Koalas have large, rubbery noses because smell is their primary survival tool. That oversized nose houses a sophisticated scent-detection system that helps them do something remarkably difficult: pick safe leaves from a genus of trees loaded with toxic compounds. But the nose does more than sniff out dinner. It also plays roles in cooling their bodies, detecting social signals from other koalas, and even guiding newborn joeys to their mother’s pouch.
Sorting Safe Leaves From Toxic Ones
Eucalyptus leaves are a terrible food source by almost any measure. They’re low in nutrients, tough to digest, and packed with chemicals that would poison most animals. Koalas have evolved to tolerate these compounds better than other species, but they still can’t eat indiscriminately. Among the roughly 700 species of eucalyptus in Australia, koalas are highly selective about which trees and even which individual leaves they’ll eat.
The nose is central to this selectivity. Before eating, koalas carefully sniff leaves to assess their chemical profile. Research published in Biology Letters found that koalas consistently avoided foliage with high concentrations of certain plant defense compounds called formylated phloroglucinol compounds, or FPCs. In feeding trials, captive koalas ate significantly less from trees with elevated levels of these chemicals. Even between two closely related eucalyptus species growing in the same area, koalas strongly preferred one over the other based on the presence of specific toxic compounds (jensenal and grandinal) that made one species a more potent deterrent, even when overall toxin levels were similar between the two.
Beyond toxins, koalas also need to evaluate moisture content, nitrogen levels, fiber, tannins, and other qualities in their food. This is a lot of chemical information to process through scent alone, and a larger nasal cavity provides more surface area for olfactory tissue to do that work.
What’s Inside the Nose
The koala’s nose isn’t just big on the outside. Internally, the nasal cavity contains bony structures called conchae (or turbinates) that increase the surface area available for warming, humidifying, and filtering air. In koalas, the ventral concha curves back on itself to form a rounded chamber called a bulla. The olfactory epithelium, the tissue responsible for detecting scent molecules, covers the deeper bony structures and the back of the nasal septum.
Interestingly, these internal structures are relatively simple compared to some mammals. The conchae are smooth, curved plates rather than elaborately scrolled structures. This suggests the koala’s scent ability comes not just from physical complexity inside the nose but also from genetic and neurological specialization in processing what it detects.
A Surprising Genetic Twist
When scientists sequenced the koala genome in 2018, they expected to find an unusually large number of olfactory receptor genes given how dependent the animal is on smell. The reality was more nuanced. Koalas have about 1,169 olfactory receptor genes, which is actually slightly fewer than other marsupials like the tammar wallaby (1,660) or the gray short-tailed opossum (1,431).
Where koalas stand out is in a different category of scent receptors. They have six genes in a specific lineage of vomeronasal receptors (V1R type), which detect nonvolatile chemicals, the kind of molecules found in scent markings and on leaf surfaces rather than floating through the air. The Tasmanian devil and opossum each have just one of these genes, and wallabies, humans, mice, and dogs have none. This expansion hints that koalas have evolved a specialized ability to read chemical information through close-range sniffing, exactly what you’d need to evaluate a leaf held right up to your nose.
Cooling Down Through Breathing
Australia gets hot, and koalas don’t sweat much. When temperatures climb above 25 to 30 degrees Celsius (roughly 77 to 86°F), koalas rely on evaporative cooling in their airways to regulate body temperature. They increase their breathing rate, allowing moisture to evaporate from the surfaces inside the nose and respiratory tract, which pulls heat away from the body.
A larger nose with more internal surface area makes this process more efficient. The nasal passages act like a built-in radiator, and the koala’s prominent nose provides the real estate needed for meaningful heat exchange. At the same time, koalas compensate for the water lost through breathing by producing more concentrated urine, conserving fluid in an environment where they rarely drink water directly.
Social Scent Signals
Koalas are mostly solitary, but they still need to communicate with each other about territory and mating. Males have a scent gland on their chest that they rub against trees to leave chemical messages. Other koalas read these signals by sniffing the marks, gathering information about the identity, sex, and reproductive status of whoever left them.
The expanded vomeronasal receptor genes in koalas are likely involved in decoding these social scents. Vomeronasal receptors across the animal kingdom are closely tied to detecting pheromones and other chemical signals from members of the same species. Having six times more of these receptor genes than their closest marsupial relatives suggests that chemical communication plays a bigger role in koala social life than their quiet, tree-dwelling reputation might suggest.
The Nose Works From Birth
A koala joey is born after just 34 to 36 days of gestation. At birth, it’s roughly the size of a jellybean, blind, hairless, and with barely formed limbs. Yet it manages to crawl from the birth canal into its mother’s pouch and latch onto a nipple, a journey guided almost entirely by smell. The newborn’s sense of smell is already functional enough to follow the scent of its mother’s milk.
This early reliance on scent continues throughout development. As the joey grows and begins sampling pre-digested eucalyptus material (called pap) that its mother produces, it starts learning the smell profiles of safe food sources. By the time a young koala leaves the pouch and begins eating leaves independently, its nose has already been trained on which chemical signatures to seek out and which to avoid. The large nose isn’t just an adult feature; it’s part of a lifelong sensory system that begins working from the very first moments of life.