Marsupials & monotremes
Mammals are split into three groups based on how their offspring develop before weaning. All mammals make milk to nourish newborns, but only one group, the eutherians, feed babies using a placenta during a long pregnancy. The other two groups, marsupials and monotremes (M&Ms), use different strategies at the end of reproduction.
So what is a placenta?
It’s a structure that provides nutrients, but with more ingredients than an energy-rich egg yolk. In true mammals (eutherians), offspring are born after a long gestation and the placenta creates an intimate link between mother and baby in the womb. Though true mammals are known as placentals, the M&M groups use placentas too: monotremes (prototherians) have a temporary structure; marsupials (metatherians) use one over a much shorter pregnancy.
What are marsupials?
Pouched mammals, though some don’t have a marsupium, Latin for ‘pouch’. The group includes more than 300 species, from koalas and kangaroos to possums and opossums.
Whereas placentals are precocious, born nearly fully formed, marsupial newborns (called joeys) are altricious – they’re born underdeveloped, the equivalent to an 8-10 week-old human embryo, and gestation lasts weeks (12.5 days in some bandicoots). Most embryonic development occurs after a foetus leaves its mother’s vagina. After birth, that baby must climb multiple times its own bodylength to reach the pouch and drink milk from a teat. Produced by a female’s mammary glands, milk contains nutrients and substances that boost infant immunity and, in marsupials, these will change to meet their needs at different stages.
And what are monotremes?
Egg-laying mammals. Monotremes aren’t viviparous (live-bearing) but oviparous, so young are sustained by yolk until they hatch. Hatchlings (nicknamed puggles) must lick the milk secreted from a mother’s skin, as females don’t have nipples. There are only five living species: four kinds of echidna plus the platypus – a semi-aquatic creature that looks like a beaver and a duck’s love-child.
Monotremes have just one orifice or ‘cloaca’ for urination, intercourse and defaecation, hence the name – monotreme comes from the Greek for ‘single opening’. It’s one of several reptilian or bird-like traits that were probably present in the common ancestor of all living mammals. But while monotremes are considered primitive, they have some sophisticated features too. Their snout (or beak/bill) can sense electricity to detect prey, for example, and platypus males have venomous leg spurs.
Where are M&Ms found?
Though associated with Australia, marsupials also live on Melanesia and a third of species occur in the Americas. Monotremes are found in Australia and New Guinea. This distribution of M&Ms reflects the fact that Australia and South America were once connected as a supercontinent, Gondwana.
Marsupials typically occupy ecological niches where placental mammals went extinct or were outcompeted. This has led to convergent evolution: for example, marsupial mice and moles look superficially similar to their placental counterparts.
When did they evolve?
Besides milk, mammals are characterised by fur, big brains and ear bones that confer exceptional hearing compared to other vertebrates. Based on DNA, evolutionary trees suggest that the first mammals originated around 200 million years ago (MYA), then 140 MYA split into two branches: one leading to monotremes, the other to ‘therians’ – a group that diverged 80 MYA to become marsupials and placentals.
Why pouches and eggs?
A complex placenta and long pregnancy has benefits, such as protecting a baby from predators and extreme temperatures. But carrying a foetus for several months has costs: it’s exhausting for females and makes mother and child vulnerable to attack or injury. Pregnancy also causes conflicts over resources and triggers immune responses because one individual is a parasite within the other’s body. So in certain environments, like Australia’s harsh habitats, natural selection may have favoured mammals that can abandon an egg or foetus to survive, until conditions are better for their reproductive success.