Guiry E, Buckley M (2018) Urban rats have less variable, higher protein diets. Proc R Soc B Biol Sci 285:. doi: 10.1098/rspb.2018.1441
(Header photo: Rat eating human food, by: Kapa65: https://www.needpix.com/photo/download/1101872/rat-color-rat-tame-rodent-animal-mammal-head-ears-fur)
Rats have shared our cities for a long time
Many recent studies have focused on trying to understand how certain animals are able to live in cities and what makes them so successful at spreading across the urbanizing globe. Even though rats have exploited human settlements for many years, relatively little is known about why there are so good at it. A prevalent theory in urban ecology is that animals that exploit human food become more successful because of the extra energy they obtain at relatively little cost. A recent study set out to ask whether the diet of 19th century (1790-1890) rural and urban Canadian rat populations differed (Figure 1).
You are what you eat: Stable isotopes show long term patterns of your diet
Biologists throughout history collected many plants and animals from museums across the world for scientific studies. However, few of these collections contain samples of rats. Because of this, researchers turned to archeological digs where they found the rat remains they needed. To study the diet of these ancient rats, scientists applied the technique of chemical fingerprinting via stable isotopes sampling.
Many chemical elements have stable isotopes, which are similar versions of the same element. As stable isotopes of various chemical elements are present everywhere a specific diet can leave a chemical signature in our bones. With this information scientists can track the sources and identify likely food sources. For studying diets, the ratio of carbon and nitrogen isotopes can help describe the plant sources and the degree of carnivory in animals.
The country rat versus the city rat
Scientists found that the diet of urban rats was less variable than that of rural rats (Figure 2). High nitrogen ratios indicated that in the 19th century city rats were consuming more meat products than rural rats. Because most rural rats had low nitrogen ratios (meaning that they’re consuming more plants and lower sources of protein), it’s likely that rural rats couldn’t access higher quality protein sources compared to their city counterparts. Interestingly, rural rats had a higher carbon ratio than urban rats, livestock, and herbivores. Differences in carbon ratios can help identify which type of plants animals are eating. Corn in particular has high carbon ratios. It’s possible that rural rats were eating corn imported to Canada in the 19th century as part of their diet.
Fat and prosperous in the city
Recently other studies have documented new urban colonizers incorporating human food into their diet. For example, Galapagos finches have developed a taste for rice, cookies and salty chips. This new study shows us that even in the 19th century when the abundance of human trash (i.e. food sources) was harder to find, animals like rats could have still been attracted to this kind of food. Access to such rich (and easy to grab) sources of energy can help in rearing more young, surviving periods of scarcity (like winters), and growing faster, which can help animals escape certain predators. What animals have you seen visiting human food sources near you?