I don't know anything about circus animals or their treatment, but there's a lot of common misconception about Zoo's, and I'm sure Circus animals as well. Most Circus performers that work with animals, I'm sure spend more of their time caring for the animals than abusing them. Why would you abuse an elephant you have to ride for hrs in practice and during a show? Do you want to die? That animal will kill you! But, I'm passionate about animals and I think they're fascinating, especially aquatic animals and marine life. So I'm arguing for the pro-Zoo side, because there's a lot people don't actually know, but instead assume to know.
Now not ALL Zoo's are like this, but a good portion of them are, at least the Zoo's that are a part of the AZA (Association of Zoo's and Aquariums). These are accredited Zoo's in the states. If a Zoo isn't accredited by the AZA, they aren't a safe place for animals to be and those are the kinds of Zoo's that yes, are probably abusing the animals and aren't taking proper care of them,
those are the ones to worry about, not the ones accredited by the AZA, those are safe. This applies to "Sanctuaries" and Animal Reserves as well, none of them are AZA accredited and only have to abide by USDA regulations which are completely lenient in comparison to AZA standards.
Everything in this post comes from people that work or have worked in these Zoo's. They are trained to take care of these animals and know what they're talking about. I'm not making these things up as I go, and it's not the product of shoddy research. This is information and facts provided by professionals that work in Zoo's across the nation, and by the AZA.
Zoo's follow the SSP (Species Survival Plan). This plan helps save endangered, wounded, and injured animals in the wild. When a Zoo accredited by the AZA saves an animal, the AZA is networked to that Zoo and can track and supervise how that animal is being treated and cared for while in captivity. They keep studybooks of bloodlines, and if a specific species is endangered and there are a male and female of that species at different Zoo's that are of breeding age and valuable genetics they can breed the two to begin the process of saving that species from extinction. A process otherwise near impossible in the wild in some cases due to poachers and such.
The SSP also incorporates a plan to re-introduce healthy animals that have been rescued, back in to the wild. But what people don't understand is not all of the Zoo animals were rescued, a lot were born in captivity as well. People argue all the time that the animals should be returned to the wild but what they don't understand is, they can't. They weren't born in the wild and would have no idea how to survive. Sending them to the wild would ultimately kill them, which is completely opposite of what Zoo's are trying to do. However, it does happen. Sometimes they are able to accomplish introduction or re-introduction to the wild safely with the animals health completely intact. Some are even adopted by wild "parents".
(Examples:
http://animals.sandiegozoo.org/animals/california-condor ,
http://www.nczoo.org/conservation/regional/RedWolvesAliRiver.html)
Another thing Zoo's do that's important is conservation. Conserving the wildlife for the betterment of the animals and their habitat. People that oppose Zoo's often have no idea what conservation is, or even participate in it. They want the animals out of the Zoo, but have little to no idea how the impact of throwing them back in to harsh conditions could ultimately kill them. An example of this kind of ignorance (and I don't use that term insultingly) is when people protest orangutans being captive, but continue to buy products that contain Palm Oil. Deforestation of Palm Oil plantations are killing off Orangutans by the thousands. They've lost nearly 80% of their natural habitat, so where else are they supposed to go? Releasing an animal back in to harsh conditions that aren't conducive to the animal's survival is more cruel and inhumane than them being in captivity. There's a lot more that goes in to re-introduction than just "setting them free".
(Example:
http://www.aza.org/conservation-commitments-and-impacts/)
People talk about the abuse the animals in Zoo's suffer. And while there are bad times, because of bad people that shouldn't be working with animals, the level people assume animals are being abused (but have little to no proof 90% of the time) is almost non-existent. The overall standard of care is incredible and most times, better than what people give their domestic pets by far. Zoo's take the nutrition of their animals highly seriously. Their diets are designed for optimum nutrition and conditioning of their bodies. And they are cared for by highly trained professional staff members that have worked with animals for years and years (they don't send a newbie to go and groom or feed the tigers.) Their health is extremely important to the Zoo, so there is always a team of world-class vets that are on-call 24/7 to be ready to care for the animal if need be.
The AZA has strict regulations about the animals treatment and care.
(Source:
http://www.aza.org/animal-husbandry-and-welfare/)
Another thing that people assume about Zoo's is that the amount of space the animals are allotted isn't enough for them to be free and content with. There's only so much space Zoo's gave give, but they make up for what is lacking in the Enrichment process.
(Source:
http://www.aza.org/enrichment/)
In the Zoo, environmental changes are made with the goal of increasing the animal’s behavioral choices and drawing out their species-appropriate behaviors, thus enhancing animal welfare. But in lament's terms, Enrichment is basically anything to stimulate the animals brain and make life in captivity not so boring for them. This can be incorporating toys, puzzle feeders, training sessions, cuddling, interaction with other animals, scent and sound, or even TV. A good example of this would be in one Zoo there's a tug-of-war game you can play with a tiger. A group stands on one side of the fence pulling the rope while the tiger pulls from the other side. People have often mistaken this practice for abuse, but it's a good form of enrichment. It allows the tiger to play, rather than lay in the dirt bored out of it's mind, and it's also a fun experience for guests.
And finally, Zoo's do a great deal of work with outreach and education of animals. So many people don't understand animals rights and just go off of assumption based on clippings they hear from other people here and there.
People are quick to pass judgement on Zoo's based on what they don't understand, but the key fact of why Zoo's are so important, is they keep animals alive and healthy.
For more information on the AZA and their policies, you can look here:
(Source:
http://www.aza.org/ad-policy/)