There are a lot of studioli in paintings of St. Jerome because he was a scholar and scholars need privacy to work. The studioli are small wooden studies, such as you can see here, a small room inside a room, or a panelled side of a room, with raised platforms to keep feet off the cold floor, nearly all with sliding piles of books, and hourglasses and pens and inkwells, more books on the floor, books toppling over in cabinets, reading stands, writing stands, astrolabes, green baize or Turkish carpets for table covers, high-backed seats to keep the draft off, prayer beads, vases, pots of flowers, a pear for snacking, scissors, glasses, notes on bulletin boards, cardinals' hats (because that is half of how you identify Jerome), clocks, birds and shells, antlers and squirrels; peacocks and quail--because scholars study nature.
The other half of how you identify Jerome is his lion--lions like kittens, lions like stuffed animals, sometimes Jerome looks like a lion, lions like old dogs, and in Durer's along with a dozing lion there is a sleeping dog, because wise men who are scholars have white dogs, in contrast to old wise women who are witches who have black cats. The best known of these white dogs is Carpaccio's Augustine, but that painting is about Jerome, too, because Augustine in his wonderful study--far grander than a studiolo--has abruptly had a vision of Jerome in death. Both of those last two wise men with white dogs are intended as portraits of Cardinal Bessarion and Bessarion is the explanation for the astrolabes that appear in Carpaccio's Augustine, and in occasional paintings of Jerome. The astronomer, Regiomontanus, made an astrolabe for Bessarion, and Bessarion, like Jerome, was a Cardinal. Most of these studiolo paintings bring you close in to the studiolo with Jerome, give you that wonderful enclosure, but the painting by Antonello da Messina showing an elevated studiolo in a setting something like a church, surrounds Jerome with privacy. In other paintings, the lion is sleeping, or guarding, or holding up his paw to have a thorn removed--at some point the story of Jerome got mixed up with the story of Androcles--and Bastiani and Carpaccio have great fun watching the lion scaring the monks.in the cloister. For Antonello, though, Jeroma has a cat among his flower pots, because scholars tend to work quietly and cats like not to be disturbed.
Far off to the right of Antonello's studiolo is the image you see above, an alert gracile lion pacing among the pillars of the cloister as if they were trees. Other studiolo lions might represent the taming of the bestial in man, the triumph of law over nature, the redemption of fallen nature, and so on and so forth, but not this lion. Antonello's lion is a metaphor for scholarship--for the deliberate and watchful stalking of information, for the pacing nature of thought, for the open spaces of the mind, for the easy interchange of views. And because it is Jerome: the lion suggests the possibility of the radical transformation of the culture from which the studiolo protects.
For much more, see the wonderful book by Eugene F. Rice, Jr., Saint Jerome in the Renaissance. and a modern architect's recreation.