Rats swimming in laboratory settings form the backbone of some of the most widely used behavioral tests in neuroscience and pharmacology. The two most prominent are the forced swim test, which measures how quickly a rat stops struggling in a water-filled cylinder, and the Morris water maze, which tracks how a rat learns to find a hidden platform in a pool. These experiments have shaped how we screen antidepressant drugs, study memory, and understand stress physiology. They have also sparked serious ethical and scientific debate about whether what we think we are measuring is what we are actually measuring.
The Forced Swim Test
The forced swim test, or FST, is one of the most commonly used animal models in depression research. A rat is placed in a cylinder of water deep enough that it cannot touch the bottom or climb out. At first, the animal swims vigorously and tries to escape. Over time, it transitions to floating with only the minimal movements needed to keep its head above water. Researchers originally interpreted this shift as “behavioral despair,” a rodent analog of the helplessness seen in human depression. Drugs that reduce immobility time, keeping the rat active for longer, were taken as evidence of antidepressant potential.1PubMed Central. The forced swim test as a model of depressive-like behavior
The test’s appeal has always been practical. It is cheap, fast, and sensitive to a broad range of antidepressant compounds. A modified version of the original protocol went further by categorizing the rat’s active behaviors into two types: swimming and climbing. This distinction turned out to be pharmacologically meaningful. Drugs that boost serotonin, like the SSRIs fluoxetine and sertraline, selectively increase swimming behavior. Drugs that boost norepinephrine, like desipramine, selectively increase climbing.2PubMed. Active behaviors in the rat forced swimming test differentially produced by serotonergic and noradrenergic antidepressants This meant the FST could do more than just flag whether a compound had antidepressant properties; it could hint at which neurotransmitter system the compound was acting through.3PubMed Central. Factors influencing behavior in the forced swim test
Other interventions besides drugs have been tested in the FST. Repetitive transcranial magnetic stimulation, a non-invasive brain stimulation technique used in human psychiatry, also reduces immobility time in rats across a range of stimulation frequencies.4PubMed. Effects of different frequencies of transcranial magnetic stimulation (TMS) on the forced swim test model of depression in rats The test has served as a quick screen for any treatment that might plausibly affect mood.
Is Immobility Really Despair?
The behavioral despair interpretation has come under sustained criticism. A growing body of evidence suggests that when a rat stops swimming and starts floating, it is not “giving up” in any emotional sense. Instead, it may be making a smart energy-conservation decision. One line of research frames the switch from active to passive behavior as an adaptive response to an inescapable stressor, a form of behavioral flexibility tied to learning and survival rather than hopelessness.5PubMed. Immobility in the forced swim test is adaptive and does not reflect depression
Another hypothesis focuses on body temperature. Rats lose heat quickly in water, and immobility may be a thermoregulatory strategy. By minimizing movement, a rat reduces convective heat loss to the surrounding water, which could improve its chances of surviving a prolonged submersion. Research consistent with this idea found that immobility patterns align better with a heat-conservation model than a despair model.6PubMed. Thermoregulatory significance of immobility in the forced swim test If immobility is partly about staying warm rather than feeling hopeless, that reframes what antidepressant drugs are doing when they “reduce immobility.” They may be altering arousal, activity levels, or thermoregulation rather than reversing a depression-like state.
These critiques matter because the FST has been used for decades to decide which drug candidates move forward in development. If the test measures something other than what we assumed, some of those decisions may have been based on a misreading of the data. The test has high “predictive validity,” meaning it correctly identifies many drugs that do work as antidepressants in humans, but the mechanism linking test performance to therapeutic effect may not be the one originally proposed.
