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It’s pretty well known that Australia is home to some strange animals, but echidnas are especially weird evolutionary misfits.

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Sources:
http://www.nationalgeographic.com.au/animals/how-the-platypus-and-echidna-lost-their-stomachs.aspx
https://www.smithsonianmag.com/smart-news/many-animals-including-the-platypus-lost-their-stomachs-180948103/
http://phenomena.nationalgeographic.com/2013/12/03/how-the-platypus-and-a-quarter-of-fishes-lost-their-stomachs/
http://www.nationalgeographic.com.au/animals/echidna.aspx
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0006070#s1
http://www.publish.csiro.au/ZO/ZO09037
https://eprints.utas.edu.au/2320/1/Nicol_et_al._2007_The_life_history_of_an_egg-laying_mammal2C_the_echidna_Ecoscience.pdf
http://www.sciencedirect.com/science/article/pii/S0531556508001629
http://reptilepark.com.au/animals/mammals/echidna/
http://www.abc.net.au/science/articles/2011/01/11/3110568.htm
http://onlinelibrary.wiley.com/doi/10.1002/jcp.1040670108/abstract
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5399094/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2441467/
http://www.ucl.ac.uk/museums-static/obl4he/vertebratediversity/monotremes.html
http://www.jstor.org/stable/10.1086/522847?seq=2#page_scan_tab_contents
http://www.environment.nsw.gov.au/resources/nature/Factsheet3Echidnas.pdf
http://ro.uow.edu.au/cgi/viewcontent.cgi?article=1591&context=hbspapers
https://www.researchgate.net/publication/290413985_Marsupial_and_Monotreme_Evolution_And_Biogeography

Image Sources:
https://commons.wikimedia.org/wiki/File:Heteropoda_venatoria_-_female_at_Mechode_Padur.jpg
https://commons.wikimedia.org/wiki/File:CSIRO_ScienceImage_10356_Superb_Lyrebird.jpg
https://commons.wikimedia.org/wiki/File:Monotreme_reproductive_organs.png
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Whether you need a domain, website, or online store, make it with Squarespace [♩INTRO ]. It’s pretty well known that Australia is home to some strange animals — everything from giant spiders to birds that can mimic chainsaws.

And most of them follow familiar patterns in biology… except a tiny order called Monotremata, with only two subgroups: the echidnas and the platypus. Monotremes are mammals, so they have mammary glands and milk, they’re covered in hair, and they can heat up their bodies. But monotremes also lay eggs.

In fact, they have a bunch of weird adaptations, and many scientists think that they branched off from other mammals, a long long time ago, like 200 million years ago. So by taking a closer look at echidnas, we can see how these unusual traits work and why they may have stuck around. Echidnas, like all monotremes, are missing something most animals rely on to digest food.

They don’t have stomachs. The digestive system is basically a series of flesh bags with different secretions, enzymes, and bacteria. It breaks down food, absorbs the nutrients, and gets rid of the waste.

At least 450 million years ago, scientists think that vertebrates were having trouble getting nutrients out of larger proteins in their diets. So to cope with these big molecules, they evolved a sack of super corrosive acid and other digestive enzymes — a stomach. But, over millions of years, there were some vertebrates that apparently didn’t need the extra help, like monotremes.

Researchers have found that genes that code for stomach acid and enzymes seem to have mutated to the point of being lost from their genome. Echidnas mostly eat ants and termites, using long sticky tongues to catch their prey, and a bony plate in their mouth to grind them up. And even without stomachs, echidnas seem to be surviving just fine.

Maybe even better than fine, because they live a ridiculously long time. There are reports of echidnas living up to 50 years in captivity, and over 40 in the wild. The platypus, for comparison, maxes out at around 17 years in captivity.

Scientists think this lifespan may be related to how their bodies manage energy. Even though they can generate heat, monotremes have some of the lowest body temperatures of any mammal. Humans sit at a comfortable 37 degrees Celsius, but the echidna only gets up to 32 degrees and down to 10 while hibernating.

Now, temperature is one variable that fits into the metabolic theory of ecology, a mathematical model that describes how an organism’s metabolic rate relates to things like body mass and lifespan. Scientists have found that echidnas have a relatively low basal metabolic rate, even compared to platypuses. That means they typically use energy more slowly.

Because echidnas are also relatively small and stay cooler, the metabolic theory of ecology predicts a longer lifespan. The thing is though, by some calculations, echidnas live almost 4 times longer than these predictions say they should. So researchers think there must be something else going on.

And it’s probably related to how their cells defend against damage. In a study from 2010, echidna skeletal muscle and liver cells were shown to be resistant to something called lipid peroxidation. Peroxidation is a chemical process that happens naturally to molecules in cell membranes, specifically fat molecules with double-bonded carbons.

These lipids are attacked by chemically reactive compounds and break down in a sort of chain reaction. And we think this is one of the key parts of aging. Now, echidnas have a lot of monounsaturated fats in their cell membranes.

And these molecules are less likely to be broken down because they only have one double bond. This sort of resistance to aging is unusual but not completely unique. Animals like naked mole rats and humans have similar cell membranes and live longer lives.

So the weirdest thing about echidnas is probably comes down to their reproduction. Males flock to females, often in ‘mating trains’ of 6 or more echidnas hoping to sow their wild oats. They build the females a trench and then fight, trying to push each other out.

And eventually, one pair mates. Every female actually has two reproductive tracts branching off a single opening. And every male has a four-pronged penis.

And, it turns out, only two of the penis heads are active during one mating session. Which I guess that makes sense — one sperm-cannon per reproductive tract. One paper suggested that this is because of competition.

Basically, the first male who mates with a female wants to beat out other sperm that might be deposited later. But we aren’t exactly sure why the four prongs exist, if they only use two at a time. It might also be competition-related.

They could use two different pools of sperm, or cut down on the refractory time between mating sessions, because the other half of their penis is ready to go. Researchers have even compared this kind of half-activation to the the two hemipenes of lizards and snakes, which are used one at a time. After an egg is fertilized, it grows inside the female until it’s about 2 centimeters across.

Then, the soft-shelled egg gets laid into a pouch, which is kind of similar to both modern reptiles and marsupials. After about 10 days the baby, called a puggle, hatches. It latches on to a mammary gland on the echidna’s belly to feed, because echidnas don’t have nipples.

After about three months, it crawls out into the world. So we’re not entirely sure how monotremes fit into the evolutionary transitions between laying eggs and giving birth to live young. And since echidnas also have a long life and weird digestive systems, it seems like they just sort of pulled from a grab-bag of adaptations.

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