The Platypus is a semiaquatic, egg laying mammal native to eastern australia. Together with the four species of echidnas, it is one of the five members of the order monotremata. Echidna are also long-billed and short-billed. Monotremes are the only mammals that lay eggs, the reason being that they were the earliest offshoot of the mammalian family tree. Monotremes are thought to have split from other mammals approximately 166 million years ago. As adults males and females are relatively the same size, males being slightly larger than females. However males have a venomous spine located on their left hind ankle. This venom is used not to kill prey like most other animals, it is instead used during the mating season to establish dominance. This evidence is backed up by the fact that males produce more venom during the mating season than any other time of the year. Platypus are exclusively carnivores and use a combination of electroreception and vibration detection to find prey, as explained below.
*This figure depicts the number of electroreceptors found in the bill of each species of monotreme.
*This figure shows the platypus’s range of electroreception. Demonstrating the fractions of millivolts it can detect at certain distances.
Electrolocation and mechanical senses:
Monotremes are the only mammals that use electroreception to find prey. This method of foraging is extremely effective because the platypus feeds exclusively on other animals. And since live prey produce electrical impulses stemming from the nervous system, it makes it very easy for the platypus to find food, even when it’s buried under the substrate. This method of locating food is highly developed, as the platypus closes its eyes and ears when underwater.
The electroreceptors are located in rostrocaudal rows in the skin of the bill and are made up of sensitive mucous glands, the platypus can determine the direction of an electrical source by comparing differences in signal strength across the sheet of electroreceptors, this explains why they move their head from side to side while foraging. [Groves pg. 1132].
The platypus also has another very sensitive method of detecting prey, mechanoreceptors. Mechanoreceptors are clusters of extremely sensitive epidermal cells, called push-rod mechanoreceptors. These clusters of epidermal cells are extremely sensitive to vibrations in the water, and are used alongside the electroreceptors to find prey. The mechanoreceptors are distributed uniformly across the bill, allowing the platypus to detect movement from all sides. Electroreceptors are linked to the trigeminal cranial nerve. Sir Everara Home was the first to discover the trigeminal nerves, which are the nerves that supply sensory stimuli to the brain from face, teeth, and the tongue. Platypus electroreception was first demonstrated in Australia and the detection strengths were measured 300 vper cm. Electroreception explains the accurate navigation through light-limited areas.
There are about 40,000 electroreceptors and 60,00 mechanoreceptors located in the bill. [Pettigrew pg. 1200]
Along with the use of electroreceptors to detect movement, the platypus uses the mechanoreceptors to also detect movement, but of a different kind. When a prey moves it emits electrical impulses as well as mechanical pressure changes, travelling back to the platypus. The platypus uses the difference in arrival times of the two signals to detect distance between itself and its prey. [Proske pg. 1189]
Fun fact: the platypus can detect electrical signals as low as 20mV per cm.
Groves, C.P. (2005). “Order Monotremata”. In Wilson, D.E.; Reeder, D.M. Mammal Species of the World: A Taxonomic and Geographic Reference (3rd ed.). Johns Hopkins University Press
Proske, Uwe; Gregory, J. E.; Iggo, A. (1998). “Sensory receptors in monotremes”. Philosophical Transactions of the Royal Society of London pg. 1187-1198
Pettigrew, John D.; Manger, P.R.; Fine, S.L. (1998). “The sensory world of the platypus”. Philosophical Transactions of the Royal Society of London. 353 (1372): 1199–1210