MONOTREMES: CHARACTERISTICS, REPRODUCTION, FEEDING, HABITAT - BIOLOGY - 2025

The monotremes are the group of mammals with more primitive characteristics is known. They are characterized by being mammals that lay eggs and by having the same duct through which they reproduce and excrete their waste: feces and urine.

Mammals are currently classified into three major groups: placentates, marsupials, and monotremes. Today only 5 species remain alive from the monotreme group, while the rest are only known through fossil records.

Monotremes. In the upper left corner there is a photograph of a platypus, the others are different species of echidnas (Source: Ypna / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) via Wikimedia Commons)

Taxonomically speaking, monotremes are classified in the order Monotreme and the subclass known as Prototheria within the group of mammals. This order only has two different families: Tachyglossidae, where the echidnas have been classified, and Ornithorhynchidae, where the platypus are classified.

Within the family Tachyglossidae are 4 of the 5 species of the order, while the fifth species is in the family Ornithorhynchidae (there is only one species of platypus: Ornithorhynchus anatinus).

All living representatives of monotremes that are alive inhabit Oceania. Echidnas are mainly in the Australian outback and on the island of Tasmania, meanwhile the platypus have only been detected on the east coast of Australia and also on the island of Tasmania.

Most monotremes are abundant in their natural habitats and only echidnas that are classified as "long-beaked echidnas" are considered endangered.

Characteristics of monotremes

In the monotreme group, there is a great diversity of unique morphological characteristics in each family. However, both families share some common and unique characteristics of the order, among which we can mention:

Monotremes and marsupials (Source: Community Archives / Public domain, via Wikimedia Commons)

- They are the only oviparous mammals, that is, they lay eggs (it is important to remember that mammals give birth to live young, which feed on the milk produced by their breasts).

- They are mammals with "cloaca". The cloaca is a hole where the "outlet" of the digestive system, the urinary system and the reproductive system converge. The order owes its name to this characteristic which means "mono" = one or only and "trema" = hole, that is to say: "a hole".

- All species of this order have, in general, a homeothermic mechanism similar to that of mammals. However, they have a lower standard temperature than that of other mammals.

- The two families in the group have a large amount of fur. The echidna is characterized by a coat particularly adapted as a defense system, as it corresponds to a complex of subcutaneous spines.

- The heart of the monotremes also has its peculiarities. This has a very large coronary vein that crosses the atrioventricular groove, is separated from the right atrium by an inversion of the serous pericardium and drains directly between the anterior and posterior vena cavae.

- The skull of the monotremes is quite "flat" and elongated, so it shares characteristics with the most "ancient" mammals.

Characteristics of echidnas

Echidnas are land mammals with a long tubular snout and long, strong, powerful claws. The entire dorsal surface of their body is covered by long spines and on the tail they have a high density of these spines.

All the spines are strongly attached to the skin of the animal and, unlike the porcupine, these spines do not eject in dangerous situations. These animals have fur between the spines and on the ventral part of their body.

A short-beaked echidna (Source: fir0002 flagstaffotos gmail.com Canon 20D + Canon 70-200mm f / 2.8 L / GFDL 1.2 (http://www.gnu.org/licenses/old-licenses/fdl-1.2.html) via Wikimedia Commons)

Among the echidnas, two morphologically different groups have been differentiated: the short-beaked echidnas and the long-beaked echidnas. The long-beaked echidnas, in addition to the long beak, have shorter spines and in much less quantity than the short-beaked echidnas.

Also, long-beaked echidnas have much thicker fur compared to short-beaked echidnas. Of these animals, 2 species have been described, while those with the short beak are represented by a species that is subdivided, in turn, into 5 subspecies.

Characteristics of the platypus

Platypus. Source: Stefan Kraft

Platypus are aquatic mammals primarily adapted to life in water. Practically its entire body is covered with quite impermeable hair, with the exception of its beak and legs.

Its legs are webbed and its beak is flattened (both structures resemble those of a duck, only the beak is longer and flattened). In their posterior region they have a wide and long tail, the length of which corresponds to approximately one third of the entire animal's body; it is flat and helps them move underwater.

The platypus does not harbor subcutaneous fat on its body, in contrast, it is all stored in the tail and constitutes about 40% of its total body fat.

All male platypus have spurs that are connected to poisonous glands and are located on the ventral part of the hind legs. These are covered by a skin sheath that only breaks when the individual reaches an age greater than 9-12 months.

Although the injection of the poison is quite painful for humans, today it is known that it is not lethal; although it is for smaller mammals such as dogs, rodents and other platypus.

