The Ticklish Laughter of Rats

Let’s go tickle some rats.” With those epic words, neuroscientist Jaak Panksepp and his undergraduate assistant, Jeff Burgdorf, went into their Bowling Green State University lab to engage in the hard work of science.

Panksepp, who had been studying play behavior in young human children as well as 50-kHz ultrasonic chirping noises made by juvenile rats during rough-and-tumble play, had just put two and two together: “I had the ‘insight’ (perhaps delusion) that our 50 kHz chirping response in playing rats might have some ancestral relationship to human laughter.1

The rest has been history, and today Panksepp is undoubtedly the world’s foremost authority on rodent tickling:

As they progressed with their research, Panksepp and his colleagues found that many of their rats seemed irresistibly drawn to tickling, chasing after the ticklers and making substantially more play chirps while being tickled than during any other behavior. But the researchers weren’t content with anecdotal observations, and over the course of several years and a number of experiments, they systematically documented a dozen separate lines of evidence suggesting that the rats’ tickle chirping corresponded behaviorally to playful laughter in young human children.2

No, I went "chirp, chirp, chirp." If I'd been laughing, it would have been "chirp, chirp, chirp."

They compiled data establishing, among other things, that certain areas of the body are particularly ticklish (the nape of the neck, for you do-it-yourselfers), that the most playful rats tend to be the most ticklish, that rats can become conditioned to chirp simply in anticipation of being tickled, that tickle response rates decline after adolescence, that young rats preferentially spend time with older ones who chirp more frequently, that the tickle response appears to generate social bonding, that chirping decreases in the presence of negative stimuli (such as the scent of a cat), that rats will run mazes and press levers to get tickled, etc. Based on their research and observations, Panksepp and his fellow researchers hypothesized that rats, when being tickled or engaging in other playful activities, experience social joy that they vocalize through 50 kHz chirping, a primordial form of laughter that is evolutionarily related to joyful social laughter in young human children.

Does it look like my name is Elmo?

It’s safe to say that the neuroscientific community did not exactly rush to embrace this hypothesis. Behavioral neuroscience can be a particularly conservative and skeptical field, one that has traditionally been extremely wary of any theorizing about emotions controlling neural processes or behavior in animals. Since subjective experiences cannot, after all, be measured directly, it has been considered far more appropriate to those functional brain activities and processes that can be scanned and measured objectively, and to simply deny or ignore the possibility that animals experience complex emotional states such as joy, at least in the context of scientific research. As Panksepp put it:

Of course, it was hard to publish this kind of work, and it was ironic that the publication of our initial manuscript was impeded by prominent emotion researchers, some of whom take pains to deny that we can ever know whether animals have any emotional feelings.3

Hahaha, we've had our little fun now, haven't we? If you tickle me again, I'll pee in your coffee.

Fortunately, time and scientific progress have been on Panksepp’s side. We have identified an increasing number of common underlying structures and processes (homologies) in the brains of humans and other mammals and, as brain scanning technologies have become more sophisticated, we have gained greater insight into neural activities triggered in connection with particular emotional experiences. While the ability to cognitively “get a joke” may depend on our incredibly advanced human neocortex, we now believe that much of the foundational brain circuitry relating to laughter, mirth, social joy, social play and emotional processing lies deep within subcortical regions, where our brains are much more similar to those of other animals.

At this point, Panksepp and his colleagues recognize that they have not definitively proven their hypothesis, but their view is essentially that they have made a reasonable case that fits their data and that hasn’t been disproved:

Until someone can offer us some data that falsifies our hypothesis, we believe our theoretical approach better reveals the true nature of the underlying processes than any intellectual scheme that simply constrains itself simply to the accurate description of the environmental and neural control of behavioral acts.4

Even acknowledging the understandable caution of neuroscientists and the obvious difficulty in drawing scientific conclusions about the subjective experiences of animals, it does seem entirely plausible (and not overly surprising) that social animals such as rats would enjoy playful romping and tickling, and that they might vocalize their pleasure in a way that was somewhat akin to basic human laughter. In fact, we hope and fully expect that, as our knowledge of comparative brain structure and function grows over time, we will see more and more studies that show clear linkages between the minds and brains of humans and other animals.


