Pavlov might have called that happy look on your dog's face a collection of conditioned reflexes, but now science is catching up with what animal lovers have always known.

According to Professor Nicholas Dodman, head of animal behavior at the Cummings School of Veterinary Medicine at Tufts School of Medicine and a regular on Animal Planet's Dogs 101 and Cats 101, until recently, scientists have generally underestimated the emotional range of animals. He says that today it is widely understood by scientists that mammals do experience primary emotions such as fear, sadness, anger, and happiness and even some secondary emotions like jealously and embarrassment-and they communicate them. Dodman says that dogs even have a sense of humor and laugh with a kind of huffing sound. He describes a study that examines how playing recordings of this laughing sound actually calms shelter dogs.

As for dogs' smiles, he points out the dogs in our slideshow, "Note that the lips horizontally retract into what's called a 'submissive smile' - a sign that a dog is non-threatening. It's an expression that disarms possible aggression, much like the human smile." Chimps, such as the group in our slideshow, exhibit what's called a "play face" - or an invitation for fun. Cats have naturally bowed mouths-like the cat in our slideshow, so Dodman says its tricky to pinpoint an actual smile, but they are emotionally sensitive, trainable, and affectionate. Among many other pets, Dodman has enjoyed sharing his home with rats, which he says are "very affectionate and intelligent." Dodman, points out that your pet might not understand the exact details of your hard day, but you probably sense it is empathetic enough to curl up and listen.

Marc Bekoff, Professor Emeritus of Ecology and Evolutionary Biology at the University of Colorado and author of The Animal Manifesto: Six Reasons for Increasing our Compassion Footprint, agrees. "People are often keener observers of animal behavior than they give themselves credit for," says the leading expert on animal emotions. Bekoff says that scientific research, for the most part, eventually confirms what animal lovers intuit and observe. Part of the lag is due to "studying animals in a box" as Dodman calls it. Dodman, who is giving a series of lectures on dog and cat behavior in November, explains that our advances in understanding the richness and depth of animal's lives is enhanced by researchers such as Jane Goodall who live with animals in their natural environments.

Bekoff points out that it makes biological and evolutionary sense for animals to experience a range of emotions and be able to show them, just as it does for humans. In a paper published by researchers from the University of Washington on rats, laughter, and joy, the authors describe how young rats vocalize when being tickled. The scientists explain that this laughter is bonding and "may have evolutionary relations to the joyfulness of human childhood laughter commonly accompanying social play." Bekoff says our emotions might not be exactly analogous to those of animals, but neither are all humans' emotions the same. "The way two siblings experience the death of a parent might not be exactly the same, but they are both experiencing grief."

Bekoff believes that our growing acceptance of animals as sentient beings based on scientific research needs to lead to legislation that provides significantly more protection of animals in labs, slaughterhouses, and entertainment. For example, a 2011 study on chimpanzees and mood disorders concluded that, "Chimpanzees display behavioral clusters similar to PTSD and depression [to humans] in their key diagnostic criteria, underscoring the importance of ethical considerations regarding the use of chimpanzees in experimentation and other captive settings." As for how this understanding affects humans, it has also, as he puts it, "increased humans' knowledge of our place in the world as mammals-unique mammals-but mammals, nonetheless."

Are the animals in our slideshow smiling? What do you think?