Marsupial misconceptions: weird mammals, placentas and pouches

I’ve now been living in Australia for almost 18 years, and I’m an unashamed convert to #TeamMarsupial. Marsupials are fascinating animals in both evolutionary and ecological terms, but at times I am surprised by how poorly-understood they are. I’ve been thinking of writing a post to address some recurring marsupial misconceptions for a while. When I saw how many marsupials were in the lineup for this year’s Mammal March Madness (more on this below) I decided that the time was right! So here we have it: eight things you might not know about marsupials, and profiles of the eight amazing marsupial species featured in Mammal March Madness 2017:


Eight things about marsupials…

1. Marsupials are mammals
2. Platypus and echidnas are also mammals, but they are not marsupials

Taxonomists group organisms together at different levels, or ranks, of classification. These progress from the extremely broad level of “domain”, to the much more specific level of “species”, as follows: domain, kingdom, phylum, class, order, family, genus, species. So a genus may include several closely-related species, a family may include several closely-related genera, an order may include several closely-related families… and so on… All living mammals are “officially” in the class Mammalia, but this is further divided into three subclasses:

  • Subclass Eutheria (almost 4000 described species) includes the mammals that most people (outside the Australo-Papuan region) are most familiar with. Humans are eutherian mammals, as are dogs, cats, dolphins, whales, bats, rodents, badgers, sheep, cows, elephants and pandas… and many others.
  • Subclass Marsupialia includes all of the marsupial groups that occur in the Australo-Papuan region (over 200 species) and in the Americas (about 100 species).
  • Subclass Prototheria is much smaller (5 species) and consists of a group known as the monotremes: the four echidnas and the singular platypus.

3. Marsupials are not the only mammals native to Australia

I’ve encountered a number of “educational” websites that say things along the lines of “eutherian mammals are absent from Australia and so marsupials were able to survive there and evolve into many different forms”. These statements can be confusing.

Yes, at various points in history, marsupials in the parts of the world that became Australia did not face the same competition from eutherian mammals as they did elsewhere. This may have contributed to the outstanding success of marsupials in the Australo-Papuan region, whereas in most other parts of the world eutherian mammals were able to outcompete marsupials.

But also… no, eutherian mammals are not completely absent from Australia. Whilst foxes, cats, rabbits, pigs, sheep, deer and many other eutherian mammals are recent introductions, Australia is also home to a unique assemblage of native eutherian mammals. These include many native rodents and bats, marine mammals that inhabit the oceans surrounding Australia, and then of course there is the dingo, which is an interesting and controversial story in its own right.

4. Marsupials do have placentas – but they’re different from “placental” placentas

Eutherian mammals are sometimes also called “placental mammals”. This is because people make the distinction between mammals that lay eggs (the monotremes), mammals that nourish their young with a placenta (eutherians) and mammals that keep their young in pouches (the marsupials – more on that below). Many people will tell you that marsupials do not have placentas, but this is a bit of an over-simplification… and also a sure-fire way to antagonise some marsupial biologists!

In eutherian mammals, embryos develop within their mother’s womb, often over many months (think about human reproduction). They do most of their development in the womb and are born at a relatively advanced stage. Whilst in the womb, the placenta connects the embryo to the mother’s blood supply, providing the embryo with nutrients and oxygen, and removes waste. The structure and appearance of the placenta varies greatly among different eutherian species.

Pregnant marsupials don’t develop a placenta in exactly the same way as eutherians do, but they do have a yolk-sac placenta in the womb that delivers nutrients to the embryo. In addition, bandicoots, quolls and wombats develop an allantochorion (a structure similar to that seen in eutherian placentas) in later pregnancy and bandicoots develop chorio-allantoic placentas very similar to those seen in eutherian mammals, but only during the last couple of days of pregnancy.

5. Marsupial babies do most of their development outside the womb

Marsupials are born at a much earlier stage in development than eutherian mammals, sometimes only a few days after conception, when their eyes, ears, limbs and organs are not fully formed. For example, the largest extant marsupial is the red kangaroo, which can weigh up to 91 kilograms, but at birth their young weigh just 1 gram! This strategy may have helped marsupials to adapt to and thrive in uncertain or variable environments.

Some marsupial newborns have relatively well-developed arms. This is because they have to crawl through their mother’s fur to find and latch onto a teat, which will provide them with nourishing milk while they complete their development. At times this can be a life or death competition, because some marsupial mothers, including quolls and Tasmanian devils, will give birth to many more babies than they have teats available, in which case only the first few to reach a teat will survive.

