NEWLY hatched turtles head straight for the sea and tadpoles recognise a predatory salamander the very first time they see one. But what may, on the surface, appear to be a primordial instinct can sometimes hide a deeper, stranger truth: that our first lessons in life come before birth itself.
Over the years, studies of young animals belonging to a range of species have pushed back the known onset of learning. “We tried earlier and earlier,” says Ludovic Dickel, who studies cuttlefish at the University of Caen Normandy in France. “In the end, we questioned embryos.”
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From this research, a trend is emerging. Birds in the egg are listening to their mothers; lambs, like human babies, can be taught about food before birth; and some embryos watch the world through their still-developing eyes. And because these are all examples of acquired knowledge, not instinct, they can also be manipulated.
Take taste. Stories of human babies developing a preference for certain foods while still in their mother’s womb aren’t uncommon (see “Is my bump wise to the world?“). Perhaps the best illustration of this is that we tend to be more tolerant of spicy food if our pregnant mothers ate a diet full of such cuisine.
Likewise, Konstantinos Fegeros’s team at the Agricultural University of Athens in Greece has shown that if a pregnant sheep is fed oregano, after birth its lamb is than if its mother is fed a regular diet.
Even chicken fetuses locked away inside eggshells get a dose of their mothers’ diet. Aline Bertin of the French National Institute of Agricultural Research in Nouzilly fed chickens a diet that was enriched with either a fish oil or soybean oil. She found that chicks born from the first group were more likely to eat unfamiliar foods after hatching .
This kind of habituation is a very basic form of learning – an example of how animals change their behaviour based on information they gather from their environment. Studies in rats show a more complex form of learning happening in the uterus.
Unlike humans, many other animals can walk, run or swim almost from the moment they are born. That makes sense. Being able to flee a predator right from the get-go is an obvious advantage in the wild. But it looks as if we can no longer put this down to instinct. Ultrasound images show that rat fetuses move their limbs in a coordinated manner, which suggests that they may be practising for the day they enter the world.
Scott Robinson at the University of Iowa in Iowa City . He performed delicate surgeries on pregnant rats to tie two legs together on each fetus, and then monitored them using ultrasound. Regardless of which limbs Robinson paired, the fetuses learned new, coordinated movements within 30 minutes.
Other experiments have accomplished a more fundamental kind of reprogramming in birds. Christopher Harshaw at Indiana University in Bloomington and Robert Lickliter at Florida International University in Miami played recordings of Japanese quail song to the eggs of American quail (pictured right). When the chicks hatched, they had a choice. From one direction, a loudspeaker played the song of American quail – the calls their mothers would have made had she been present; from the other came the warblings of a Japanese quail. The chicks’ prenatal experience trumped whatever instincts they might have had: they preferred to spend time near the calls of the wrong species.
Life or death
Experiments such as these underline how critical prenatal information-gathering is. In Australia, superb fairy wrens must either learn something while stashed away inside the egg or face certain death. Their nests are plagued by parasitic cuckoos that lay their own eggs inside a wren clutch a few days before it hatches. Diane Colombelli-Negrel at Flinders University in Adelaide showed that to thwart the invaders, mother wrens sing a unique password song to their eggs. Her own eggs have weeks to learn the song, but the cuckoo chicks don’t, and that’s their downfall: the mother wren will leave to die most chicks that don’t integrate the password into their own song once they hatch.
You might think that none of this is as impressive as the learning demonstrated by Ivan Pavlov’s dogs, who famously learned to associate the sound of a bell with food. “After a few sessions, the dogs started salivating as soon as they heard it, even though no food was presented,” says Maud Ferrari at the University of Saskatchewan in Canada. Ferrari and her colleagues recently demonstrated this classic form of associative learning in unhatched frogspawn.
“Many species get scared when they smell that one of their own was attacked,” says Ferrari. “They can smell chemicals leaching out of the injured body. So to teach a prey about a predator, we can pair the alarm cues with the cue of a specific predator. The next time it detects the predator cues, it knows to be scared of it.”
