The Platypus: Sensing the Body Electric

pattiPatti was one of my favorite Beanie Babies. Her bright magenta body and yellow webbed feet exuded a certain sunny optimism, and her strange resemblance to a flattened duck endowed her with an undeniable silliness. I remember feeling a bit confused as to whether she was a real creature or more akin to Mystic the unicorn. Also, if platypuses were real, were they actually pink?

Platypuses are indeed real, and as it turns out, have a fascinating nervous system. In fact, the neuroscientist in me is possibly even more excited about platypuses than the Beanie-Baby-loving child was!

Platypuses: Fake News?

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[1] An engraving of a drawing of a platypus made by Captain John Hunter

Apparently I wasn’t the only one who originally harbored doubts about whether platypuses are real. These strange mammals are found only in the freshwater lakes and streams of Australia, and Westerners did not encounter them until the late eighteenth century. Captain John Hunter, the governor of New South Wales—then a British penal colony—sent a bit of platypus skin back to Britain in 1798 with a drawing [1]. Some platypus skin eventually made its way to George Shaw at the British museum, and Shaw echoed my flattened-duck observation, albeit far more eloquently: “Of all the Mammalia yet known it seems the most extra-ordinary in its conformation; exhibiting the perfect resemblance of the beak of a Duck engrafted on the head of a quadruped. So accurate is the similitude, that, at first view, it naturally excites the idea of some deceptive preparation by artificial means” [2-3]. Yes, the platypus was so weird-looking that some thought the taxidermist must have been playing a prank, sewing some webbed feet and a large beak onto a furry mammal [1].

So platypuses are real, but are they interesting beyond their wacky shape? Undeniably, yes. (Although not because of their color… they are not bright magenta as my childhood-self had feverishly hoped, but rather a dull, dark gray.) Like other mammals, they have fur and produce milk. However, they have some very distinct reptilian characters as well. Female platypuses lay eggs instead of giving birth to live young, and male platypuses generate a nasty venom during breeding season which they use to incapacitate their competition for the lady platypuses. Platypus reproduction gets even more dicey when you consider that, instead of having an X and Y chromosome, male platypuses have 5 X chromosomes (all distinct!) and 5 Y chromosomes [4]! Another point on the list of oddities, and one that ends up relating to neuroscience? Their hunting prowess seems downright magical.

It’s like they have ESPN or something

Early observers of platypuses were dumbfounded by the creatures’ ability to hunt prey in dark water while not using their eyes, ears, or sense of smell. Platypuses have folds of skin that cover their eyes and ears while they swim, and their nostrils form a waterproof seal [5]. Could platypuses have a sixth sense?! Well, sort of. Platypuses are able to sense electric currents, a phenomenon referred to as “electroreception.” As with the more well-known senses, electroreception starts with an external stimulus (in this case an electrical current in the water). The current activates a receptor on a sensory neuron, which then transmits the information to the platypus’ brain.

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The mechanoreceptor and electroreceptor, adapted from [6].

The awkwardly large bill of a platypus is actually teeming with sensory receptors—about 40,000 electroreceptors that can sense current, as well as about 60,000 mechanoreceptors that sense physical movement [6]! (As a comparison, the human tongue has about 10,000 taste buds.) Electroreceptors are arranged in daisy chain-like patterns, with open nerve endings converging and popping out into mucous gland pores in the bill, where they have direct access to currents in the water. The mechanoreceptors are of the “push-rod” variety; physical disruptions in the water push little outshoots of skin (rods) back and forth, a signal that then gets transmitted to the brain.

When the platypus is moving along the bottom of a stream, it brushes against lots of stones, gravel, and mud that stimulate its mechanoreceptors. But once in a while, the swimming platypus will also get a jolt from its electroreceptors, meaning that it has encountered another living organism! All living beings give off a teeny bit of electricity—even algae—and those that use skeletal muscles to move emit a decent bit of current… when the neurons signal to the muscles to move, they create an electric potential in all of the relevant muscle cells. When in the vicinity of a platypus, this current emitted from a fish, insect, or worm becomes an “eat me” signal!

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[6] The stripes of electroreceptors and mechanoreceptors on the platypus bill

The ~100,000 electro- and mechanoreceptors on the platypus bill are beautifully arranged in a striped pattern—stripes of electroreceptors alternating with stripes of mechanoreceptors [7]. This striped pattern exists to a certain extent in the brain as well [8]. The neurons in the platypus’ primary sensory cortex (the neurons that receive the electrical and mechanical information from the sensory receptors) are also in somewhat of a striped formation, bearing a striking resemblance to ocular dominance columns found in the visual cortex of many animals, including humans.