6. Marsupials have some weird reproductive strategies

It’s a bit subjective I know, but from a human perspective, some marsupial reproductive strategies really are just weird. For example, tammar wallabies have synchronised reproduction, meaning that all females give birth to their young at the same time, on or around January 22nd. Each female will mate again immediately after giving birth, and hold this second embryo in a dormant state called “embryonic diapause” for the next 11 months, before development resumes in time for birth around January 22nd the following year! In fact, embryonic diapause occurs in most kangaroos and wallabies, and some other marsupials. Essentially, this means that embryonic development pauses just a few days after conception, and resumes weeks or months later. In some species this may allow females to delay the birth of their offspring until good resources are available.

Then there are the dasyurids, like the many species of antechinus, whose reproductive strategies lead to headlines like “Antechinus go out with a bang” and “Doing it to death: suicidal sex in ‘marsupial mice’“…

7. Not all marsupial pouches are the same and not all marsupials have a pouch

The name marsupial comes from the word “marsupium”, which is a Latin word meaning “pouch” derived from the Greek “marsipos”, which means “purse”. Most female marsupials do indeed have pouches, which they use to protect their young. If you picture a marsupial with a pouch, you probably picture a kangaroo, which has a forward-opening pouch that opens towards the female’s head. But some marsupials, including wombats, koalas, devils, bandicoots and Virginia opossums, have backward-opening pouches. Wombats spend a lot of time in burrows and digging, so a pouch that opens towards the female’s tail is a lot less likely to fill up with dirt and rubble. Other marsupials have much shallower pouches, more like a fold of skin than a deep pocket, while some marsupials, like the short-tailed opossum, don’t really have a pouch at all.

long-footed-potoroo-young-in-the-pouch
A long-footed potoroo joey in the pouch

8. Marsupials can make their own antimicrobials

You could be forgiven for thinking that a marsupial’s pouch would be a rather unpleasant place – after all, how much dirt and grime accumulates in your pockets? Is an external pouch really a sensible place to grow a baby?!? Despite this potential for exposure to all sorts of dangers and diseases, pouches clearly do work and have been working for marsupials for a very long time. And now we are starting to understand why. In exciting work over the last few years, Australian researchers have discovered previously unknown types of antimicrobials in marsupial pouches, including those of the tammar wallaby, the koala, the brushtail possum and the Tasmanian devil. In a piece of great timing a new review paper on antimicrobial protection of marsupial pouch young, by Yuanyuan Cheng and Kathy Belov, was published online a few days ago.


#TeamMarsupial 2017

If you aren’t familiar with Mammal March Madness, I highly recommend you check it out. It is simply one of the most entertaining science communication events out there, with a combination of science, art, wildlife photography and natural history. Follow #2017MMM on Twitter – there is still time to fill out your bracket before the fun starts next Monday. To encourage you to support #TeamMarsupial, I’ll now share a brief profile of each of the eight marsupial contenders in the 2017 contest. My money is on the wombat or the quoll…

Order Diprotodontia (kangaroos, wallabies, possums, wombats, koalas…)

1024px-sthn_hnose_wombat3
Southern hairy-nosed wombat – image credit Kalyob

The southern hairy-nosed wombat (Lasiorhinus latifrons) lives in semi-arid areas of southern Australia. The IUCN lists this species as Near Threatened because population sizes have declined and sub-populations have become isolated. Southern hairy-nosed wombats are threatened by habitat loss, competition for food with introduced rabbits and domestic animals, and in some areas, sarcoptic mange.

 

burrowing_bettong_danielaparra
Burrowing bettong – image credit Daniela Parra

The burrowing bettong (Bettongia lesueur) was once one of the most widespread Australian mammals, but it had become extinct in mainland Australia by the 1960’s. The IUCN now lists this species as Near Threatened, with a long list of threats to burrowing bettong survival including introduced predators and rodents, changes to fire regimes, habitat loss and diseases. There are some great videos of the world’s only burrowing kangaroo on ARKive.

 

quokka_matthiasliffers
A quokka on Rottnest Island – image credit Matthias Liffers

The quokka (Setonix brachyurus) has an international reputation for cuteness. Despite this, the IUCN lists this species as Vulnerable because it only occurs in fragmented populations in a small area of south-west Western Australia, and may be at risk from habitat clearing and introduced foxes.

 

adult-bear-cuscus
An adult bear cuscus

The bear cuscus (Ailurops ursinus) is found on Sulawesi (Indonesia) and neighbouring islands. The IUCN lists this species as Vulnerable, as it is threatened by habitat loss, hunting and the pet trade. The bear cuscus is poorly-understood, but it seems to be a diurnal species that feeds on leaves in the forest canopy.

Order Dasyuromorphia (devils, quolls and other cute little vicious things!)

tiger_quoll_pierre_pouliquin
A spotted-tailed quoll – image credit Pierre Pouliquin

The tiger quoll, or spotted-tailed quoll (Dasyurus maculatus) is the largest marsupial carnivore on mainland Australia (and worldwide is beaten only by the Tasmanian devil). You seriously do not want to mess with an angry tiger quoll! Entirely subjectively, quolls are also simply the best looking marsupials out there (see ARKive photos). The spotted-tailed quoll is listed by the IUCN as Near Threatened for a number of reasons, including habitat loss, competition with introduced cats and foxes, and direct persecution by humans.

