The Weird and Wonderful Platypus

Sophie Teall explores some of the more surprising features of this unusual mammal.

 Image credit: Klaus, Flickr, CC BY-SA 2.0

Imagine a glowing creature with venomous spikes that stalks its prey in the night with eyes shut tight. It may sound like a thing of legends (or nightmares), but this creature is very much real. It is also surprisingly adorable, covered in dense fur and with a cartoonishly large beak and webbed feet. The platypus may be famous for looking like a fusion of several different animals – indeed, Aboriginal tales describe them as the offspring of a duck and a water rat – yet somehow their outward appearance is not the strangest thing about them.

Native to Australia, platypuses are a member of a group of mammals known as monotremes. The only other members of this group are echidnas, small hedgehog-like animals with long snouts. While other mammals give birth to live young, both platypuses and echidnas lay eggs. Once the eggs hatch, like other mammals, female monotremes will provide their young with milk until they can start searching for and catching food themselves. However, things get even stranger than the platypuses ability to lay eggs.

Hunting Blind

Hunting in murky waters at night, most senses become useless to a platypus. Once in the water, their nostrils close and their eyes and ears are temporarily covered by folds of skin. This allows them to protect their eyes and prevent any water from entering the nasal cavity or ear canals. Despite this, platypuses are still able to locate prey hidden in muddy river beds with remarkable accuracy.

The bill of a platypus is covered in thousands of receptors. Some of these are pressure receptors which can detect movement within the water. The others, electroreceptors, pick up the electrical signals produced by muscle contractions. As a platypus moves through their watery world, they rapidly move their head from side-to-side to determine the direction and the distance of their prey from the information gathered by these receptors. The shape of their beak plays a role in this too, as its large surface area gives a higher sensitivity to the exact direction of their prey. It has been observed that if a platypus accidentally swims over the source of a signal then they will quickly turn back to investigate the area. 

Platypuses are not completely unique in their use of electroreceptors. Water is a better conductor for electrical signals than air, so many aquatic species such as the Guiana dolphin can use electricity to find prey. Given platypuses’ watery hunting ground, it makes sense that they too would use the detection of electric signals for hunting. Interestingly, both the long-beaked and short-beaked echidnas have receptors in their snouts. These species either forage for food in damp leaf litter or humid ant nests. However, with the ability to utilise other senses, the snouts of these animalscontain significantly fewer electroreceptors than the 40,000 found on the platypus bill. 

The use of electroreceptors was not without trade-offs. Historic relatives of the platypus appear to have been toothed and fed closer to the water surface. Modern-day platypuses lack this feature and instead have to collect gravel in their mouths to help grind food down. Comparisons of the skulls of the modern platypus and its historic relative suggest that to be able to use electroreceptors with such precision to feed in murkier waters, the space in the skull for teeth became restricted to the point where they were lost.

Cute Yet Deadly

A second platypus trait that is rare among mammals is their use of venom. Male platypuses have a single sharp spur on the back of each rear foot. These claw-like structures allow platypuses to pierce the skin and inject venom via the wound. They are incredibly strong and able to support the full bodyweight of a male platypus, with people who have been pierced needing assistance to physically remove the animal.

The venomous claw-like spike found on the rear heel of a male platypus 
Image credit: Stowe Boyd, Flickr, CC BY-SA 2.0

The venom of a platypus contains multiple proteins, many of which are similar to those found in other venomous species such as snakes, although these proteins seem to have evolved independently of one another. While this venom can kill other animals such as dogs, it is non-fatal to larger animals such as humans. However, it has been described as incredibly painful and will temporarily incapacitate the recipient. Curiously, it does not appear to be fatal to other platypuses, as living males have been found with multiple puncture wounds. 

The presence of puncture wounds on other individuals indicates that males may primarily use their spurs during the mating season while fighting other males. Both males and females are born with spurs, but only males retain them into adulthood, suggesting that they are not necessary for activities that both males and females participate in, such as hunting. Additionally, the production of venom in males increases during the breeding season along with aggression levels and the frequency of puncture wounds.  

Glow-in-the-dark

Considering how unique and wacky these mammals are, maybe the fact that they glow does not even come as that much of a surprise. A recent study found that when platypuses are observed under UV light, their fur fluoresces with shades of purple, blue and green. This glow has been observed in plants, birds and other nocturnal mammals such as flying squirrels, but this is the first record of fluorescence in a monotreme. Due to being observed in both wild and museum samples, it appears to be a genuine property of platypus fur, and not just a result of museum preparation. 

There is currently uncertainty as to why platypuses glow. It is, however, possible to rule out a couple of possibilities. Some other animals, such as bluetits, show a difference in degree of fluorescence between males and females. Females will show a preference for mates with a brighter fluorescent patch, and so this trait is selected for due to mate choice. In contrast, fluorescence is equal in both male and female platypuses, indicating that this feature is unlikely to be linked to seeking a mate. In fact, it is unlikely to play any large role in platypuses interacting with each other owing to their reliance on navigating the world without their eyes. Instead, it may be used to avoid predation at night by helping the platypus to blend in with the fluorescent plants in its environment. Alternatively, it may serve no modern function, and may simply be a historical trait that platypuses have managed to retain. 

While the platypuses’ combination of fascinating traits  certainly makes them stand out, they also offer a fascinating glimpse into the evolution of mammals. The presence of both egg laying and milk production in monotremes indicates that lactation in mammals evolved before this group split off from the rest of the mammals, over 166 million years ago. The glow observed in platypuses and other nocturnal mammals may be an ancestral trait that only some mammals have kept, and the venomous spikes found on males appear to be the last case of an ancestral trait that has been lost in all other mammals. With their unique lifestyles, platypuses are full of surprises. 

Written by Sophie Teall and edited by Ailie McWhinnie.

Sophie is a final year undergraduate biology student, specialising in Zoology. Find her on Twitter @sophteall and LinkedIn @Sophie Teall.

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