Sensory Systems

Vision

The platypus eye is small (6 millimetres in diameter) and equipped with a round pupil. The internal structure of the eye is typically mammalian in most respects but includes some reptilian features, such as the presence of double (as opposed to single) retinal cone cells used to perceive colour. The lens resembles those found in otters and sea lions, in being fairly flat at the front and much more curved at the back. This shape reduces the eye’s ability to see fine detail, but improves underwater vision.

Interestingly, the platypus rarely uses sight when submerged – its eyes normally close automatically as soon as it dives. One possible explanation is that the platypus’s ancestors relied on underwater vision more extensively than the modern species does.

Hearing

Platypus ears resemble those of other mammals in most respects but do have a few primitive features, such as the fact that the ear region is encased in cartilage rather than bone. The platypus ear is most sensitive to sound frequencies around 4 kilohertz (exactly the same as in humans) but can hear frequencies as high as 15 kilohertz.

Smell and taste

Aquatic mammals typically don’t rely much on smell to find food or detect predators, as chemical cues tend to be washed away by water. In the case of the platypus, only about half as many genes are linked to standard odour detection as compared to most land-based mammals.

However, scientists have been intrigued to learn that the platypus has an exceptionally large number of genes coding for specialised smell receptors in the vomeronasal (or Jacobsons) organ –  paired pouch-like structures located in the roof of the mouth.

Vomeronasal organs are found in both reptiles and mammals and are mainly important in social communication – detecting odours produced by other individuals of the same species. Accordingly, these receptors are likely to be used by a platypus to track chemical signs left to mark territorial boundaries or advertise reproductive status, though it’s possible they may also help the platypus find its prey underwater.

The platypus has two grooves at the back of the tongue which are lined with sensory papillae (tiny projections) which are believed to be used to taste food.

Bill sensory receptors.

The skin of a platypus bill holds tens of thousands of specialised sensory structures providing information needed to navigate underwater and capture prey. Receptors known as “push rods” are sensitive to touch or pressure, either as an outcome of solid objects contacting the skin or water movement. Nerves are activated when the tip of a push rod receptor is displaced by as little as 20 microns (0.00002 metres), which means a platypus can detect the movements of edible invertebrates such as freshwater shrimp or crayfish at a distance of 15-20 centimetres, simply by sensing the associated movement of water.

The bill surface is also thickly dotted with acutely sensitive electroreceptors (“sensory mucous glands”), which respond to the tiny amount of electricity generated when the muscles of aquatic invertebrates contract. Because electricity moves so rapidly through water, the tail flick of a shrimp will be recorded a fraction of an instant earlier by bill electroreceptors as compared to push rods, providing a way for a platypus to judge the distance to a prey item.

Further reading:
Pettigrew, J.D., Manger, P.R. and Fine, S.L.B. (1998). The sensory world of the platypus. Philosophical Transactions of the Royal Society of London, Biological Sciences 353: 199-1210.

Proske, U., Gregory, J.E. and Iggo, A. (1998). Sensory receptors in monotremes. Philosophical Transactions of the Royal Society of London, Biological Sciences 353: 1187-1198.

Scheich, H., Langner, G., Tidemann, C., Coles, R.B. and Guppy, A. (1986). Electroreception and electrolocation in platypus. Nature 319: 401-402.