What is an animal?

It’s a surprisingly difficult question, if you really think about it. Here’s Merriam-Webster’s definition:

any of a kingdom (Animalia) of living things including many-celled organisms and often many of the single-celled ones (as protozoans) that typically differ from plants in having cells without cellulose walls, in lacking chlorophyll and the capacity for photosynthesis, in requiring more complex food materials (as proteins), in being organized to a greater degree of complexity, and in having the capacity for spontaneous movement and rapid motor responses to stimulation

Right.

But any 4-year-old knows what an animal is, and she certainly doesn’t define it in terms of cells or proteins or degrees of complexity. So how, then, do humans develop this understanding of animal-ness?

According to a weird and fascinating new study, babies expect animals to not only exhibit certain behaviors (like intentional movements) and have particular physical features (like fur), but also to have guts. I mean that literally: Babies apparently find it odd to see an animal that’s hollow.

The study focused on 8-month-old babies, far too young to talk. Like a lot of research on pre-verbal infants, this one gauged what the babies were thinking by measuring how long they looked at an array of objects. When babies see something surprising, they tend to look at it longer than something they were expecting to see.

The researchers, led by Renée Baillargeon of the University of Illinois at Urbana-Champaign, presented the babies with many kinds of toys. Some were furry, some were smooth. Some of the toys moved by themselves, such as a can that bounced across the floor. Some were “agentive”, such as a can that quacked to start a “conversation” with one of the researchers.

After the babies had familiarized themselves with all of the toys, the researchers then picked up and rotated the objects, revealing that some of them were hollow. The babies looked significantly longer — as if to say, Umm, what?! — at toys that were self-propelled, agentive, and hollow than they did at toys that were self-propelled, agentive, and not hollow. They didn’t have this reaction to toys that were only self-propelled and hollow, or only agentive and hollow.

The researchers also wanted to test whether insides mattered with furry toys. Other studies had suggested that by 7 months old, infants start using fur as a clue that an unfamiliar object is an animal that might be expected to move on its own. The new study also measured the infants’ looking times after showing them a can covered in brown beaver fur compared with a box covered in tan paper. The babies looked longer at the can when it was furry, self-propelled, and hollow than when it was furry, stationary, and hollow. And they didn’t seem to care if the box, whether moving or stationary, was hollow.

The researchers interpret these findings to mean that infants expect animals, in particular, to have insides, but don’t have that expectation for other objects. They say this bolsters something called the “innards principle,” first proposed by Baillargeon’s co-author, Rochel Gelman, in 1990. The innards principle says that we are born with the notion that things that move by themselves must have something inside of them to facilitate that motion.

Lots of research on toddlers and children has lent support to the innards principle. One 1995 study found, for example, that children younger than 8 expect the insides of animals to be different than the insides of machines. Kids also seem to understand that animals need their insides to function: Kindergarteners know, for example, that a dog can’t bark if it loses its insides, and that animals need to eat and drink to keep their insides working properly. The new study, though, is the first to show that an understanding of so-called “vitalistic biology” or “folk biology” appears at a very young age.

View Images By Don Burkett, via Flickr

To me, the strangest thing about the innards principle is that infants seem to recognize that animals have insides without having any specific knowledge about what, exactly, is in there. As Frank Keil of Yale University pointed out in a commentary about the study published this week, if infants know anything about insides, it would be about non-animal ones: “The average infant has minimal firsthand experiences with road kill, surgical dissections, slaughterhouses, or other graphic displays of animal interiors,” Keil writes. “In fact, they would be expected to surely have far fewer of such experiences than for many simple devices and toys that they can be quite skilled at breaking open.”

So what, then, could explain the innards principle (assuming it’s true)? How would this odd and specific skill have evolved?

The researchers put forth several possible explanations, but the one I like best boils down to predators and prey. Both predators and prey are animals, and they have exerted a pretty powerful selective pressure over the course of human evolution: If we don’t recognize a predator, we die, and if we don’t recognize prey, we die.