Diversity
The subclass
Prototheria
contains the egg-laying mammals, which are the most ancestral forms in the class
Mammalia
. There are only three extant species grouped into two families and a single order,
the
Monotremata
. Despite bearing fewer species than most mammalian genera, the prototherians are
so unique among mammals that there is little question that they represent a distinct
and ancient branch of the mammmalian family tree. However, it is not clear how monotremes
are related to the two other major lineages of mammals, marsupials (
Metatheria
) and placentals (
Eutheria
). Some evidence supports the hypothesis that prototherians form a clade with the
marsupials, while other evidence suggests that prototherians are sister to a clade
containing both marsupials and placentals.
Prototherians probably split from the lineage leading to other mammals sometime in
the Mesozoic. They retain many characters of their therapsid ancestors (for example,
a complex pectoral girdle, laying of eggs rather than bearing live young, limbs oriented
with humerus and femur held lateral to body, and a cloaca). The skulls of monotremes
are almost birdlike in appearance, with a long rostrum and smooth external appearance.
Modern monotremes lack teeth as adults; sutures are hard to see; the rostrum is elongate,
beak-like, and covered by a leathery sheath; and lacrimal bones are absent. Monotremes
have several important mammalian characters, however, including
fur
(but they lack vibrissae), a four chambered heart, a single
dentary
bone,
three middle ear bones
, and the ability to
lactate
.
Geographic Range
Monotremes are restricted to Australia and New Guinea. Their fossil record is very
poor; the earliest fossil attributed to this group is from the early Cretaceous. A
fossil from Argentina suggests that the monotremes were more widely distributed early
in their history.
- Biogeographic Regions
- australian
Habitat
Prototherians are either terrestrial (
Tachyglossidae
) or primarily aquatic (
Ornithorhynchidae
). Their terrestrial habitats include deserts, sandy plains, rocky areas, and forests
in both lowlands and mountains. Platypuses inhabit lakes, ponds and streams; they
shelter in burrows along the banks and spend much of their time foraging in the water.
- Habitat Regions
- temperate
- tropical
- terrestrial
- freshwater
- Terrestrial Biomes
- desert or dune
- savanna or grassland
- forest
- scrub forest
- mountains
- Aquatic Biomes
- lakes and ponds
- rivers and streams
- Other Habitat Features
- riparian
Physical Description
Besides the absence of teeth, lacrimals, and obvious sutures, prototherians share
a number of skeletal characteristics. On the skulls, the
jugals
are reduced or absent, the
dentary
is a slender bone with only a vestige of a
coronoid process
, the angle of the dentary is not inflected medially (unlike that of marsupials),
auditory bullae
are missing (part of the middle ear is enclosed by tympanic rings), and much of the
wall of the braincase is made up by the petrosal rather than the
alisphenoid
(unlike all other modern mammals). Postcranially, the skeleton of prototherians is
also unique among mammals. It is a fascinating mosaic of primitive characteristics
inherited from therapsids but found in no other living mammals, and modifications
probably related to the burrowing habits of modern prototherians. Their
shoulder girdles
are complex, including the standard components of modern mammals (
scapula
and
clavicle
), but also additional elements including
coracoid
,
epicoracoid
, and
interclavicle
. The scapula, however, is simplified, lacking a supraspinous fossa. The shoulder
girdle is much more rigidly attached to the axillary skeleton than in other mammals.
Femur
and
humerus
are held roughly parallel to the ground when the animal walks, more in the fashion
of therapsids and most modern reptiles than like modern mammals.
Ribs
are found on the neck (cervical)
vertebrae
as well as the chest (thoracic) vertebrae; in all other modern mammals, they are
restricted to the thoracic region.
Another interesting skeletal characteristic of prototherians is the large
epipubic bones in the pelvic region. Epipubic bones were originally thought to be
related to having a pouch, but they are found in both males and females. They also
occur in all species of marsupials, whether a pouch is present or not (not all marsupials
have a pouch). It is now thought that epipubic bones are a vestige of the skeleton
of therapsids, providing members of that group with extra attachments for abdominal
muscles to support the weight of the hindquarters.
Prototherians are endothermic, but they have unusually low metabolic rates and maintain
a body temperature that is lower than that of most other mammals.
