Behold the Three-Toed Skink, who can pick live birth or eggs

June 2, 2020 8:45 PM

In the broadest of terms, animals are generally born one of two ways*: egg laying (oviparity) and live birth (viviparity), which raises the question: When and why did live birth evolve? Enter the Australian three-toed skink (Saiphos equalis), which can it both lay eggs and bear live young, and can do both within a single litter of offspring. Egg Laying or Live Birth: How Evolution Chooses (Quanta Magazine).

Cited research articles:

• Emergence of an evolutionary innovation: Gene expression differences associated with the transition between oviparity and viviparity (Molecular Ecology, abstract only)
• Evolution of vertebrate viviparity and specializations for fetal nutrition: A quantitative and qualitative analysis (Journal of Morphology, abstract only / Research gate, full article), which states that live birth evolved at least 121 separate times in reptiles alone
• Terrestrial Origin of Viviparity in Mesozoic Marine Reptiles Indicated by Early Triassic Embryonic Fossils (PLOS ONE, open access), in which researchers describe the fossil of an ichthyosaur that died while giving birth, indicating that indicates the ichthyosaur inherited live birth from an even more ancient land ancestor
• Early origin of viviparity and multiple reversions to oviparity in squamate reptiles (Ecology Letters, abstract only / Researchgate, full letter)

* As noted in the Wikipedia article Modes of reproduction, there are three traditional modes, with oviparity and viviparity joined by ovoviviparity, traditionally covering mechanisms which span the modes of oviparity and viviparity.

In 2001 and 2012, Thierry Lodé suggested that "the traditional typology spanning three classes does not seem to be adequate to clarify the level of parental investment," and proposed five classes instead. See: Oviparity or Viviparity? That Is the Question… (Pub Med, abstract only / Researchgate with full article).

posted by filthy light thief (10 comments total)

I love how complex Nature is, and how it just loves to set up rules and then do that thing where it's all "yeah, but..."
posted by hippybear at 9:06 PM on June 2, 2020 [1 favorite]

I remember our first vacation together. HE wanted to go to Spain and well, I wanted live birth or eggs.
posted by Lipstick Thespian at 9:49 PM on June 2, 2020 [1 favorite]

Live birth or eggs?

[Monotremes] hold my beer
posted by Fiasco da Gama at 10:29 PM on June 2, 2020 [5 favorites]

Strange, they don't look like the Easter Bunny
posted by kleinsteradikaleminderheit at 2:35 AM on June 3, 2020 [3 favorites]

Enter the Australian three-toed skink (Saiphos equalis), which can both lay eggs and bear live young, and can do both within a single litter of offspring.

Of course it can. Because Australia.
posted by The Bellman at 7:15 AM on June 3, 2020 [4 favorites]

I love how complex Nature is, and how it just loves to set up rules and then do that thing where it's all "yeah, but..."

No. Nature does not "set up rules." Nature does what it does, and the "rules" are human attempts to describe that, in terms of regularities or symmetries.

I think there is an argument to be made (if hand-waving in the direction of Gödel's Theorem were allowed) that descriptions are inherently incomplete and so it's impossible to construct one (if "descriptions" are finite anyway) that doesn't break down in to "yeah, but..." at some point. At least in the general case, where what's being described is the physical universe and the description is in a natural human language.
posted by Aardvark Cheeselog at 10:54 AM on June 3, 2020 [2 favorites]

This is an awesome post. My undergraduate research advisor did a lot of work on this topic. She had a Ph.D. student at the time who had rigged up some complex incubation boxes that would allow him to incubate lizard eggs under varying conditions of hypoxia, to better understand how constraints of oxygen supply play a role in the evolution of viviparity in lizards. I remember that work fondly.
posted by biogeo at 11:44 AM on June 3, 2020 [1 favorite]

This is a great post for the science, and also I hope one day someone feels the wonder and love about me that this guy feels about skinks.
posted by centrifugal at 12:17 PM on June 3, 2020

skinks: snakelings with legs? So elegant shiny carbon color. Excellent model for brooch design. Also my ultra villainess hair clip. Or anything, really.
Great post, thank you.
posted by winesong at 1:28 PM on June 3, 2020

Aardvark Cheeselog: Yes, of course you are correct.

But for most of us, with our education, we're taught there are Ways Nature Does Things, and there are rather sharp divides, of which there examples of being crossed, and we are taught those also. For the most parts, mammals don't lay eggs. Some do, that's amazing, that's taught. It's the "the exception makes the rule" rule of life. The "Yeah, but..." of my comment was only that, nothing more. I didn't go for "hold my beer" because it's not like we've discovered nature having honeybees birth meadows of clover.

But for most of the broad categories we're taught in common biology education across much of the US educational system (I cannot speak to other countries much), it's broad strokes and you get a lot of "all these things are like this" with a few "except for these which do this", and as an education it's interesting enough to keep up engagement, I think.

I'm sorry if you felt I was somehow trying to set up some kind of strict methodology. I was more speaking to how large families of animals are taught as having basically similar characteristics, but there are enough examples of how that doesn't hold for ALL examples of those families, and that those exceptions are common enough that they are worthy of note and not shoving over into a dark corner of "here there be the other".
posted by hippybear at 8:55 PM on June 3, 2020