VATICAN CITY — Given the enormity of the task before them, it's no surprise that most visitors to St. Peter's Basilica completely miss some of the most curious characters in the massive church's artwork: the animals.

The artists who worked on St. Peter's over the centuries used animals not only for traditional and serious purposes, like symbolically depicting the Holy Spirit as a dove, but also as an outlet for humor or even vanity.

To see the lighter aspects requires sharp eyes and a willingness to press against walls, get down on hands and knees, or stand on tiptoe.

Put the guidebooks away. They're usually no help.

One little-known example of artistic vanity can be found on the funeral monument of Pope Clement XIII, in the right wing of the basilica behind the main altar. An 18th-century sculptor who helped carve the monument's two life-sized lions was so proud of his work that he fashioned the left-hand animal's rear end into the face of an elephant, his trademark signature. The lion's tail forms the elephant's trunk.

One of the basilica's most famous creative geniuses, 17th-century Gian Lorenzo Bernini, also incorporated lighter elements into his immense and intricate four-columned bronze altar canopy, which stands more than 90 feet high. Crawling on the base of the two rear columns are several real lizards that he bronzed, with one taking a bite out of a bronzed scorpion. (Another personal touch is the artist's bronzed rosary and holy medals, which rest several feet away.)

Given the vast number and variety of animals in the basilica, one official laughingly calls it “Noah's ark.” The appellation has a double meaning in Italian because the last name of the cardinal who heads the basilica, Virgilio Noe, translates as “Noah.”

Yet the topic is also the subject of serious scholarship. One of the basilica's chief art historians, Daniele Pergolizzi, has teamed up with an animal specialist to research the basilica's flora and fauna. They hope to publish a book in coming months.

Pergolizzi, who gets noticeably excited when the topic comes up in conversation, is particularly interested in tracking new animal species that began populating the basilica's art after the discovery of America. He says the phenomenon shows up mainly in different varieties of birds.

Art experts recently focused attention on the figure of a dog that often appears in ancient representations of St. Peter. According to the basilica's July newsletter, the depictions are based on scenes described in the apocryphal “Acts of St. Peter.” The animal enters the text as the apostle's friend and defender against lies, at one point even assuming a human voice to help Peter denounce a sorcerer's deceptions.

According to other legends, the dog followed Peter to his death by crucifixion, and is shown in some basilica artwork standing mournfully at the foot of Peter's cross.

One such example, on the church's bronze central door, is St. Peter's crucifixion scene in which the barely discernible figure of a dog looks on. A dog occupies a more central spot in a 15th-century funeral monument to Pope Paul II, which is now housed in the basilica's administration office.

One reason animals are so popular with artists is their ability, through symbolism and reference to legend, to pack a lot of significance into a limited space.

The dog, for instance, represents ardent and unfailing faithfulness. One of its most famous uses in the basilica is in a huge sculpture of St. Dominic. A perky little dog carrying a torch in its mouth accompanies the famous preacher, symbolizing his fidelity to Christ. A Latin name play on Dominic's religious followers, “Dominicanes,” means “dogs of the Lord.”

Bernini's lizards, at first glance a simple expression of artistic high spirits, also carry Christian significance. Because the cold-blooded creatures hibernate, they are used as an image of the resurrection. The lizard chewing on the scorpion, a symbol of sin, serves as a sign of spiritual rebirth.

Fantastical creatures, like dragons and wide-mouthed dolphins, peer out of almost every shadowy corner in the basilica. A massive gold unicorn hovers 80 feet up amid a grouping of sculptures representing the virtues; it signifies purity because of the creature's mythological affinity for female virgins.

Much of the animals' symbolism is lost on modern visitors because common cultural references have changed over the centuries.

But the meaning of some representations baffles even art experts. One example is found on the basilica's central entry, known as Filarete's Door after its 15th-century creator.

In the gloom at the very bottom of the door's inner face sits a small panel depicting Filarete and his six assistants. On either side rides a knight, one on a donkey and the other on a camel.

Pergolizzi, the basilica's art specialist, speculates that the camel represents the East, since much of the door's artwork is dedicated to the Council of Florence, which attempted to unite Eastern and Western churches.

But he won't even hazard a guess about the significance of two little pigs that play at the feet of the camel.

“Art critics haven't yet turned their attention to that panel,” he said, adding with a note of relish, “there's still much work to be done.”