Singer would also claim that there is no actual utility in abusing animals in this way, and that it does not promote the greatest good for the greatest number for either the animal or human communities. Even if an individual animal has no inherent rights (as Tom Regan would claim), the human being has a moral duty to respect all animals with the capacity for suffering.
It is important to alleviate suffering, and therefore important to liberate the animals in their collective condition of oppression in an exploitative zoo. Tom Regan would argue that the exploitative zoo infringes on the rights of the individual animals -- as Regan does claim that animals have the same rights as human beings. Therefore, it is more likely that Regan would denounce all zoos including those designed for species preservation -- because Regan would not place any perceived utility before rights. Regan's thesis is based on a belief that animals have inherent value, worth, and consideration. Even if the zoo is helping critically endangered species, it does so at the expense of the dignity and intrinsic worth of the individual animal. Although animals cannot express their desires in ways other than the fight or flight response, humans can assume that no animal would voluntarily live in a zoo.
Both Singer and Regan would argue that there is an express ethical duty to treat animals with respect and without inflicting any pain. Singer would say that any animal that has the capacity for suffering should be spared from that suffering. If a zoo ensures that their animals are being treated well, and if the zoo can demonstrate that their actions are carried out in accordance with altruistic, albeit utilitarian, principles, then Singer would support zoos. Regan might prefer to open all the cages and let the animals choose for themselves.