Before there were cities, there were squirrels. Before there were cars and trucks, there were squirrels. Before there were french fries and pizza crusts and half-eaten candy bars, there were squirrels.

Squirrels used to live in their world, now they live in ours. What does that mean for them?

That’s a question researchers around the world are trying to answer. Squirrel Week XI starts today with a look at how scientists are comparing the behavior and physiology of urban squirrels with that of their country cousins.

Our first stop is in North Carolina, where Rebecca Rimbach spent a good part of last year staring at outdoor trash cans on the campus of Duke University, where she is a postdoctoral associate in evolutionary anthropology.

“You see them eat pizza, fries or chicken wings,” Rimbach said. “We know that’s not healthy for humans. I wondered if we could see any effects on their health.”

Rimbach selected five trash bins to observe, carefully noting every time a squirrel entered one, how much time it spent inside and what it emerged with. She also observed squirrels in a forest environment close to Durham, N.C.

At both locations, she trapped squirrels and, working with veterinarians, anesthetized them and assessed their health. She noted their gender, measured their body mass index, checked for parasites and took blood samples.

Rimbach is still analyzing the data. She was surprised at the similarities between the two populations. Most of the parameters were roughly the same, though forest squirrels were harder to catch than urban squirrels. (She surmises they were unaccustomed to metal and thus were suspicious of the traps.)

“The only thing I see so far really sticking out is a difference in blood glucose,” she said. “Squirrels on campus have higher blood glucose levels.”

That’s not good in humans — diabetes is one possible outcome of hyperglycemia — and it may not be good in squirrels.

Similar research is being conducted at the University of Guelph in Ontario, Canada.

“We’re looking at how the human-modified landscape impacts the stress of gray squirrels, and also looking at how that stress might change different aspects of their physiology,” said Alannah Grant, a master’s degree student in the school’s Newman Lab.

Squirrels are trapped on the campus and at the Rare Research Reserve, a natural forest 15 miles away. Researchers take blood, hair and fecal samples, each of which holds information on levels of the stress-related hormone cortisol.

Grant said that, broadly speaking, urban squirrels react better to stressful situations than rural squirrels.

Her focus is now on exploring how stress affects a squirrel’s ability to form new neurons — a process called neurogenesis — in an area of the brain called the hippocampus. The hippocampus is closely linked to memory formation. Memory is important to squirrels because of what they do with a lot of their food: cache it, literally squirreling it away to eat later.

While we often think squirrels use their sense of smell to find buried acorns, Grant said something more complex is going on. She’s comparing rates of neurogenesis in squirrels from different environments and looking at whether a heightened ability to generate neurons is passed from mother to offspring.

We know that post-traumatic stress syndrome can interfere with memory formation in humans. Does that happen in squirrels? And does a human-built environment cause more stress in the animals?

Of course, plenty of people don’t care much for squirrels and may not see the point. In other words: Why do this?

“I think from a conservation perspective, it’s really quite important that we understand the wildlife we share our cities with and also understand what is making them so successful,” Grant said.

Or not successful. Said Grant: “As urbanization goes on, animals either leave, die out or adapt. Gray squirrels have adapted. From a scientific perspective, it’s really quite new. It might be a new example of evolution. We’d like to see what’s going on, what impact we’re having.”

Understanding that impact can help limit negative consequences for wild animals — which, despite the close association they’ve developed with humans, squirrels remain.

“People shouldn’t feed them,” said Duke’s Rimbach. “They are wildlife. They should be left alone.”

Tomorrow: Squirrel Week continues with the story of how a viral video of baby squirrels changed the life of an English wildlife photographer.

Twitter: @johnkelly

For previous columns, visit washingtonpost.com/john-kelly.