by Greg Mayer
One of the standard things we learn about animals are their modes of reproduction: budding, egg-laying, live-bearing, etc. And one of the standard things we “know” about modes of reproduction is that mammals are live-bearing, and reptiles lay eggs. Neither of these things we “know” is true, though– they are generalities, with exceptions. The platypus and its cousins the echidnas are fairly well known as egg-laying mammals, but that many lizards and snakes are live-bearers is not well known. Lizards and snakes are actually quite adept at evolving viviparity: over 100 instances of independent (i.e. convergent) evolution of live-bearing are known among lizards and snakes, versus only a single (or perhaps two) instances in mammals.
For many years, our foremost student of reptilian live-bearing has been Daniel Blackburn of Trinity College in Connecticut. In a paper in press in the Journal of Morphology, he and Alexander Flemming of Stellenbosch University report the most mammal-like placenta yet found in a reptile.
In most placental reptiles, exchange of nutrients, gases, and wastes occur through juxtaposition of fetal and maternal tissues, but not by direct contact with maternal capillaries. In the African skink Trachylepis ivensi, they have now found that this does occur, a condition previously thought to occur only in mammals. Money quote:
Histological study shows that this species has evolved an extraordinary placental pattern long thought to be confined to mammals, in which fetal tissues invade the uterine lining to contact maternal blood vessels.
This species of skink is not very well known. Blackburn and Flemming did their histological studies on a small series of preserved specimens housed in the scientific collections of the Port Elizabeth Museum in South Africa.
h/t Dominic, Matthew Cobb
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Blackburn, D.G. 2006. Squamate reptiles as model organisms for the evolution of viviparity. Herpetological Monographs 20: 131-146. (abstract)
Blackburn, D.G. and A.F. Flemming. 2011. Invasive implantation and intimate placental associations in a placentotrophic african lizard, Trachylepis ivensi (Scincidae). Journal of Morphology in press. (abstract)
Blackburn, D.G., L.J. Vitt and C.A. Beuchat. 1984. Eutherian-like reproductive specializations in a viviparous reptile. Proceedings of the National Academy of Science (USA) 81:4860-4863. (pdf)
So is there an endogenous retrovirus involved?
Nature abhors a pigeon-hole.
MC: Lizards and snakes are actually quite adept at evolving viviparity: over 100 instances of independent (i.e. convergent) evolution of live-bearing are known among lizards and snakes, versus only a single (or perhaps two) instances in mammals.
Since vivipary was present pretty close to the base of the mammal clade, there obviously were not many opportunities for mammals to evolve that. If they’d evolved egg-laying, or something really weird (spores?), that’d have been interesting.
Now, birds — there’s an oviparous group that has conspicuously not evolved the live-bearing habit. For reasons that seem obvious, I guess. But, bats fly and give birth to live young, so maybe it’s not so obvious.
Lizards and snakes are actually quite adept at evolving viviparity
fish, too.
example:
http://www.sciencemag.org/content/298/5595/1018.abstract
Evolution of viviparity involves loss of eggshell. Oviparous squamate embryos get most of their calcium from egg-yolk, while archosaurs and turtle embryos obtain much of their calcium from the egg-shell.
It’s a pity, a penguin the size of a blue whale would be cool.
Evolution of viviparity involves loss of eggshell.
not in fish it don’t.
It’s a pity, a penguin the size of a blue whale would be cool.
that’s not actually the primary metabolic reason why we don’t find giant sea birds.
it’s only a reason we don’t find giant eggs.
I think this is fascinating, & thanks for writing about it Greg. But I have always wondered why or if any dinosaurs were viviparous, though from watching the BBC’s Planet Dinosaur (very good if biased to the giants!) it seems that even the largest laid eggs which is amazing to me – how many would have been crushed by the adults in the process?
But not all birds flies, so that can’t be the hinder (if there is one).
TL: But not all birds flies, so that can’t be the hinder (if there is one)
But the ones that don’t fly are relatively few (c. 50-100?, out of 7000 modern sp.), so the chances are reduced. It’s not an absolute hinderance, since bats are live-bearing yet fly. But, surely it a barrier of some magnitude. Carrying around extra weight seems to be something active flyers gen. don’t do.
Still, bats even have to carry their young for a while after they’re born, I believe.
Aah, the skinks: one of my favorite rock bands of the sixties….
Scorpions bear live young.
(subscribing)
Things to consider
– Bats possess different phylogenetic characteristics than birds, assuming birds are convergent of reptiles while bats are of mammals.
– “Embryonic diapause” as reasoning for an absence of viviparity in turtles and birds. Eggs retained in the uterus do not continue developing but develop only after they are deposited. Michael S. Y. Lee and Richard Shine.
– Marine reptiles, such as the Plesiosaurs, have been found in the fossil record giving live birth around the Mesozoic. (depending on your definition of DINO this guy could apply)
– The “loss” of egg-shell in the transition between oviparity and viviparity may not be a necessary step. Considering, it could simply be that shell deposition was delayed and subsequently suppressed all together. Donald w. Tinkle and J. Whitefield gibbons.
I am writing a paper with emphasis on viviparity in squamate reptiles. I happen to stumble on this site and couldn’t help but post my findings.