Animal Embryos Can Sense Predators and Food While Still Inside the Egg

In 1997, a team of scientists reared several chicken eggs in two different incubators: one was a normal, sterile incubator, while the other one was infused with the sweet, fruity aroma of strawberries. Sometimes, they even rubbed the experimental group with strawberry aromas straight on the shell.

Once the chicks hatched, much to the scientist’s amusement, the few baby chickens that had been exposed to strawberries while still embryos loved the scent and the taste of it: they liked strawberry-flavored water and spent more time in strawberry-scented spaces.

Loving strawberries doesn’t have much of an impact on a chicken’s life, but since then, a growing body of research across the animal kingdom started noticing that many oviparous animals start being aware of their surroundings, predatory risks, and food availability while still an embryo by feeling, smelling, and seeing through their eggs. 

“We have long thought of eggs not as having a large suite of capabilities,” says Karen Warkentin, a biologist at the University of Boston who studies embryonic learning in frogs. And while obviously, in some ways, that’s true — they’re not very mobile and not that developed — eggs are still subject to the laws of evolution.

“Doing the same thing all the time under different circumstances is often a bad idea,” says Warkentin. “Responding appropriately to your environment is an evolutionarily beneficial, fitness-enhancing, success-improving thing to do.”

Since there are way more eggs than there are mature animals, and a lot of them die, natural selection for survival through the egg stage is likely extremely powerful, says Warkentin. And now, researchers wonder whether it could also help animals adapt to bigger, systemic threats like climate change.

“I suspect that there are lots of other embryos that also use multiple kinds of information [to learn about their surroundings],” says Warkentin. “It's just not as well documented.” 

Some embryos learn from their surroundings so they can usefully change their looks and behaviors once they’re out of the egg.

Ringed salamander (Ambystoma annulatum) embryos can sense the chemicals of other predatory salamanders in their waters and, after hatching, are more likely to try to stay still and look for shelter to avoid being prey. The common frog (Rana temporaria) hatches with tweaked features like shorter, stubbier bodies and more flared tail fins to enhance their ability to fend for themselves when they’ve sensed predatory beetle larvae around them as embryos.

Although the tadpoles of the Mexican spadefoot toad (Spea multiplicata) are still wriggling around in their gelatinous eggs, unborn, they already know they’ll need to grow strong jaws once they hatch. The embryos have sensed whether the water they’re in is replete with fairy shrimp, their favorite snack, or not, according to a study from 2023. And they have adjusted the size and strength of their jaws so they’ll be more formidable hunters once they’re out.

Pharaoh cuttlefish (Acanthosepion pharaonis) develop inside transparent egg sacks, which helps them see predators and learn about new ones. When scientists reared pharaoh cuttlefish embryos in the same tank as unassuming clownfish, initially, the embryos stayed put. But when they started squirting ink inside the tank at the same time as the clownfish was slipped in — grown cuttlefish release ink when they need defense — the cuttlefish embryos, albeit still in their eggs, started learning that clownfish could be a predator. They started camouflaging in the background while in their embryos, making themselves hard to spot.

Red-eyed tree frog embryos (Agalychnis callidryas) can hatch from their eggs early if they sense there's a snake around that's about to go in for the kill.  The embryos sense the vibrations of the snake slithering around them and can plop out of their eggs early to escape a possible pounce. This is something Warkentin first discovered in the ‘90s. They can do the same if they sense egg-famished wasps in their vicinity or an egg-killing fungus.

“They use information from a whole range of senses,” says Warkentin. “It's a really kind of rich, multifaceted sensory world.” And it changes really rapidly. Warkentin will be studying embryos at a certain stage of their development, and they can perfectly sense the lack of oxygen, but they have no idea if a snake is slithering nearby. “There is an onset of different information streams as they're developing, and they use more and more information as they develop,” says Warkentin.

Not all tree frogs are masters of this skill, though. Gliding tree frog embryos suck at escaping from egg-eating snakes. They only manage about 9 percent of the time, compared to their red-eyed counterpart's whopping 77 percent success rate. When researchers transplanted gliding tree frog embryos into the eggs of the more successful species, they figured out that eggs that are thick and gelatinous are much better for escaping predation than the thinner and stiffer gliding tree frog ones. After the transplant, gliding tree frog clutches nearly tripled their escape success.

Read More: 6 Unusual Traits of Animal Evolution

But threats don’t always have to be totally imminent; violent ones such as climate change are also something that embryos are learning to adapt to, according to a growing body of research. Frogs that live on land can hatch early to avoid heatwaves, for instance. They can also hatch early to avoid dry spells, and new research published by Warkentin’s team in 2024 suggests they can do that thanks to their ability to sense a build-up in the concentration of ammonia in their surroundings.

While scientists don’t yet have data on whether this is an evolved response to increases in heat and aridity due to recent climate change or just “a lucky combination,” says Warkentin,  it’s something embryos can use to their advantage.

A warming, drying climate “is going to kill a lot of eggs,” says Warkentin. “But if there’s any reduction in mortality for individuals that have better sensitivity, it may offer some, some scope.” 

“It might help,” says Warkentin. “We don't know how much.”

Read More: When Environments Change Faster than Species Do

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• Physiology & behavior. Chemosensory learning in the chicken embryo

• Vocabulary.com. Oviparous

• Biologist at the University of Boston. Karen Warkentin

• Proceedings of the Royal Society B. Learning by embryos and the ghost of predation future

• Canadian Journal of Zoology. Predation-induced effects on hatchling morphology in the common frog (Rana temporaria)

• Biology Letters. Frog hatchlings use early environmental cues to produce an anticipatory resource-use phenotype

• Learning and Behavior. Awareness of danger inside the egg: Evidence of innate and learned predator recognition in cuttlefish embryos

• Animal Behaviour. Wasp predation and wasp-induced hatching of red-eyed treefrog eggs

• Ecology. Egg-Killing Fungus Induces Early Hatching of Red-Eyed Treefrog Eggs

• Integrative Organismal Biology. Egg-Clutch Biomechanics Affect Escape-Hatching Behavior and Performance

• Integrative Organismal Biology. Heat-Induced Hatching of Red-Eyed Treefrog Embryos: Hydration and Clutch Structure Increase Behavioral Thermal Tolerance

• Journal of experimental zoology. Part B, Molecular and developmental evolution. Elevated ammonia cues hatching in red-eyed treefrogs: A mechanism for escape from drying eggs

Sofia Quaglia is a freelance journalist writing about all things science and how we talk about it. Her work has appeared in the New York Times, National Geographic, The Guardian, New Scientist, and more. She’s on a mission to visit the entire planet by spending each month in a different country, so she’s been living on the road since 2021.