Ecology in the Concrete Jungle
The Effects of Urbanization on Mammal Body Size
Background
One of the most fascinating aspects of biology is the incredible variation in scale between organisms. Each organism’s respective size allows it to better survive in its specific niche. The towering giraffe uses its height to reach tree leaves and detect predators, while the microscopic bacteria in our intestines help us digest food.
Animal body size and mass are important ecological characteristics that can be studied to reveal insights into the natural world. Bergmann’s Rule is the tendency for organisms to be larger in colder climates compared to their equatorial counterparts.
The underlying mechanism for this observation revolves around heat. Larger animals have more surface area, and thus lose heat faster. Think of the difference in warmth between gloves and mittens: gloves have much more surface area and thus lose heat faster than mittens — which explains why the Eskimo people stick with mittens.
Wait … don’t elephants have large bodies and live in warm climates? To compensate for their massive sizes, elephants have drastically increased their surface areas with tons of wrinkles — no pun intended! This allows them to efficiently dissipate the massive heat their large bodies generate, and thus avoid overheating in the subtropical regions of Africa and Asia.
Because of how EZ it is to measure and the insights it can provide, body size has been a major focus of ecological study in the last half-century. A recent study — the focus of this article in fact — analyzed datasets of mammal species measurements to study the how climate and urbanization impact mammal body size. The emergence of cities provides ecologists with an opportunity to observe how animals respond to new environments in real time. Urbanization isn’t a variable that they can measure, so they used (human) population density.
Summary
The researchers chose to study mammals because they have diverse body sizes, life history strategies, ecological niches, and, most importantly, lots of data to work with that has been recorded over time. An abundance of data already available provides the “gas” for the study. You can’t really look for patterns if you don’t have any data. Luckily, the datasets that they compiled contained over 140,000 records of body measurements across more than 100 mammal species.
They focus on three main hypotheses for the potential effects of urbanization on mammalian body size:
1. Urban Heat Island Effects — Due to human activity and buildings, cities tend to be warmer than the surrounding wilderness which creates “urban heat islands,” which simulate the effects of warmer climates regardless of geographic location. Animals in the warmer urban heat islands are predicted to be smaller according to Bergmann’s Rule.
2. Habitat Fragmentation — Urbanization breaks up larger areas of natural land into smaller, more isolated patches, which changes the ecological pressures on an individual. Fragmentation is predicted to increase body size in smaller species and decrease it for larger species. Smaller species in an isolated environment, such as a sewer, are more protected from predation and thus have more competition within their species in this new, isolated environment. Foster’s rule predicts that animals in an isolated environment will develop larger body sizes. Larger species, on the other hand, must grow smaller to avoid human detection. For instance, a grizzly bear that was once the alpha of a large forest wouldn’t last long on the streets of Manhattan!
3. Resource Availability — Cities also have many more resources available to animals, such as food from trash and water from plumbing. This is predicted to increase body sizes in cities. You can make an analogy to how the modern food industry has allowed humans to grow larger over the past few centuries. Look at Dwayne “The Rock” Johnson’s diet: he doesn’t get that massive on 2000 calories.
Now to the results!
Each of the hypotheses mentioned above have logical effects on mammalian body size, but some may be stronger than others. First, let’s look at Bergmann’s Rule. One major finding of the paper was that — unsurprisingly for a seasoned biologist — body characteristics are complicated. Body mass isn’t just determined by temperature. They found that other variables, such as population density, had a much larger effect on mammal size. Cities actually ended up increasing the average body size of a mammal species, which shows that hypothesis #1 is less powerful than #3.
Even more surprisingly, they found that mammals in urbanized cold areas have larger bodies than their rural counterparts, which means that urbanization is a stronger force than Bergmann’s! Urban environments give animals opportunities for reliable food, water, and shelter access, which decrease energy costs and increase growth rates. They note, however, that these findings may not apply to all North American mammals equally. Only species that can adapt to the city landscape are able to take advantage of these opportunities.
The Habitat Fragmentation hypothesis (#2) held up for smaller mammals but not for larger ones. Reminder: the habitat fragmentation hypothesis predicted that smaller mammals would get larger, and larger mammals would get smaller. Both groups were longer overall and had lower body weights, but large mammals in cities changed proportionally more. Growing more slender allows animals to move more quickly and exploit the abundant resources better. To visualize, compare a F1 racing car to a VW Beetle! The adaptation to decrease in length for rural mammals may be an adaptive response to avoid detection.
In conclusion, this study reveals how temperature and urbanization are important factors contributing to mammalian body size. Just like most other things in biology, however, there are still more complex factors. Future ecological studies need to consider other human disturbances besides climate change. This paper has major implications for conservation management as climate change and urbanization continue to accelerate. Further collection of trait data for different species at the individual level is needed to allow future research into the large-scale patterns affecting wildlife in the urbanizing world.
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