Humans have always been fascinated by the species contained within the order monotremata (monotremes). These mysterious and odd animals have captured our hearts and our imagination. Australian aboriginals once believed that the platypus was the offspring of a lonely female duck and a male water-rat. In 1799, when the first platypus arrived in Europe, many believed it was a hoax. It was only after careful examination by scientists like Dr. George Shaw that the platypus was recognised as a new species and not as the result of clever stitching. Since 1799, lots has been learnt about the platypus, the short-billed echidna and the long-billed echidna. We know know that monotremes are egg-laying mammals with many secrets we have yet to discover and thus our fascination with these animals continues.
What is a monotreme?
Monotremes are defined by a combination of mammalian and therapsid ancestral characters. Characters worth noting are listed below.
Characters that establish monotremes as mammals:
- Four chambered heart
- Body Covered in fur
- Single dentary bone
- Three middle ear bone
Therapsid Ancestral Characters:
- Oviparious (they lay eggs)
- Complex pectoral girdle
- Testes housed within the abdominal cavit
Characters that distinguish monotremes from other mammals:
|Character||Monotremes||Marsupials and Placental Mammals|
|Cloaca|| A single opening in which urine, feces and eggs are deposited before being expulsion from body. Similar to the mammalian uterus.||Have at least two distinct openings for expulsion of feces and urine and for the birth of young. Have a true uterus.|
| Leathery bill|| Present. It is used to detect prey.|| Not present.|
|Nipple|| No true nipple; milk is secreted through the skin of the abdomen.|| Milk is collected into a single duct and exits body through the nipple.|
| Spur on hind limb|| Present in males. Platypus can secrete venom through spur.|| Not present.|
|Ear||Primitive development of the inner ear bone, it is less coiled.|| More highly coiled inner ear.|
|Teeth|| Lack of teeth in adults, instead horny pads develop.|| Present in Adults.|
|Gait||Reptile-like; limbs are aligned laterally with respect to body.||Limbs are aligned below body.|
Phylogenetic Relationship: Monotremes, Marsupials and Placental Mammals
The phylogenetic relationship between monotremes (monotremata), marsupials (marsupilia) and placental mammals (eutheria) has been the subject of recent debate. The theria hypothesis, the traditional view, states that monotremes diverged from marsupials and placental mammals first then the marsupials diverged from the placental mammals. This theory has been largely supported by morphological data. One noteworthy exception is the similar pattern of tooth replacement seen in monotremes and marsupials by Kühne in 1973. This pattern provides the evidence for the marsupionata theory, first proposed by gregory in 1947. The marsupionata theory proposes that a common ancestor of both monotremes and marsupials diverged from eutherian mammals, followed by the divergence of the monotremes and the marsupials into separate groups. Since the majority of evidence supported the theria hypothesis, it was largely accepted. Diagrams showing the proposed phylogenies of both theories are included below.
© 2007 Margaret Thibodeau
In 1997, Axel Challenged the traditional theria hypothesis, providing evidence for the marsupionata hypothesis. His analyis of 12 mitochondrial protein-coding genes united the marsupials and the monotremes as sister groups. Axel compared the mtDNA of the platypus (monotreme), two marsupials, 21 placental mammals and 3 outgroups. The maximum likelyhood method using the quartet puzzling delivered the most consistent bootstrap values in support of the marsupionata hypothesis (94.3 - 99.9%) Other analyses had bootstrap values of 31-100%.
In 2005, van Rheede published an extensive analysis of nuclear DNA in mammals in order to once and for all confirm the therian hypothesis. He stated that Axel's analysis from 1997 was flawed due to base compositional bias. Van Rheede's analysis showed bootstrap values in support of the therian hypothesis from 55-100%. In the same analyses, they considerably higher than the highest provided molecular and genetic evidence for the therian hypothesis. Debate continues as to the correct phylogenetic relationship between the monotremes, the marsupials and the placental mammals but it seems that the therian hypothesis is more commonly accepted.
Currently three living species of monotremes are recognised. They are the Ornithorhynchus anatinus (duck-pilled platypus), the Zaglossus bruijni (the long-beaked echidna) and the Tachyglossus aculeatus (short-beaked echidna). The following chart includes some general information on each species.
| Common Name||Duck-Billed Platypus||Long-Beaked Echidna||Short-Beaked Echidna|
Species: Ornithorhynchus anatinus
Species: Zaglossus bruijni
The platypus has a streamlined body plan much like that of an otter, a tail similar to a beaver's tail and webbed feet. It also has a leathery duck-like bill that it uses to detect prey.
Adults typically range in length from 47.5 to 60.0 cm and in mass from 1.30 to 2.30 kg.
The smaller of the two echidnas. The body is covered in spines that are typically yellow with black tips and fur underneath. The snout is usually half the length of the head. Males have a spur on the hind foot but it lacks venom.
Adults can reach a length of 53 cm, with a 6cm tail and they range in mass from 2.5 to 6 kg.
The long-beaked echidna can be distinguished from the other monotreme by its long leathery snout. Covered in black or brown fur that may conceal the spinnes underneath.
They have an average body length of 45.0 to 77.5 cm and typically range in mass from 5 to 16.5 kg.
Live in freshwater of Eastern Australia.
Live in diverse geographic areas including deserts, meadows and forests. They can be found in Australia, Tasmania and New Guinea.
Typically live in the mountain forestso of New Guinea but can also be found in high altitude alpine meadows.
The Platypus; A possible link between bird and mammals
Chromosomes are very useful in determining relationships between various groups of animas. Their numbers, shape, size and contained genetic sequences can be used to determine how closely or how distant to groups are related. The following section discuses the link between the platypus may form between mammals and birds through the research of Franz Grützner.
Key differences exist between mammalian and avian sex-determining chromosomes. Typically mammals have two sex chromosomes, either XX in females of XY in males. Also, the Y chromosome contains the sex-determining gene SRY. Birds lack the SRY gene but instead the DMRT1 is suspected of being the sex-determining gene.
By labelling individual chromosome, Grützner determined that the platypus has 10 sex chromosomes, 5X5Y in males and 10X in females. The presence of the X an Y chromosomes is a mammalian character and gives further evidence for the classification of the platypus as a mammal. Meanwhile, the absence of the SRY gene and the presence of a gene similar to DMRT1 on the platypus' X5 suggests a link to the birds. Much research must still be done to fully understand the platypus' sex chromosomes but a possible evolutionary link has been identified.
Monotremes in Zoos
The Platypus: Taronga Zoo, Australia
The platypus breeding program at Taronga Zoo is run by Margaret Hawkins, a behavioural biologist, and by Adam Battaglia, an Australian mammals division zookeeper. It is an extremely sucessful program. Since 2003, they have sucessfully bred two sets of twins and a single birth making them the first zoo to hatch eggs succesfully three times. By creating a captive environment much like the natural habitat, they have had great sucess. They have been working hand in hand with Healseville Sanctuary, trading knowledge. They have also been exchanging platypuses in hopes of creating a sustainable captive population.