The supraorbital gland is a specialized salt gland in certain animal species, located in the skull just above the eye sockets. This gland is an adaptation for life in saline environments where animals ingest large amounts of salt. Its presence allows these creatures to thrive in habitats that would otherwise be inhospitable.
Function in Salt Regulation
The primary function of the supraorbital gland is osmoregulation, the maintenance of a stable balance between salt and water. These glands excrete excess sodium chloride from the bloodstream, functioning more efficiently than the kidneys. This active process requires significant energy to move salt against its concentration gradient into the gland’s tubules.
Blood flowing through capillaries within the gland releases salt into secretory cells. These cells use ion transport proteins to pump sodium and chloride ions into a central canal. This creates a highly concentrated brine that can be five times saltier than the animal’s body fluids. This fluid is then excreted, allowing the animal to consume salty prey and drink seawater without risking cellular damage.
This physiological process is triggered by osmoreceptors that detect an increase in blood salinity. When activated, the gland rapidly begins salt extraction. The efficiency of this system is a clear example of adaptation to environments where fresh water is scarce and salt is abundant.
Animals with Supraorbital Glands
Supraorbital glands are famously associated with marine birds, which spend much of their lives at sea. Species like albatrosses, gulls, petrels, and penguins rely on these glands to process salt from marine life and seawater. This allows them to travel vast distances over the ocean, far from fresh water. The excreted salt solution often drips from their nostrils or is sneezed out.
The glands are not exclusive to birds and have evolved independently in several reptile lineages. Sea turtles, for example, possess large salt glands to maintain their internal salt balance in the ocean. Marine iguanas of the Galápagos Islands use their nasal salt glands to sneeze out excess salt from their diet of marine algae.
Crocodilians, including crocodiles, alligators, and caimans, also possess these glands. While many crocodilians live in freshwater, some, like the saltwater crocodile, inhabit brackish and marine environments. Their supraorbital glands are modified lingual glands on their tongues necessary for expelling accumulated salt, enabling them to thrive where other reptiles cannot.
Debunking “Crocodile Tears”
The phrase “crocodile tears” is used to describe a false display of emotion, originating from the belief that crocodiles weep for the victims they are devouring. While crocodiles are observed with fluid streaming from their eyes during feeding, this discharge is not remorse. It is the byproduct of their supraorbital glands at work.
The physical act of capturing and consuming prey is believed to trigger this visible excretion. The immense pressure exerted by the crocodile’s powerful jaw muscles during a bite, combined with the hissing and forceful breathing that often accompanies feeding, is thought to force the saline fluid out through ducts that open near the eyes.
This biological process has been misunderstood for centuries, giving rise to a persistent myth. The tears are not emotional but physiological, a visible manifestation of the gland’s function. The phenomenon is a mechanical result of feeding pressures acting on the salt gland’s drainage system, not a display of crocodile compassion.