A behavioral assay that forces lab animals to swim for several minutes or until they sink is coming under increasing criticism from scientists and animal rights activists for being potentially both uninformative and cruel, according to a recent report in Nature (July 18). The forced-swim test, which involves making a rodent try to stay afloat in a tank of water and monitoring how much time it spends immobile, has long been a staple technique in research on depression and other mood disorders, but is now being phased out or discouraged by an increasing number of research institutions, funders, and pharmaceutical companies.
“The National Institute of Mental Health has for some time been discouraging the use of certain behavioral assays, including the forced swim and tail suspension test, as models of depression,” Joshua Gordon, director of NIMH, tells Nature in a statement. “While no single animal test can capture the full complexity of a human disorder, these tests in particular are recognized by many scientists as lacking sufficient mechanistic specificity to be of general use in clarifying the neurobiological mechanisms underlying human depression.”
Developed in the 1970s, the forced-swim test involves putting a mouse or rat into a tank of water too deep for the animal to stand in, and monitoring how long the animal tries to swim or escape. The test is typically halted after six minutes for mice and after up to 15 minutes for rats—or earlier if the animal begins sinking.
Traditionally, researchers have associated immobility in the water with a lack of drive, equivalent to the symptoms of human depression. But that interpretation has been criticized in recent years. Researchers have pointed out that water temperature and other factors seem to affect the results, and that rodents may stop swimming not because they’ve run out of motivation, but because they learn they’ll be lifted out of the tank if they sink.
Animal rights group People for the Ethical Treatment of Animals (PETA) recently upped its pressure on NIMH to halt funding for projects relying on the test, writing to the agency on July 12 that the tests “create intense fear, anxiety, terror, and depression in small animals,” without generating useful data, Nature reports. The group has also targeted pharmaceutical companies, and reported earlier this month (July 3) that Roche Pharmaceuticals had committed to end its use of the test, joining a number of other major pharma companies including AbbVie and Johnson & Johnson.
Critics of the test hope that better, in vitro techniques such as organoids can take the place of the forced-swim test and other behavioral assays that create stress in animal models. But some researchers argue that behavioral tests will always be necessary to study human behavioral disorders, and until better ones are found, the tests remain “crucial” for some scientific questions, Gordon tells Nature.
Clarification (July 23): This article has been updated to reflect the fact that the test is conducted for a finite amount of time—usually six minutes for mice—but is halted before that if the rodent sinks.
Catherine Offord is an associate editor at The Scientist. Email her at firstname.lastname@example.org.