Fishing for an explanation

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[6] A hypothesis of bimodal neurons

What is the purpose of the stripes? It seems likely that the organization evolved to help platypuses locate their prey in dark waters. Researcher Jack Pettigrew proposed that in the sensory cortex, there are neurons that sit on the border of the stripes. They don’t respond to any particular form of sensory information but rather they are connected to both their electroreception and mechanoreception neighbors (Pettigrew calls them “bimodal” neurons). Some bimodal neurons get excited when the electrical and mechanical inputs are close together in time, some get excited when the inputs are far apart in time, and they exist this way on a spectrum [6].

What would this allow? Say a fish is relatively far away from the platypus. The electric current emanating from the muscles of its flapping fins can travel at a faster frequency than the mechanical disruption of the water, so the time between the two inputs would be a long one. As the fish comes closer, the signals would get closer and closer together in time. If Pettigrew’s bimodal neurons exist, the platypus would be able to judge the distance of its prey based on the time difference between the two signals. And while the theory has never been proven, sensory systems across species exhibit many examples of parsing information from the delay between different signals.

Researchers have observed platypuses reliably making jerking movements with their heads in the direction of an electrical stimulus in the environment. Somehow they seem to know the direction of the prey. It has been proposed that the platypus can discern the directionality of a source of electricity based on the delay in between the signal hitting one side of the curved bill and hitting the other side [6]. This is similar to how humans detect which direction a sound is coming from—our brains process the delay in time between the signal entering one ear versus the other.

The platypus: a key to understanding evolution?

The platypus is marvelous in the eyes of science not only because of its physical and biological oddities and hunting abilities, but also for the simple reason that it is a monotreme. There are only two surviving families of monotremes, platypuses and echidna, which means that among all extant mammals, platypuses are one of two most distantly related to humans. Other mammals diverged from the egg-laying monotremes over 160 million years ago in the Jurassic period (yes, that is Jurassic as in when dinosaurs were around) [9].

evolution

By studying the platypus—its genes, its pattern of development, its anatomy (particularly its brain anatomy!), researchers can hope to better understand how mammals evolved and what genes and traits have been preserved across millions of years and how basic brain circuits have been modified in different lineages [10-11].

A poetic ending

It would be remiss to not end with Ogden Nash’s wise words about our furry, very very distantly related friends:

I like the duck-billed platypus

Because it is anomalous.

I like the way it raises its family

Partly birdly, partly mammaly.

I like its independent attitude.

Let no one call it a duck-billed platitude.

–Ogden Nash

References

Cover photo from National Geographic 

  1. Griffiths M. Platypus research 1798-1998. Philos Trans R Soc Lond B Biol Sci. 1998 Jul 29;353(1372):1059-61. Review. PubMed PMID: 9720104; PubMed Central PMCID: PMC1692302.
  2. Shaw, G. 1799 The duck-billed platypus. In The naturalists’ miscellany, vol. 10. London.
  3. Hall, Brian K. “The Paradoxical Platypus.” BioScience, vol. 49, no. 3, 1999, pp. 211–218. JSTOR, JSTOR, http://www.jstor.org/stable/10.1525/bisi.1999.49.3.211.
  4. Grützner F, Rens W, Tsend-Ayush E, El-Mogharbel N, O’Brien PC, Jones RC, Ferguson-Smith MA, Marshall Graves JA. In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes. Nature. 2004 Dec 16;432(7019):913-7. Epub 2004 Oct 24. PubMed PMID: 15502814.
  5. “Platypus” National Geographic. https://www.nationalgeographic.com/animals/mammals/p/platypus/
  6. Pettigrew JD. Electroreception in monotremes. J Exp Biol. 1999 May;202(Pt 10):1447-54. Review. PubMed PMID: 10210685
  7. Manger PR, Pettigrew JD. Ultrastructure, number, distribution and innervation of electroreceptors and mechanoreceptors in the bill skin of the platypus, Ornithorhynchus anatinus. Brain Behav Evol. 1996;48(1):27-54. PubMed PMID:8828862.
  8. Krubitzer L, Manger P, Pettigrew J, Calford M. Organization of somatosensory cortex in monotremes: in search of the prototypical plan. J Comp Neurol. 1995 Jan 9;351(2):261-306. PubMed PMID: 7699113.
  9. Bininda-Emonds OR, Cardillo M, Jones KE, MacPhee RD, Beck RM, Grenyer R, Price SA, Vos RA, Gittleman JL, Purvis A. The delayed rise of present-day mammals. Nature. 2007 Mar 29;446(7135):507-12. Erratum in: Nature. 2008 Nov 13;456(7219):274. PubMed PMID: 17392779.
  10. Warren, Wesley C et al. “Genome analysis of the platypus reveals unique signatures of evolution”  Nature vol. 453,7192 (2008): 175-83.
  11. Krubitzer L. What can monotremes tell us about brain evolution? Philos Trans R Soc Lond B Biol Sci. 1998 Jul 29;353(1372):1127-46. Review. PubMed PMID: 9720110; PubMed Central PMCID: PMC1692304.

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