There they came trudging along, straight upright on stubby legs, shoulders swinging back and forth with each step. Fuzzy at first, their dark silhouettes started to come into focus on the screen just as I was eating my first bite of popcorn.

Hobbits, maybe? No. We already knew from the movie's title that they would be birds. Then Morgan Freeman's otherworldly voice informs us that these beings are on a long and difficult journey in one of the most inhospitable places on Earth, and that they are driven by their "quest for love."

Since the documentary "March of the Penguins" has become one of the sleeper hits of the summer, I guess I'm not the only one to have been mesmerized.

I've long known the story of the emperor penguins, having told it to generations of biology students as a textbook example of adaptation, but to see the sheer beauty and wonder of it all come into focus on the screen still took my breath away because film technology has finally allowed us to (virtually) enter an exotic world, and yet one that is real.

As the movie continues, everything about these animals seems on the surface utterly different from human existence; and yet at the same time the closer one looks, the more everything also seems familiar.

Stepping back and viewing from the context of the vast diversity of millions of other organisms that evolved on the tree of life — grass, trees, tapeworms, hornets, jellyfish, tuna, green anoles and elephants — these animals marching across the screen are practically kissing cousins to us.

Like many others who loved the movie, I admired the heroics of both the birds themselves and the intrepid camera crew that braved the inhumanly hostile environments of the Antarctic. But as a research biologist who has spent half a century studying the behavior and cognition of animals other than ourselves, I also admired the boldness of the filmmaker, Luc Jacquet, to face down the demon, if not the taboo, of anthropomorphizing his subjects.

Which brings me back to Freeman's use of the word "love" in the context of the penguin's behavior. The unspoken rule is that this four-letter word is to be applied only to one creature on Earth, homo sapiens. But why?

A look at the larger picture shows this presumption of exclusivity is utterly unproved. In a broad physiological sense, we are practically identical not only with other mammals but also with birds — muscle for muscle, eye for eye, nerve for nerve, lung for lung, brain for brain, hormone for hormone — except for differences in detail of particular design specifications.

Functionally, I suspect love is an often temporary chemical imbalance of the brain induced by sensory stimuli that causes us to maintain focus on something that carries an adaptive agenda. Love is an adaptive feeling or emotion — like hate, jealousy, hunger, thirst — necessary where rationality alone would not suffice to carry the day.

Could rationality alone induce a penguin to trek 70 miles over the ice in order to mate and then balance an egg on his toes while fasting for four months in total darkness and enduring temperatures of minus-80 degrees Fahrenheit and gusts of up to 100 mph?

And bear in mind that this 5-year-old penguin has just returned to the place of its birth from the sea, and thus has never seen an egg in its life and could not possibly have any idea what it is or why it must be kept warm. Any rational penguin would eventually say, "To hell with this thing, I'm going back for a swim and to eat my fill of fish."

And that, of course, would be the immediate end to the evolution of rationality in emperor penguins (and perhaps to the evolution of the penguins as a species). Adaptation and adherence to an unconscious genetic program driven by passions and appetites are as vital as they are often incredible.

Even humans, the most rational of all species, require an overpowering love to do the remarkable things that parents do for their children.

The penguin's drive to persist in proximally bizarre behavior in the face of what must otherwise be overpowering temptations to do otherwise also suggests that they love to an inordinate degree. Where they differ from us is that they can "love" an egg as much or more than a peeping fuzz-ball of a hatchling.

In the past half-century, the hidden reality of nature has been revealed as never before. Our general perceptions, though generally lagging behind, are now catching up. We are becoming weaned from the make-believe world of Walt Disney's "Bambi."

Is that why high-tech documentaries like "Microcosmos," "Winged Migration," "March of the Penguins" and, in a slightly different vein, Werner Herzog's new "Grizzly Man" are catching on?

I suspect that the new breed of nature film will become increasingly mainstream because, as we learn more about ourselves from other animals and find out that we are more like them than supposed, we are now allowed to "relate" to them, and therefore to empathize.

Paradoxically, the cartoonish anthropomorphism of "Bambi," although it entertained the youngsters, blocked rather than promoted an understanding of animals. In "Bambi" we do not see other creatures. Instead, we are presented humans with antlers, and with our thought and speech.

Nature is the greatest show on Earth, and reverence for life requires acknowledging the differences between ourselves and the animals as well as seeing our relatedness. Sometimes that involves walking a tightrope, and missteps can result in tragedy.

Take grizzly bears. Most of us rightly fear them (no less an instinct than love). Those who don't feel fear and consort with them — like Timothy Treadwell, the subject of "Grizzly Man" — stand a good chance of being eliminated from the gene pool.

There are so many stories to tell. Furthermore, the "actors" in these dramas do not perform for any money. They are at home in the most inhospitable (for us) places on the planet, and not only do they behave as if they are oblivious to the camera, they probably ARE oblivious.

If we gain more exposure to the real — and if the producers and studios invest half as much care and expense into portraying animals as they do into showing ourselves — I suspect the results will be as profitable, in economic as well as emotional and intellectual terms, as the "March of the Penguins."

Bernd Heinrich, emeritus professor at the University of Vermont, is the author of "The Geese of Beaver Bog."