Knowing Animals Through Urine
Years ago, a banner from a school went viral online. It read, "having known each another since childhood" (從小便相識). Whoever made the banner had intended to say, "having known each another since childhood" (「從小」便相識), but netizens interpreted it as "knowing each other through urine" (從「小便」相識), leading to much amusement. Indeed, while humans can only "know each other since childhood", animals genuinely have the ability to "know each other through urine". For many animals, urine is not simply metabolic waste they expel from the body; it is a crucial tool for communication and even mating!
Animal urine contains pheromones, which are chemical scent signals that many animals use to communicate with their own kind. Some animals can even convey their social status, or complex information such as their physical condition and reproductive state through urine. African Lions (Panthera leo) and Common Squirrel Monkeys (Saimiri sciureus) from South America are typical examples of animals that can identify one another through the scent of urine. They are able to distinguish each other's gender, age, social group and whether they are familiar individuals, etc., so as to adjust their behaviours accordingly. Urine also plays an important role in reproduction. Ungulates such as deer, elk, and goats even spray urine on themselves to highlight their social status and physical condition, attracting the opposite sex.
Sizing Up an Opponent
Scientists have discovered a species of cichlid, Neolamprologus pulcher, that uses urine to assess their opponents of the same species during confrontations. The team injected a fluorescent agent into the fish to facilitate observation of their urine. They then separated a large and a small fish with a transparent partition. In half of the set-ups, the partition had holes that allowed water (and urine) to pass through; in the other half, the partitions had no holes in them. When the two fish noticed each other, they displayed hostility by approaching with their fins flared, followed by urination. In set-ups where the partition had holes, the smaller fish would shy away from the confrontation. Conversely, in set-ups where the partition had no holes, the smaller fish would launch an attack against the larger one. The experiment showed that in the set-ups where the partition had holes, the smaller fish would detect the urine of the larger fish and recognise the superior strength of its opponent, causing it to falter in its aggression and surrender. Conversely, where the partition had no holes, the smaller fish remained aggressive and initiated an attack, as it was unable to receive signals in the urine of the larger fish.
Finding a Mate
During courtship, a female American Signal Crayfish (Pacifastacus leniusculus) would release urine, which would excite a male upon contact and trigger in him an immediate desire to mate. However, the female would not accept the male's advances right away, but would instead engage in a "duel" with him to determine whether his physical strength and capabilities met her standards. During the fight, the female would continually urinate, sending the male complex messages that both "invite" and "challenge" him. Her purpose is to continuously tempt and challenge the male until she is satisfied he is the ideal one. Through this "peeing tactic", the female better understands the male, and finds a mate with whom she can produce the best offspring.
In another study, a research team collected urine from dominant male mice and raised female mice in an environment exposed to such urine. Remarkably, the team found that the brains of these female mice experienced a 25% increase in new neurons in two specific areas - the regions responsible for smell and learning. Scientists believe this neural growth helps female mice more effectively interpret scent signals in urine, enabling them to determine whether a male mouse is a high-quality mate.
Beyond using urine to find mates, animals also use it to ward off their "rivals". By spraying their urine onto a mate, they leave a scent mark warning other males to stay away. This habit is seen in animals such as rabbits, squirrels, foxes and porcupines.
Marking their Territory
Many animals exhibit territorial behaviour, often using urine as a means to define territorial boundaries. Generally speaking, the area of activity of different individuals or groups of animals tend to overlap, and they rely on the scent of urine to identify each other's territory, which they then try to evade, rather than invading and getting chased out.
In particular, coyotes, wolves and domestic dogs often mark their territory with urine and are able to discern the identity of their peers from the scent of urine. We often see dogs sniffing around trees or walls before urinating in those spots. When they detect the smell of urine from another dog, they will be able to tell if it is male or female, or whether it poses a territorial threat. Once a dog establishes its territory, it urinates to mark it and reinforces its claim by urinating repeatedly in the same area.
Bears and bisons also use urine to mark their territory. However, instead of urinating directly on their territory, they first urinate onto the ground and then smear the urine-soaked mud onto tree trunks.