ResearchBlogging.org1Panksepp, J. (2007). Neuroevolutionary sources of laughter and social joy: Modeling primal human laughter in laboratory rats Behavioural Brain Research, 182 (2), 231-244 DOI: 10.1016/j.bbr.2007.02.015.

2Panksepp, J., & Burgdorf, J. (2003) “Laughing” rats and the evolutionary antecedents of human joy?. Physiology & Behavior, 79(3), 533-547. DOI: Panksepp, J., & Burgdorf, J. (2003). “Laughing” rats and the evolutionary antecedents of human joy? Physiology & Behavior, 79 (3), 533-547 DOI: 10.1016/S0031-9384(03)00159-8.

3Panksepp & Burgdorf (2003).

4Panksepp (2007).

Cornered Rat Waves Poisoned Tool, Attacker Flees in Terror!

Screams the tabloid headline…

Is this the plotline for a sequel to The Planet of the Apes in which mistreated lab rats rebel against cruel animal experimenters?

No, it’s actually an accurate (ok, a bit sensationalized) description of the way in which a small African rat has opportunistically found a way to deploy a poison tool (yes, tool, see below) to defend itself from predators.

For years, observers had suspected something was up with the African crested rat (Lophiomys imhausi): it moves sluggishly, acts fearlessly – practically inviting predators to attack it – and twists around to display boldly-patterned black and white bands along its flanks when it’s excited or threated. Some have speculated that these displays could be designed to mimic the appearance of the spiny porcupine or skunk-like zorilla, and over the years there have been reports suggesting that the crested rat may be poisonous, based in part on anecdotes about dogs retreating in fear from the small rodents or showing signs of having been poisoned after crested rat run-ins.

The mystery of the crested rat was cleared up last week, when a team of researchers led by Jonathan Kingdon of the University of Oxford’s Department of Zoology, published their findings about the rat’s unique set of defenses online in the Proceedings of the Royal Society B1.

Poisonous Defense

The researchers found that the crested rat gnaws and chews the roots and bark of local Acokanthera schimperi trees, which contain a substance called ouabain that is used in a traditional African arrow poison known to be capable of killing elephants by amplifying heart contractions. In chewing on the bark and roots, the rat creates a thick gel-like mixture of saliva and plant toxins, which it proceeds to slather onto the distinctively colored fur along its flanks. Here’s a video of the crested rat in which it briefly displays some grooming behavior:

As it turns out, the hairs of the fur in crested rat’s flank-area are highly specialized and extremely well-suited to deliver this self-applied poisonous mixture. These hairs are essentially perforated cylinders containing fiber-like strands that act as wicks, rapidly absorbing the slobbery, poisonous gel and drawing it up by capillary action. When the researchers chemically analyzed the hairs by infrared spectroscopy, they found strong evidence that that they were indeed absorbing and wicking up ouabain from the saliva mixture. Here’s another video of the hairs doing showing off their wicking abilities (that’s red dye in the video, not blood!):

Properly armed with this potent poison and benefited by some additional physical adaptations (an armored skull, enlarged vertebrae, and dense and thick skin), the crested rat enjoys a suite of defenses that allow it to stare down many a predator. The research paper describes the crested rat’s behavior when threatened:

Flaring of the fur is triggered by external interference or attack on the animal, whereupon white and black banding of the longer hairs on either side of the lateral line effects outlines of the tract in a bold white and black ‘target’ design. An aggravated rat pulls its head back into its shoulders and turns its flared tract towards its adversary as if actively soliciting an attack. This display may or may not be accompanied by vocalizations.

No, you don’t want to mess with Lophiomys imhausi.

The researchers characterize the crested rat’s poisonous defense as “toxicity by acquisition” never before reported for a placental mammal, noting that the closest mammalian analogy may be European hedgehogs, who are known to slather their spines with a mixture of toad venom and saliva, presumably to increase the pain and discomfort that their spines can inflict. By contrast, they point out that there’s no evidence that the crested rat needs to create any kind of a wound; rather, the would-be predator is poisoned when it bites – or even just mouths – the crested rat.

