Rats don’t cry the way humans do, but they do produce something that looks alarming: a reddish-brown discharge around their eyes and nose that’s often called “red tears.” This substance isn’t blood, and it isn’t triggered by sadness. It’s a pigment called porphyrin, produced by a gland behind the eye, and its presence usually signals stress, illness, or pain. Rats also express distress through ultrasonic vocalizations that humans can’t hear, which function as their true emotional “crying.”
What Red Tears Actually Are
Behind each of a rat’s eyes sits a structure called the Harderian gland. This gland primarily produces lipids that lubricate the eye, but it also synthesizes a pigment called porphyrin. In small amounts, porphyrin is normal. It appears as golden to dark brown material, and rats typically groom it away before you ever notice it. Porphyrin production naturally increases with age, so older rats tend to show more of it.
When porphyrin builds up faster than a rat can groom it away, it creates visible reddish-brown crusting around the eyes and nose. This condition has a clinical name: chromodacryorrhea. The discharge travels from the eyes down through the tear ducts to the nose, which is why you’ll often see staining in both places. It looks startlingly like blood, but it’s chemically distinct. One way to tell the difference is to shine a UV light (a Wood’s lamp) on the discharge. Porphyrin fluoresces a bright pinkish-red under UV light, while blood does not.
Why Porphyrin Production Increases
A small amount of porphyrin staining after sleep is perfectly normal, similar to the “eye gunk” humans wake up with. Visible buildup throughout the day, or heavy crusting that the rat isn’t cleaning, is a different story. The most common triggers fall into a few categories.
Stress is the leading cause. Overcrowding, a dirty cage, loud environments, loneliness, or a recent move can all push porphyrin production up. Chronic physiological stress in rats reliably causes chromodacryorrhea, making red tears one of the earliest visible warning signs that something in a rat’s environment isn’t right.
Illness and infection are the other major triggers. Respiratory disease is one of the most common health problems in pet rats, and red tears are often among the first signs. If you notice increased porphyrin along with sneezing, wheezing, nasal discharge, or labored breathing, a respiratory infection is likely. A rat coronavirus called sialodacryoadenitis virus (SDAV) directly inflames the Harderian gland itself, causing a spike in secretions. Bacterial infections involving Mycoplasma are another frequent culprit.
Pain also drives porphyrin output. Any condition causing ongoing discomfort, from dental problems to internal illness, can produce visible red tears. Because rats instinctively hide signs of weakness, porphyrin staining may be the only external clue that something is wrong.
Reduced grooming plays a role too. A healthy rat cleans porphyrin away almost as fast as it appears. When a rat feels too sick, depressed, or lethargic to groom, the normal baseline of porphyrin simply accumulates and becomes visible. So sometimes the issue isn’t overproduction at all. It’s that the rat has stopped keeping up with it.
How Rats Actually Express Emotion
While red tears aren’t an emotional response, rats do have a rich system for expressing how they feel. They just do it at frequencies humans can’t hear. Rats communicate through ultrasonic vocalizations, sounds pitched well above the range of human hearing, and researchers have mapped these calls to specific emotional states.
Distressed or anxious rats emit calls at around 22 kHz, a long, low-frequency ultrasonic tone. These calls show up in situations involving fear, social isolation, and chronic pain. Notably, these vocalizations don’t signal pain directly. They express the anxiety and emotional suffering that accompanies painful or threatening experiences. Baby rats separated from their mothers produce related calls that represent infantile anxiety, and these calls can be reduced with anti-anxiety medications.
Happy rats produce a completely different sound: rapid 50 kHz calls that researchers have compared to human laughter. These chirps are abundant during play fighting among juvenile rats and appear when rats anticipate something rewarding. The contrast between these two call types gives researchers, and increasingly pet owners with bat detectors, a window into a rat’s emotional world that’s far more nuanced than the visible red tears.
Normal Porphyrin vs. a Problem
A trace of reddish-brown around the nose after a nap is routine, especially in older rats. You can usually wipe it away with a damp cloth, and if it doesn’t reappear heavily throughout the day, there’s little to worry about.
The signs that point to a real problem include porphyrin that keeps building up during waking hours, staining that appears on the fur of the forepaws (where the rat has been wiping its face), and discharge visible around both the eyes and nose simultaneously. When combined with any of the following, something beyond normal secretion is going on:
- Respiratory signs: sneezing, rattling or crackling sounds when breathing, nasal discharge
- Behavioral changes: lethargy, hunched posture, loss of appetite, reluctance to move
- Grooming changes: a coat that looks rough, unkempt, or puffed up
- Swelling: puffiness around the eyes or neck, which can indicate SDAV infection or a dental abscess
Reducing Porphyrin Staining
Because stress and poor husbandry are the most common triggers, environmental changes often make a noticeable difference. Rats are social animals that do poorly alone, so housing them with at least one compatible companion reduces chronic stress. Keep the cage clean but avoid strongly scented bedding, especially cedar or pine shavings, which release compounds that irritate the respiratory tract. Adequate ventilation matters too: ammonia from urine buildup is a major respiratory irritant in enclosed cages.
A quiet, consistent location for the cage helps, as does a predictable routine. Rats are sensitive to disruption, and even a change in room or household noise levels can temporarily increase porphyrin output. Providing hiding spots gives rats a sense of security that measurably lowers stress responses.
If environmental improvements don’t reduce the staining within a few days, or if respiratory or behavioral symptoms are present, the underlying cause is likely medical rather than situational. Respiratory infections in rats are common, progressive, and respond best to early treatment.