Regulatory Pushback and the Shift Away From the FST
Ethical and scientific concerns have converged to push the FST toward retirement in several countries. The UK, guided by the National Centre for the Replacement, Refinement and Reduction of Animals in Research, now rarely approves FST proposals unless researchers can demonstrate that no alternative exists. Ethics committees routinely reject or demand revisions to study designs that include it.7Frontiers in Animal Science. Re‐evaluating the forced swim test: ethical, scientific, and regulatory drivers for validated alternatives Regulatory bodies from the UK to India now restrict the test, citing poor translatability to human depression. Alternatives gaining traction include the sucrose preference test, which measures a rodent’s interest in a sweet reward as a proxy for anhedonia, and newer approaches using human stem-cell-derived neuron models that bypass animal testing altogether.8Frontiers in Animal Science. Re‐evaluating the forced swim test: ethical, scientific, and regulatory drivers for validated alternatives
The pharmaceutical industry has also started stepping back. Several major drug companies announced in the late 2010s and early 2020s that they would no longer use the FST in internal research. The combination of scientific doubt about what the test measures and growing public concern about animal welfare has made it harder to justify, particularly when the test involves obvious distress to the animals involved.
The Morris Water Maze
If the FST puts rats in water to measure mood, the Morris water maze puts them in water to measure memory. Developed in the early 1980s, the maze consists of a circular pool filled with opaque water. Somewhere below the surface sits a small platform the rat can stand on. The rat cannot see the platform from the water’s surface, so it has to learn its location using visual cues around the room: posters on the walls, the position of a door, the shape of a lamp. Over repeated trials, a healthy rat learns to swim directly to the platform instead of searching randomly.9PubMed Central. Morris water maze: procedures for assessing spatial and related forms of learning and memory
The original procedure also included variations for studying different aspects of memory. Working memory versions change the platform location each day, forcing the rat to update its mental map. Non-spatial versions test whether rats can learn other kinds of discrimination tasks in the same apparatus. The speed with which rats pick up these tasks made the water maze attractive for pharmacological research and studies of brain lesions.10Journal of Neuroscience Methods. Developments of a water-maze procedure for studying spatial learning in the rat
Much of what we know about the hippocampus and spatial memory comes from water maze experiments. The hippocampus, a seahorse-shaped brain structure deep in the temporal lobe, is critical for forming and retrieving spatial memories. Blocking certain receptors in the hippocampus after a rat has already learned the platform location can actually prevent the normal decay of that memory, suggesting these receptors play a role not just in learning but in the natural process of forgetting.11PubMed. Post-acquisition hippocampal NMDA receptor blockade sustains retention of spatial reference memory in Morris water maze
How Aging Changes Water Maze Performance
The water maze has become a standard tool for studying cognitive decline. When researchers compare young rats to aged ones (typically around two years old), the older animals consistently take longer to find the platform and use less efficient search strategies. Young rats tend to swim in a fairly direct path, while aged rats often rely on indirect approaches, circling the pool or following the wall before stumbling onto the platform. These indirect strategies can still get the job done, but they reflect a reduced capacity for precise spatial navigation.12PubMed Central. Aged rats learn Morris Water maze using non-spatial search strategies evidenced by a parameter-based algorithm
Not all aged rats decline equally. When large groups of old rats are tested, some perform nearly as well as young animals, while others are clearly impaired. Researchers classify these as “cognitively unimpaired” and “cognitively impaired” aged rats, and then look for neurobiological differences between the two groups, differences in receptor density, gene expression, or brain structure that might explain why some individuals age more gracefully than others.13PubMed. Neurotensin receptor levels as a function of brain aging and cognitive performance in the Morris water maze task in the rat This individual-differences approach has made the water maze valuable for testing potential cognitive enhancers. Compounds like phosphatidylserine, a fatty molecule found in cell membranes, have been shown to improve escape times in aged rats, suggesting some degree of neuroprotective activity against age-related cognitive decline.14PubMed. Krill phosphatidylserine improves learning and memory in Morris water maze in aged rats
Swimming as an Exercise Model
Beyond behavioral testing, swimming is widely used as a form of controlled exercise in rat physiology research. Rats can be trained to swim for progressively longer bouts, making it possible to study the effects of chronic aerobic exercise on the heart, brain, and metabolism. One of the best-documented effects is exercise-induced cardiac hypertrophy: the heart gets bigger and stronger. Trained rats develop hearts roughly 20% heavier than sedentary controls, with improved pumping efficiency and better mechanical performance.15Comparative Biochemistry and Physiology Part D: Genomics and Proteomics. Proteomic adaptation to chronic high intensity swimming training in the rat heart Detailed hemodynamic measurements confirm that this is a healthy form of hypertrophy: stroke volume and ejection fraction go up, while the heart’s stiffness stays the same and relaxation actually improves.16PubMed. Rat model of exercise-induced cardiac hypertrophy: hemodynamic characterization using left ventricular pressure-volume analysis
At the molecular level, swim training activates a specific signaling pathway in the heart that drives this beneficial growth. Swimming exercise increases the activity of key proteins in a cellular growth cascade and regulates small RNA molecules called microRNAs that fine-tune gene expression in heart tissue.17PubMed. Swimming exercise training-induced left ventricular hypertrophy involves microRNAs and synergistic regulation of the PI3K/AKT/mTOR signaling pathway This molecular detail matters because it helps distinguish “athlete’s heart,” a benign form of enlargement, from the pathological hypertrophy seen in heart disease.
