If you’re an embryo inside of an egg, hatching is a major milestone in your life It’s bigger than like your first  job, bigger than your first car, bigger than not getting a blue  ribbon at the science fair.

I am not over it. One moment, you’re enjoying the cozy  comforts of a house full of food.

And the next, it’s time to leave and learn just how hard staying alive is. But I am curious what is it that  determines when that happens? Are you really just chilling out the whole time, counting down the days while completely oblivious to the outside world?

Well, it turns out hatching is super complex. And over the past few decades, scientists have learned how many animals can not only sense what’s happening  in the world outside their eggs, but also use that information to  hatch at the exact right moment. [♪ INTRO] Embryos aren’t necessarily  on their own when it comes to finding the right time to hatch. In some species, parents can actually give their offspring a little nudge in the right direction.

For example, dancing fiddler  crabs time their mating so their larvae can hatch at higher, nighttime tides. That might give them an advantage against any predators lurking in the area. But that might suggest that the embryos themselves don’t have a say in when they hatch.

The mother crab lays them, turns on a proverbial egg timer, and  goes about her crab life. And it is true that eggs usually take  a certain amount of time to hatch. But around the 1990s, scientists  began to find more and more evidence that embryos can  actually sense the world around them to figure out whether or  not it’s a good time to hatch.

That “good time” will be different depending on what species you’re talking about. But one is definitely “Oh no  a predator is about to eat me and I am much too small to  deal with that right now”. For example, the flatworm Phagocotus gracilis loves to eat the larvae of streamside salamanders.

These salamander eggs are laid by the hundreds, and when they all hatch at roughly the same time, gangs of flatworms will swim  around, targeting single larva in a coordinated attack  and devour it within minutes. But if a larva is larger and more developed, it can do a better job fending off that attack. In other words, it’s worth  hatching as late as possible.

And back in 1993, scientists  reported that salamanders could delay their own hatching if they  sensed flatworms were in the area. It was one of the first demonstrations that eggs could mount their own kind  of anti-predator response. But while streamside salamanders  might delay their hatching to avoid getting nommed on, other  animals go in the opposite direction.

The red-eyed tree frog lays its eggs  on leaves that hang above ponds. And a paper published in 1995 found that when those eggs are attacked by snakes, they hatch early to give the  tadpoles a chance to escape. Good for them.

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Going back to the red-eyed  tree frog, their eggs can also hatch early when their surroundings are flooded. They’re basically at risk of  dying due to lack of oxygen, like you or I would be when drowning. And other animals, like sand  dollars, can hit “pause” on the whole thing until the waters  around them are the right salinity.

But whatever the reason, how  exactly do these different species and their eggs figure  out the right time to hatch? Well, it depends a lot on the species and what exactly they’re trying to adapt to. For the streamside salamander eggs who wanted to “cook” a little longer,  they were likely sensing and responding to chemicals produced  by their flatworm predators.

When researchers set up a  screen to keep the eggs separate from the flatworms, they found  that the eggs could still delay their hatching. And that suggests  that it’s the flatworm chemicals that floated through the  screen and alerted the eggs. Since red-eyed tree frogs can  hatch early for multiple reasons, they’ve got multiple kinds of sensors.

Some can raise the proverbial alarm if an egg stays underwater too long. And to escape a hungry snake,  they can sense the predator’s vibrations using special sensory  organs called neuromasts. These organs are made of hair-like cells, similar to the ones that help us hear.

When they’re tadpoles, the frogs  will rely on those hair cells to detect movement in water.  Including movements caused by things that want to eat  them at that stage of life. These neuromasts are so good at doing their job, they can distinguish between vibrations that come from snakes and those that are due to rain! Meanwhile, vibrations also play an important role in coordinating hatching.

Take the brown marmorated stinkbug, for example. When one egg hatches, the vibrations from the hatching sets off nearby eggs to hatch too. And this is important because one of the stinkbug’s predators is… the stinkbug.

Yes, the individual stinkbugs that hatch earlier will cannibalize unhatched eggs. So for stinkbug embryos, there’s  a strong incentive to get cracking when you “hear” the other eggs hatching. Now, it’s not clear how the stinkbug embryos are able to sense and respond to vibrations.

But the researchers behind this  work have speculated that they’re using similar vibration-detection  pathways that exist in adult stinkbugs. And in general, exactly how  embryos translate these signals into the decision to hatch at  a particular time is unclear But in 2024, scientists turned to a research workhorse to get an answer. Not an actual workhorse, though.

Horses don’t lay eggs. This is a work fish. Specifically, they studied zebrafish embryos.

And they used our old friend  CRISPR to pinpoint a molecule called Thyrotropin-releasing  hormone or TRH to its buddies. They found that when a baby  zebrafish is ready to emerge, its brain releases a bunch of TRH. The hormone then travels through the blood to a group of large cells  called the hatching gland.

Which, as the name suggests, basically exists just to get the fish out of its egg. Once the TRH arrives, the  gland releases a cocktail of enzymes that then dissolve the egg envelope. But if the fish can’t produce TRH, say, because they’ve been CRISPR’d  not to, they never hatch.

Plus, the scientists were able  to watch TRH in action in a distantly related fish called  the medaka, showing that this process might be common to almost  all the fish species alive today. Now, we still don’t know what  gets the egg to form the neural connections that drive the  production of TRH in the first place. And we also don’t know how universal  this is among other fishes, let alone in the wider animal kingdom.

But this work gives us a sense of some of the molecular players involved  in the hatching process. Life inside an egg certainly  can’t prepare embryos for all of the challenges they  face in the outside world. But science has uncovered several ways they can figure out what’s going on  out there before they hatch.

Even if what’s going on out there  is a sibling that wants to eat them. [♪OUTRO]