Platypus: Sensational Swimmers
Nature is so full of wonderful creatures that it must be hard to focus on one for special study. However in recent years, a strange assortment of animals have enjoyed a brief moment of scientific attention. In each case the occasion for this special fame was the publication of the genome of that organism. The genome consists of details concerning the DNA molecules in each cell of the organism. DNA, or the genetic information, is made up of four special molecules (called nucleotides) strung together. It is the order of the nucleotides, like beads on a string, which determines the information content of the DNA for each particular organism.
While all such information may be interesting in its own right, scientists choose which organisms to sequence (document the order of nucleotides in the DNA) based on evolutionary theory. Thus bacteria, yeast, round worms, fruit flies, rats, dogs, apes and humans have all enjoyed their moment of fame. In May 2008, the genome of Australia’s platypus was published. This creature however is justly famous anyway, but the genome studies have helped focus attention on why this is so.
When British naturalists first saw a pelt of a platypus, they were sure it was a hoax. Eventually naturalists discovered that this animal was real and that it lays eggs, although it suckles its young with genuine mother’s milk. It seemed as if this creature was a strange jumble of bird, reptile and mammalian (feeds milk to young) characteristics. More careful study however reveals that this organism is actually a beautifully designed organism. The genome study further emphasizes this fact.
Research since the platypus was discovered, has turned up only two species of the faintly similar Echidna, also native of Australia. Thus the duckbilled platypus remains a highly unusual creature. Not only its appearance, but many aspects of its biology are unique. These small animals (up to 60 cm long) spend most of their time under water. Indeed they are unable to find food on land. Amazingly however, they swim blind, deaf and without the normal opportunity to detect chemicals since flaps cover their eyes, ears and nose while they are submerged. Recent research however has revealed that they have some unique abilities to compensate for lack of sight, hearing and smell.
Once the genome data has been collected, there is nothing obvious to show what stretches of DNA contain genes of interest. Faced with endless arrangements of nucleotides, how do scientists “read” the information? Initially, what scientists did, was to identify coding for certain basic proteins. Gradually they built up a computerized repertoire of DNA coding which identifies important genes in at least in one organism. Then when they wish to study a different organism, they use huge computers to look for similar stretches of DNA in the new organism.
Fancy mathematics allows the computer to decide whether similar sequences are close enough to represent the same gene or not. Since the genomes of many organisms have now been documented, scientists now have a large collection of nucleotide sequences which code for important genes. This provides an opportunity to compare the new organism with other creatures. Does it have similar genes or different ones? This analysis certainly reveals interesting things about the platypus.
Obviously the creature needs special hardware and talents designed for navigation since its ears, nose and eyes are closed under water. Back in 1985, German scientist Henning Scheich discovered some highly unusual properties of the platypus. This animal reacts to weak electrical fields in water. What this scientist did was to bury a small charged battery under a brick in the water. In addition, he placed a similar, but dead battery under another brick. The platypus dislodged the brick sitting on top of the charged battery, but ignored the other brick/battery site. Later, the platypus avoided a mesh screen placed in front of a charged battery, but it collided with a screen placed in front of a dead battery. Further studies have amply confirmed that platypus have electroreceptors in their bills.
Since the late 1980s, scientists have discovered that there are two kinds of electroreceptor and one type of touch receptor in the platypus snout. Over the main surface of the bill there are oblique arrays of pores which are mucous-filled. The mucous serves to enhance transmission of a signal to the nerve at the bottom of the pit. The bill of the platypus has 40,000 such electroreceptors.
The push-rod mechanical (touch) receptors in the bill are remarkable in their own right. Inside the pore is a compacted column of skin which can rotate about its base or move up and down. These very sensitive touch receptors are similar to the highly unusual touch receptors in the nose of the star-nosed mole. The organ of touch in the snout of the star-nosed mole is so sensitive, that the information obtained from it is almost as detailed as vision. This animal also spends most of its time foraging for food under water. Until recently, scientists knew of no other creatures with as sensitive a sense of touch. Now it appears that the mechanoreceptors in the bill of platypus are of even more sophisticated design.
There is yet another interesting feature of these sensory pores on the bill of the platypus, each is surrounded by petal-like skin flaps which open when the animal is under water. When the animal emerges from the water however, tiny sphincters around each pore close the flaps so that the sensors will not dry out.
The platypus hunts small organisms found near the sediments of lakes, ponds and rivers. Apparently these small victims generate weak electrical fields. Scientists suspect that the platypus knows how far away an electrical source is, whether it is moving, and in what direction. The remarkable thing is that these talents of platypus are so unique. As far as electrical sensing is concerned, some fish also exhibit this ability. However in the case of fish, the sensors are all over the body and they are not nearly so sensitive. But platypus has more talents yet! One might have imagined that platypus would not need much in the way of a sense of smell since their noses are closed under water. This conclusion is partly right and partly wrong.
As far as genes for normal smell (chemical receptors) are concerned, the genome project shows that platypus has a reduced number. However there are chemical receptors called vomeronasal receptors which may be located in the mouth or the nose and surprise, surprise, platypus has the largest variety of vomeronasal type 1 receptors known. At 950 different types, the platypus has 50% more than the mouse. Moreover the chicken and a lizard have no such receptors at all, nor do people.
The platypus thus has very special electrical, touch and chemical receptors. The article on the platypus genome in Nature (May 8/08 pp. 175-183) discusses the large number of genes which code for the special chemical receptors. The article however makes no mention of genes for electrical and touch receptors. Obviously there must be quite a number of genes in the platypus coding for these sophisticated sensors. The sequence (order) of nucleotides however does not come with labels identifying which sections code for electroreceptor components or anything else. Scientists need an already established standard order of nucleotides coding for such genes from another, not too different creature. Since these talents are highly unusual however, no comparison with a similar gene in a similar creature can as yet be made. Thus we don’t hear about how many genes code for these other special talents of the platypus.
What recent studies show is that this animal is not a jumble of features from a broad assortment of organisms, but rather a wonderfully integrated collection of unusual anatomy and attributes. Certain features may remind us of birds and reptiles, but the similarities are merely superficial. The platypus truly is unique in its navigational abilities and in all the other features. Obviously this unusual creature was designed to pursue its unique but effective lifestyle. It is evident that the creation of the platypus involved finesse and exquisite attention to detail. We should be very grateful for the opportunity to learn about this wonderful creature from Australia!
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