“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
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
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.
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 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
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.
MONOTREMES AND PEOPLE
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.