"Monotreme" means "one opening" and refers to the single rear orifice, or opening, that these animals have for getting rid of wastes, laying eggs, and mating. The lower intestine, excretory system (system that gets rid of wastes), and reproductive system all end at this opening, called the cloaca (kloh-AY-kah). This feature is common in reptiles and birds but extremely rare among mammals.
Trying to describe a "typical" monotreme (MAHN-ah-treem) is difficult, since the only two living types, the platypus and the echidna (ih-KID-nah), do not look much alike at first glance. The platypus is built in a streamlined manner, like an otter, has soft fur, and its snout resembles a duck's bill, while the echidna looks like a pudgy, waddling watermelon covered with fur and sharp spines, with a narrow, hornlike snout. Although echidnas may look overweight, most of the soft tissue mass that might be mistaken for blubber is muscle, lots of it. The platypus is semiaquatic, hunting animal food underwater but sheltering in a dry burrow, but the echidnas are land animals that forage, or search, in the soil for insects and worms.
Adult platypus are about the size of house cats, while echidnas range from twice to three times as large as a house cat. An adult platypus weighs from 3 to 5 pounds (1.4 to 2.3 kilograms), and its adult head and body length runs 12 to 18 inches (30 to 46 centimeters), the tail adding another 4 to 6 inches (10 to 15 centimeters). The short-beaked, or short-nosed, echidna can grow up to 14 pounds (6.4 kilograms), with a head and body length of up to 21 inches (53 centimeters), the stubby tail adding another 3 or 4 inches (7.6 to 10 centimeters). The long-beaked, or long-nosed, echidna weighs up to twenty pounds, with a head and body length ranging from 18 to 31 inches (45 to 77.5 centimeters), while the tail, like that of the short-nosed echidna, is a mere stubby shoot. Male platypus and male echidnas are larger than females.
Platypus and echidnas are often called "primitive" because they carry a number of reptilian, or reptile-like, characteristics along with typically mammalian features. Ever since the first discovery of monotremes by Europeans in the late 1700s, zoologists, scientists who study animals, have been busy studying this mix of details in order to place the monotremes properly in the framework of mammalian evolution. Even more confusing is that the living monotremes have a number of modified, or changed, features all their own, examples being the snouts of platypus and echidnas.
The most well-known and special feature of the monotremes, and the one that seems most reptilian, is that the females lay eggs rather than giving live birth. Monotremes are the only living, egg-laying mammals. Other characteristics that platypus and echidnas have in common are similar skeletons and highly modified snouts equipped with nerves whose endings are sensitive to pressure and to natural electricity. Monotremes have fur, but not whiskers, while the echidnas, in addition to fur, have sharp, defensive spines, which are modified hairs, scattered over their backs and sides.
Monotremes walk in a reptilian manner, like alligators and crocodiles. Like the arms of someone in the middle of doing a pushup, the upper bones of monotreme forelimbs and hindlimbs go straight out from the body, horizontal to the ground, and the lower limb bones go straight down. Other lines of mammal evolution have abandoned this clumsy sort of movement and now carry their entire legs vertically beneath their bodies. Zoologists are not yet sure if the push-up style of legs and walking in monotremes is something left over from their reptilian ancestors or if they are more recent changes to fit their lifestyles.
Another odd monotreme characteristic is that male and female platypus, and male echidnas, have short, sharp, hollow, defensive spurs on the inner sides of the ankles of their rear limbs. The spurs of the male platypus connect with poison glands and are fully functional as stingers.
Monotremes are found in Australia and New Guinea. Platypus are found in Australia, including the southern island of Tasmania. Echidnas are found in Australia, Tasmania, and New Guinea. Fossil evidence from sixty-three million years ago confirms that monotremes once lived in South America, dating back to a remote time when the continents of Australia, Antarctica, and South America were closer to one another and connected by dry land.
Platypus live alongside bodies of fresh water, in tropical and temperate (mild) regions of eastern Australia. Echidnas live in most of the wet and dry biomes of Australia, and in the lowland and highland tropical forests of New Guinea.
Platypus hunt underwater, snagging and eating various small water creatures. The short-beaked echidna shovels soil and tears up logs for ants and termites, while the long-beaked echidna digs up and eats mainly earthworms.
BEHAVIOR AND REPRODUCTION
The most well-known feature of monotremes is their method of reproduction. They are the only living mammals in which females lay eggs instead of giving live birth. The length of time the egg remains within the mother is short, only twelve to twenty days. While the egg is still within the mother's oviduct (the tube leading from the ovaries to the cloaca), the tissues of the oviduct secrete a shell onto the egg, as happens in birds and egg-laying reptiles. The monotreme eggshell is soft and leathery, and porous enough to soak up nutrients secreted into the oviduct from the mother's circulatory system.
The embryo begins its development before the egg is laid. When the mother lays her egg, the embryo has already developed to about the same degree as a newborn marsupial. The eggshell is leathery, like a reptile's, spherical, and small, 0.5 to 0.6 inches (13 to 15 millimeters) in diameter, or the size of a grape. After about ten days of the egg's incubation, the young hatches by tearing at the shell by means of a temporary egg tooth on its snout. When the youngster is fully hatched, it nestles close to the mother and feeds on her milk. The young are weaned at four to six months of age.
