The duck-billed platypus is quite the strange mammal. Because it has a bill like a duck, lays eggs like a bird, has poisoned spurs, and swims around in the water with webbed feet and a paddle-like tail, it’s often called “God’s joke.” The platypus is just one of the remaining species belonging to a most astonishing order of mammals: the monotremes.
Only two families of monotremes are still around today: the duck-billed platypus and the echidnas. The order shares several obvious characteristics with reptiles and birds, making them quite different from both placental mammals (like us) and marsupials (which were talked about previously). The word “monotreme” literally means “one opening,” which is a characteristic feature: similar to birds and reptiles, they have the same opening for fecal matter, urine, and reproduction, called a cloaca. Also like birds and reptiles, monotremes lay eggs, although their eggs are uniquely rubbery and rather small.
Monotremes have retained and modified these and other reptilian traits from the Jurassic period, 200 million to 145 million years ago, which is when they diverged evolutionarily from all other mammals. Marsupials and placental mammals are much more similar to each other; they shared common ancestors until 90 million to 65 million years ago.
Because few species of monotremes exist anymore, their evolutionary past is difficult to decipher, but it is thought that monotremes originally evolved in Australia and spread to South America and Antarctica, which were all connected at the time. They have since gone extinct everywhere except Australia and some nearby islands. Their geographic isolation from placental mammals on Australia may have allowed them to survive there.
Monotremes should not be thought of as precursors to the other mammalian groups, but a branch that diverged from the others at an earlier point in history. They appear to have more in common with marsupials than placental mammals. Certain brain features and the act of “premature” birthing are shared between monotremes and marsupials. Additionally, like marsupials, echidnas have a pouch for their young.
Aside from the clearly reptilian traits of laying eggs and having a cloaca, monotremes share many other more subtle reptilian traits as well. They have a lower body temperature than most mammals (around 90 degrees Fahrenheit), several reptilian bone features, chromosome compilations suggesting greater reptilian influence, and sperm and testes structures that are similar to reptiles’. Of particular note, all monotremes have several distinct skull features in common: high-domed craniums, dental similarities including a lack of teeth in adults, and elongated rostrums, or “snouts.” Overall, this leads to monotremes having heads with a rather bird-like appearance.
Today, only five species of monotremes exist, three of which are critically endangered. The duck-billed platypus (Ornithorhynchus anatinus) is the only living member of its family (Ornithorhynchidae). The echidna family (Tachyglossidae) has four existing species, including three endangered long-beaked echidna species and one short-beaked species. At least two other monotremes families have been deciphered from fossils, but they are now extinct, along with many genera.
The duck-billed platypus, as might be expected of an animal that looks like a cross between a duck and a beaver, is very well adapted to life in the water. Swimming around in the freshwaters of Australia and Tasmania, the semi-aquatic platypus hunts small invertebrates, fish, and amphibians. Although its “bill” looks like a duck’s, it is very different; the bill actually helps it sense prey. Soft and pliable, the bill amazingly has electroreceptors, enabling it to sense electrical fields generated by muscle contractions. The platypus is very reliant on this for catching food; it even closes its eyes underwater. To help with its semi-aquatic lifestyle, the platypus has webbed feet and a broad, flat tail that acts as a rudder.
The reproduction process and early life of the platypus is also quite unique. The platypus uses its clawed feet to create a burrow. While both genders do this, females make deeper burrows for reproduction, which reach 20 to 30 meters long with a nest at the end. Unlike marsupials and echidnas, the platypus does not have a pouch, and once it lays its eggs it curls around them. When the young are hatched, they drink milk that is secreted from mammary glands and pores on their mother’s fur. (Monotremes don’t have nipples.) The young remain in the burrow for several months. While they have teeth when first born, platypus loose their teeth and instead develop keratinized pads.
Another quite remarkable platypus feature is its poisonous spurs; it is the only known poisonous mammal. Only the males have these poisonous spurs, which are on the hind leg ankles and are connected to a poison gland. When the animal feels threatened, it actually throws the poison out on to the intruder, even moving the spurs for a better angle if need be. While the poison usually does not kill the threat, it can cause excruciating pain.
Echidnas are spiny, terrestrial monotremes, living mostly in forests, though some reside in deserts. They can be found in Australia, Tasmania, and New Guinea. Like the platypus, the echidna’s very long “beak” also has electroreceptors. The short-beaked echidna, known as the “spiny anteater,” digs into anthills and termite nests to eat the residents. It is possible that masses of these insects can generate a detectable electrical field, although this is still an area of much-needed scientific investigation. Like the platypus, echidnas are equipped for digging. Whereas the short-beaked echidnas dig into these insect nests, the long-beaked echidnas unearth earthworms or rummage through forest leaf litter. Male echidnas have spurs on their ankles like the platypus does, but the spurs do not release poison and their function is unknown.
Despite both being labeled monotremes, echidnas have several features quite distinct from the platypus. Echidnas have a pouch in which they incubate their eggs. They secrete milk into parts of the pouch to nourish their young, who stay inside the pouch for weeks after hatching. Echidnas use a long, sticky, fast-acting tongue to capture prey, which they then grind against tough pads inside their mouth; they have no teeth. Additionally, for defense, echidnas can roll themselves into a ball, shielded by their spines, much like a hedgehog.
Conservation efforts are being made to preserve the few existing species of this most bizarre group of mammals. Currently the three species of long-beaked echidnas, which are only found in New Guinea, are all critically endangered. By better understanding these animals, researchers can discover not only how to best ensure their survival for future generations to appreciate, but can also learn more about our own ancient evolutionary history, through both our similarities and differences with the monotremes.
For further reading on monotremes, see Wikipedia’s “Monotreme” and George A. Feldhamer, Lee C. Drickamer, Stephen H. Vassey, and Joseph F. Merritt’s book Mammalogy: Adaptation, Diversity, and Ecology.
Biology Bytes author Teisha Rowland is a science writer, blogger at All Things Stem Cell, and graduate student in molecular, cellular, and developmental biology at UCSB, where she studies stem cells. Send any ideas for future columns to her at email@example.com.