Monkey man

"He didn't often write in books so this one is comparatively rare," he says, before reading aloud the dedication: "I hope you won't hold this drivel against me - it really is a pale mask as I hope you will discover." Morris looks not unlike the elderly Brando, and seems to enjoy impersonating the actor's distinctive voice. Their relationship began shortly after Morris presented a television series called The Human Animal in the mid-1990s. He had just arrived home from Africa when the phone began ringing. "I'm afraid I didn't take it seriously," he says. "But then the voice said, I want to discuss the banality of evil', and I realised it wasn't a wind-up." Their friendship lasted until the actor's death in 2004, a lonely Brando calling from Los Angeles night or day and talking for hours. "A lot of people said he was very difficult but I found him charming," says Morris, "although I did see the Godfather in him once when we'd been out and he was mobbed by paparazzi."

Pins jut out of a world map on the wall. Morris has visited 90 countries, the latest being a clutch of South American nations last year. On a table next to the map there is a collection of crystal globes - one made of "the sludge that human life came from"; another fashioned from fossilised coral. He tries to buy at least one new globe per foreign trip. When I ask him if they are easy to track down he looks horrified. "Oh no ... If you're collecting something it musn't be easy. It's like a goal in football. Someone suggested we should make the goalposts wider so there would be more scoring. Ridiculous ... They should be extremely difficult to get because that's what makes them so exciting."

Ostensibly, we should be discussing Morris's new book The Naked Man, a study and reflection on the male sector of the species: from head to toe, through the ages, from jungle-dweller to urban monkey. The book tackles conundrums such as why men shave their faces, and why, unlike other mammals, they have no bones in their penises. Morris wants to plug the book but his heart is not fully in it. Its predecessor, The Naked Woman, was a far more attractive proposition. "I'm more interested in women than men to be honest so I enjoyed writing The Naked Female enormously. I learned so much about females, although it did make me wish I'd known as much when I was 18. When the publishers asked for a sequel it was obvious what it had to be, even though I was quite reluctant to write it. But then I started discovering things about the human male and it became exciting."

At least part of Morris's love of women stems from his childhood, when he was raised by females due to his father's protracted illness. An army captain who was gassed in the trenches during the first world war, Morris Sr withered slowly, writing children's stories that would never be published. By the time he died, the second world war had already begun. The gloom had a profound impact on Morris. "As a child they put me in a little cadet uniform and I knew that, if I ever got past sixth form while the war was still on, I would have to go out and kill people," he says. "That's just what adults did and that's one of the reasons I turned to other animals and became a zoologist."

By the time he completed his doctorate at Oxford - on the sex life of the stickleback - he had already met his wife, Ramona, who would work with him behind the scenes during his 11 years presenting Zoo Time for Granada TV. He points to a beautiful black and white photograph, taken in the 1960s. Ramona looks at the viewer, a glass in her hand, a python wrapped casually around her wrist. "She loves any animal that's persecuted," says Morris. "Crows, snakes ..." The couple met at the party of an eccentric art collector, James Bomford, when Morris was 21. During a game of hide and seek, Morris followed Ramona and her art critic wooer into a room, hiding under a bed. When the lights went on he discovered he'd been fondling the art critic's hand and not hers. She appreciated the gesture.

Later, when catching wild rabbits for an experiment, Morris would drive through a field in the darkness with Ramona on the bonnet of his car. With the creatures caught in the headlights, he would slam on the brakes, propelling Ramona through the air to pounce on the rabbits. "I was lucky to find someone who was such an exciting personality," he says. "Now that we're both old, it's a more intense love than ever before because we've become almost like one person. It's an extraordinary relationship, and we're both still completely surprised by it."

Another impulse driving Morris has been painting, which he does in the second room - "my right hemisphere" - of his studio. His early surrealist works now sell for around £8000 at auction. The walls teem with books on modern and tribal art - "I've never been interested in what's in-between" - and surround a triptych reminiscent of works by Dali and Joan Miro, with whom Morris once exhibited. Amorphous shapes in odd colours suggest life, but not as we know it. "I wanted it to feel like these creatures really existed but not on our terms," he says, "and that goes for all the paintings I've done over the last 60 years. If I want an organism to have a purple body instead of a green body that's up to me. It's an enormous escape from what I do next door, where I try to record our world as accurately as I can. Here, I can be as imaginative as I like."

