Australia is a long way from anywhere and has been for a very long time. The landmass definitively separated from the supercontinent of Gondwana around 40 million years ago and, since then, has existed – as a big blob in the middle of an even bigger ocean – in glorious geographical isolation.
This isolation, combined with a diverse range of habitats and shifting global temperatures, tipped evolution on a unique trajectory.
Now Australia is home to a high percentage of species that are found nowhere else. Around 85 per cent of the continent’s mammals, 93 per cent of its reptiles and 94 per cent of its frog species are unique to the continent.
When the separation happened, there were no land-living placental mammals in Australia, but there were ancient lineages of marsupials and monotremes (egg-laying mammals).
These survived and diversified, leading to the evolution of the 300 or so modern marsupial species, including bandicoots and quolls, and the two species of living Australian monotreme, the platypus and the short-beaked echidna.
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Here, ‘weirdness’ abounds. The platypus, for example, seemingly has the beak of a duck, the body of an otter, and the tail of a beaver.
It lays leathery eggs like a reptile, yet feeds milk to its young like a mammal, via specialised pores in its abdomen that leak the lickable liquid on to its fur. Males have venom-laced ankle spurs, while the beaks are sensitive to electricity.
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Echidnas are covered in spikes and can also detect electricity, but it is their reproductive system that grabs the ‘weird’ limelight. Males have a distinctive bendy, four-headed penis, while females have two separate vaginal openings.
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Not to be outdone, some male marsupials, including opossums, sport a bifurcated penis, which delivers sperm to both of the female’s two permanent vaginal canals. Females also develop a third, temporary vagina, which they use for giving birth.
And let’s not forget, marsupials are big on pouches. These provide a safe haven for their tiny babies, which are born under-developed and embryo-like after the briefest of gestation periods.
To the uninitiated, all of these features may seem weird. But view them through an evolutionary lens and they make perfect sense.
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How do these weird features benefit Australia’s weird animals?
Each and every ‘weird’ feature represents a solution to the problem of survival, which in turn is related to the animals’ lifestyle and habitat, which are themselves related to their home continent of Australia.
Swimming with eyes shut and nostrils closed, electrosensitive cells in its bill enable the platypus to detect the electrical signals generated by the muscular movements of its prey.
Glands in the adult male’s upper thigh make venom, which is then released via its ankle spurs, and used as a weapon when fighting other males for territory or mating rights.
Echidna spikes, meanwhile, are defensive, while males use just two of the four heads of their penis at a time, then alternate them with each successive copulation.
It’s thought that this helps to increase the chances of successful fertilisation, especially as females mate with multiple males during the breeding season.
The female echidna’s own idiosyncratic reproductive system has evolved in lockstep with that of her suitors’ and, while a similar set up has evolved in opossums, the addition here of a third, temporary birthing vagina facilitates the passage of the tiny embryonic marsupial from womb to pouch, where it can continue to develop in safety.
Animals in Australia are truly some of the weirdest, yet most wonderful, on the planet – there really is nowhere else quite like it.
Top image: a platypus swimming through a log. Credit: Martin Harvey/Getty Images
