The capacity for anxiety is a deeply ancient and conserved emotional state across the animal kingdom, overlooking the common belief that anxiety is a uniquely human experience. This state, which evolved to promote survival, is a natural response to uncertainty and potential threat. Anxiety manifests through observable behaviors and measurable physiological changes in common household pets and exotic wildlife. Understanding this breadth provides insight into the shared biological foundations of emotional life.
How Scientists Define Anxiety in Animals
Scientists differentiate anxiety from simple fear by examining the nature of the threat. Fear is a reaction to an immediate, present threat, triggering an acute “fight or flight” response. Anxiety, conversely, is the apprehension or uneasy anticipation of a potential future threat or danger, often generalized and without an immediate, identifiable trigger.
The assessment of animal anxiety relies on three primary categories of indicators. Behaviorally, researchers look for signs like avoidance, excessive vigilance, and repetitive or compulsive actions, such as pacing or excessive grooming. Physiologically, anxiety is tracked through changes in the autonomic nervous system, including increased heart rate, rapid breathing, and heightened levels of stress hormones.
The third indicator involves neurobiological markers, which examine the underlying brain activity. In controlled research settings, scientists measure the activation of specific brain regions and the presence of neurotransmitters associated with stress. This framework allows for diagnosing and studying anxiety-like states in non-human animals.
Documented Anxiety in Domesticated and Wild Mammals
Companion animals frequently exhibit anxiety disorders, with separation anxiety in dogs being one of the most widely studied examples. Dogs suffering from this condition may engage in destructive chewing, urinating or defecating indoors, and excessive vocalization, such as howling or whining. Physiologically, these dogs show a significantly higher mean heart rate during separation compared to non-anxious dogs, and elevated salivary cortisol levels are often present.
In cats, generalized anxiety often presents as changes in grooming habits, leading to bald patches from excessive licking, or inappropriate elimination outside the litter box. Anxious felines may also become hyper-vigilant, pace restlessly, or hide, sometimes displaying physical signs like dilated pupils or increased respiratory rates. Beyond domestic species, primates, particularly chimpanzees, can exhibit signs comparable to Post-Traumatic Stress Disorder (PTSD) and depression following traumatic events like maternal separation or social isolation. Golden snub-nosed monkey mothers exposed to infanticide attempts show a heightened responsiveness to alarm calls and a significant reduction in prosocial behaviors, mirroring hyperarousal and social withdrawal symptoms.
Rodents, such as mice and rats, are extensively used in laboratory models to study anxiety, often through behavioral tests that rely on their natural avoidance of open spaces. In the elevated plus maze test, an anxious rodent will spend less time exploring the open arms and more time in the closed, protected arms. This type of avoidance, known as thigmotaxis, is a key metric for measuring anxiety-like behavior in research.
Evidence of Anxiety in Non-Mammalian Species
The experience of anxiety is not limited to mammals. Birds, especially parrots, frequently develop feather-damaging behavior (feather plucking), considered an abnormal repetitive behavior linked to psychological distress. This self-mutilation often begins as a displacement behavior in response to chronic stress from an inadequate environment, such as a lack of social interaction or a change in routine.
Reptiles, once thought incapable of complex emotions, also display signs of anxiety. Studies on red-footed tortoises, for example, show that individuals exhibit more anxious behavior when exposed to a novel environment. This includes physiological changes like increased heart rate during handling and behavioral responses such as refusing to eat or hiding.
Fish demonstrate measurable anxiety-like states, often assessed by changes in their ventilation rate (opercular beats per minute). An increase in this rate is a physiological indicator of stress and disturbance in fish species like the Nile tilapia. Exposure to novel or threatening stimuli can cause fish to alter their swimming patterns, sometimes exhibiting erratic darting or increasing their tendency to avoid the center of their tank, a behavior similar to the thigmotaxis seen in rodents.
Shared Biological Mechanisms Underlying Animal Anxiety
The widespread presence of anxiety across diverse animal groups is rooted in a conserved neurological architecture. The limbic system, a network of brain structures involved in emotion and memory, plays a central role, particularly the amygdala. This region functions as the brain’s alarm center, processing threats and initiating the anxiety response in creatures from fish to primates.
This response is chemically mediated by a shared array of neurotransmitters and hormones. Serotonin, dopamine, and GABA (gamma-aminobutyric acid) are all conserved neurochemicals that modulate anxiety levels across species. Disruptions in the balance of these systems are linked to pathological anxiety in both humans and animals.
The physical manifestation of chronic anxiety is largely controlled by the hypothalamic-pituitary-adrenal (HPA) axis. When a threat is perceived, the HPA axis is activated, leading to the release of glucocorticoid stress hormones, such as cortisol in mammals and corticosterone in reptiles and birds. This common biological circuitry explains why anxiety can be observed across such a phylogenetically broad spectrum of life.