Gut microbes in toddlers may shape curiosity and exploration

Young rats given gut microbes from exuberant human toddlers became more exploratory than peers that received microbes from inhibited toddlers or a neutral control.

The experiment suggests that the bacteria we host in early life may help shape curiosity and risk-reward behavior. This could be one route by which childhood microbiomes shape aspects of personality.

Gut microbes and toddler behavior

Mounting research ties the gut microbiome to physical health and mood, with patterns linking certain bacterial communities to higher risks of depression and anxiety.

Cause and effect have remained murky, but interventions offer clues. Transplants from people with depression can induce depression-like behavior in rodents.

Early human trials, meanwhile, suggest that fecal transplants may ease symptoms in some patients.

Testing temperament in toddlers

To probe temperament rather than diagnosed disorders, Anna Aatsinki from the University of Turku in Finland and colleagues worked with 27 toddlers aged about two and a half.

The team used a standard temperament assessment and a playful “bubble gun” task to gauge behavior across a spectrum from inhibited to outgoing.

“We couldn’t really study things like anxiety disorder in 2-year-olds, but we thought there might be behavior differences we could look at; if they are, for instance, behaviorally inhibited versus very outgoing and extroverted,” Aatsinki told New Scientist.

Based on the assessments, 10 children were classified as exuberant and eight as inhibited and introverted.

From these, the researchers selected a balanced subset – four exuberant and four inhibited toddlers, half boys and half girls – and collected stool samples.

Toddler gut microbes affect rats

After adding glycerol as a preservative, the team transplanted these samples into 53 young rats. A separate control group received glycerol alone after their bowels were cleared to make room for the incoming microbiota.

The rats were then put through a series of standard behavioral assays that measure willingness to explore novel spaces and objects.

Animals that received microbiota from exuberant toddlers spent more time investigating their surroundings. They also showed greater exploratory drive than rats given a control transplant or microbiota from inhibited toddlers.

The result suggests that community-level differences in human toddler gut bacteria can transfer to rodents and bias behavior toward or away from exploration.

Examining the gut-brain pathway

To examine possible brain mechanisms, the team analyzed gene activity in neural tissue. One striking pattern emerged: rats colonized with microbiota from inhibited toddlers showed reduced activity in dopamine-producing neurons.

Dopamine is central to reward processing and motivation, and lower dopaminergic tone is often linked to reduced novelty seeking and cautious behavior.

That constellation of findings points to a gut-brain pathway that modulates curiosity, reward, and motivation via the dopamine system.

Microbes guide, not decide

The authors are careful not to oversell the microbes’ reach. Temperament in adulthood is heritable to a substantial degree, with genetics explaining a large share of variance.

But the environment matters too, and the microbiome is one environmental layer among many that could shift tendencies within genetic bounds.

Crucially, the rat results don’t prove that toddler microbes cause exuberance in children.

The arrow could point the other way: outgoing kids might interact differently with people, places, and foods, and those habits could sculpt a distinct microbial community.

The transplant design shows that whatever those microbial differences are, they can influence behavior when placed in a new host.

Early-life bacteria guide behavior

The study strengthens the case that early life microbiomes do more than aid digestion. They may calibrate neural circuits for motivation and emotional reactivity.

The next steps will likely include identifying specific microbial taxa or metabolites that correlate with the exuberant versus inhibited profiles.

Moreover, experts will be testing whether targeted probiotics or diet can replicate the behavioral effects and tracking how stable these microbiome–behavior links are across development.

Because the transplants altered gene activity in dopamine neurons, future work can also hunt for causal molecules. These may include short-chain fatty acids or amino acid derivatives that cross from gut to brain and tweak dopaminergic signaling.

Tracking behavior through microbes

Longitudinal studies in children, paired with careful dietary and environmental data, could help untangle whether microbes precede temperament shifts or simply mirror them.

For now, the message is both modest and intriguing. Childhood gut communities carry behavioral signals robust enough to transfer across species, nudging rats toward more or less exploration.

That doesn’t mean microbes write our personalities. But it does suggest they help edit the draft – especially early in life, when brains and microbiomes are both still finding their form.

A preprint of the study can be found on bioRxiv.

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