“How about a good sweat sample from the humans?” I offered. “As long as they don’t wear deodorant, we could have people do a workout and wipe their armpits with a gauze pad.”

There was reluctant agreement with this plan, but this immediately raised the issue of who the canine and human “donors” would be.

The pack versus not-pack question became one of familiarity. To Callie’s nose, all the scents in the house were familiar to her: me, Kat, Helen, Maddy, Lyra, and even Callie’s own scent. This was her dog-human pack. Melissa and I would be at the scanner already, and our scents would pervade the scanner, setting a backdrop against which other smells could be measured. Ideally, we needed scents from other people in our households to serve as the “familiar human.” It would have to be Kat’s sweat for Callie and Melissa’s husband’s for McKenzie.

We would also need a comparison for not-pack, or unfamiliar, scents. We would need scents from strange dogs and strange humans. We drew a chart on the lab wall and listed all the humans in the lab, along with their dogs, and cross-referenced them against whether they had met Callie or McKenzie. Andrew’s American Eskimo, Mochi, had never been to the lab. She quickly emerged as the leading contender for “strange dog.” Plus, she urinated whenever she got excited, and Andrew would have no problem getting a urine sample from her. Since Callie and McKenzie had met all the humans in the lab, we still needed some “strange humans.” Considerable discussion ensued about the logistics of getting fresh sweat on scan day, as well as the possibility that the dogs had already been exposed to the smells of spouses, girlfriends, and boyfriends inadvertently as scents carried on lab members or, as the cops say, “on their person.”

In the end, I convinced a neighbor to donate her sweat to be the “strange female” as a control for Kat’s sweat. Kat’s kickboxing coach agreed to donate his sweat to be the “strange male” control for Melissa’s husband.

Timing was critical. Everything depended on getting samples as fresh as possible. For the dogs, that meant morning pee, which, we reasoned, would be the most concentrated of the day. For the humans, they needed to get a good sweat going, which could be mopped up from their armpits. Each of the human donors had been instructed to not shower or wear deodorant for the twenty-four hours prior to sample collection. Everyone was provided with sterile gauze pads, gloves, and a specimen bag to place the sample in.

As usual, the scanner was booked for one p.m. We would need all the samples at the lab by noon so that Andrew, who had volunteered for pee-pad duty, could prepare them for the experiment. He would have to cut the pads into strips with sterile scissors and carefully attach each sample to a six-inch-long cotton swab. Each swab would be numerically coded. This way, neither Melissa nor I would know the identity of the samples and inadvertently cue the dogs during the experiment. Only Andrew would know the code.

That morning, Kat and I took Callie and Lyra for their walk. I trailed the dogs, looking like a crime scene investigator, wearing purple surgical gloves and toting specimen bags. Callie loved to pee on her walks. As soon as she caught the scent of what I presumed was another dog, she would squat and dribble out some urine. She had a peculiar way of doing it, though. Her bottom never quite made contact with the ground. Instead, she sort of hovered and continued walking, giving the appearance of a duck waddling. Callie never left pee spots. She left pee trails.

Her urination habits made it easy to collect a pee sample. As Callie tracked a scent, she intensified her sniffing of a location on our neighbor’s lawn. I knew she was about to pee and had my pee pad ready. As soon as she squatted, I thrust the pad beneath her lady parts and was rewarded with a warm yellow stain. Callie looked over her shoulder at me. Hey! What are you doing back there?

Lyra was more difficult. The deep fur, stained and matted around her butt, appeared less clean than Callie’s. Plus, Lyra peed more conventionally for a female dog: back straight, butt in contact with the ground. The best I could do was wipe her bottom right after she peed. It was enough.

Poor Andrew. We had to lock him in a closet while he cut up all the samples. We couldn’t have the dogs getting whiffs of all those great smells before the experiment. After an hour of cutting up urine and sweat pads, Andrew emerged.

“Are you all right?” I asked.

He waved me off. “I just need to get some air.”

By now, the excitement of parading the dogs across the quad to the hospital had worn off, and only lab members who actually had a job to do on the Dog Project accompanied us. I still got a thrill out of the walk, though.

Like a well-oiled machine, everyone took up their positions at the scanner. Melissa and McKenzie were there, of course, and they relaxed in the control room until it was their turn. Andrew set up a test-tube rack on a plastic worktable at the rear of the magnet. He inserted the cotton swabs, business end down, in each tube.

For each dog, Andrew had prepared seven swabs: the four combinations of strange and familiar humans and dogs, plus an intermediate category of “acquaintances.” Callie and McKenzie were acquaintances. They knew each other, but there was no reason to expect that they viewed each other as part of their pack. We would present their scents to each other for this category. In this fashion, we would have a continuum of familiarity from stranger to acquaintance to household member. Using lab members’ sweat, we created the corresponding human acquaintance category. Finally, for a baseline, we used the dogs’ own urine as a “self” category.

With Callie in the scanner, the shimming and localizer sequence took less than a minute. She knew the routine. For the functional scans, we modified the hot dog experiment. Instead of holding up a hand signal for ten or fifteen seconds, Andrew would hand me a swab, and I would hold it in front of Callie’s nose for a few seconds. She would continue holding still to allow enough time for the hemodynamic response to peak, and then I would reward her with a hot dog treat. It was the same as before except we would insert a smell during the middle of the repetition.

To get her used to a cotton swab being shoved in front of her face, Callie and I had practiced this at home. The first few times she backed away, but she soon realized nothing bad was going to happen and just sniffed.

With the functional scans running, she performed flawlessly. Each swab was presented eight times in random order. It took two functional runs lasting six minutes each, and then we were done. Another four hundred scans in the can.

McKenzie, on the other hand, was not having a good day. She did not like the smells or the swabs coming at her. From the control room, I could see that her brain images were moving. Even though we acquired nearly five hundred images, most were unusable. She would have to come back another day after more training with the swabs.

Like we did in the hot dog experiment, Andrew and I analyzed the smell data individually for the two dogs as well as combining their brains. Both analyses yielded surprising results. When we combined their brains, we were able to identify the parts of the brain that reacted to the different smells in both dogs. This showed the common regions of activation. In contrast, the individual analyses told us how the dogs reacted differently. We focused on two comparisons.