An Evolutionary Disaster: The Platypus
Livy Jong
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The year is 1798, news has reached London that a mole-like animal with a duck bill and webbed feet has been discovered. At this point, this creature goes against every known fact of biology. Is it possible that a chimera of a duck, otter, and mole exists? In fact, its scientific name, assigned in 1803, Ornithorhynchus paradoxus, means “paradoxical bird-snout”.
The platypus is one of five species of monotremes (egg laying mammals) that still exist, with the others being variations of echidnas, the only close relative to the platypus. Similarly to other mammals, monotremes have hair on their bodies and produce milk to feed their young. However, many major differences are to be seen between monotremes and other mammals. Firstly, monotreme, by definition means “one opening”. This refers to the single duct, called the cloaca, which is the only opening for the reproductive, digestive and urinary tract. The cloaca is seen to be an evolutionary predecessor to placental mammals as they possess many openings, in females especially (i.e. Vagina for reproduction, Urethra for urinary anus for digestive).
Multiple cloacae seem to have a far superior evolutionary advantage in terms of reproduction. For example, in males, separate openings for sperm and feces would ensure that sperm amount is optimized and isn’t lost during defecation. A possible explanation for the lack of cloacae in platypus is the fact that newborn are laid as eggs. An evolutionary advantage of separate orifices for newborn and excretion is to prevent infection as the offspring would be very vulnerable to disease. However, with a protective layer such as an egg shell, this advantage may not be as necessary and therefore not developed.
Another unique feature of monotremes is the lack of teeth in adults. This is quite peculiar as the platypus’ diet consists mainly of crustaceans such as shrimp and crayfish, which have a tough outer shell. Fossil evidence and young platypuses do possess teeth, more specifically a tribosphenic set of molars. This suggests that the platypus has evolved away from possessing teeth optimal for grinding and shredding for a reason. The leading theory as to why platypuses have stopped developing teeth is due to their electrolocation features. Electroreceptors are located in rows on the skin of the platypuses' duck-like beak. The electroreceptors allow the platypus to locate anything that gives off an electric signal, determining the direction by the difference in strength of each receptor. With this feature working in convergence with their mechanoreceptors, platypus are able to pinpoint their prey by calculating the distance through the difference of signal arrival between the electroreceptor and the mechanoreceptor.
Diagram showing the use of electroreceptors and mechanoreceptors with reference to location.
Electrolocation is highly advantageous to platypus as the majority of their prey is found on the bottom of the streams. With electrolocation, platypuses are able to easily recognize living things amongst the sedimentary stream floor. Nonetheless, the electrolocation of monotremes remains mostly a mystery and the evolution of it is still difficult to justify.
This article has barely scratched the surface of the iceberg that is the platypus. Many other questionable features of the platypus such as biofluorescence, venomous spurs, and a heavily complex genome make the animal so much more intriguing. As research continues to unfold, we hope the platypus would tell us more about its evolutionary history and more importantly why it still exists despite its obscure features.
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