All male prototherians have spurs on their ankles that are presumed to be used in
fighting and in defense. In one family (
Ornithorhynchidae
), a groove along the spur carries poison secreted by adjacent glands.
- Other Physical Features
- endothermic
- homoiothermic
- bilateral symmetry
- venomous
Reproduction
Little is known about the mating systems of
prototherians
. They are solitary for most of the year, coming together only to mate. During the
mating season,
duck-billed platypuses
are found in pairs, but despite these observations, platypuses are not likely to
be monogamous because males do not associate with females post-copulation, nor do
they provide any parental care. Female
short-nosed spiny echidnas
have been observed with several males at a time, which may reflect a polygyny or
polyandry. Even less can be inferred about the mating systems of
long-nosed spiny echidnas
because so little is known about their basic behavior and biology.
Prototherians
are seasonal breeders. Typically, the breeding season lasts 1 to 3 months between
July and October. At least one species (
duck-billed platypuses
) perform somewhat elaborate courtship behaviors prior to copulation.
The eggs layed by monotremes are small (13 to 15 mm diameter) and covered by a leathery
shell. The number of eggs laid is small, usually 1 to 3, and they are placed in the
mother's pouch. They contain a large yolk, which is concentrated at one end of the
egg, very much like the yolk of a bird's egg. Only the left ovary is functional in
the platypus, but both produce eggs in the echidna. Like the eggs of birds, monotreme
eggs are incubated and hatched outside the body of the mother. Incubation lasts about
12 days. The young, which are tiny and at a very early stage of development when they
hatch, break out of the eggs using a "milk tooth. They are protected in a temporary
pouch in echidnas but not platypuses. They are fed milk produced by mammary glands;
the milk is secreted onto the skin within the pouch and sucked or lapped up by the
babies. Weaning takes place when the young are 16 to 20 weeks old.
- Key Reproductive Features
- iteroparous
- seasonal breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
- sexual
- oviparous
The parental investment of male
prototherians
appears to consist entirely of acquiring mates and fertilizing a female's eggs. All
other investment and parental care is provided by females. Young are born in a highly
altricial state and require considerable care and protection from their mothers. As
mammals, females produce milk and nurse their young.
Echidnas
develop a brood pouch on their abdomen within which eggs and hatched young develop
for nearly two months. Young are weaned by about three months of age.
Platypuses
do not have a brood pouch, and instead lay their eggs in deep, complex burrows on
the banks of streams and ponds. Young develop within the burrow and are weaned after
3 months.
- Parental Investment
- altricial
-
pre-fertilization
- provisioning
-
protecting
- female
-
pre-hatching/birth
-
provisioning
- female
-
protecting
- female
-
provisioning
-
pre-weaning/fledging
-
provisioning
- female
-
protecting
- female
-
provisioning
Lifespan/Longevity
Little is known regarding the natural lifespan of
prototherians
; however, they can live several decades in captivity. In at least one case, a
short-nosed echidna
lived 50 years.
Behavior
Prototherians
are primarily solitary animals and at least one species (
Tachyglossus aculeatus
) is territorial. Activity patterns vary among species, and even among populations
of
T. aculeatus
; prototherians may be diurnal, crepuscular or nocturnal.
Echidnas
are fully terrestrial and eat mainly ants, termites, and worms whereas
platypuses
spend much of their time foraging in the water for a wider variety of invertebrates.
All three species are exceptional diggers, using powerful limbs to dig shelters or
to quickly escape from predators. In addition to digging their way out of trouble,
echidnas can roll up and erect their spines as a defense mechanism. If insufficient
food is available, prototherians may enter temporary torpor or more prolonged periods
of hibernation when food is scarce in the winter.
- Key Behaviors
- terricolous
- fossorial
- natatorial
- diurnal
- nocturnal
- crepuscular
- motile
- hibernation
- solitary
- territorial
Communication and Perception
Hearing, olfaction, touch, and vision are all important to some degree in
prototherians
. Hearing and sight are well developed in
platypuses
and moderately well-developed in
echidnas
. The sense of touch is perhaps most important to a platypus that is searching at
the bottom of a stream for food or an echida that is rooting through the earth for
termites or worms. Platypus bills and echidna snouts are extremely sensitive organs
that are essential to effective foraging. Platypuses may even use electrical stimuli
to locate prey. Olfaction is well-developed in echidnas and may be used in individual
recognition. Prototherians occasionally produce some simple vocalizations, but their
function is unknown.
