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My tortoise is always glaring at me. Whether I’m feeding him carrots, stroking his shell, or casually watching him from afar, all I get in return is his eternal stink eye.

I’ve tried my best to please Linus since spotting him at a pet store chain that shall not be named. He sat in a stuffy, glass tank under fluorescent lights, and hid in his shell when customers peered in or tapped on the glass. His food dish was almost empty, with just a few hard pellets left of commercial tortoise food. The girl working there told me these unappetizing supplements gave tortoises all the nourishment they need (for the record, that is incorrect). I decided then and there Linus was coming home with me.

I built him an open wooden terrarium, replete with hiding spots, sunning rocks, and dishes overflowing with leafy greens. Of course, a life of captivity isn’t ideal for any tortoise, but my artificial oasis had to be better than a glass prison in a loud store.

I want to believe Linus is happy with me, but with that impassive, craggy face it’s impossible to tell. So I consulted the almighty Google: “How do I know if my tortoise is happy?” There were a lot of answers. One user said tortoises like being cuddled. I was skeptical, but sat Linus on my lap hoping we could enjoy a good book together. He promptly pooped on me. Another user said to let them roam free around the house. That experiment also ended in poop.

I couldn’t find a satisfactory explanation. There was plenty of advice on how to keep a tortoise healthy, but healthy isn’t the same as happy. So, how do we know if our pets—especially the prehistoric ones—are content?

It’s a question scientists have explored for centuries. In 1872, Charles Darwin published a whole book on the subject, asserting that humans and many animals express emotions in similar ways—if anything, emotions actually evolved before our species did. If this is true or not, philosopher Thomas Nagel said, we’ll never know. In his paper “What is it like to be a bat?” Nagel wrote that even if we spent our days “hanging upsidedown by one’s feet in an attic,” it’s impossible know how a bat feels—not even Batman has that kind of inner access.

But just because we may not know how an animal feels, doesn’t mean they don’t feel something. Dolphins, cows, chimpanzees, dogs, and even squirrels have similar emotional processing centers in the brain as humans, so at the very least, they would experience basic emotions like fear, anger, grief and joy. One group of scientists was so sure of this, they signed the Cambridge Declaration on Consciousness in 2012, acknowledging that all mammals and birds (and some invertebrates, like octopuses) are conscious creatures. We’ve all witnessed heartfelt and heart wrenching displays of animal emotions, like when Damini the elephant reportedly died of grief following the death of her pachyderm partner at the zoo. Or when Christian the lion joyfully embraced the men who reared him after spending nine months apart.

https://www.youtube.com/watch?v=btuxO-C2IzE

If you don’t cry watching this video, then you’re the one without emotions.

But some reject the concept of animal emotions altogether, like behaviorist B.F. Skinner. He argued even human feelings were a farce, and emotions in any species are “fictional causes to which we commonly attribute behavior.” John Watson, another well-known behaviorist, claimed these reactive mental states were simply physical responses to stimuli.

The naysayers aggravate Marc Bekoff, professor emeritus of ecology and evolutionary biology at the University of Colorado, who says animals are undeniably deep, emotional creatures—just like people.

“The real question is why these emotions have evolved, not if they’ve evolved,” he says. “One thing we have direct access to in both humans and non-humans is behavior. We can use it as a good measure of what animals are feeling.”

No two animals are exactly alike, which makes behavior extremely difficult to measure. And for pets we think we know better than ourselves, it’s easy to draw conclusions. A dog wagging its tail is happy. A cat purring is content. But that’s not always the case. Dogs have bitten people while wagging their tails. Cats can purr when they’re injured. Past studies found their vocals vibrate at a frequency shown to promote healing in bones and muscles—it’s the same frequency in what’s called vibration therapy, which was first used on astronauts in the Soviet space program to help build bone density and prevent atrophy of muscles. This adaptation could explain why cats suffer less from affiliations like osteoporosis, and seem to have nine lives.

Although there’s no barometer for behavior, the link between action and emotion likely isn’t limited to humans. In a study published earlier this year, scientists tested how dogs reacted when owners gave food to a realistic fake dog. Brain scans of the more aggressive canines showed an active amygdala, the same area of the brain that lights up when humans experience jealousy.

Other research studied sound as an indicator of positive emotions in animals. Mathilde Stomp, a Ph.D. student at the University of Rennes, found that horse snorts could serve a higher purpose than just clearing out nostrils. In a study that came out this week in the journal PLOS One, Stomp measured 500 snorts of horses in various living conditions. Stomp says horses living in a more natural environment snorted much more frequently than riding school horses confined to single stalls.

“We observed repeatedly that snorts were very frequent in places where horses were in good welfare conditions, but also when moving to new pleasant situations, such as going to a new rich pasture,” she says. “This could not be just due to air quality as there were few of them produced in some dusty stables.”

Stomp says these unspoken sounds are reflections of contentment, similar to when we humans sigh when we’re feeling fulfilled.

And it’s not just horses that blow out a bunch of hot nose air when they’re in high spirits—Stomp has seen rhinos and tapirs do it, too.

