Kinda funny to think about because leaf-eaters tend to have larger more complex gut microbiomes. Your gut microbiome has been called "the second brain" because the majority of neurotransmitters (including over 90% of all seratonin) originates there.

It almost seems like there's a tradeoff between the two "brains"

This would explain why pandas, koalas, and sloths (all leaf-eaters) are often thought of as "dumb" lol

The neurotransmitters in the gut do _not_ make it across the blood-brain barrier, and there certainly isn’t a direct tradeoff between behavioural complexity and the stew of monoamines in your gut.

Prucalopride and other gut motility meds that contain neurotransmitters often have depression/suicide warnings, so some sort of information transmission is happening, even if it's not that the transmitters directly cross the barrier.

>Kinda funny to think about because leaf-eaters tend to have larger more complex gut microbiomes. Your gut microbiome has been called "the second brain" because the majority of neurotransmitters (including over 90% of all seratonin) originates there.

Leaf eaters also tend to have larger jaw muscles and attachments for same.

That tends to limit the size of the brain case, as that space is needed for the chewing apparatus and supporting skeletal features[0].

[0] https://www.nature.com/articles/news040322-9

Fascinating, thanks!

I remember seeing a lecture by an anthropologist who showed a graph that mapped the size of different primates vs what percent of their day they spend just chewing food. Chimps spend about 60% of their days just chewing for example. The trendline was a surprisingly effective predictor for other primates. Except for humans. Based on our mass we would expect humans to spend around 70% of our day just chewing. Instead it's less than 10%

Another point of evidence that supports the hypothesis put forth in the OP

Can't really explain elephants in that case.

Elephants actually do eat a lot of fruit as well. But yeah it was definitely a playful hypothesis and not a serious proposal. I'm sure if I researched it for even 10 minutes I could find a lot of examples that disprove it

That's funny. If I had a bigger brain I too would munch on fruit instead of leafs. I think they have it backward causation wise.

If you had a bigger brain, you would automatically have access to fruit? That seems backward causation wise