Swimming with dolphins is an effective treatment for mild to moderate depression, say researchers in this week’s BMJ.

Their findings support the theory of biophilia, which shows how human health and wellbeing are dependent on our relationships with the natural environment.

The study was carried out in Honduras and involved 30 patients diagnosed with mild or moderate depression. Half were assigned to the experimental group and half to the control group.

Over a two-week period, participants in the experimental group swam and snorkelled in the water with dolphins for one hour a day. Participants in the control group were assigned to the same water activities, but in the absence of dolphins, to control for the influence of water and the natural setting.

All participants discontinued antidepressant drugs or psychotherapy at least four weeks before entering the study, and were not allowed to take drugs during the study. Depression scores were measured before the study and at the end of treatment.

Although some participants dropped out of the study, the average severity of the depressive symptoms was more reduced in the experimental group than in the control group.

Animal facilitated therapy with dolphins is more effective than water therapy in treating people with mild to moderate depression, say the authors. Despite some study limitations, the effects exerted by the animals were significantly greater than those of just the natural setting.

The echolocation system, the aesthetic value, and the emotions raised by the interaction with dolphins may explain the mammals’ healing properties, they suggest.

Three months after the study, participants in both groups also reported lasting improvement and did not require treatment. This suggests that in patients with mild or moderate depression, using drugs or conventional psychotherapy may not be necessary when biophilic treatment with animals is used, they conclude.