Fat and happy in the city

Lyons J, Mastromonaco G, Edwards DB, Schulte-Hostedde AI. 2017. Fat and happy in the city: Eastern chipmunks in urban environments. Behav. Ecol. 0:1–8. https://doi.org/10.1093/beheco/arx109

As human population grows, we are building new cities and expanding on existing ones, making urban habitats a fascinating area of study.  Over time, more wildlife is expected to either live within the city environment or be heavily surrounded by it. In many cases city life can be detrimental to plants and animals. However, some animals seem to thrive in this environment and increase in numbers because of it. Understanding more about how city life either aids or hurts plants and animals is important in the effort to build cities in a way that minimizes species extinction.

Eastern chipmunks as a model for urban wildlife studies

An example of an animal abundant in cities is the eastern chipmunk (Tamia striatus). They live throughout most of North America’s forests and cities in the easternmost parts of the continent. Chipmunks are small rodents that often explore large areas of their habitat to search for food and store their findings in small caches. This food searching and storing behavior makes them an ideal species to study to ask questions about how urban and forest populations might differ because of habitat change. For example, if food is plentiful in urban habitats, chipmunks might not need to travel as much to find food, and they may attain better body condition (i.e. have more body mass) and experience less stress associated from traversing risky environments. These questions were the focus of a study done by a research team from Laurentian University in Ontario, Canada.

Eastern chipmunk eating food provided by humans (Pexels CC)

How might urbanization influence chipmunks?

The researchers compared the behavioral and body condition of urban and forest populations of chipmunks in Canada. They studied how behaviors differed by providing boxes mounted with cameras, which chipmunks could enter at will. Once a chipmunk entered the box, its behavior was recorded for three minutes. Afterwards, they took hair and fecal samples to measure the levels of the hormone cortisol. This hormone is a great indicator of stress: animals that experience more stress will have higher levels of cortisol. By taking hair and fecal samples, researchers could evaluate long and short-term accumulation of cortisol respectively.

To compare chipmunk behavior, researchers measured 8 different behaviors that chipmunks exhibited while inside the boxes. They summed the total time the chipmunks spent moving inside the box (locomotion), investigating holes in the box (head dip), scanning the box, grooming, climbing, observing while only standing on their back legs (rearing), standing inside a hollow tube before entering the box (latency), and stillness.

Urban chipmunks move less, are fatter and less stressed:

Data showed that behavior did not differ very much between the forest and urban populations, though urban chipmunks spent less time moving and grooming. Researchers point out that this could be an indicator of an urban personality. Other studies have found that animals that move less often explore smaller portions of their habitat. Maybe food is plentiful in the city, so chipmunks don’t need to move as much or explore large distances in this habitat.

Fig 1: Comparison of time spent engaging in behavior for chipmunks from urban (blue) and natural (red) habitats. Time spent allotted to behaviors did not differ significantly between habitats. However, there was a trend towards less locomotion and more latency in urban habitats (Lyons et al., 2017).

 

Researchers also found that chipmunks in the city were heavier than forest chipmunks. This could be because food is more plentiful and city chipmunks use less energy to find food. Interestingly the biggest difference in weight was between city females and forest females. Although males in the city were also heavier, this difference wasn’t as pronounced as in females. This sex difference could be because females have higher energy demands, particularly during lactation periods. Furthermore, by gaining more weight, female chipmunks in the city may also be more successful at rearing and producing more offspring.

 

Stress is a good indicator for the risk animals experience in their habitat. For example, if predators are plentiful you might expect that animals have higher levels of stress. This study found that urban chipmunks had less stress than forest populations. This finding was validated through cortisol (more cortisol means more stress) collected from chipmunks’ feces.

Cortisol levels were not different when sampling the hair. Cortisol deposits in the hair represent stress accumulated over longer time periods. Over longer time spans the level of stress may reach equal levelsbetween these two habitat types. Researchers also pointed out that it’s possible that over longer time spans chipmunks may move between forest and urban habitats. Further work is needed to tease out these differences in short and long-term stress levels .

Implications for future urban ecology work:

Overall, this study highlights how some wildlife can thrive in urban conditions. Particularly, fatter females could produce more offspring and be able to provide them with more nourishment during lactation. Lower short-term stress levels in the city could be due to less risk from predators and less foraging to find food in this environment.

More research should be done to find out if urban chipmunk populations can achieve higher numbers because of increase reproductive success and lessening of risk and stress. These types of comparative studies can help identify what features of the urban environment are beneficial or detrimental to plants and animals. This information can be used to inform our city planners to decide where to expand cities and how to expand them in a way that doesn’t harm plants and animals.

 

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Kevin Aviles Rodriguez

I am in the process of completing my PhD in Ecology and Evolutionary Biology at the University of Massachusetts in Boston. I am interested in human environmental changes as natural experiments to test hypothesis about the evolution of animals. Specifically, I study small lizards known as anoles and how living near human households impacts their ecology and behavior. I love fieldwork because often it takes me away from the cold and towards the sunny beachy islands that I love the most.

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