (Puckett et al. 4742). Since they are agile climbers, these rats inhabit top places and are found in any area that supports a vegetarian diet. Rattus thrives in tropical regions but is primarily driven out of more temperate zones. However, it is successful in cold climates. These rats are believed to be introduced through human travel overseas to all continents, including India and Indo-Malayan nations.
Rats eat and contaminate animal feeds and foodstuffs, damaging packaging materials and containers where feed and food are stored. They also cause problems by gnawing on wooden structures and electrical wires and tearing up insulation in walls and ceilings for nesting. Rats may also undermine building slabs and foundations with their burrowing activities and gnaw on all kinds of materials, including lead and copper.
Since rats carry a host of deadly pathogens, they are a public health threat, causing severe illness in humans. Rat infestation is a widespread and serious issue in New York City. The economic, health, and social impacts of rat infestation may be significant (Richardson, para. 3).
Better management strategies should reduce their population. Some conditions contributing to the rat invasion include accumulated water, garbage, and trash not kept in rigidly covered rat-resistant waste material containers, which are all ordinary in New York. However, efforts to control rat populations in urban environments rarely result in the complete eradication of the rodents (Houser, para. 1). Effective strategies for rat control include foundations and walls, vermin-proof and sanitary practices, and population control.
Maintaining high standards of cleanliness around and in buildings helps decrease the availability of food and shelter for brown rats in New York City. Well-kept and neat woodsheds, garden tools, drains, and household items will reduce the area’s suitability for rats and ease their detection (Richardson). Trash, garbage, and garden leftovers should be collected regularly, and all garbage receptacles should have tight-fitting covers.
Another measure includes sealing openings and cracks in buildings, sewers, drain spouts, and water pipes. Trapping is one of the most effective and safest methods of controlling rats than poison baits. The baits should be placed behind objects and walls, in dark corners, and in areas where rat signs, including their droppings, have been seen (Richardson). Thus, comprehensive rat control should focus on sustainable and long-term goals and decrease populations to tolerable numbers using different tools, such as contraceptives, to reduce fertility.
Conclusion
Rats in New York City have witnessed a rapid advancement. Urbanization exposes rat populations to novel environments and selection pressures that might alter structural traits within a population. The urban rats are very different from their rural counterparts since finding shelter, food, and warmth in rural settings is challenging. The main factors contributing to the strange evolution of New York rats include ample sources of food, large amounts of garbage, and the city’s dense population, which offers them an ideal environment to thrive.
Natural selection is the main factor contributing to rats’ rapid evolution in urban settings, including New York City. Given that rats in urban environments are so closely linked with human urban dwellers, subsequent studies should address whether rats respond to the pressures of urban settings that humans face. The remarkable commonality between urban rats and human dwellers is their diet. Human meals contain increased proportions of highly refined fats and sugars that contribute to several public health issues. Some of these health problems apply to rats, too, and have devastating effects. This study found that phenotypic plasticity and genetic adaptation are primary drivers of success for rats in urban environments.
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