It's difficult to imagine a more entertaining and educational family outing than a visit to a zoo. Zoos help open our eyes, minds and hearts to the wonders of the natural world.

America's first zoo opened in Philadelphia in 1874. Today, the Department of Agriculture licenses 2,400 "animal exhibitors," of which 212 are accredited members of the Association of Zoos & Aquariums (AZA), an organization that requires high standards of animal care and conservation.

AZA's high standards are important, and here's why: Wildlife populations are declining at an alarming rate around the world. Experts predict that by 2050 more than half of the planet's species could become extinct due to climate change and loss of habitat.

So it is becoming increasingly evident that zoos must be more than just entertaining attractions. No longer simply modern-day arks, most of the country's accredited zoos are vigorously proactive in conserving wild animals, restoring habitats and breeding and reintroducing endangered species.

Probing the ranks of America's accredited zoos, we've identified the following eight that standout among the best of America's zoological parks — listed from East to West.

1. Bronx Zoo

New York, New York

One of America's most historic zoos, the Bronx Zoo dates back to 1899. It's also the country's largest urban zoo home to more than 6,000 animals of 650 species spread out across 265 acres. The zoo has worked diligently to get away from caging its animals in favor of natural habitats such as JungleWorld and the Himalayan Highlands.

On the entertainment side, the zoo recently opened a 4-D theater where viewers are sprinkled with real snowflakes and mist. There's also the Treetop Adventure a ropes course that leads visitors through a canopy atop the park. The newest addition at the Bronx is the Butterfly Garden where visitors can mix with some of nature's best works of art.

2. National Zoo

Washington, D.C.

Part of the prestigious Smithsonian Institution, the National Zoo is scenically situated in Rock Creek Valley and was one of the first major zoos to be located in a spacious landscaped setting. More than 400 species of animals roam among the tree-shaded trails threading through the 163-acre park.

The National Zoo is a prime visitor attraction as well as a noted center for animal conservation, research and education. Its most popular exhibits include the Asia Trail (home to the only giant pandas on the East Coast), Great Cats, Elephant Trails and the Reptile Discovery Center, where visitors can watch voracious giant Komodo dragons feed.

3. Zoo Miami

Miami, Florida

Although there seems to be a lot of confusion surrounding its name it's sometimes listed as Miami MetroZoo or Miami-Dade Zoological Park & Gardens one thing is for certain: This is the only wildlife park in the United States located in a subtropical climate. The hot, humid weather and jungle-like environment seems to please most of the 900 animals that freely roam the park's 280 acres.

For visitors, the best and most comfortable way to get around Zoo Miami's 100-plus exhibits is to join a Safari Tram Tour where expert guides provide commentary on the zoo's creatures. At the Kaziranga Camp Rhino Encounter, a nominal fee gives visitors a rare opportunity to touch and feed a pair of massive Indian rhinos.

4. Columbus Zoo & Aquarium

Columbus, Ohio

Columbus Zoo is home to more than 7,000 animals representing about 800 species and draws nearly 2.5 million visitors annually. It has a worldwide reputation and has been regularly cited as one of America's top zoos. This success is largely attributable to the efforts and influence of noted animal expert and TV personality Jack Hanna, who from 1978 to 1993 led the zoo's development and continues to serve as its director emeritus.

The animal exhibits, scattered over 588 acres, are divided into eight regions of the world, ranging from polar regions to the heart of Africa. Far out of the ordinary (for Ohio) are Manatee Coast, the largest exhibit of Manatees outside Florida; and Discovery Reef, an indoor 100,000-gallon coral reef exhibit. It is home as well to one of the country's best primate exhibits and is recognized worldwide for its lowland gorilla-breeding program.

5. Audubon Zoo

New Orleans, Louisiana

Managed by Audubon Nature Institute and nestled among 58 acres of ancient oaks and lush gardens in historic Uptown New Orleans, Audubon Zoo offers an exotic mix of nearly 2,000 creatures from around the globe, hands-on animal encounters and engaging educational programs.

For the most part, its animals are exhibited in their natural habitats, a factor that has led to Audubon's consistent ranking among the country's best zoos. Its most highly acclaimed habitat, Louisiana Swamp, invites visitors to explore an authentic swamp environment home to a congregation of mysterious white alligators and, ever so appropriately, the rustic Cypress Knee Cafe serving spicy Cajun dishes.

6. St. Louis Zoo

St. Louis, Missouri

A top attraction in St. Louis (second only to the city's landmark Gateway Arch), the St. Louis Zoo occupies 90 acres in the heart of sprawling Forest Park. Recently voted Best Zoo and America's Top Free Attraction, the St. Louis Zoo is dedicated to connecting people to animals, and it's renowned for its innovative approaches to animal management, wildlife conservation and education.

Annually, nearly 3 million visitors enjoy free admission to experience the zoo's 17,000-plus animals, many of which are rare and endangered. Its recently-added children's zoo is a big hit with younger devotees who can pet fuzzy koalas and feed nectar to dive-bombing lorikeets.

7. Henry Doorly Zoo & Aquarium

Omaha, Nebraska

Nebraska doesn't seem a likely location for a world-class zoo, but that's precisely the case. In 2014, Trip Advisor proclaimed Henry Doorly Zoo the "world's best zoo," topping San Diego and Spain's Loro Parque, based on an algorithmic assimilation of millions of reviews for 275 major zoos worldwide.

Internationally renowned for its leadership in animal conservation and research, Doorly is equally acclaimed for its visitor appeal. It is Nebraska's top paid attendance attraction and has welcomed more than 25 million visitors during its 40-year existence.

It is home to several of the largest zoo exhibits in the nation and the world. Its Cat Complex is the largest of its kind in North America; Kingdoms of the Night is the world's largest nocturnal exhibit; the Lied Jungle is one of the world's largest indoor rain forests, and the Desert Dome houses the world's largest indoor desert.

8. San Diego Zoo

San Diego, California

It would be safe to say that San Diego is America's best-known zoo, and it certainly ranks among the two or three finest and most respected zoos in the world. If one were to combine the 3,700 animals of 650 species living at the 100-acre Balboa Park facility with the nearly 3,000 beasties representing 300 species roaming the zoo's 1,800-acre Safari Park in Escondido, 30 miles north of the city, it is clear that San Diego Zoo protects and exhibits more animals than any zoological organization in the world.

Popular exhibits at Balboa Park include Northern Frontier with its polar bears and reindeer; Elephant Odyssey, featuring both African and Asian species; and Panda Canyon, home to three giant pandas. San Diego is one of only four zoos in the country to exhibit these loveable creatures and was the first in the world to produce a giant panda through artificial insemination.

Safari Park was designed to help breed endangered species and has seen some 20,000 animals born within its borders including 176 rhinos, one of the world's most endangered large mammals.