THE room's decor includes a selection of Chinese operatic masks, the protective "eyes" from Maltese fishing boats, and at least a dozen ceramic cats, beckoning, in Japanese style, paw down. Morris bought them at the Goruki-Ji Temple in Tokyo, which is dedicated to the worship of cats. The legend goes that two samurai were saved when a feline beckoned them towards the temple, seconds before lightning struck the spot where they had been standing. Morris has a biological explanation. "Cats are really sensitive to thunderstorms and when they get disturbed they wash their faces, which looks like the Japanese beckoning motion. I've no proof of course but it gave me a tremendous kick to put those facts into the story."

Critics of Morris would pounce on such a statement with their claws out, arguing that he has built a career on presenting unsubstantiated claims as pure fact, subverting science in the interest of sensationalism. Yet none of his books has stirred the same controversy as The Naked Ape, despite presenting much the same argument Charles Darwin had advanced more than a century earlier. Classifying humans as apes, albeit without the full covering of hair, ruffled many feathers - particularly among the church establishment. Morris is still flabbergasted that the book caused such a furore, although the publicity helped it sell 12 million copies. Prior to its publication, Morris had finished working as the presenter of Zoo Time, and had just taken up directorship of the Institute of Contemporary Arts (ICA) in London. Following its success, he packed up and moved to Malta with Ramona for five years, buying two Rolls-Royces, a 27-room manor and a yacht, rattling though the money in a lavish Mediterranean lifestyle. He can't remember the exact figures but says in today's money it would have been millions.

"It wasn't squandering the money but employing it," he says. "I was still only in my late 30s and was able to do what I'd always wanted to do, which was paint and write. If I'd been cautious and invested the money I needn't have worked again, I could have sat and looked at the sunset, but who wants to do that? I wanted to entertain my friends and have a wonderful time, then go back to work, and that's exactly what I did."

Much of the research for Manwatching - the book that popularised the concept of body language - was done in Malta, although the writing was done in the studio where we are today. Morris didn't plan to stay in the property for so long but says he is unlikely to move now. He turns 80 later this month and, as a lifelong atheist, his thoughts are increasingly on mortality and what he expects to be a dreamless sleep.

"I would love for there to be another form of existence," he says. "I'm not immune to those thoughts. I don't relish the idea that things are going to stop, and for me they're going to stop pretty soon because I'm past my sell-by date." He pauses, points to a book he has written on canines. "It would be wonderful to be a dog because you'd have no concept of death." Humans have it harder, he says, due to our capacity for language - the future tense in particular and the awareness of our inevitable full stop. "The force of religion comes from the animal fear of extinction, and the need to dampen our fear. All over the world, cultures developed ideas of an afterlife that meant when you died you went on and lived in another world somewhere. It's had a tremendous impact on our development. There wouldn't be any suicide bombers if they didn't believe in heaven."

BUT Morris remains optimistic. His faith in humankind has come full circle from the pessimism of his youth, which is heartening considering he has spent a lifetime studying human behaviour. Despite being unpredictable as a species, he thinks that life - even in the context of nuclear weapons, overcrowding and global warming - will prevail. He predicts that the next big innovation in technical terms will be anti-gravitational cars, a far-flung idea, but no more so than the internet would have seemed when he was a young man. And, in biological terms, he predicts immortality.

"It's slightly terrifying to say, but death is not inevitable. Our built-in obsolescence is a genetic device to keep our species pliable. The one factor that causes death is the increasing inefficiency of cell replacement and eventually some biologist will be able to knock out that factor. Given the problems we have with overcrowding already, the whole thing will be a really interesting nightmare, although I won't be around to see it."

He picks up a black-and-white portrait of his younger self, with more hair, but still naked in comparison to Congo, the celebrity chimp from Zoo Time. Back in the 1950s, Morris gave Congo a pencil and encouraged him to draw what would later become his famous fan-shaped etchings. The resulting exhibition on "the birth of art" in 1957 at the ICA was a huge success. Joan Miro bought one of Congo's works, as did Picasso. When he was asked by a journalist in his French studio how the picture made him feel, Picasso left the room, before returning, arms swinging low, and biting the journalist. Morris remembers it as a fascinating period that inspired the one big project he has ambitions to complete.

"I would love to write a book on the origin of art," he says. "I've been planning it for years but it always gets pushed back by other things. How did art begin? How did we become an artistic animal? It's an extraordinary phenomenon and one of the greatest unsolved mysteries." He puts the portrait of man and his closest relative back on the floor, and wheezes. "I want to write that one before I conk out."