Field of Science


Decapitated Worms Regrow Heads with Memories Still Inside

How good are you at remembering something you learned two weeks earlier? What if during the intervening 14 days, your head was removed? One flatworm isn't bothered by this scenario. After growing back its entire head and brain, it picks off pretty much where it left off.

The planarian is a modest little flatworm, the kind of common microscope denizen you might find in a Gary Larson cartoon. What's remarkable about it is its ability to regenerate. The whole body can regrow, head to eyespots to tail, from even a tiny fragment of the original animal.

Tal Shomrat and Michael Levin at Tufts University built a computerized apparatus for training planarians. Back in the 1960s, an intriguing line of research had suggested that the worms might be able to retain memories after decapitation. But researchers had done their training and testing by hand, a cumbersome method that led to inconsistent results. ("The process of training worms by hand is very time-consuming," Levin says, probably understating it.) Ultimately, the topic was abandoned. Now, with a totally automated procedure, Shomrat and Levin hoped to study planarian memory with less error and greater numbers of worms.

First, their worms spent 10 days getting familiar with one kind of environment, either a regular petri dish or one with a rough floor. They were fed abundantly so that they'd learn a positive association with their home environment. Then, for testing, they were put in a rough-bottomed dish with a little spot of food in the center and a light shining on it. Planarians like to stick to the periphery, and they hate light, so they needed to overcome both aversions to get the food. As expected, worms that were more familiar with the rough dishes reached the food sooner, as measured by video tracking.

When the researchers tested the worms again 14 days later, they found that the worms trained on a rough-bottomed dish were still more comfortable with it than the other worms. This memory seemed to last for at least two weeks. Perfect—that's just enough time for a planarian to lose its head and grow it back.

The worms were relieved of their heads. The scientists made certain that no bit of brain survived. Then, after the worm stumps had painstakingly re-headed themselves, the planarians went back into the testing chamber.

The memory wasn't there right away. But Levin and Shomrat found that if they gave all the worms one quick training session before testing, worms who'd previously been familiarized with rough petri dishes reached the food significantly faster than the other worms. The training session "basically allowed the worms to refresh their memory of what they had learned before decapitation," Levin says. In other words, their memories had survived the loss and regrowth of their heads.

Levin doesn't know how to explain this. He says epigenetics may play a role—modifications to an organism's DNA that dial certain genes up or down—"but this alone doesn't begin to explain it."

It's a mystery, Levin says, how a chemical tweak somewhere outside of a worm's brain can later be translated into information, such as the knowledge that a bumpy environment means food is nearby. "We don't have an answer to this," he says. "What we do show evidence of is the remarkable fact that memory seems to be stored outside the brain."

Image: Shomrat and Levin.

Tal Shomrat, & Michael Levin (2013). An automated training paradigm reveals long-term memory in planaria and its persistence through head regeneration The Journal of Experimental Biology : 10.1242/​jeb.087809


  1. I don't know much about flatworms, but could this be some kind of combination of muscle memory + epigenetics?

    Is it possible that this "training memory" is really calculated on-the-spot based on other bodily factors when the brain is determining how to approach an activity?

    Just some thoughts that popped into my head.

  2. One obvious question is whether all the flatworms' neurones are actually in the head. No, really.

    Invertebrates often have nervous systems with considerable nos. of neurones in 'ganglia' within others parts of the body. So removing a flatworm's head may not actually be removing all or even most its 'decision making nervous system', as the 'head end' neurones may only control a certain sub-set of what the animal does.

  3. How about the possibility that the new brains were more receptive to 'training'?
    That, on the face of it, seems a much simpler explanation than memories retained through decapitation but needing to be refreshed?!

    1. The "refresher" session was the same for both groups of worms--everyone got one session in the rough-bottomed dish. So if they were just more receptive to training, you'd expect both groups of worms to respond to the refresher session.

    2. Ah, I misread that - I didn't spot there were untrained beheaded worms in there too :)

  4. My first reaction is that in some way, the sense organs in the skin have been trained to have a pleasant feeling on a rough underground. If this 'pleasant' message is sent to the new brain, it is also more likely to be less fearful. So the memory would be partly in the skin that feels the roughness, and maybe already interprets it?

  5. If I spend my days walking barefoot on gravel, my feet get tough. Thus, I can more easily walk on gravel.

  6. lots of interesting suggestions by others here. - I wonder what the results are for the reverse. Let the heads grow a new body then test their memory, this may sound obvious but it would shed some light on the explanation.

    Prof Ian Bell
    Institute of Backyard Studies‎

  7. This is new? Wasn't this done during the 1950's using the Y maze? and several other variants?

  8. This is amazing, its like the opposite of Hermann Ebbinghaus' forgetting curve. In his experiment he tested how long it took himself to forget information after different lengths of time. this experiment is similar because the worms were trained to learn a different atmosphere in 14 days then their heads were removed (which took the same amount of time to grow back as the training session)and they had to see if they would remember or forget the atmosphere.

  9. "If I spend my days walking barefoot on gravel, my feet get tough. Thus, I can more easily walk on gravel."

    Very true. This apparent retention of ability could simply be through gaining of some mechanical/structural capacity during training sessions that does not have anything to do with the worm's nervous system. With their experimental design, relating this to "memory" in its behavioral sense is far fetched.

  10. Could this finally explain why some people say my brain is in my trousers?
    I thank you!
    I like the idea of the new brain receiving stronger pleasure signals from the acostomed sense organs than the unaccustomed, but what do I know, I'm just here to make wise cracks.

  11. sounds like muscle memory for me..

    1. and by the way the link to the paper doesn't work... I tried searching for the paper on webofknowledge but couldn't find it too.

    2. Malcolm, the link in the text (3rd paragraph from the bottom) should take you to the paper at JEB. (It's not open access, though.)


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