You would think an embryo in an egg could relax. They can’t eat, or go anywhere; what can the world ask of them, besides that they grow? A decade ago it would have seemed ridiculous to talk about “embryo behavior.” Now, though, we know that even embryos have things to do.
Turtle embryos bask
Turtle eggs aren’t incubated like bird eggs to keep them warm. The mother turtle lays her eggs in a suitable place, buries them, and leaves them to their own devices. The embryos then simply have to make due with whatever temperatures happen to occur where they are.
Despite being trapped inside their eggs, the turtles embryos aren’t passive about this: they actually move around inside the eggs, pressing themselves against the warmest side of the egg. Du et al. (2011) tested this by heating turtle eggs from one direction for a while, then moving the heating source. The turtle embryos moved inside the eggs to follow the heat.
Look at that photo: these are not fuly-formed turtles about to hatch. These are truly embryos—shell transparent, legs just little nubs—and yet they are already moving around in response to the world.
They have good reason to want to bask. The length of time that a turtle needs to develop before it is ready to hatch depends upon its temperature. Even in eggs less than 5g (very small), one side of the egg may be almost a full degree Celsius warmer than the other. This temperature difference adds up to days of incubation for the turtle: stay on the warm side, and you’ll hatch days earlier than you otherwise would. That’s a few fewer days to be vulnerable to nest predators—which could be the difference between life and death.
Du et al. also suggest that the turtle embryos might be able to better survive dangerous heat waves (even a turtle can get too hot) by moving to the cooler side of the egg. Not all that impressive, maybe—until you consider that these little guys have are still working on growing their own brains! And yet those half-formed brains are already making judgements about heat and movement.
Superb Fairy-wren embryos learn
Pshaw, say the Superb Fairy-wrens. Turtles just move? We learn calls in the egg.
A female Superb Fairy-wren begins giving an “incubation call” about ten days after laying her eggs. This isn’t a call to a mate or any other adults: it’s a call to her embryos inside their eggs. By the time the chicks hatch, about five days later, they have learned her call and mimic it in their own begging calls. Each female has a unique incubation call, and so her chicks have begging calls that identify them as her own.
But…why? She knows they’re her chicks already—they’re in her nest!
This bird is why:
Yes, it’s an exceedingly handsome bird; but that doesn’t mean the wren has to like it. That shiny, stripey cuckoo lays an egg in the wren’s nest, and tosses out one of the wren’s eggs so that the total number of eggs stays the same. Then, when the cuckoo chick hatches, it pushes all the wren chicks out of the nest so that it can get all of the food the wren parents bring for itself.
The wrens, as you can imagine, are not in favor of this. But it’s hard to prevent it: the cuckoo chick starts out looking like a wren chick. The wrens can’t tell the difference.
So they came up with a password—a wren-only password: that incubation call. The wren chicks hear that call for five days while they’re in the egg, and hatch out knowing it. The cuckoo chick takes less time to incubate before hatching: he hatches after 11-12 days, not 15 like the wrens. So since Momma Wren only starts calling on day 10, and stops calling as soon as the chicks hatch, the cuckoo only hears the call for one or two days before he hatches. He can try to mimic it—and the cuckoo chicks do try—but they don’t do as good a job as the wrens, because they haven’t had as long to learn it.
If the female wren comes to the nest to discover that she only has one chick, and that chick doesn’t do a good job of saying the password, she leaves. She starts a new nest, lays new eggs, and doesn’t spend any effort feeding that duplicitous cuckoo chick.
The wren embryos not only learn their mother’s call; they can distinguish between Superb Fairy-wren calls and the calls of other species. (Good thing—you wouldn’t want to accidentally learn a Kookaburra laugh instead of your mother’s password.) Too, once they have heard one Superb Fairy-wren’s calls several times, they can tell the difference between that call and the call of a different Superb Fairy-wren. (Colombelli-Négrel et al. 2014 determined this by monitoring the heart rates of the embryos. Embryos’ hearts beat slower for the calls of their own species compared to other species, and for the calls of familiar individuals compared to novel individuals.)
There are still people who don’t know that birds have individual recognition at all. If you meet one of them, be sure to tell them about the Superb Fairy-wren: the bird that has individual recognition before it even hatches out of its egg.
Colombelli-Négrel D, Hauber ME, Robertson J, Sulloway FJ, Hoi H, Griggio M, Kleindorfer S. 2012. Embryonic learning of vocal passwords in Superb Fairy-wrens reveals intruder cuckoo nestlings. Current Biology 22:2155-2160.
Colombelli-Négrel D, Hauber ME, Kleindorfer S. 2014. Prenatal learning in an Australian songbird: habituation and individual discrimination in superb fairy-wren embryos. Proceedings of the Royal Society B 281: 20141154. http://dx.doi.org/10.1098/rspb.2014.1154
Du W, Zhao B, Chen Y, Shine R. 2011. Behavioral thermoregulation by turtle embryos. Proceedings of National Academy of Sciences 108(3):9513-9515.