Swimming with dolphins appears to help alleviate mild to moderate depression, researchers have found.
Dolphins are increasingly being used in therapeutic treatments
A University of Leicester team tested the effect of regular swimming sessions with dolphins on 15 depressed people in a study carried out in Honduras.
They found that symptoms improved more among this group than among another 15 who swam in the same area - but did not interact with dolphins.
The study is published in the British Medical Journal.
All the volunteers who took part in the trial stopped taking antidepressant drugs or undergoing psychotherapy at least four weeks beforehand.
Half the volunteers swam and snorkelled around dolphins for one hour a day over a two-week period.
The others took part in the same activities, but without dolphins around.
Two weeks later, both groups showed improved mental health, but especially so among patients who had been swimming with the dolphins.
The researchers say dolphins' aesthetic value, and the emotions raised by the interaction may have healing properties. Some have speculated that the ultrasound emitted by dolphins as part of their echolocation system may have a beneficial effect.
The Leicester team believe that using animals in this way could be a productive way to treat depression and other psychiatric illnesses.
Researcher Professor Michael Reveley said: "Dolphins are highly intelligent animals who are capable of complex interactions, and regard humans positively.
"Some people who are depressed may have issues with other humans, and might respond more positively to other types of interaction.
"We need to remember that we are part of the natural world, and interacting with it can have a beneficial effect on us."
Dolphin therapy is already used to help children undergoing rehabilitation for a range of conditions.
Shared brain system
Dr Iain Ryrie, research programme director at the Mental Health Foundation, said that humans and dolphins shared a limbic brain system that plays a key role in regulating many of the body's physiological and emotional processes.
He said: "Emotional contact is a biological need for mammals, stimulating their limbic systems, ensuring the suckling response and providing gentle encouragement toward ever more maturity.
"As humans we are hard-wired to need touch and to be connected to others, something that differentiates us from reptiles say, who don't have a limbic communication system and who are not suckled.
"So it's possible for humans to make loving relationships with many different mammals because of this biological/social similarity."
Dr Ryrie said research had shown the symptoms of depression could be ameliorated by pet assisted therapy.
The technique had also been shown to aid young people with attention deficit hyperactivity disorder, and older people with dementia.
"Animals, and especially mammals, can favourably change our social dynamic, which is typically one of withdrawal and increasing isolation among people with depression.
"Swimming with and caring for dolphins as a group activity in a vacation context is very likely therefore to alleviate depression."
However, he said researchers would probably do better to focus their efforts on animal interactions that were more readily available closer to home.