But designers of the 15-year-old Tropical Forest didn't figure on Little Joe, a 5-foot-tall, 300-pound adolescent gorilla who, like human teenagers, was increasingly restless with life at home. Little Joe, 11, was able to scale a steep wall and cross the moat because he has not yet gained the weight to fill out his long arms and body.
That's when startled zoo volunteers spotted Little Joe with nothing between him and the public except a potted plant. Zookeepers quickly evacuated the one family in the Tropical Forest that morning, allowing Joe to tour the forest exhibit building, returning to the ape enclosure on his own 10 minutes later.
Though no one was hurt, the incident alarmed zoo staff since there had never been a successful gorilla escape before.
Joe's brief brush with freedom in August underscores a growing problem at American zoos: an increasing number of young male gorillas who are both agile enough and restless enough to challenge the security systems that hold them.
While the growth is only part of an overall 38 percent increase in the captive gorilla population since the 1980s, zookeepers say the adolescent males present the most problems.
"It's a challenge and a growing challenge in North American [zoos]," said John Linehan, president and CEO of Zoo New England in Boston, which runs the Franklin Park Zoo. The zoo has acquired three male gorillas since 1998.
In many zoos, including Franklin Park, the gorillas live in a single group of males and females rather than in gorillas' more natural grouping, a harem where a single sexually mature male mates with several females. The young adult males, called bachelors, can start to become a social problem between the ages of 10 and 19 as they start to become more interested in the females.
Victor Camp, zoo director at St. Paul's Como Zoo in Minnesota, compares the adolescent ape problem to the "bar male syndrome:" The guys in the bar are getting along fine, playing pool or whatever, until the women arrive and their whole attitude changes. "They start strutting their stuff," Camp said.
When males start to act out, the best solution is to remove them from the larger group. But, because most zoos do not have multiple long-term holding areas for gorillas, they have trouble nipping conflicts in the bud.
As a result, bachelors can become restless, and at times, physical. A young gorilla escaped from the Como Zoo in 1994, leaping nearly 12 feet onto a boulder and jumping across a moat to climb out. Another male gorilla, named Hercules, broke loose at the Dallas Zoo in 1998. Hercules bit a zookeeper and then dragged her down a hallway.
Breakouts are not the only problem. The combination of massive size and an immature mind can be dangerous. Camp said that while males are not aggressive by nature, they can unintentionally hurt each other by posturing, throwing their arms at each other, or accidentally brushing up against large canine teeth.
For instance, Kitombe, a 17-year-old male at Franklin Park, deeply ripped another gorilla's calf muscle during a skirmish several years ago.
In a way, the zoos are victims of their own success: Captive breeding programs have worked almost too well, boosting the number of captive gorillas in the United States from 270 to 375 over the last 20 years. But while zoos can increase the birthrate, they cannot control the gender of baby gorillas.
"The problem we have in a captive environment is that there are an equal number of males and females," said Tara Stoinski, manager of conservation programs for Zoo Atlanta in Georgia.
Unfortunately, Species Survival Plan, the group in charge of managing endangered animal populations in US zoos, does not offer much guidance because zoologists are just starting to understand captive gorilla behavior.
Zoo Atlanta, which just finished a two-year study of adolescent males at seven institutions, is about to issue a report suggesting that the bachelors be grouped together, but away from females. "The data suggests that bachelor groups are good for males," Stoinski said, keeping them socially engaged without as much sparring.
In the meantime, zoos employ a number of methods to control problem behavior and to keep the peace among their gorilla groups. The design of the enclosures themselves is key in minimizing dangerous encounters.
"The basic premise is that the animals need to get away from each other," Camp said. "You try to create avenues of escape where one [gorilla] can't be cornered by another."
The Como Zoo built a bridge in its gorilla enclosure so that animals could run in a circle. If a chase occurs, they eventually tire out. The Franklin Park Zoo built in climbable walls to offer escape routes to gorillas trying to avoid confrontation.
To prevent future escapes, the Boston zoo will also add hot wires to the top of the enclosure.
In some cases, zoos may use psychotropic drugs to handle aggressive behavior. "If the aggression is too severe and is potentially a threat, the zoo may provide some sort of medication to the animal," Camp said. "We haven't done that."
Linehan said that the Franklin Park Zoo experimented with low dosages of antidepressants and anti-anxiety medications about five years ago. "We didn't see any positive effects so we discontinued use," he said.
Looking ahead, institutions building new gorilla exhibits are creating two areas, one for the family group and one for the bachelor group.
"Probably we're going to need double the number of all-male groups in the future," said Stoinski, who advocates moving adolescent males into bachelor groups while they are quite young. "The longer we wait, the harder it's going to be integrating them."
For the time being Little Joe will remain with the family group at Franklin Park, but the dynamics are changing. Little Joe continues to be more aggressive and dominant among his peers, while Kitombe is ready to breed for the second time and take on the father role of the group.
Linehan said that the exhibit groups might change or Joe may have to be moved to a new zoo in the future.
The group dynamics for the Franklin Park Zoo gorillas have been stable for five years, Linehan added. "It's been such a joy to watch them grow, and not worry about the challenges," he said. "Now we will just have to work through it."
© Copyright 2003 Globe Newspaper Company.