“In both cases, the production of the snorts in these species was associated to positive situations: while foraging for the rhinos, and during close social contact between a mother and her young for the tapirs,” she says.

We can’t say with surety snorts are satisfactory—in horses, it can also be a signal danger is approaching. So many of these behaviors can serve dual-purposes when it comes to expressing emotion. But there are some biologist Bekoff says can only be interpreted as joyful, like the greeting ceremonies he saw while studying Adélie penguins in Antarctica.

“These penguins don’t have expressive faces at all, but when they start vocalizing and kissing one another, you can feel the joy,” he says. “There’s no other conclusion than that they’re happy to see each other.”

Maybe it has to do with our own rogue emotions, but it seems we associate traditionally lovable animals—like our wobble walkers in the far south—as being more emotive than creatures that give us the creeps. Take New York City’s Public Enemy No. 1: the rat. Unlike a golden retriever puppy or a penguin reunion, you don’t often hear, “Look how happy those rats look!”

But scientists have shown rats exude just as much “happiness,” if not more, than other mammals. In the ’90s, neuroscientist Jaak Panksepp made a surprising discovery: rats loved being tickled. When rats play, they emit high frequency chirps undetectable by the human ear. Using special equipment to isolate the sounds, Panksepp discovered when he tickled the rats, their feverish chirps were akin to giggling.

https://www.youtube.com/watch?v=j-admRGFVNM

Who knows if they were actually giggling, but Panksepp argued it couldn’t be a noise of displeasure or fear; those are lower frequency sounds you may have heard yourself if you’ve witnessed the carnage of rats fighting over a chicken bone. And when Panksepp put his hand in the cage, the rats would run straight for it, eager for a tickle. When he pulled his hand out, the rats would hover on their hind legs searching for the omniscient tickle monster.

A dog, cat, or rat might not be able to tell us they’re happy, but in the right context, behaviors like a wag or a tickle indicate they probably are. But what about other species who don’t seem to express any emotion at all, like my stoic slowpoke Linus?

Matt Evans, a biologist at the Smithsonian’s National Zoo, says even with a trained eye, emotions in cold-blooded creatures are still a mystery. But cold blood doesn’t necessarily mean a cold heart.

“It’s very difficult to quantify emotions or feelings, or even try to gauge illness when it comes to reptiles,” he says. “It’s not that they don’t have those things, they just display them differently than mammals.”

Evans has seen crocodiles and even the fearsome komodo dragon exhibit what could be construed as playful behavior (this video of a komodo dragon playing tug of war is totally worth watching). Other reptiles—like the 550 pound Aldabra tortoise—appear to enjoy tactile sensations like having their neck or shell rubbed.

“They’ll push their neck out when you rub it, and hold it out almost like a dog would,” Evans says. “They also seem to perk up when you walk into the room—they’ll stand up to turn around and look at you.”

Even the oldest animals in the world enjoy a good neck rub.

I’ve noticed this with Linus. He’ll stretch his neck out and arc his head towards me when I talk to him. Sometimes he’ll even walk toward me if I’m near his enclosure. I would like to think it’s because he loves me so, but I think it’s simply because I’m the keeper of the carrots. And that’s the tricky part about animal emotions. It’s so easy to anthropomorphize, which is what it’s called when we assign human emotions or traits to other animals.

Examples of this are Denver the guilty dog (although, that poor dog looks incredibly remorseful), and the cockatoo who loves Elvis. Yes, it’s tempting to assume a dog is making a pouty face because they know they did a bad thing and feel badly for it, and it’s tempting to think a wiggly animal is getting genuine joy from its owner’s rendition of the King of rock n’ roll’s greatest hits. But some scientists argue animals can’t experience secondary emotions like guilt, and are just reacting to an owner’s body language and voice—maybe the cockatoo hates Elvis, but can’t help bopping along with his animated owner strumming away on the guitar. But how do we really know?

Happiness is still a bit elusive, but lab tests can prove the opposite. Like all good science, it involves poop. Janine Brown, head of the endocrinology lab at the Smithsonian Conservation Biology Institute, tests levels of cortisol—the “stress” hormone—in fecal samples to find out if animals are sick or anxious.

Brown says it’s especially helpful for our hard-to-get friends like Linus, or species where it would be disadvantageous to show weakness in the wild. These tests can also indicate if an animal’s enclosure isn’t up to snuff. Brown analyzed the poop of more than half the captive clouded leopards in North America, and found stress levels were much higher in enclosures lacking hiding places and tall trees. Zoos altered the enclosures to better fit the leopards’ natural habitat, and their stress levels dropped.

“It drove home the importance of building enclosures that met their needs based on natural history,” she says. “We were able to make a pretty dramatic change in how we managed leopards just based on a very simple poop study.”

Like scientists 100 years before her, Brown is also trying to crack the happiness code by studying animal hormones like oxytocin and prolactin. For now, Brown says the best way to learn about animal emotions is through behavior. And we may never know what it’s like to be in another animals hooves, paws, or claws—but maybe we were never meant to.

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