We usually think of smell as one of the five senses and a generally passive process. Odorants drift into our noses; receptors detect them and send signals to our brains. However, more so than vision or hearing, smell is an active process involving many groups of muscles. Animals can control the rate at which odors enter the nose by the way they sniff. Sniffing involves muscle movements in the face and the nose. It requires movements of the diaphragm to control the rate of air movement. And there is likely some control over the fine hairs inside the nose. This means that for smell in particular, we would also expect to see the involvement of parts of the brain that control movement.
If the scent of a dog activated the brain in the same pattern as the scent of a human, then that would tell us that dogs lumped us in the same category as them. If, on the other hand, dog and human scents caused different patterns of activation, then we would know that dogs have different categories for us and them.
Like the hot dog experiment, the dogs wouldn’t have to do anything except hold their heads still, and they were already pros at that. We would hold up cotton swabs in front of the dogs and let the scent drift into their noses. Later, we could analyze the fMRI data to see which parts of their brains reacted to different scents.
This experiment presented certain logistical complications. Where would we get the scents and how would we get them? These questions became the topic of heated debate in the lab, especially as it became apparent that everyone would have to give something for the cause.
“So let me get this straight,” Andrew said. “We are going to present scents from dogs and humans to Callie and McKenzie.”
“That’s right,” I said.
“What kind of scents?”
“Well,” I said, “we all know what dogs do when they greet each other.”
Andrew didn’t like where this was going. “You’re suggesting a butt wiping?”
“I don’t think there is any other way.”
Lisa chimed in and offered an alternative: “Dogs sweat from their paws. You could get scents from there.”
“But we would also pick up all sorts of smells from where the dogs walked,” I said. “Besides, dogs go right to the butt. As far as they’re concerned, that is where the good stuff is.”
“Are we talking about a butt wiping or something more substantial?” asked Andrew.
It was a good question. A swabbing of the perianal area would probably do the trick, but there was good evidence that urine would be a more powerful signal. Dogs can differentiate their own urine marks from those of other dogs, suggesting that dog urine contains unique pheromones that are the equivalent of doggie fingerprints.
“I think we need urine,” I said.
“What about the humans?” Andrew asked.
“If you’re doing it for the dogs,” Lisa said, “I think you should do it for the humans.”
Andrew and I looked at her aghast.
“What?” she said. “The first thing Sheriff does is stick his nose in someone’s crotch.”
Although Lisa had a point, there were some boundaries that we couldn’t cross. Besides, what she was suggesting could be construed as biohazardous waste by the university lawyers.
“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.