Reproduction

The reproduction of monotremes is very similar to the reproduction of marsupials, with the difference that female monotremes do not have a uterus or vagina. The copulatory organ of males consists of a tubular penis that only fulfills the function of reproduction, that is, it is not an organ that is part of the excretory system.

The urethra of the penis connects directly with the urogenital sinus, opposite to the excretory vas deferens.

There are two testicles in males and they are internal, although only one of them has been shown to be functional.

In echidnas, mating occurs between April and September, while in platypuses between July and October, in the interior of Australia, since in Tasmania the reproduction occurs during the month of February. In this video you can see two platypus specimens mating:

Monotremes are usually solitary individuals, but in the reproductive season, echidnas can be seen forming "lines" or "trains" of up to 11 males following a female. The time from the line to the act of mating can last from 7 to 37 days.

Platypuses do not enter a reproductive state until their fourth year of maturity. At this point they mate several times over several days. It is a general rule that monotremes (both platypus and echidna) take care of their young after birth (from hatching of the eggs).

Monotremes do not have nipples, so they excrete the milk that nourishes their young from two areas called “milk patch” or “areola”. This zone contains between 100 and 150 individual pores through which the milk flows. The newborn sucks milk directly from the mother's skin or hair.

Feeding

Echidna species with shorter beaks consume ants, termites, and some small invertebrates such as earthworms and beetle larvae. Long-beaked echidnas mainly consume earthworms, small centipedes, and underground cicadas.

Platypus feed on freshwater invertebrates such as beetles, snails, crustaceans, flies, and larvae of Lepidoptera and Diptera. These usually submerge between 30 and 140 seconds to catch their prey in the water.

A platypus feeding (Source: robertpaulyoung / CC BY (https://creativecommons.org/licenses/by/2.0) via Wikimedia Commons)

Platypus can slow their heart rate and remain submerged for up to 10 minutes while feeding on the bottom of lakes more than 5 meters deep.

All monotremes are nocturnal and can spend 12 hours a night feeding. These animals consume between 13 and 28% of their body weight in food daily.

During lactation periods, mothers can consume up to 100% of their body weight in one night of feeding, since the calves can consume up to 70% of the mother's body weight through milk. Here we see a platypus feeding:

Habitat

Equines inhabit various ecosystems in Australia, New Guinea and the island of Tasmania, depending on the species. Short-beaked ones inhabit plains and deserts in the Australian outback, where they spend their lives devouring termites and insect larvae.

Long-beaked echidnas can be found in cloud forest forests and in the mountains. These, having a more varied diet, have a greater range of geographic dispersion.

Platypus inhabit freshwater streams, lakes and ponds in eastern Australia and on the island of Tasmania. These were first observed by biologist John Hunter and the governor of the penal colony at the time, Port Jackson.

Examples of monotreme species

There are 3 species of echidna very abundant in nature. One is commonly known as the short-beaked echidna or Tachyglossus aculeatus, which is divided into 5 subspecies. The first of these is Tachyglossus aculeatus acanthion, which inhabits much of the Australian territory.

Another of these is Tachyglossus aculeatus aculeatus, which lives in eastern New South Wales, Victoria south of Queensland-Australia. The third is Tachyglossus aculeatus lawesii, which lives only in New Guinea.

The fourth subspecies is Tachyglossus aculeatus multiaculeatus, it lives in South Australia and lastly is Tachyglossus aculeatus setoso, which is exclusive to the island of Tasmania.

The other two species of echidnas that exist are Zaglossus bartoni and Zaglossus bruijnii. Z. bartoni is characterized by five claws on its front legs, while Zaglossus bruijnii has only three. Both species are unique to New Guinea.

Platypus are only represented by the species Ornithorhynchus anatinus found along the east coast of mainland Australia and on the island of Tasmania. It is very sensitive to the intervention of bodies of fresh water, which is why it generally prefers bodies of fresh water far from civilization or, in other words, spaces little intervened by man.

References

1. Graves, JAM (1996). Mammals that break the rules: genetics of marsupials and monotremes. Annual review of genetics, 30 (1), 233-260.
2. Griffiths, M. (2012). The biology of the monotremes. Elsevier.
3. Holz, P. (2014). Monotremata (Echidna, Platypus). Fowler's Zoo and Wild Animal Medicine, Volume 8-EBook, 8, 247.
4. Jenkins, FA (1989). Monotremes and the biology of Mesozoic mammals. Netherlands Journal of Zoology, 40 (1-2), 5-31.
5. Pascual, R., Archer, M., Jaureguizar, EO, Prado, JL, Godthelp, H., & Hand, SJ (1992). First discovery of monotremes in South America. Nature, 356 (6371), 704-706.