So, is the crested rat just a fascinatingly well-adapted defender, or is it a full-fledged tool user?

Tool Use

Tool user! (We at AnimalWise are never shy about making pronouncements … or speaking about ourselves in the “royal we.”)

Even poisonous rats like carrots! (photo credit: Susan Rouse)

Although not mentioned in the research report, the crested rat’s deployment of the plant toxins does indeed qualify as “tool use” as defined in Benjamin Beck’s Animal Tool Behavior, the most complete catalog of tool use in animals. The original 1980 version contained what remains one of the most widely-accepted scientific definitions of the term:

[T]he external employment of an unattached environmental object to alter more efficiently the form, position, or condition of another object, another organism, or the user itself, when the user holds or carries the tool during or just prior to use and is responsible for the proper and effective orientation of the tool.2

In 2011, this treatise was substantially revised and updated, and now contains the following definition:

The external employment of an unattached or manipulable attached environmental object to alter more efficiently the form, position, or condition of another object, another organism, or the user itself, when the user holds and directly manipulates the tool during or prior to use and is responsible for the proper and effective orientation of the tool.3

While it’s not all that much fun wading through the definitions (would they read better in verse?), the authors themselves make it clear that they would consider the crested rat’s “self-anointment” behavior to be tool use: the bark/roots are “unattached environmental objects,” the crested rat uses them to provide itself with a more efficient defensive position, it holds (carries) and manipulates the tool, and is responsible for properly and effectively orienting it.

In fact, the authors have come up with what they call modes of tool use, including several – Affix (attaching an object to the body with adhesive), Apply (attaching a fluid or object to the body without adhesive) and Drape (placing an object on the body temporarily) – which are directly applicable here.4

Moreover, considering only rodents (there are other examples elsewhere in the animal kingdom), the authors specifically call out a number of additional examples of “Affix, Apply, Drape” tool use by self-anointers: rice-field rats that apply the anal gland secretions of the weasel, one of their predators, presumably for concealment purposes; and California ground squirrels, rock squirrels, and Siberian chipmunks that anoint themselves with the scent of rattlesnakes by chewing shed snakeskin, applying dirt (substrate) the snake has been contacted with, and/or anointing themselves with snake urine, all most likely for “olfactory camouflage” purposes.5

So, there you have it. The crested rat is bold, it’s brave, it’s poison, and it’s a tool user!


1Kingdon, J., Agwanda, B., Kinnaird, M., O’Brien, T., Holland, C., Gheysens, T., Boulet-Audet, M., & Vollrath, F. (2011). A poisonous surprise under the coat of the African crested rat Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.1169.

2Beck, B.B. 1980. Animal tool behavior. New York: Garland (as quoted in Shumaker, Robert W.; Walkup, Kristina R.; Beck, Benjamin B.; Burghardt, Gordon M. (2011-04-28). Animal Tool Behavior: The Use and Manufacture of Tools by Animals (Kindle Locations 299-301). JHUP. Kindle Edition).

3Shumaker, Walkup; Beck & Burghardt 2011 (Kindle Locations 372-375).

4Id. (Kindle Location 601).

5Id. (Kindle Locations 1934-1943).

Perchance to Dream…

Do you ever wake up and feel like you’ve spent the whole night replaying a tape of the stresses of the day before? Well, at least you didn’t spend you didn’t spend that day running through mazes. Oh, you did? In that case, you might want to grab a chunk of cheese and sit down for some comfort eating with a friendly rat who can commiserate with you.

Matthew Wilson, an MIT professor of neuroscience, has been studying rats as they work and sleep for years, and has found out that they, too, replay their daily activities as they sleep.

Rat dreaming of running in circles... (MIT image)

In groundbreaking research published in 2001 in the journal Neuron1, Wilson and his colleague Kenway Louie were given an unprecedented glimpse into the dreams of rats by studying rats’ brain activity while they ran through mazes and then later on while they slept. (To clarify, the rats – not the researchers – were the ones who ran through the mazes. Sorry to disappoint you.)