 

Order Peramelemorphia (bandicoots and bilbies)

Macrotis_lagotis_JudyDunlop
Greater bilby – image credit Judy Dunlop

The greater bilby (Macrotis lagotis) is a wonderful animal, and you MUST watch some of the bilby videos on ARKive. Those ears! That tail! I’ve written a little about this species before, in my post on bandicoots. Sadly I fear it will not do so well in Mammal March Madness, just as it has not done so well in real life. Once the bilby was found across much of mainland Australia, in arid and semi-arid habitats, but now it survives in just a few protected locations. The IUCN lists this species as Vulnerable, predominantly because of the threat posed by introduced predators and changes to fire regimes.

Order Notoryctemorphia (the two species of marsupial moles)

notoryctes_biodiversity_heritage_library
A marsupial mole image from the Biodiversity Heritage Library

The southern (or central) marsupial mole (Notoryctes typhlops), also known as Itjaritjari, swims inside sand dunes in central Australia, where it catches and eat insects. Now that is an impressive lifestyle! Also you MUST check out the photos of an Itjaritjari eating a gecko on their ARKive page. Although we know very little about many aspects of this species biology, the IUCN lists it as Least Concern, because it is still widespread and relatively abundant in suitable habitat.

Order Didelphimorphia (opossums)

robinsonsmouseopossum_emammal
Camera trap photo of a Robinson’s mouse opossum from eMammal

Robinson’s mouse opossum (Marmosa robinsoni) seems to have uncertain taxonomy and may in fact not be a single species, but rather a complex of three or four seperate species. These opossums occur across parts of Panama, Colombia and Venezuela, as well as several Central American islands. They are listed by the IUCN as Least Concern, because of their widespread distribution, large population sizes and tolerance to some level of disturbance.

Advertisements

6 Comments Add yours

  1. artlikker says:

    Thanks for the useful info and links etc Anna, I used some in my artist diary at https://www.facebook.com/artlikker/?ref=aymt_homepage_panel hope you don’t mind. I have only every seen the inside of a marsupial pouch once, a Burrowing Bettongs (working as a volunteer). The outside of pouches look cute but the inside seemed a bit grubby for babies! The paper you linked re antibacterial secretions makes perfect sense. Please keep the detail re marsupials coming. The more we know about them the more we will value and treasure them.

    1. Anna MacDonald says:

      Glad you found it useful – and no problem! I love your numbat 🙂

  2. Simon says:

    Lovely article. It is distressing that so many of these beautiful species are at risk of extinction. What’s the best way the help?

    1. Anna MacDonald says:

      Hi Simon, thanks for the feedback. You are right, it would be a tragedy to see more marsupial extinctions. There are many factors that threaten wildlife and obviously some species will be more susceptible to some threats than others, but there are some common themes. In Australia some of the greatest threats are introduced species (predation by foxes or cats, competition from rabbits…) and habitat loss (caused by land clearing, changing fire patterns, climate change…). It isn’t a fun read, but I wrote about a lot of these threats here: https://wildlifesnpits.wordpress.com/2015/01/26/an-australia-day-post-on-australian-wildlife-conservation/

      At a large scale it can be difficult for a single person to know how to help. We really need a change of attitude, so that society as a whole takes wildlife conservation seriously. There are some interesting articles in the Conversation on this:
      https://theconversation.com/to-save-australias-mammals-we-need-a-change-of-heart-27423
      https://theconversation.com/invasive-predators-are-eating-the-worlds-animals-to-extinction-and-the-worst-is-close-to-home-64741
      https://theconversation.com/mourn-our-lost-mammals-while-helping-the-survivors-battle-back-36126

      At the local scale there are lots of little things we can do. For example, in Australia one really important thing if you have a pet cat is to desex it and don’t let it roam outside. If you have a garden, create habitat for wildlife (tree hollows / nest boxes, native flowering plants for wildlife food, shrubs for animals to shelter from predators…). Think about what chemicals might wash into local waterways (which are habitat and water sources for wildlife) from your property (garden fertilisers, pesticides, car oil from driveways…). Don’t collect dead trees and branches for firewood in the bush – these are important wildlife habitat. Support local conservation initiatives or nature reserves, or some of the organisations that work to conserve wildlife across the country. In many places you can volunteer to help with wildlife surveys or weed eradication blitzes. Then of course there are all of the individual actions we can take in our efforts to prevent climate change. Hope that all makes sense!

      Anna

  3. I have a marsupial species in my comic so you are calling them weird too which offends me. Marsupials and monotremes are not weird. They’re normal. You’re speciesist for calling every last one of them weird.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s