Swimming is good for your body but swimming with dolphins is good for your soul.

The healing power of dolphins has been widely promoted, but in the first controlled trial researchers have shown that an hour a day in the water with the sociable aquatic creatures is an effective treatment for mild to moderate depression, and better than swimming with other humans .

Psychiatrists from the University of Leicester compared two groups of patients with depression, half of whom swam and snorkelled with dolphins while the other half spent the same time snorkelling with each other on a coral reef in the absence of the dolphins.

In the study, at the Roatan Institute for Marine Sciences in Honduras, all participants stopped taking antidepressants or psychotherapy at least four weeks before the start of the treatment and their depression score was measured.

After two weeks, results showed the group who had swum with the dolphins had improved significantly more than the control group.

Three months after the study, participants reported lasting improvement in their symptoms which did not need treatment.

The authors say the natural setting of the island with its coral reef was an important factor in the treatment.

But they add: "The effects exerted by the animals were considerably greater than those of just the natural setting.

The echo-location system [the sounds the dolphins emit to navigate] the aesthetic value and the emotions raised by the interaction with dolphins may explain the animals' healing properties." The finding confirms the importance of biophilia, the recognition that human health and wellbeing are dependent on our relationships with the environment, they say.

The study is in the British Medical Journal, which has a themed issue on human and animal health.

A separate article notes that half of the households in the UK own a pet and 90 per cent of owners regard their pet as a valued family member.

Pet ownership provides social benefits and emotional support and some research suggests it may mirror the benefits of human relationships, the authors say.

And restoring health through contact with nature, known as ecotherapy, may involve working with wildlife, on conservation projects or in a garden, according to a third paper in the BMJ.

Ambra Burls, a senior lecturer in mental health at Anglia Ruskin University in Essex said that smaller animals such as squirrels and owls had been used successfully in therapy for children with emotional and behavioural problems.

A project to develop and maintain a community garden in west London had helped in the rehabilitation of mentally ill people.

"Since the Industrial Revolution we have become detached from nature and we all have a need to be in close contact with nature.

It is not just because it is healthy in a physical sense but because we are a part of it."English Nature has a strategy to reconnect people with nature after research showed that activities from walking to drystone wall building reduced stress, fatigue and depression.

A spokesman said: "The activities quite strongly enhanced mood.

Drystone walls last for 200 to 300 years which provide a good metaphor for endurance."