In the last post, it was pointed out that the platypus has a unique way of swimming: it swims using its fore limbs and uses its hind feet as rudders. There is something else interesting about the platypus when it swims: it closes its eyes, ears, and nose.[1] The eyes and ears close because of grooves lying behind the base of the bill. There are two grooves, one on each side of the head, and each groove contains an eye and an ear.[2] These grooves then close to shut the eye and ear together.
As a side note, the platypus has no external ears. It has a pair of holes in its head which conduct sound to the inner ear, but there is no auricle. The auricle, also called the pinna, is the external ear. It is the part that we normally think of as the ear. If you take a human and remove his auricles, he would be left with a pair of holes in his head. That is basically the situation with the platypus.
Now, while auricles are common in mammals, and not found in any other tetrapod, they are not found in every type of mammal. A few mammals, particularly those that are aquatic or semi-aquatic, lack auricles. For example, whales, sea cows, and seals all lack auricles. So while the lack of an auricle may sometimes be cited as a “primitive” feature of the platypus, it is not a universal feature of mammals.
With all of its sense organs cut off, how does the platypus navigate while underwater? That is where its electroreceptors come into play. These were mentioned a couple of posts back: they are one of the purposes of the bill. The soft bill has a number of electroreceptors, tiny structures that are sensitive to electrical changes. Since nerve conduction and muscle contractions are associated with a movement of ions, the movement of a muscle creates a small charge. The electroreceptors can pick up these changes and home in on prey. For a platypus, prey consists of shrimp, insect larvae, and clams.[3] These can be dug up out of the mud on the bottom of a river using its bill.
Using electroreceptors to find prey is sometimes called a “sixth sense,” and it is most commonly identified with sharks. Sharks and rays do indeed have a number of electroreceptors across their bodies, mostly located in their heads, which are used to detect prey. However, sharks are not the only animals to have them. A number of bony fish, such as elephant trunk fish and the electric eel, use them as well. The latter is most famous for its ability to stun prey using electricity, but it also uses electricity to detect its prey in the first place.
I believe that the platypus is the only vertebrate other than a fish that uses electroreceptors. It is, after all, only really useful in water, where electricity can be conducted easily, so it would not be useful to most mammals which are land dwellers.
When food has been caught, the platypus then chews it up using tooth plates in the back of its mouth. The extant platypus does not have teeth, or at least, the adult does not. While a young platypus has molars, they are eventually replaced by horny plates that grow on top of them.[4]
The eletroreceptors set the platypus appart from other mammals. Another interesting characteristic that makes it distinct from most other mammals is the venom found in the spurs of the male platypus. I say “distinct from most mammals” because there are a few other mammals that produce and use toxins, such as the soledon and a few types of shrews. Some people regard the slow loris as venomous, but that is debatable as the toxicity of a slow loris appears to be due to an allergen rather than a true poison.[5] Moreover, the toxin may be used for scent communication rather than intentionally for inflicting harm on prey or predators.[6]
Anyway, back to the platypus. Males have a pair of spurs on their hind feet. In terms of location on the foot, it is similar to that of the spurs of a rooster. The spur is hollow and connected to a gland that produces a poison.[7] Interestingly, the venom produced by this gland is seasonal, meaning that production increases during August, which is also when the males are preparing to mate.[8] It is believed that these spurs are used by males when they fight over territory and females.[9] Interestingly, while the poison of a platypus has been reported to kill dogs (though that is when it is injected directly into the bloodstream as opposed to being stabbed by a platypus spur), one of its primary purposes appears to be simply to cause pain.
Finally, I want to talk a little bit about the mammary glands of the platypus. These were mentioned in the last post. They certainly have them: they are, after all, one of the defining characteristics of mammals. Some people claim that the mammary glands of the platypus are simply modified sweat glands that secrete milk onto the females abdomen. The young then get the milk by lapping it off of the mother’s abdomen. Neither of these claims are true. Female platypuses have mammary glands that are structured and organized much like the mammary glands of other mammals. There are a few differences, however. They are covered with hair, including the “nipples,” which are actually flat, unlike the nipples of most mammals. Nevertheless, the milk produced is normal mammal milk and not some fatty sweat. Moreover, the release of milk appears to be stimulated by the young proding the “nipple” in much the same way that other mammals release milk when their young suckle.[10] I am not sure exactly how the young get the milk. Young platypuses are referred to as sucklings, so it appears that they actually suckle at the “nipple,” as opposed to lapping it up.Hopefully, it is still apparent that while the platypus has distinctly mammalian features, it has some unique traits that are not found in most mammals. In the next post, we will talk a little bit about fossils that relate to platypuses, which will actually show that platypuses are not quite as distinctive as they appear to be.
Thoughts from Steven
[1]Griffiths, Mervyn (1988) “The Platypus” Scientific American 258(5):84-91
[2]Grant, Tom (2013) Platypus, CSIRO Publishing, Collingwood, Australia, pg. 5
[3]Griffiths, Mervyn (1988) “The Platypus” Scientific American 258(5):84-91
[4]Gregory, William King (1947) “The Monotremes and the Palimpsest Theory” Bulletin of the American Museum of Natural History 88(1):1-52
[5]Krane, Sonja; Yasuhiro Itagaki; Koji Nakanishi; Paul Weldon (2003) “‘Venom’ of the slow loris: sequence similarity or prosimian skin gland protein and Fel d 1 cat allergen” Naturwissenschaften 90:60-62
[6]Hagey, Lee; Bryan Fry; Helena Fitch-Snyder (2007) “Talking Defensively: A Dual Use for the Brachial Gland Exudate of Slow and Pygmy Lorises” in Primate Anti-Predator Strategies Sharon Gurskey and K. A. I. Nekaris, eds. Springer, Boston, Massachusetts, pg. 253-272
[7]I’m going to get on my soap box here. I am fully aware that “poisonous” means “if you bite it, you die” while “venomous” means “if it bites you, you die.” Some people really like to be particular about the distinction between poisonous and venomous animals, jumping at every opportunity to correct someone who uses the wrong word. The problem is, “venom” falls under the dictionary definition of a “poison” (see https://www.merriam-webster.com/dictionary/poison for example). Thus, a venomous animal is also a poisonous animal, so I will be using the two terms interchangeably as they apply to the platypus.
[8]Griffiths, Mervyn (1988) “The Platypus” Scientific American 258(5):84-91
[9]Gerritsen, Vivienne (2002) “Platypus poison” Protein Spotlight 29
[10]Grant, Tom (2013) Platypus, CSIRO Publishing, Collingwood, Australia, pg. 25
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