MONOTREMES – [Monotremata]


“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, excre-

tory 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 semi-

aquatic, 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 echid-

nas 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 cen-

timeters), 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, zool-

ogists, 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 confus-

ing 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 liv-

ing, egg-laying mammals. Other characteristics that platypus and

echidnas have in common are similar skeletons and highly mod-

ified 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 evo-

lution 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 fe-

male 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. Platy-

pus 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 con-

firms that monotremes once lived in South America, dating

back to a remote time when the continents of Australia, Antarc-

tica, 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 low-

land 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.


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 devel-

oped 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 platy-

pus 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 under-

bellies, 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 elec-

tricity, 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 electrore-

ceptors 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 los-

ing the electrosense while platypus have developed it into one

of nature’s most complex sensory systems.


The special features of monotremes that set them apart from

other mammals make them subjects of fascination and curios-

ity. 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 ani-

mals to extinction. Strict laws within Aus-

tralia 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 pro-

tected by law in Australia. Platypus are fairly

plentiful in their somewhat limited area.

Short-nosed echidnas are plentiful and wide-

spread, because they can live in many differ-

ent 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.

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, in-

cluding 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.