Most of the social communication happening in a backyard at night is invisible to humans. When a Virginia opossum moves through a yard, following a route it has used for weeks, it is reading a record written entirely in chemistry — deposits left by previous opossum visits, information laid down on branch surfaces, fence rails, and raised objects that remain chemically legible long after the animal that left them has moved on. This scent-based communication system is the primary channel through which opossums, a largely solitary species, track one another and navigate shared territory without the sustained social contact that more gregarious mammals rely on.
Gland Types and Location
The Virginia opossum possesses several distinct scent-producing structures, each associated with different communication contexts and behavioral patterns.
The sternal gland is the most prominent. Located on the midline of the chest, it is a patch of glandular skin that produces a complex lipid-and-protein secretion. In males, this gland is visibly larger and its output increases markedly during the breeding season. The secretion has a distinctive odor — musky and faintly sweet — that is detectable to humans at close range, but is far more informationally dense to other opossums than its simple smell implies to us.
Cheek glands flank the facial region and contribute secretions smeared onto surfaces during facial rubbing behavior. These glands are less studied than the sternal gland but appear to encode individual identity information that other opossums respond to with investigative behavior when they encounter the deposit.
Cloacal glands associated with the posterior opening produce secretions deposited incidentally during urination and defecation. These deposits are less deliberate than sternal or cheek marking but contribute to the chemical record left along an opossum's established travel route.
What Information Scent Encodes
Chemical analysis of opossum secretions, combined with behavioral studies tracking how opossums respond to scent samples from different individuals, has established that these deposits convey several categories of information. Individual identity — which specific animal left the mark — appears to be encoded in the precise chemical profile of the secretion, which varies between individuals in ways that are apparently distinguishable to other opossums. Sex is encoded as well; male and female secretions differ in compound ratios, and opossums show reliably different behavioral responses to male versus female marks.
Reproductive state is the most behaviorally significant information in sternal gland secretions. Males in breeding condition produce higher output from the sternal gland, and females in estrus produce secretion profiles that elicit extended investigation and route-following behavior from males. Age and body condition may also influence secretion chemistry, though the evidence for this in Virginia opossums is less fully established than for some other scent-marking mammals.
Marking Behavior in Practice
The characteristic marking behavior associated with the sternal gland is the chest rub. An opossum pauses at a favored surface — a branch junction, a raised root, a fence post or deck rail — and presses its chest against the surface, rocking slightly and rotating to transfer secretion across a small contact area. The motion is deliberate and unhurried, typically lasting between two and ten seconds, and is often repeated at the same surface across multiple visits on consecutive nights.
Opossums show consistent preferences for certain surface types. Rough, slightly elevated surfaces that retain secretion without immediately wicking it away are favored over smooth flat ground. Branch surfaces at a height of one to three feet above the ground receive marking attention more consistently than objects at ground level. Established travel routes accumulate deposit history over weeks or months, building a chemical record that functions as a persistent information layer overlaid on the physical landscape.
In male Virginia opossums, sternal gland size and secretion output peak between January and October, corresponding to the active breeding period. Outside this window — during late fall — gland activity decreases noticeably. Captive opossum studies have documented measurable differences in gland area and secretion composition between breeding-season and non-breeding-season males, confirming that the gland's output is hormonally regulated rather than constant throughout the year.
How Other Opossums Respond
When an opossum encounters a fresh scent deposit along its route, the behavioral response depends on what information the deposit carries. A male encountering a female estrus deposit will typically pause, investigate thoroughly with extended nasal contact at the mark, and may alter its movement direction to follow the route the female has recently taken. A male encountering another male's mark during the breeding season may intensify its own marking in the same area, apparently overmarking the deposit with its own secretion to establish chemical precedence.
The solitary nature of opossums means that these scent exchanges substitute for the direct social interaction that more gregarious species use to establish familiarity, status, and reproductive coordination. Two opossums sharing overlapping home ranges may exchange scent information across the same surfaces for weeks without ever physically encountering each other — a distributed conversation conducted entirely in chemistry, written and read along the routes where their territories intersect. The landscape itself, marked and re-marked with each passage, becomes the medium through which opossum social life is conducted.