Ferrari’s team wanted to see if wood frog embryos, safe inside their gelatinous eggs, could do the same as adults and associate a smell with danger. Every afternoon for five days, they exposed frogspawn simultaneously to the smell of crushed tadpoles and the odour of a tiger salamander. Later, when the tadpoles were a couple of weeks old, they exposed them again to the salamander smell. Despite never having met a salamander, the tadpoles that had been exposed to the cues while in the egg froze – a typical response seen in frogs to avoid being noticed by predators. .
But this wasn’t the limit of their abilities. “The fact that they learned per se wasn’t that surprising,” says Ferrari. “Cool, but not really unexpected. Lots of embryos learn things. What was more surprising was how sophisticated their learning can get.”
Salamanders usually hunt during the afternoon, when air temperatures and their bodies are warmer, so to make the experiment realistic, the researchers exposed the embryos to the predator cues at that time of day. Later, these tadpoles froze only if they were faced with salamander cues in the afternoon. But if eggs were “trained” in the morning instead, the tadpoles would only respond to the cues in the morning. “To learn that something only shows up at a certain time of day, you need repetition, time to find a pattern. So to learn after just two or three exposures, that’s pretty amazing,” says Ferrari.
“Rat fetuses learned new movements within 30 minutes”
Some embryos even watch the world go by from the safety of their eggs. That’s the case with late-stage cuttlefish embryos, whose eggs are transparent. “The embryo is very reactive,” says Dickel. “When you move your hand in front of it, it changes its colour.”
Dickel and his team left eggs to develop alongside live crabs, with the two species separated only by a transparent pane of glass. The team had previously shown that newly hatched cuttlefish prefer shrimp to all other foods, but when Dickel’s hatched subjects were given a choice of prey, they lunged at the crabs instead. The team believes that the cuttlefish spy on the crabs through their embryonic eyes. To confirm this, the group plans to repeat the experiment with videos of crabs, instead of live ones.
“In the last week of development, they’re changing the colour of their skin into different camouflage patterns,” says Dickel. “It’s probably a kind of practice: in the egg they’re protected from predators, so they can make mistakes. They can learn to detect, to feel, to smell.”
Experiments like these show that unborn creatures are taking stock of the environment they will soon inhabit. As their senses come online, they process and remember the cues that filter through the walls of their embryonic nursery to give them vital preparation. So be careful what you do around frogspawn – it’s taking note.
Is my bump wise to the world?
Human fetuses glean information from the world on the other side of their mum’s tum. By looking at how newborns move their heads and speed up or slow down their sucking on a rubber sensor when presented with stimuli, you can tell what grabs their attention and deduce from this what they’ve already become familiar with before birth.
Language
Babies start responding to different sounds from 20 weeks. Our mother’s voice is the one that’s loudest inside the womb, and we are born preferring it to others, and our mother’s tongue to other languages. The melody of our first cries are even . And studies reveal that children born to bilingual mothers .
Music
Your musical tastes may have begun to develop in the womb. In one study, third trimester babies were repeatedly played Twinkle Twinkle Little Star. They were played the tune again at birth and four months later, but this time with some wrong notes introduced. Babies who had been trained in the womb responded much more than others who hadn’t.
Taste
Strong flavours from spicy or garlicky food diffuse into the amniotic fluid, which fetuses start swallowing at nine weeks, sampling it with their developing taste buds. And they remember these tastes: newborns offered garlicky food for the first time will accept it with gusto .
Seeds of cleverness
Learning has been spotted throughout the tree of life – and not just in animals. seems to show that pea seedlings can match Pavlov’s dogs for learning to associate two things with each other.
Seedlings were planted in Y-shaped pots with two possible openings to grow out of. For a few hours on three consecutive days, the seedlings were trained with a light source and a fan. In some pots, the fan and light were aimed at the same opening, in others, at different openings. Control plants were grown with just a light source.Then the lights were switched off and a fan was aimed at one opening. Control plants ignored it and grew towards the light’s last position. But most trained plants grew according to their previous experience of the fan: plants that were used to wind coming from the same direction as the light grew towards the fan; those used to opposite cues grew away from it.
The authors say this shows that plants are capable of associative learning. “We have a lot to learn about learning,” says Monica Gagliano at the University of Western Australia.
This article appeared in print under the headline “Early learners”