The brain benefits from swimming exercise too. Rats exposed to chronic mild stress, a model of depression involving weeks of unpredictable minor stressors, show reduced levels of brain-derived neurotrophic factor (BDNF) in the hippocampus. Swimming exercise reverses this decline, boosting BDNF expression back toward normal levels.18PubMed Central. The Impacts of Swimming Exercise on Hippocampal Expression of Neurotrophic Factors in Rats Exposed to Chronic Unpredictable Mild Stress BDNF supports the survival and growth of neurons, so this finding connects exercise to neuroplasticity in a way that may partially explain the antidepressant effects of physical activity. Even maternal exercise matters: when pregnant rats swim regularly, their pups show enhanced hippocampal neurogenesis and better short-term memory compared to pups from sedentary mothers.19PubMed. Maternal swimming during pregnancy enhances short-term memory and neurogenesis in the hippocampus of rat pups
The Stress Hormones Behind the Swim
Any time a rat is placed in water involuntarily, a stress response kicks in. The hypothalamic-pituitary-adrenal axis ramps up, flooding the bloodstream with corticosterone (the rodent equivalent of cortisol in humans). This happens whether the water exposure is a brief FST session or a longer swim stress protocol. What makes it interesting is how different rat strains respond. Wistar-Kyoto rats, which are often used as a model of anxiety and depression-like behavior because of their naturally high immobility in the FST, also show exaggerated corticosterone and ACTH secretion during swim stress compared to Sprague-Dawley rats. Their stress hormones spike higher and take longer to come back down, possibly because of reduced sensitivity in the feedback system that is supposed to shut the stress response off.20Psychoneuroendocrinology. Amplified behavioral and endocrine responses to forced swim stress in the Wistar–Kyoto rat
Sex differences add another layer. Forced swimming alters brain corticosteroid receptors differently in male and female rats, and when swim stress is layered on top of prior restraint stress, the feedback mechanisms behave differently depending on sex. Female rats show decreased serotonergic activity in the hippocampus and hypothalamus after the FST, while males show increased serotonergic activity in the hypothalamus. These neurochemical sex differences map onto behavioral differences as well, such as head-swinging behavior that appears predominantly in males.21PubMed. Sex differences in behavioral, neurochemical and neuroendocrine effects induced by the forced swim test in rats The broader point is that what looks like one test producing one measurement is actually an interaction between the stressor, the animal’s sex, its strain, and its prior experience, all filtering through the same behavioral output of swimming or floating.22Neuroendocrinology. Forced Swimming Differentially Affects Male and Female Brain Corticosteroid Receptors
Water Temperature, Tank Size, and Other Hidden Variables
One of the most underappreciated aspects of rats swimming experiments is how much the results depend on seemingly minor procedural details. Water temperature is a major variable. Rats behave quite differently at different temperatures: they are more active in cooler water and more immobile in warmer water, though very cold water (around 19°C) can actually increase immobility again, probably because the cold itself becomes overwhelming. The generally accepted sweet spot is around 25°C.23Thrita Journal of Neuron. Direct and Indirect Factors Affecting the Forced Swim Test to Investigate the Level of Depression in Rodents
Studies directly comparing different temperatures have found that rats in 20°C and 25°C water try to escape faster than those in 30°C water, and the colder groups develop more severe hypothermia afterward, with core body temperature taking 30 to 40 minutes to return to normal. At 30°C, body temperature barely drops. Stress hormones go up regardless of temperature, but the pattern is cleaner at warmer temperatures because the cold itself is no longer acting as an additional confound.24PubMed. Impact of water temperature and stressor controllability on swim stress-induced changes in body temperature, serum corticosterone, and immobility in rats
