They climbed to the very top of the mesh enclosure surrounding Chimpanzee Forrest, hung by one arm and gawked at the goings-on in a nearby construction site.
They banged thunderously on the glass window separating them from human visitors, tickled themselves with tufts of grass, and chased one another around the compound as the thermometer rose to a balmy 57 degrees.
Another, surely an adolescent member of the tribe, slouched on a rock and snacked moodily on a biscuit with a far-away look in his eyes.
"That's Jack," said Carey Riccardone, mammal collection and conservation manager, indicating her favorite. "He's my boy. He's kind of simple, and he doesn't realize how strong he is."
Springtime — in January?
With the unseasonably mild weather this weekend — the high temperature for Sunday is forecast to reach the lower mid-60s, at least 20 degrees above normal for this time of year — animals of all kinds ventured outside.
According to zoo officials, 1,142 visitors had walked inside the entrance gates as of 1 p.m. By comparison, last Saturday the high was 45 degrees and the total attendance for the entire day was a mere 265, zoo spokeswoman Jane Ballentine said.
Much like their human counterparts, the chimps, which hail from Africa, are kept inside when it gets below about 50 degrees, because chilly temperatures leave them vulnerable to disease. Sometimes they aren't allowed outside again until spring bulbs are in bloom.
Chimps with cabin fever aren't good company. Some get listless and dull. Others can become aggressive, and more fights than usual break out.
To keep the highly intelligent primates from succumbing to the doldrums during their winter confinement, Riccardone and her staff enroll them in the zoo's version of chimp school. They construct puzzles for the apes to solve and build toys that encourage behaviors the beasts would normally engage in the wild.
Riccardone loves the looks the chimps get on their faces when they are trying to figure out the answer to a particularly vexing puzzle.
"They're just so intelligent," she said. "They really do work at trying to learn what you want them to learn."
But this weekend, none of that was necessary. The great, big variable world as viewed from the enclosure's mesh ceiling — with its stream of traffic and occasional visits by a bird or even a squirrel — provided stimulation enough for Jack, Louie, Joyce and even the zoo's top banana, the alpha male Kasoje.
"The warm weather means a break for us, too," Riccardone said.
Elsewhere in the zoo, Badu, the African lioness, sunned regally on a hot rock with just the end of her tail twitching rapidly to indicate more than a passing interest in the sea gull (yes, a sea gull, perhaps attracted by the smell of the penguins' meal of fish) circling overhead. The prairie dogs popped in and out of their burrows like wind-up toys.
And from over on Rock Island, there came a raucous hee-hawing, a cacophonous braying. It sounded as though a herd of donkeys had somehow leaped the fence, crossed the moat, and become marooned on the boulders.
But, no, it was just the so-called "Jackass" penguins, a black-footed variety that hails not from subzero Antarctica but from the moderate climes of South Africa. On warm winter days, it seems, a young penguin's fancy turns to thoughts of love.
"They're calling for their mates," said Jen Kottyan, the zoo's avian collection and conservation manager. "In South Africa, the seasons are reversed, so it's summertime there. This is their breeding season."
Normally on a frosty January morning, the calls of the lovesick birds would most likely be confined to their indoor pens — at least until their keepers could break apart the ice coating the outdoor moat so the birds could take a dip.
"Of the 18 different species of penguins," Kottyan said, "only three live at the South Pole. The black-footed penguins are well-suited to Baltimore's climate. They love the warm weather. But when it's below freezing, we have to keep them inside."
The penguins' island vacation is likely to be short-lived. Come Monday, it's back to the holding pen and that claustrophobic, cooped-up feeling.
According to the National Weather Service forecast, temperatures will drop sharply once the weekend ends. By Tuesday, highs are expected to be in the 40s.