Spit: the smell of survival

The Australian outback can reach blistering temperatures as high as 50 degrees Celsius during the day and its landscape often lacks trees, making it is hard to find shade. This makes it extremely difficult for animals to survive in, mainly due to the dangers of dehydration and overheating. Overheating damages enzymes, which are important for digestion and many key biological processes, causing them to denature (lose their specific structure) and become non-functional; meaning that the animal may die.

In order to survive in such hostile conditions, the species that live in the outback have evolved many physiological and behavioural adaptations that allow them to cope with high temperatures and often, limited access to water. The most obvious behavioural adaptations are restricting activity to the night (nocturnal) or to the twilight (crepuscular) hours, when the temperature is much cooler. Most physiological adaptations are designed to increase the amount of heat energy lost by an organism and a striking example of this is having extremely large ears. Although this may look comical to us, it is actually a very clever way of cooling down since large ears have a very high surface area, from which the body's excess heat energy can be lost and are very thin. This high surface area to volume ratio makes them perfect for losing heat energy and they contain many blood vessels, which continuously carry the hot blood from the animals core to their ears. Thus, losing heat energy via the ears is extremely efficient and makes it possible for them to remain cool enough to survive. In addition, it also improves their hearing as large ears can 'collect' more sound waves than smaller ones, helping them stay alert for the sounds of predators or prey, respectively.

A bilby in Australia. Its large ears lose a lot of heat energy, helping it to survive in the extremely hot conditions of the Australian outback.

Controlling the times of activity and possessing adaptations like large ears are fine for smaller animals, such as biblies, but kangaroos are large and very active. A particular problem they have is that hopping, their main method of locomotion, is very energy expensive and produces lots of waste heat energy. Thus, these common methods of controlling body temperature would be inadequate to allow kangaroos to survive in such extreme conditions by themselves. So, to increase the amount of heat energy they lose, kangaroos use a novel behavioural trait - they lick the insides of their forearms. Although this may seem strange, it is highly efficient at losing heat energy and works by the same principles as sweating, which makes sense since kangaroos, like many mammals, are unable to sweat and rely on panting as a method of water loss instead. The process loses heat because the water on the skin absorbs heat energy, which it uses to evaporate. As the water leaves the skin, it carries the energy it has absorbed with it, cooling the skin down. Therefore, kangaroos can lose an enormous amount of heat energy via 'spit bathing' as it is called, helping them to stay cool

The importance that spit bathing plays for the survival of kangaroos is indicated by the highly developed salivary glands in their mouth. Of particular importance is the the parotid salivary glands, which are very large and are twice as heavy in relation to a kangaroo's body size than in other ruminants, such as sheep. They also have an extremely large number of serous-type secretary cells that constantly produce saliva, which is unusual because in most animals they are mainly active when food is actually being eaten. Thus, kangaroos can constantly produce large quantities of saliva.

A red kangaroo lounging in a hole that it has dug in the shade beneath a tree. Note its large ears and long thin forearms, perfect for losing heat energy.

However, there is a major problem with using saliva to cool themselves down - it wastes valuable water in an environment where it is already scarce. How then do kangaroos manage to keep enough water to maintain their blood pressure, whilst using enough to cool down? Firstly, kangaroos are able to concentrate their urine by absorbing much more water back from the filtrate in their kidneys than humans are able to do, which reduces the volume of urine that they produce and thereby, limits water loss. Secondly, they slow down their metabolism during the hottest parts of the day so that their body uses up less water in respiration.

Thus, kangaroos are able to survive in the hot, arid conditions of the Australian outback, despite their large size and active lifestyle.