The sight of tears streaming down a human face is universally understood as a signal of deep emotion, whether from sadness, joy, or frustration. This visible expression of inner turmoil is often cited as a defining human trait, leading to the belief that no other creature weeps in the same way. While animals experience profound emotions, determining if their grief or fear manifests as a flow of lacrimal fluid requires examining the biology of tears. Science suggests the answer depends entirely on how the term “cry” is defined.
Defining the Human Phenomenon: Emotional vs. Reflexive Tears
Human tear production is categorized into three distinct types, each with its own function and chemical signature. Basal tears are produced constantly, forming a thin, protective film that keeps the cornea lubricated and nourished, and contain proteins like lysozyme for antimicrobial defense. Reflex tears are secreted in a sudden flood in response to irritants, such as cutting an onion or getting dust in the eye, working to flush out foreign bodies.
The third category, emotional tears, sets human crying apart from the rest of the animal kingdom. These tears are triggered by psychological distress or intense emotional states, and their composition differs notably from basal and reflex tears. Emotional tears contain a higher concentration of protein-based hormones, specifically stress hormones such as prolactin and ACTH, and the natural painkiller leucine enkephalin. This unique chemical profile suggests emotional weeping serves a biological function by releasing stress-inducing compounds, helping to restore emotional equilibrium.
Animal Tears: Biological Necessity vs. Emotional Expression
Producing basal and reflex tears is a universal trait shared by nearly all land animals, from reptiles to mammals. For creatures like dogs, primates, and horses, tears are purely a functional necessity for eye health, serving to lubricate the ocular surface and clear away debris. If a steady flow of tears is observed from an animal, it is typically a sign of a physical problem, such as an infection or a scratched cornea, rather than a display of sadness.
The idiom “crocodile tears” highlights a common misinterpretation of this biological function. Crocodiles produce copious tears, but this lacrimation is a physiological response, often triggered by the pressure exerted on their tear glands during feeding. Scientific evidence has not documented any non-human animal producing tears with the distinct chemical composition—the high concentration of stress hormones—found in human emotional weeping. While animals have tear ducts and tear fluid, they lack the neurochemical link that translates deep emotion into a visible, chemically unique flow of tears.
Non-Tear Distress Signals in the Animal Kingdom
Despite the lack of emotional tears, animals possess complex, non-lacrimal systems for expressing profound sadness, grief, and distress. In African elephants, for example, a distressed individual will emit a low-frequency rumble or trumpet, and nearby elephants often engage in consolation behavior. They use their trunks to gently touch the troubled individual or even place the tip of their trunk in the other elephant’s mouth, a gesture believed to be the equivalent of a comforting hug.
Chimpanzees, our closest living relatives, exhibit behaviors that suggest a human-like response to loss and bereavement. Mothers have been observed carrying and grooming the lifeless bodies of their deceased infants for days or weeks, an act researchers suggest is an analog of human grief. When a group member is distressed, other chimpanzees offer reassurance through affiliative expressions, such as grooming and body kisses, which alleviate the recipient’s state of arousal. These behaviors reveal that the motivation to signal and receive comfort is not limited to humans; only the method of communication is different.
Why Humans Evolved Emotional Crying
The evolutionary development of emotional crying is largely attributed to its function as a reliable social signal. Tears are a non-verbal display of vulnerability that effectively communicates a need for support, which is important for a highly social species. The blurred vision caused by tears temporarily impairs the crier, making the display an honest signal of helplessness that solicits aid and promotes prosocial behavior from others.
This visible sign of distress is a powerful bonding mechanism that strengthens social ties within a group. The chemical composition of emotional tears also suggests a self-soothing function. The expulsion of stress hormones, coupled with the release of endogenous opioids, helps regulate the body’s response to intense emotional arousal. This combination of chemical relief and social support has cemented emotional crying as a unique feature of the human experience.