''Omigosh, look at him! He is too cute!''

''How adorable! I wish I could just reach in there and give him a big squeeze!''

''He's so fuzzy! I've never seen anything so cute in my life!''

A guard's sonorous voice rises above the burble. ''OK, folks, five oohs and aahs per person, then it's time to let someone else step up front.''

The 6-month-old, 25-pound Tai Shan - whose name means ''peaceful mountain'' - is the first surviving giant panda cub born at the Smithsonian's zoo. And though the zoo's adult pandas have long been among Washington's top tourist attractions, the public debut of the baby in December has unleashed an almost bestial frenzy. About 13,000 timed tickets to see the cub were snapped up within two hours of being released, and almost immediately began trading on eBay for up to $200 a pair.

Panda mania is not the only reason 2005 proved an exceptionally cute year. In summer, a movie about another black-and-white charmer, the emperor penguin, became one of the highest-grossing documentaries of all time.

Scientists who study the evolution of visual signaling have identified a wide and still-expanding assortment of features and behaviors that make something cute: bright forward-facing eyes set low on a big round face, a pair of big round ears, floppy limbs and a side-to-side, teeter-totter gait, among many others.

Cute cues are those that indicate extreme youth, vulnerability, harmlessness and need, scientists say, and attending to them closely makes good Darwinian sense.

The human cuteness detector is set so low, researchers said, that it deems cute practically anything remotely resembling a human baby or a part thereof, and so ends up including the young of virtually every mammalian species.

The greater the number of cute cues that an animal or object possesses, or the more exaggerated the signals may be, the louder and more italicized are the squeals provoked.

Even as they say a cute tooth has rational roots, scientists admit they are just beginning to map its subtleties and source. New studies suggest that cute images stimulate the same pleasure centers of the brain aroused by sex, a good meal or psychoactive drugs like cocaine.

Advertisers and product designers are forever toying with cute cues to lend their merchandise instant appeal, monkeying with the vocabulary of cute to keep the message fresh and fetching.

That market-driven exercise in cultural evolution can yield bizarre if endearing results, like the blatantly ugly Cabbage Patch dolls, Furbies, the figgy face of E.T., the froggy one of Yoda.

Madison Avenue may adapt its strategies for maximal tweaking of our inherent baby radar, but babies themselves, evolutionary scientists say, did not really evolve to be cute. Instead, most of their salient qualities stem from the demands of human anatomy and the human brain and became appealing to a potential caretakers' eye only because infants would not survive otherwise.

Human babies have unusually large heads because humans have unusually large brains. Their heads are round because their brains continue to grow throughout the first months of life.

Baby eyes and ears are located comparatively far down the face and skull. Baby eyes are also notably forward-facing, and all our favorite Disney characters also sport forward-facing eyes, including ducks and mice, species that in reality have eyes on the sides of their heads.

The giant panda offers another case study in accidental cuteness. Although it is a member of the bear family, a highly carnivorous clan, the giant panda specializes in eating bamboo.

And, inside the bear's large, rounded - cute - head, said Lisa Stevens, assistant panda curator at the National Zoo, are the highly developed jaw muscles and the set of broad, grinding molars it needs to crush its way through some 40 pounds of fibrous bamboo plant a day.