Food Habits
All
prototherians
are carnivorous, with their diets consisting of various invertebrates.
Platypuses
forage in the benthos of lakes and streams, using their sensitive bills to find prey.
They are generalist predators, whereas
echidnas
specialize on either ants and termites (
Tachyglossus
) or worms (
Zaglossus
). Both species of echidna are powerful diggers and use their claws and snouts to
root through the earth to find food.
- Primary Diet
-
carnivore
- insectivore
- eats non-insect arthropods
- molluscivore
Predation
Little is known about the predators of
prototherians
or how predation impacts prototherian populations. With their robust spines,
echidnas
are certainly well-protected from threats. To deter potential predators, echidnas
erect their spines, roll into protected balls, or rapidly dig a hole or enter a crevice,
exposing only their spines.
Ecosystem Roles
Prototherians
may significantly impact populations of their prey; this may be more true for
echidnas
because they specialize on only a few prey types rather than eating a little bit
of many different species. Because they are adept diggers, prototherians create and
modify habitat for other organisms.
Platypuses
in particular can exavate extensive burrows on the banks of freshwater lakes and
streams. Prototherians are hosts of various parasites (e.g.
trypanosomes
in platypuses and
hepatozoans
in echidnas).
- Ecosystem Impact
- creates habitat
Economic Importance for Humans: Positive
One of the three species within
Prototheria
(
Zaglossus bruinji
) is eaten by the indigenous people of New Guinea. Hunting pressure has been so great
that this species is now threatened with extinction.
- Positive Impacts
- food
Economic Importance for Humans: Negative
Prototherians
have no known negative impact on people, except perhaps for the pain a
platypus
can cause if it delivers venom with its spur. If unmolested, platypuses will not
attack humans.
- Negative Impacts
-
injures humans
- bites or stings
Conservation Status
At least two of the three species of
prototherians
are threatened with extinction or were at some point in the recent past.
Long-nosed echidnas
are currently listed as endangered by the International Union for the Conservation
of Nature (IUCN) redlist (www.redlist.org). Habitat destruction and heavy hunting
pressure have severely limited populations of this species. In the past,
platypuses
were declining rapidly due to their overexploitation in the fur trade, but recent
conservation efforts have helped populations rebound considerably.
Additional Links
Contributors
Tanya Dewey (editor), Animal Diversity Web.
Matthew Wund (author), University of Michigan-Ann Arbor, Anna Bess Sorin (author), Biology Dept., University of Memphis, Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.
- Australian
-
Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- temperate
-
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
- tropical
-
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
- terrestrial
-
Living on the ground.
- freshwater
-
mainly lives in water that is not salty.
- desert or dunes
-
in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.
- tropical savanna and grassland
-
A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.
- savanna
-
A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.
- temperate grassland
-
A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.
- forest
-
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
- scrub forest
-
scrub forests develop in areas that experience dry seasons.
- mountains
-
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
- riparian
-
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
- endothermic
-
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
- bilateral symmetry
-
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
- venomous
-
an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).
- iteroparous
-
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
- seasonal breeding
-
breeding is confined to a particular season
- sexual
-
reproduction that includes combining the genetic contribution of two individuals, a male and a female
- oviparous
-
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
- altricial
-
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
- fossorial
-
Referring to a burrowing life-style or behavior, specialized for digging or burrowing.
- natatorial
-
specialized for swimming
- diurnal
-
- active during the day, 2. lasting for one day.
- nocturnal
-
active during the night
- crepuscular
-
active at dawn and dusk
- motile
-
having the capacity to move from one place to another.
- hibernation
-
the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.
- solitary
-
lives alone
- territorial
-
defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement
- visual
-
uses sight to communicate
- tactile
-
uses touch to communicate
- acoustic
-
uses sound to communicate
- chemical
-
uses smells or other chemicals to communicate
- visual
-
uses sight to communicate
- tactile
-
uses touch to communicate
- acoustic
-
uses sound to communicate
- chemical
-
uses smells or other chemicals to communicate
- food
-
A substance that provides both nutrients and energy to a living thing.
- carnivore
-
an animal that mainly eats meat
- insectivore
-
An animal that eats mainly insects or spiders.
- molluscivore
-
eats mollusks, members of Phylum Mollusca
References
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