Animals that are not domesticated are generally much worse at adapting to conditions imposed by humans than are farm and companion animals. It's true that some circus animals are kept in good conditions but too many are not.
Elephants may have shackles on their legs for most of their lives and other animals are tied or kept in small cages. Shackled, tied, or closely confined animals spend long periods showing stereotypies, repeated movements with no apparent purpose, indicating poor welfare.
Elephants may swing their heads and trunks or make other rocking movements, and big cats pace. Closely confined circus animals may also be unresponsive to events around them.
In humans, stereotypies and extreme unresponsiveness are indicators of depression and of other disorders associated with inability to have control of environmental impacts. The welfare of circus animals showing such abnormalities is poor for the same reason.
The cages used for animals while travelling are often small in comparison with good zoo conditions and the animals may be confined in them for many weeks or months. All circus elephants, lions, other big cats, bears and some horses and hoofed animals are kept in close confinement for all or much of the year.
The welfare of closely confined or shackled circus animals is so poor that the practice should be prohibited.
All animals used in circus acts are subject to training by circus staff. It is possible to train some animals such as dogs and horses, using kind methods. However, some circus trainers have been shown to use harsh methods, using physical violence or electric goads that elicit an extreme response. Indeed, many of the "tricks" performed by circus animals are unlikely to be carried out without the use of severe punishment during training. Such punishment leads to pain and fear.
The more dangerous the animal, the greater the likelihood that significant pain and fear will be caused to it. For example, elephants are typically trained to be ridden using a knife pushed into the neck if they do not obey, at least when the rider feels at risk. To stay safe, trainers must provoke fear in lions, tigers and leopards. Dangerous animals should not be trained for public entertainment purposes.
Horses and dogs can be trained humanely to carry out some activities but the likelihood that cruel procedures will be used increases with the difficulty for the animals of performing as the trainer wishes.
Circus trainers generally want to demonstrate more and more extreme "tricks" so the use of even these species will often involve some cruel training. A further problem for many animals is that they have to put up with large groups of noisy people, especially during performances but sometimes also in their travelling cages if these can be visited by the public. Some of the effects on the animals of the roar of the crowd are obvious but the situations would probably be found to be distressing to most of the animals if this were measured scientifically. The only way to avoid all of these problems would be to prohibit the use of animal acts.
Donald Broom is professor of animal welfare, Department of Veterinary Medicine, University of Cambridge