I was in San Diego with my mom at the beginning of November and while looking for things to do the San Diego Zoo was a no brainer. My husband and I were there for our honeymoon awhile back and I remembered it being awesome. The zoo is 100 acres and houses more than 3,700 rare and endangered animals. This zoo is very unique as it also features over 700,000 different plant varieties.
Interesting Zoo Facts:
-The zoo originally opened in 1923 with admission costing 10 cents for adults.
-San Diego zoo has won numerous awards for its exhibits and its breeding efforts.
-The zoo also operates the San Diego Zoo Safari Park which someday I hope to attend.
-The koala breeding program has been very successful. The Outback area of the zoo is home to 20 koalas with the San Diego zoo having 30+ other koalas on loan to other zoos in the US and Europe. This is the largest colony of koalas outside of Australia.
-Cape Fynbos habitat within the Africa Rocks area features a 200,000 gallon pool with depths of up to 13′ and underwater viewing.
-The San Diego Zoo is one of a few zoos to sucessfully raise sunbirds from eggs, both parent raised and hand reared.
-San Diego Zoo has many animal ambassadors which help guests to get an up close and personal view of the animals. One of these is a 20# Boa which is over 6′ long. His name is Manja. Manja is one of only 32 Madagascar ground boas living in the U.S.
-The domestic dog is on display along with a cheetah in the urban jungle area of the park. The dog helps to reassure the cheetah that there is nothing to be worried about due to its naturally calm state.
-Elephant walking study is taking place at the zoo. Some elephants may be seen with a tracker on their ankles which allow step count and activity to be tracked. It is anticipated that this data will help to show elephant behavior and ensure adequate care is provided.
-The oldest residents of the zoo are the Galapagos tortoises which have been in the collection since 1928.
-The zoo often releases ladybugs to help with pest control.
-Terrace lagoon pond is home to 10+ koi fish, along with some tiny mosquito fish and a bluegill or two. The pond is 18,500 gallons and is vacuumed once a week. The water is treated with ozone gas to remove contaminants.
-The lions each are trained to their names which allows the keepers to easily call one lion to be inspected.
-Currently they do not have any California sea lions on exhibit. Oddly enough the sea lions can be found off the coast of CA and were originally sold to other zoos to acquire more exotic species.
Enclosures:
I love the design of the enclosures at the San Diego Zoo. This is easily one of the best features of the zoo. Each enclosure is specifically designed with the occupant in mind while trying to eliminate as many cages, fences and bars as possible. San Diego zoo was one of the first zoos which created a moat around some enclosures to completely eliminate the barriers.
Below is an image which shows how natural the barriers are. 
This photo (below) shows the panda enclosure and all of the space and activities that the panda has available. There are lots of areas for the pandas to forage for food while still view-able to the public.
Here is a photo which shows a natural rock wall and waterfall as the barrier to the public.
This zoo does have many open enclosures, however not all are without fences or glass. Here is an example of one of the monkeys cages. The fencing is very minimal and does not distract from the experience in any way. Most times the fencing is so minimal that it disappears completely.
The bonobos exhibit like most exhibits in the Lost Forest area was filled with lush plants, rock outcroppings, a waterfall and many twisted palms for the bonobos to play on.
The hippo enclosure was uniquely designed to withstand the force of a 2 ton hippo moving at 15mph towards the glass. The glass itself is 2.25″ thick and 110′ long of underwater viewing. Luckily the water maintains the perfect temperature for the hippos without being heated or cooled thanks to San Diego’s temperate weather. The day we were there was the new baby hippo’s first day on exhibit. The tilapia help to keep the water clean, along with a rather large water filtration system.
Below is a photo of a portion of the Africa Rocks area. This is one of the newer exhibits of the zoo. It actually was not even fully open while we were there. Although it appears from the updated map that it was fully opened a week or two after we visited.
The penguins habitat includes a nesting area, cobblestone beach and rockwork that mimics the granite boulders found in South Africa.
Transportation within the zoo
Buses-We managed to keep all of the areas missed to one path from the top so we used the bus near the end of the day to go back and hit that area. We still ended up missing the Discovery area as I thought it was a kid area.
Skyrail- The gondola provides a very nice view of the entire park. It also helps to take you from one end of the park up to the top of the hill. We found it helpful to take the skyrail up to the top right away so we were working downhill for most of the day.
Walking- This is a large zoo and it was built on a hill so be prepared with good walking shoes and be ready to walk.
The zoo also offers a guided bus tour. We did not take this tour, but many people have mentioned that it is a good way to get familiar with the entire park.
Notable Animals
Pandas- The San Diego zoo is one of 4 zoos in the United states to house giant pandas. These pandas are on loan from China as a part of a breeding project. It is very cool to see these creatures, even though they do sleep most of the day. Around the zoo you will find 70+ different kinds of bamboo which are harvested for the pandas.
Orangutan- Ken Allen was an orangutan who was born at the zoo in 1971. He has become famous for escaping his enclosure 3 times even under the watchful eye of his keepers. Surprisingly, when he escaped he was docile and would just wander throughout the zoo to visit other animals.
Additional Information:
http://www.sandiegohistory.org/journal/2016/april/san-diego-zoo-100-years/
https://www.tripsavvy.com/san-diego-zoo-facts-2937398
http://theanimalfacts.com/go-2-zoo/san-diego-zoo/
https://en.wikipedia.org/wiki/San_Diego_Zoo
http://animals.sandiegozoo.org/animals/giant-panda
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The Frog Lady 
It’s a great zoo for sure. I’ve been there a few times over the past 50 or so years. You can spend a couple days there and if you add Balboa Park with its museums you might be able to spend a week.
Lee