Female echidnas and platypus may lay up to three eggs at a time, but one is normal, and monotreme females usually bear and raise only one young per year. Females do all the raising of the young. Except during the mating season, individual platypus and echidnas of both sexes lead solitary lives.
A platypus mother incubates her eggs by curling her tail and holding the eggs between the tail and her warm underbelly. She incubates and nurses her young in a "birth chamber" burrow, which she digs and lines with moist leaves and water plants to maintain humidity. Echidna mothers form simple, temporary pouches by constricting special long muscles of their underbellies, and in which they incubate the eggs and later carry the developing young.
The monotremes are unique in yet another way. They are the only mammals to carry a sensory system that detects electricity, along with their usual senses of sight, hearing, etc. The platypus bill contains tiny electroreceptors, specialized sensory nerve endings arranged in rows along the length of the bill, on the upper and lower surfaces. These detect electricity from the muscular systems of underwater animals that the platypus hunts, and even from the electricity created by water as it flows over rocks on the bottom of the lake or river. The electroreceptors are located together with mechanoreceptors that detect underwater turbulence. Together, the two senses allow the platypus to put together a three-dimensional "picture" of its underwater hunting territory.
The bills of echidnas also have electroreceptors, though much fewer than in platypus. Biologists have confirmed the platypus's use of the electrosense, while this has not been found working in echidnas. Most likely the echidnas are gradually losing the electrosense while platypus have developed it into one of nature's most complex sensory systems.
MONOTREMES AND PEOPLE
The special features of monotremes that set them apart from other mammals make them subjects of fascination and curiosity. Nearly everyone has heard about the platypus and knows that it is an egg-laying mammal. The reptilian features of the living monotremes provide a valuable window back in time to when reptiles were evolving into mammals.
Platypus fur was once a valued commodity because of its softness and fine texture. Hunting of the platypus in the late 1800s and early 1900s nearly drove the animals to extinction. Strict laws within Australia now protect platypus and echidnas, and the animals are fairly abundant today.
Echidnas in New Guinea are sometimes considered pests because they dig up gardens and farmland in their unending search for ants, termites, and earthworms. Habitat loss threatens the long-nosed echidna because it is confined to upland New Guinean forest, a limited habitat. The New Guinean echidnas are also hunted for food.
Platypus and short-nosed echidnas are protected by law in Australia. Platypus are fairly plentiful in their somewhat limited area. Short-nosed echidnas are plentiful and widespread, because they can live in many different types of biome. Long-nosed echidnas are Endangered, and under serious threat in New Guinea from loss of habitat and being hunted for food with the help of trained dogs.
One of the shortest telegrams ever sent was the one that confirmed the fact that platypus and echidnas lay eggs instead of giving live birth. Aboriginals and white settlers had been asserting this for decades, but it seemed so improbable that zoologists insisted on proof. The Scottish zoologist William Hay Caldwell traveled to Australia in 1884 to study platypus and echidnas in the wild. Aboriginals, with their excellent tracking skills, helped by catching the animals in the wilderness and bringing them to Caldwell. When he finally did confirm that echidnas and platypus are egg-layers, he sent the following telegram, on September 2, 1884 to the British Association for the Advancement of Science, which was holding its annual meeting in Montreal: "Monotremes oviparous, ovum meroblastic." The words meant that monotremes lay eggs, and the eggs have large yolks, like birds' eggs.
Probably the most serious problem facing these animals is being hunted, killed, and eaten by carnivorous mammals introduced to Australia and New Guinea by Europeans, such as dogs, cats, rats, and foxes. Native animals prey on the monotremes as well, including some of the larger lizards and the dingo, a breed of dog that the ancestors of the Aborigines brought with them when they colonized Australia thousands of years ago.
FOR MORE INFORMATION
Augee, M. L., ed. Platypus and Echidnas. Australia: Royal Zoological Society of New South Wales, 1992.
Moyal, Ann. Platypus: the Extraordinary Story of How a Curious Creature Baffled the World. Australia: Allen & Unwin Pty Ltd, 2002.
Pascual, Rosendo, et al. "First Discovery of Monotremes in South America." Nature 356, no. 6371 (April 1992): 704–706.
Krubitzer, L. "What Can Monotremes Tell Us About Brain Evolution?" Philosophical Transactions of the Royal Society of London, Biological Sciences 353, no. 1372 (July 1998): 1127–1146.
Pettigrew, J. D., P. R. Manger, and S. L. B. Fine. "The Sensory World of the Platypus." Philosophical Transactions of the Royal Society of London, Biological Sciences 353, no. 1372 (July 1998): 1199–1210.
Pettigrew, J. D. "Electroreception in Monotremes." Journal of Experimental Biology 202, no. 10 (1999): 1447–1454.
Vergnani, Linda. "On the Trail of Scientific Oddballs (Peggy Rismiller Studies Echidnas)." The Chronicle of Higher Education 48, no. 11 (2001): A72.
Australian Platypus Conservancy. http://www.totalretail.com/platypus (accessed on June 29, 2004).
"Links for Platypus and Echidnas." Department of Anatomy & Physiology, University of Tasmania, Hobart. http://www.healthsci.utas.edu.au/medicine/research/mono/References.html (accessed on June 29, 2004).
Pelican Lagoon Research Centre (for echidnas and other animals). http://www.echidna.edu.au/index.html (accessed on June 29, 2004).