The platypus bill is covered by smooth skin with a soft, suede-like texture and (unlike a duck’s bill) is quite pliable and fleshy around the edges. The bill’s upper surface is uniformly dark grey (below right); its lower surfaces can either be uniformly grey or quite mottled (below left).
The skin of a platypus bill is thickly dotted with specialised sensory receptors that provide the information an animal needs to navigate underwater and capture its prey. More than 40,000 “push rods” distributed across both the upper and lower bill (especially at the edges) are sensitive to touch or water pressure (Manger and Pettigrew 1996). Nerves are activated when the tip of a push rod receptor is displaced by as little as 20 microns (0.00002 metre). This means that a platypus bill can detect movement by prey such as a freshwater shrimp from a distance of at least 15-20 centimetres (possibly up to 50 centimetres), simply by sensing associated water movement (Pettigrew et al. 1998).
The upper and lower bill also contain tens of thousands of electroreceptors (Proske et al. 1998) that can register the tiny amounts of electricity generated when the muscles of invertebrate prey species contract in the water (Scheich et al. 1986). It has been experimentally established that platypus electroreception can detect both direct current (DC) and alternating current (AC) (Proske et al. 1998). Because electricity travels so rapidly through water, the tail flick of a shrimp is recorded a fraction of an instant earlier by bill electroreceptors than by push rods, thereby providing a mechanism for a platypus to judge the distance to its prey (Pettigrew et al. 1998).
Photos courtesy of Ann Killeen (right); APC (left)
LITERATURE CITED
Manger PR and Pettigrew JD (1996) Ultrastructure, number, distribution and innervation of electroreceptors and mechanoreceptors in the bill of the platypus, Ornithorhynchus anatinus. Brain Behavior and Evolution 48, 27-54.
Pettigrew JD, Manger PR and Fine SLB (1998) The sensory world of the platypus. Royal Society of London Philosophical Transactions B 353, 1199-1210.
Proske U, Gregory JE and Iggo A (1998) Sensory receptors in monotremes. Royal Society of London Philosophical Transactions B 353, 1187-1198.
Scheich H, Langner G, Tidemann C, Coles RB and Guppy A (1986) Electroreception and electrolocation in the platypus. Nature 319, 401-402.