Last Night's TV: Special Needs Pets, Channel 4

It was Dr Johnson who endeared himself to generations of dog lovers and feminists by comparing a woman preaching to a dog walking on its hind legs ("It is not done well; but you are surprised to find it done at all" – not one of his funniest, but it sticks).

I thought of him at the start of Special Needs Pets, which featured one brief shot of a dog walking on its hind legs owing to the absence of the two at the front, and rather more footage of dogs with prosthetic legs or wheels fitted at the back.

Laurence Turnbull's film captured a certain confusion in our views of the animal kingdom, without quite escaping that confusion itself. Once upon a time, a cat or a rabbit didn't have to have much more than a stiff cold to get the vet reaching sympathetically for the hypodermic and making soothing noises about it all being for the best; but these days we're not so confident that their lives are disposable, as Zoey, owner of Ethel the rabbit, said, "Do we put disabled people down just because they can't walk? No, we don't. So why should we put disabled animals down when they're not in pain?" Hence when Ethel was stricken with paralysis, Zoey kept her going, fitting her with a bunny nappy to keep her from getting soaked in her own waste. Ethel dragged herself around, scrabbling for grip with her front paws, back legs splayed helpless to one side; Zoey had her fitted with a custom-made cart, her rear half suspended by a harness from a pair of wheels, giving her the look of some peculiar piece of laboratory sadism. Later, she took Ethel along to Jeanette, an animal chiropractor. (What next? A fish osteopath? Cat pilates?) Jeanette prodded at Ethel's spine and explained that "within a day or two you may see a change. Or may not see a change." I wondered if there were any other options, but as it turned out, two was enough, and it was the second one.

Incontinence is a major problem with pets, even the healthy ones. We also met James, a banker, and his cat, Henry, rescued from the side of the road, where he'd come off much the worse from an encounter with traffic and nursed back to health but needing to be "expressed" several times a day. James demonstrated, holding Henry over the lavatory while he stroked his abdomen firmly, producing some musical yowling and other, less musical things. At night, though, Henry needed a nappy to maintain his self-respect.

Turnbull visited individuals who had seen the commercial potential: Karen in Newcastle upon Tyne, who has a business making nappies for pets (Catnaps? Kitty-Kack? If she wants to use either of those, I'll expect a fee), and Bob, who makes wheelchairs for dogs. We saw Bob's results with Katie, a Jack Russell who had inner ear problems and who hadn't been able to go for a proper walk for years because she kept falling over. Both Katie and Sue, her owner, were putting on weight owing to lack of exercise. But with a pair of wheels on the back, like stabilisers on a bike, she flitted round his garden like a puppy – anthropomorphising animals is reprehensible, I know, but there aren't many humans who express joy more clearly than a dog can. Dr Glyn Heath popped up, an expert in prosthetic limbs for people with a sideline in animals – he showed off a leg for a labradoodle, and a smaller one for a honey buzzard, including a movable claw for roosting. What was the smallest animal he's ever done? A rat. (Oh, good, because we want rats to get around more easily, don't we?) The largest? He could have done a decent brachiosaur if available, but these days it would be an elephant, "Which we are currently pursuing." If you're going to pursue an elephant, one that needs a false leg is probably a good choice.

Parrots, though: they're in a different league – fearfully intelligent animals, apt to go off their trolley for lack of stimulus. Try talking about the news with them, or teach them Sudoku. There was Fred, an African grey, depressed since his owner, George, died, and pecking himself to nothing; and Trouble the cockatoo, psychotically attached to his owner, pulling at his feathers until he bled. But the star was Poppy – deceptive, since he was male. Poppy's neurosis manifested itself in frenzies of masturbation. Ruth, his owner, showed off the mechanical furry rabbit that was his favourite sex toy – Poppy used to ride on his back and masturbate on his ears. He would put on a courtship display aimed at Ruth – "And then I'd feel rotten because I couldn't fulfil his needs." Turnbull voiced the question hovering in every viewer's mind: "Has he ever masturbated on you?" Or maybe just in my mind. "On my head," Ruth admitted, after which she gave in and Poppy had hormone implants. "Do you want to see what he does with a chicken bone?" Ruth asked, teasingly. What? What does he do with the chicken bone? But at that point, a commercial break intervened and we never found out. I need a Dr Johnson to make sense of it all.