To investigate what happens in the brain during rapid eye movement (REM) sleep, the type of sleep associated with dreaming, the researchers recorded the activity of neurons in the hippocampus (the area of the brain known to be critical to the formation and encoding memories) of four rats, both while the rats ran around circular mazes and then afterwards during REM sleep.

What they found out was striking.

As the rats ran through the mazes, the neurons fired in distinctive patterns that were dependent on where the rats were within the mazes. Then, when the researchers took comparable measurements of the rats’ brain activity later on during the rats’ REM sleep after a hard day of maze running, they found that the rats played back exactly the same neuron activity patterns as had occurred when they originally performed their tasks. More specifically, in 20 of the 45 REM sleep sessions that the researchers measured, they could detect prolonged periods (tens of seconds to several minutes in length) during which the same spatially-correlated hippocampal neurons fired in the same order, with the REM patterns essentially repeating the daytime patterns at approximately the same speed.

During REM sleep, we could literally see these rat brains relive minutes of their previous experience. It was like they were watching a movie of what they had just done.

In his terrific blog The Frontal Cortex2, Jonah Lehrer noted that Wilson was astonished by these results, quoting him as saying, “During REM sleep, we could literally see these rat brains relive minutes of their previous experience. It was like they were watching a movie of what they had just done.”


More recently, Wilson and Daoyun Ji, a postdoctoral associate, extended these findings in research published in Nature NeuroScience3. In this newer study, the researchers focused on brain activity during slow-wave sleep (SWS), often referred to as deep sleep, a stage of sleep not characterized by dreaming but thought to be important to long-term memory formation. The researchers wanted to learn more about how the brain consolidates long-term memories during SWS, and whether it replays visual images from daytime experiences as part of the process. To test these matters, the researchers focused on the interaction between two separate areas of the brain: the hippocampus and the visual cortex, which is responsible for processing visual information.

As before, the researchers measured brain activity in four (presumably different!) rats as the rats ran in alternating directions through figure-eight shaped mazes, and then repeated the same measurements while the rats slept both before and after their maze-running sessions. This time, though, the researchers measured activity in both the visual cortex and the hippocampus.

Once again, the findings are notable.

As the rats ran through the mazes, neurons in both brain areas, the visual cortex and the hippocampus, acted similarly, firing in distinctive patterns that were dependent on where the rats were within the mazes. During subsequent SWS periods, the rats replayed these same firing patterns and sequences in both brain areas, much as they had done in the earlier experiment on hippocampus activity during REM sleep. Moreover, at all times, during maze-running activity and later on as the rats replayed their memories during periods of SWS, the brain activity in the visual cortex and the hippocampus were highly correlated.

By linking the visual cortex to this coordinated memory replay process, the researchers were thus able to show that not only were the rats replaying their daytime memories during sleep, but that they were reliving the same sensory experiences, the exact visual images that they had seen during their maze running!

Do these studies provide insight into the neurobiology of sleep, dreams and memory in humans and other animals? All mammals have similar brain structures that seem to operate similarly, and the research was designed to help us gain a better understanding of our own memory formation behavior, but it never hurts to ask these sorts of questions.

Assuming that these findings do have relevance for other species, then it may well be that when your Golden Retriever paddles his feet, rolls his eyes and twitches during sleep, he is indeed reliving that epic battle he recently had with an evil, slobber-covered tennis ball.


1Louie, K., & Wilson, M. (2001). Temporally Structured Replay of Awake Hippocampal Ensemble Activity during Rapid Eye Movement Sleep Neuron, 29 (1), 145-156 DOI: 10.1016/S0896-6273(01)00186-6.

2The Frontal Cortex, “The Neuroscience of Dreaming,” December 19, 2006.

3Ji D, & Wilson MA (2007). Coordinated memory replay in the visual cortex and hippocampus during sleep. Nature neuroscience, 10 (1), 100-7 PMID: 17173043.

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