Why do sewer rats grow so much larger than pet rats do?

Rats have long been seen as pests, associated with filth and disease. But sewer rats, which thrive in the world‘s sewers, have been observed to grow to unusual sizes. These giant rats, which can grow to be as large as cats, have been a source of both fascination and horror for many people. But why do sewer rats grow much larger than pet rats do? Understanding the differences between the two types of rats can help explain this phenomenon.

rat close-up isolated on white background
rat close-up isolated on white background

Primary differences between sewer rats and pet rats

One of the primary differences between sewer rats and pet rats is their diet. Pet rats are typically fed a diet of commercial pet rat food, which may also include fresh fruits and vegetables. On the other hand, sewer rats usually feed on discarded food, as well as anything else they can scavenge. This diet often includes proteins and fats that pet rats don’t have access to, which can lead to a faster growth rate. In addition, sewer rats are rarely given any medical care, further contributing to their larger size.
Another factor that can affect the size of rats is the amount of space they have to move around. Sewer rats often live in large colonies, while pet rats are usually kept in cages. This gives sewer rats more space to explore and scavenge for food, further contributing to their larger size. Additionally, the lack of regular cleaning in sewers can provide a more hospitable environment for rats to thrive in. All of these factors can lead to a larger size for sewer rats.

Rat’s ability to adapt quickly

Researchers have long been fascinated with the rat’s ability to adapt quickly and thrive in a variety of situations around the world. One phenomenon that continues to draw scientific interest is the great variation in size observed among wild rats from different parts of the world, as well as between wild rats and the pet rodents that are bred in captivity.


Wild Rats Adapt to Their Environments

Why do sewer rats grow so much larger than pet rats do?

The wild rat is found all over the world, but varies greatly in size depending on where it lives. For example, the heaviest rat on record—a male from New York City named Ralph—weighed 32 ounces (907 grams). Ralph was 21.8 inches (55 cm) long, including his tail.

A much smaller species of native rat lives on the island nation of Bougainville in Papua New Guinea and reaches only about 11 ounces (310 g) as an adult. In between those two extremes are rats who live elsewhere around the world, from Norway to Indonesia.

The heaviest rats tend to be found closer to the equator where it is warmer year-round, whereas those from northern regions average a bit less insize.

Allometric scaling

Because wild rats are found in so many different habitats it is no surprise that they vary widely in size—a phenomenon known as “allometric scaling.” Smaller species of mammals tend to have higher surface area to volume ratios, for example, which reflects an adaptation to the warmer climates where smaller species tend to live. More food can be stored internally and transported or hunted more easily by a small animal with a high surface area/volume ratio, compared to a larger one.


So what about pet rodents?

Many people assume that pet rodents should closely resemble their wild counterparts so as not to be at a disadvantage living in human-dominated environments such as homes and backyards. However, there is quite a bit of variation even among captive-bred pet rodents. Rats and mice can come in a variety of colors, often depending on the species. In addition, the average size is generally smaller than their wild counterparts. A typical adult rat or mouse from any given breed will weigh about twice as much as its wild counterpart, which reflects a decrease in surface area/volume ratio compared to their wild relatives.

Captive breeds

What’s even more striking is that captive breeds typically have a longer tail length relative to body size despite also being fatter overall. This suggests that for pet rodents there has been strong selection toward larger body sizes at the expense of tail length—a phenomenon rarely seen in nature. The increased fattiness may be due to both captive breeding as well as artificial light patterns in indoor cages, because fatness is associated with the amount of time an animal spends resting rather than moving around.


Pet Mice and Rats Have Shorter Tails

Both mice and rats are less active (and thus tend to store more food as fat) when they are kept in cages that include a running wheel—presumably because this allows them to maintain their fat reserves while still having opportunities for exercise. This also seems to be true for its wild counterparts; the only study to investigate this found that access to a running wheel reduced activity levels in wild rodents, which was probably beneficial from an energetics standpoint. If pet rodents have been selected toward greater fattiness due to captive breeding practices or artificial lighting patterns, then it would make sense that they also have shorter tails for their body size.

Allometric scaling can provide important insights into the way different species live and has been used to study everything from the effects of climate change on animals to how we might expect mammals of a given size to behave. We don’t know what proportion of variation in tail length or fatness is explained by the size and shape of an animal, which means there could be other factors at work. But so far it seems clear that many rodent species—both wild and domesticated—are adapted to their environments not just in terms of behavior but also morphology, such as body shape and tail length, specifically. What this tells us about humans is less clear. Maybe we’re just bigger versions of the same basic story. Maybe we’re different—but then, so are our pet rodents.

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