The platypus’s eye is small (6 millimetres in diameter) and has a round pupil. The structure of the eye mostly resembles that of other mammals but has some reptilian features, such as double retinal cone cells used to see colour (Grant 1989). The lens is fairly flat at the front and much more curved at the back, like those found in otters and sea lions (Sivak 1980). This shape presumably improves the acuity of platypus vision underwater.
Interestingly, the platypus rarely uses sight when submerged – its eyes normally close automatically as soon as it dives (Grant 1989; Pettigrew et al. 1998). This presumably occurs to help protect platypus vision, and makes sense given that the platypus feeds mainly at night and often in murky water. However, submerged animals can decide to open their eyes when dealing with potentially dangerous circumstances (Serena et al. 2016). When swimming or resting on the water surface, a platypus very much relies on vision as a way to detect predators (Grant 1989) – a platypus will often dive instantly if a bird (even a small, harmless one) swoops overhead.
The platypus ear is located in a muscular groove which normally pinches shut to enclose and protect the ears underwater (Grant 1989). Platypus ear structure is generally like that of other mammals, with a few primitive features. For example, the ear region is encased in cartilage rather than bone (Murray 1984). Platypus hearing is most sensitive to sound frequencies around 4 kilohertz (exactly the same as in humans) but can detect frequencies as high as 15 kilohertz (Pettigrew et al. 1998).
Photo courtesy of Barry Baker
LITERATURE CITED
Grant TR (1989) Ornithorhynchidae. Pp. 436-450 in Fauna of Australia (DW Walton and BJ Richardson, eds) Australian Government Publishing Service: Canberra ACT.
Murray P (1984) Furry egg-layers: the monotreme radiation. Pp. 571-583 in Vertebrate Zoogeography and Evolution in Australasia (M Archer and G Clayton, eds) Hesperian Press: Carlisle WA.
Pettigrew JD, Manger PR and Fine SLB (1998) The sensory world of the platypus. Royal Society of London Philosophical Transactions B 353, 1199-1210.
Serena M, Grant TR and Williams GA (2016) Reducing bycatch mortality in crustacean traps: effect of trap design on platypus and yabby retention rates. Fisheries Research 175, 43-50.
Sivak JG (1980) Accommodation in vertebrates: a contemporary survey. Current Topics in Eye Research 3, 281-330.