| The uncanny ability of platypuses to locate a shrimp on a muddy river bottom at night is due to the bills and brains of these unique mammals being specially equipped with electro-sensors that allow them to home in on the electrical signals of prey, Monash scientists believe.
Professor Uwe Proske and Dr Ed Gregory from the Department of Physiology have studied the data available on platypus physiology and speculate that the row of nerves in platypus bills, wired to the touch and electro-sensing part of the platypus brain, work like a short-range radar system to pick up the electrical signals of fish, shrimp, crayfish and other prey.
Their speculations about platypus electro-receptors have been published in the journal Comparative Biochemistry and Physiology.
"Electro-sense in animals has evolved at least twice," Professor Proske said. "The first time was in fish -- sharks, rays and other non-bony fish as well as in electric bony fish. The second time was in platypuses and echidnas. The electro-sensory system in the platypus evolved entirely independently of that in fishes."
Professor Proske said platypuses appear to have developed touch receptors in their bills, similar to those in human skin, as part of their electro-sensory system. "Some cells in the platypus bill respond to touch and some to weak electric fields, weaker than anything humans can feel. These cells send their respective messages to the brain."
It is important that the electro-receptors be sensitive to the slightest change in electric field, as the platypus doesn't rely on its other senses when hunting -- its eyes, ears and nostrils are closed when it dives, looking for prey on the bottom of a stream or pond.
"If you look at the surface of the platypus brain, about two-thirds of the sensory part of the brain is connected to the bill," Professor Proske said.
"One of the surprising things we found were brain cells that responded both to signals coming from the touch cells and from the electro-sensory cells. So does the platypus feel touch when it detects an electric field?
"We think that as a shrimp swims, it generates an electro-myogram such that when its tail is flicked, it produces a signal the platypus can detect. So, as the platypus is swimming in the dark, the first thing it detects is the electro-signal. Then, at a delay, there is a small disturbance of the water as a result of the tail flick, and the disturbance triggers the touch nerve endings in the bill.
"It has been speculated that there are cells in the brain that are tuned to respond to sensory signals coming from the bill at a preferred interval. The interval response tells the platypus how far away the prey is.
"The system is obviously effective, as a platypus can capture half its body weight in food every night."
Although Professor Proske is particularly interested in how the electro-sensory system of the platypus works, these ideas could have practical applications.
Just as the use of sonar by porpoises was developed into a practical application by the US navy, it is possible that knowledge of how the membranes in a platypus nerve fibre are affected by a weak electric signal could be used to detect objects underwater that produce electric signals.
Contact:
uwe.proske@med.monash.edu.au
Ph: +61 3 9905 2526 | 