Tank dimensions matter too. Standard FST protocols call for a water depth of 30 to 60 centimeters and a tank diameter of 20 to 30 centimeters. If the water is too shallow, the rat can touch the bottom and brace itself, which changes the behavioral dynamics entirely. If the tank is too wide, the rat may swim to the wall and use it as a resting point. These details rarely make it into popular descriptions of the test, but they can dramatically affect the data, which is one reason results sometimes fail to replicate across laboratories.25Thrita Journal of Neuron. Direct and Indirect Factors Affecting the Forced Swim Test to Investigate the Level of Depression in Rodents
The Diving Reflex in Rats
Most swimming experiments keep the rat’s head above water, but when a rat does submerge, something remarkable happens. Underwater submersion triggers the diving response: the rat stops breathing, its heart rate drops sharply through a parasympathetic reflex, and blood vessels in its limbs constrict to shunt blood toward the brain and vital organs. This reflex, sometimes called the most powerful autonomic reflex known, is present across mammals. Research has shown that the hemodynamic changes during voluntary diving in rats closely resemble those of naturally diving marine mammals, making the rat a useful laboratory model for studying the physiology of breath-hold diving.26PubMed Central. The rat: a laboratory model for studies of the diving response
This finding is surprising given that rats are not aquatic animals. But the reflex is deeply conserved across mammalian species, and it activates automatically on facial contact with water. Understanding it in rats has implications for human medicine, particularly for conditions like cardiac arrhythmias triggered during swimming or cold-water immersion.
Wild Rats Are Very Different Swimmers
Nearly all published rat swimming experiments use domesticated laboratory strains: Wistar, Sprague-Dawley, Long-Evans, and Brown Norway rats. These animals have been bred in captivity for dozens or hundreds of generations, and their relationship with water has changed dramatically as a result. When researchers directly compared wild-type rats with these common lab strains in a setup where animals could choose whether to enter water, the differences were stark. Most lab rats refused to cross water voluntarily, limiting themselves to sniffing the surface, dipping a paw in, or drinking from it. Wild rats, on the other hand, readily swam across a 60-centimeter water crossing, dove beneath the surface, and explored underwater. Spontaneous diving was observed only in wild rats. Not a single lab animal displayed that behavior.27PLoS ONE. Species Specific Behavioural Patterns (Digging and Swimming) and Reaction to Novel Objects in Wild Type, Wistar, Sprague-Dawley and Brown Norway Rats
This raises an interesting question about what we are really measuring in lab-based swimming tests. A wild rat encountering water is likely drawing on a repertoire of natural behaviors: foraging, escaping predators, navigating its environment. A lab rat placed in a cylinder of water has none of that context. Its response may be shaped more by the novelty and inescapability of the situation than by any analog of wild swimming behavior. The domestication process itself may have altered traits like boldness, stress reactivity, and comfort in water, which are exactly the traits the FST and water maze are designed to probe.
Swimming After Spinal Cord Injury
Swimming has also found a niche as a rehabilitation tool in rat models of spinal cord injury. Because water supports the animal’s body weight, rats with hindlimb impairments can still move their legs in a swimming motion even when they cannot walk on land. Researchers studying task-specific locomotor retraining found that healthy rats show stable swimming kinematics over weeks of daily practice, meaning the movement does not change with experience once the animal is acclimated. After spinal cord injury, however, swim retraining led to meaningful improvements in hindlimb range of motion and limb positioning, even though the speed of leg movements remained slow.28PubMed Central. Swimming as a model of task-specific locomotor retraining after spinal cord injury in the rat The buoyancy of water makes swimming a naturally lower-threshold motor task, which means it can serve as a bridge toward overland locomotion in recovery protocols.