The Philadelphia Zoo is letting some animals take a walk on the wild side -- sort of.



Species like monkeys, lemurs and orangutans can now use a set of enclosed trails on the 42-acre zoo to leave the confines of their exhibits and travel throughout the property.



The network of protected pathways is designed to enrich the animals' lives by giving them the ability to explore and see and smell new things. It's also aimed at enhancing the visitor experience, allowing zoo-goers to see creatures in unexpected places -- such as in the trees.



"We believe that the opportunity to travel, to explore, to choose to go toward things that are interesting, move away from things, really control their own experience ... is going to be incredibly enriching for the animals in our care," said Andrew Baker, the zoo's chief operating officer.



Last week, the zoo opened the Great Ape Trail, which allows orangutans to roam along an overhead walkway through a tree grove. Small monkeys and lemurs have used the Treetop Trail for about a year, scampering through mesh enclosures suspended about 11 feet to 16 feet off the ground.



"They get to see everything that's happening in the zoo," general curator Kim Lengel said. "They get to travel, they can climb trees, and they really take advantage of it. And if they're tired of it, they go back inside."



The 700-foot-long Treetop Trail snakes around the zoo's main outdoor plaza, and visitors have been surprised and delighted to see a red-capped mangabey in their midst.



Linzie Neary recently brought her 21-month-old twins and 4-year-old son to the zoo and spotted a monkey using the trail. Neary described it as a more realistic experience than seeing the animals behind glass. She praised zoo officials for the initiative.



"I like that they're trying to bring us closer," she said.



Steve Feldman, spokesman for the Association of Zoos and Aquariums, a national accreditation group, said that while the concept for such trails is not new, "the way that Philadelphia is doing this on a campus-wide basis is sort of taking this trend to the next level."



And he described Philadelphia's plan to study animals' reactions as "cutting edge."



Baker said zoo officials are gathering both scientific and anecdotal evidence to gauge the effect of the trails on animals' well-being. Hormones in fecal samples can indicate stress levels, he noted, and cognitive bias testing can assess animals' optimism and pessimism.



"You can't just ask them whether they like it or not," Baker said.



But the initial response appears promising. Lengel said she was "shocked" at how quickly the orangutans embraced the Great Ape Trail.



Baker said the zoo plans to spend $2 million to add another 1,000 feet to the Treetop Trail next spring; hippos, giraffes and zebras will be among the beneficiaries of a third trail that's still in development. A time-share arrangement will eventually allow bears and big cats to use the Great Ape Trail.



Over next decade, Baker said, the zoo hopes to have about a mile-and-a-half of trails linking exhibits with similar habitats. That could mean visitors will have to track down animals' locations using smartphones because the animals won't necessarily be in their "home" exhibits, he said.



"The experience will be different from visit to visit," said Baker.

