Hungry wild polar bear prefers playing with a chained sled dog to eating it.

It sounds like a paradox.

How could play - defined as "apparently purposeless activity that's fun to do and pleasurable" - be vital for grim survival in such an often random and dangerous world?

- And not just play in childhood, but throughout life.

- And throughout life not only in humans but in all sorts of animals, including hungry polar bears, chained sled dogs, rats, cats, otters, migratory birds, and, just maybe - yes - ants.

All sizes. One research team, led by ethologist Robert Fagen, now professor emeritus at the University of Alaska, spent 15 years sitting in trees in Alaska and Western Canada to observe and document how bears play in the wild. They found that bears who played more often and more successfully throughout adulthood had longer and healthier lives, and thus left more offspring behind.

As for insects at play - scientists including famed Harvard myrmecologist Edward O. Wilson and Gordon Burghardt, author of "The Genesis of Animal Play," have described activities including the mutual grappling of and tussling with mandibles - a sort of rough and tumble play in off hours at the anthill - that looks, in any case, like playful practice at the very least.

What might be the connection between play and survival?

"Play is one of the brain's best forms of exercise," says Stuart Brown, author of "Play - How It Shapes the Brain, Opens the Imagination and Invigorates the Soul."

The exploratory and risk-taking nature of play - including the healthy "rough and tumble play" that can sometimes frighten protective parents - opens the brain to new ideas, Brown told ABC News.

Play gives a brain the experience and thus the courage to search outside the box, he says, - to try out new ways of doing things in an unpredictable world that constantly keeps presenting new kinds of menacing problems and obstacles to survival.

"Play keeps minds and brains flexible," says Brown, one of the founders of modern play behavior studies. He started his career as an MD and practicing psychiatrist, and is now the director of the National Institute for Play in Carmel Valley, California.

A growing number of scientists and other professional researchers are amassing evidence that, in all kinds of creatures, an innate impulse and ability to play - goof off a little, be curious, or just have what looks to even the most jaundiced human gatherer of data like having some aimless and enjoyable fun - has been favored by evolution down through the eons.

It seems to be an attribute that somehow helps animals survive longer, and thus be more likely to pass on their fun-loving genes.

And it's not only true of play among the members of the same species.

Invitations to play - play signals - work also between some species,  and not only between humans and domestic dogs with eager eyes and wagging tails.

Take the remarkable case of the hungry polar bear and the vulnerable chained sled dog shown at the beginning of the Natures' Edge video below.

As Brown explains, the bear's winter-long play deficit seemed to trump his wild hunger.

According to those on the scene, the polar bear even hung around and played with the chained sled dogs for a week, before finally ambling off toward his seal-hunting grounds to break his long winter's fast.

The fact that non-verbal invitations to play are clearly understood between some species is an enticing clue for philosophers who are trying to describe animal intelligence and the nature of consciousness itself.

There are now countless scientific studies documenting how different kinds of "tame" or (in the wild) "habituated" animals, including even such species as the supposedly vicious and reclusive North American wolverine, can develop rambunctiously playful relationships with humans.

Such back-and-forth play between humans and all manner of pets usually include joyful mutual activities such as light wrestling and hide-and-seek - that are "apparently purposeless but fun and pleasurable" and that are triggered by a variety of non-verbal "play signals."

It suggests to some thinkers that different species, including humans, may share a common mental experience of self-awareness and awareness of others that is both highly intelligent (whatever that may mean)  and fully conscious (whatever that may mean.)

And if you're a pet owner, it may, of course, be something you have felt in your own guts when responding to a play-invite from, say, a super-energetic ferret, or a bouncing mutt who's trying to get you to play a game of chase or tag that the two of you will probably make up as you go along in that creativity of exploration that is, says Brown, one of the essential hallmarks of play.

'Play Deficit' May Also Offer Insight Into Murderers and Even Terrorists

The new study of play behavior may even, say researchers, help explain why some people become murderers and terrorists, and others don't.

They report that the opposite of play - an unnatural lack of play activity, or even worse, the constant suppression of play by a parent or adult group or political leader - may increase the likelihood of violent behavior.

Having a severe "play deficit" at any age is associated in a number of studies with a mild but chronic depression, and in some cases even with mass murder.

As Brown recounts in the video below, Charles Whitman, who suddenly seemed to snap and then killed his wife and mother before climbing the tower at the University of Texas in Austin with a rifle and shooting many more people, suffered a severe suppression of natural play behavior and shared a similarly dismal "play history" with a number of other murderers.

Play Keeps Brains Healthy and Socially Adept

In addition to exercising mental flexibility and encouraging the ability to seek out a variety of options - in humans and other animals - he says play also produces such life-nurturing rewards as a sense of community, a sense of belonging, plus a variety of mental attributes including empathy and optimism.

Stuart Brown says he does some of his best thinking in the tree house study where his did the interview for this short Nature's Edge segment.

Take a look:

The arrival of the World Wide Web also means that we're now being constantly flooded with new examples from around the globe of play behavior among animals wild and otherwise.

Here, for fun (of course) is just one of the latest.

It's gone so viral that you may well have seen it - but if not, take a peek at this brief video of the snowboarding crow:

Video: Crow "Snowboarding" on Roof  apparently in Russia:

We invite you to follow our weekly Nature's Edge Notebook on Facebook  and on Twitter @BBlakemoreABC

This is the sixth in a series on animal intelligence and the science of play behavior :

1-   " Hunting With a Most Endangered Hunter - Dateline Botswana" At:    

                   http://abcn.ws/rI6Obx             (Nature's Edge Notebook #11)

2-  "Dogs Use Subway, Cat Takes Bus…" At:

                   http://abcn.ws/zXPpDd            (Nature's Edge Notebook #12)

3-  "Who Needs Words? Crows?  You? Wild Gorillas? Alison Krauss? …" At: 

                   http://abcn.ws/wM25PJ           (Nature's Edge Notebook #13)

4-   "How Would a Prairie Dog Describe You? Just Ask One!" At:

                    http://abcn.ws/yIZ9it               (Nature's Edge Notebook #14)

5-  "Dolphins Reported Talking Whale in Their Sleep: Freud's 'Royal Road to the unconscious'  may have surfaced at a pool in France." At:

                     http://abcn.ws/yXpddT          (Nature's Edge Notebook #15)