Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Lizards are moms too (Image: Philipp Wagner)

Species: Trachylepis ivensii

Habitat: Angola, Zambia and the Democratic Republic of the Congo, being very hard to find

In evolution, as in life, some things are easier than others. It seems to be pretty straightforward to evolve complex eyes, which have turned up dozens of times.

Similarly, for some groups of animals it’s easy to stop laying eggs and start giving birth to live young. Backboned animals have evolved live birth no fewer than 132 times, and nowadays a fifth of lizards and snakes give birth. Human mothers may disagree, but live birth is clearly not that difficult.


What is difficult, however, is nourishing unborn young the way mammals do. A female mammal allows each embryo to burrow deep into the wall of her womb, where it takes nutrients straight from her blood. This intimate arrangement was long thought to have only evolved once, in mammals.

Not so. It now appears that it evolved at least twice: once in mammals, and once in an obscure African lizard called Trachylepis ivensii.

Mother lizard

These lizards don’t look like rule-breakers. T. ivensii are fairly typical skinks, one of around 1200 species. Adult females grow to 9 to 14 centimetres long, plus tail. They are rarely seen: only a few specimens have been collected. “We don’t know much about them,” says Daniel Blackburn of Trinity College in Hartford, Connecticut.

Blackburn began studying T. ivensii in earnest after his colleague Alexander Flemming of Stellenbosch University, South Africa, found nine females preserved in a museum. Blackburn and Flemming worked together to dissect them and find out how their young developed.

They focused on the oviducts – tubes that run from the ovaries to the outside. Live-bearing reptiles release eggs into the oviducts, where they develop into babies before being born. The question is, how were the developing embryos fed while in the oviduct?

All live-bearing reptiles have a basic placenta, but unlike its mammalian counterpart the embryo doesn’t get much food that way. It can’t: although it nestles up against the oviduct wall, the embryo remains inside a remnant of eggshell that acts as a barrier. Instead, it is nourished by a large yolk.

A very few reptiles, including T. ivensii, break this rule. Their eggs are small, with little yolk, so they must get lots of food from their mothers via the placenta. But only T. ivensii allows the embryo to implant itself in the oviduct wall. “It’s unprecedented,” Blackburn says.

Feed me!

Blackburn and Flemming found 42 embryos preserved in the females. More advanced embryos had shed their shells and attached themselves to the oviduct wall.

It works like this. Cells on the outside of the embryo send out extensions that burrow between the cells of the oviduct wall and then swell into knobs. These knobs then produce more embryonic cells, which spread beneath the outer layer of the wall. Eventually, the original wall cells are sloughed off, and the oviduct is lined with cells from the embryo.

This arrangement means that embryonic cells are pressed right up against the mother’s blood vessels, where they can take in lots of nutrients. That’s why the eggs don’t need much yolk.

But the arrangement has its problems, Blackburn says. An embryo in close contact with its mother’s blood risks being attacked by her immune system. Male embryos could also be “feminised” by her sex hormones. That might explain why full-scale placental feeding has evolved so rarely.

The only reptiles that come close to T. ivensii belong to a South American skink called Mabuya. Their placentas are complex and transfer plenty of nutrients, and there is some evidence of embryonic cells being able to invade the oviduct wall in a limited way. Another African species, Eumecia anchietae, also feeds its young entirely through its placenta.

Neither shows as much intimacy as T. ivensii, however, and Blackburn also says their placentas are “fundamentally different”. That may mean placental feeding has evolved in skinks not once, but three times. Clearly, in evolutionary terms these reptiles have the knack.

Journal reference: Journal of Morphology, DOI: 10.1002/jmor.11011

Read previous Zoologger columns: The fearsome jaws of a mini movie monster, Stealth millipede wears living camouflage, Dozy hamsters reverse the ageing process, World’s nicest bird murders chicks, Architect mouse builds a food mansion, The amphibious fish that mates with itself, The world’s smartest insect, The most athletic ape in the canopy, The monkey that really gets brotherly love, Bullied boobies develop brain of a bully, How deaf-mute frogs talk to each other, No brain, but at least it’s got personality.