In a study hot off the press, Gilles de Meester and colleagues examine the phylogenetic distribution of brain size across squamates (lizards and snakes; you can find a reference and a link to the study at the bottom). In it, the authors explore the hypothesis that larger brains evolved to allow organisms to better manage environmental complexity, through enhanced cognition and behavioral flexibility. Despite years of hypothesis testing on the subject, there is no clear consensus about its validity. De Meester et al. join the quest and investigate the relationship of brain size in 171 squamate species (including 8 anoles!) to habitat type and degree of sociality. The punchline is that snakes are the pea-brains of the squamate world. Unexpectedly, there was a strong positive relationship between degree of sociality and brain size, such that solitary species had the largest brains. And, perhaps less supported but still a trend; arboreal species generally have the largest brains, while fossorial species (those that burrow and live in the leaf litter) have the smallest.
From De Meester et al.: Ancestral state reconstruction of relative brain size (residuals of the brain to body mass regression) along the nodes and branches of the phylogenetic tree of 171 species of Squamata. Sphenodon punctatus is included as an outgroup. Species with positive residuals (blue) have large brains relative to their body size, whereas species with negative residuals (yellow–red) have small brains relative to their body size. Results were visualized using the contMap function in R (package phytools; Revell, 2012)
But, I hear you say, what of the anoles? Well, Neotropical species had the largest brains of any biogeographical region, and anoles specifically are exceptionally big brained. In fact, on delving into the supplementary material — in which De Meester et al. provide wonderful access into the brain size data that they accumulated — it reveals that Anolis stratulus, the Puerto Rican trunk-crown spotted anole, has the relatively largest brain size of any squamate!
Here is a crude figure I just whipped up from the De Meester et al. dataset. As it shows, anoles perform very well in the brain size department relative to both all squamates and within lizards specifically. Although the American green anole (A. carolinensis) does let the team down slightly…
You can read the study in full following the link below!
Gilles De Meester, Katleen Huyghe, Raoul Van Damme. 2019. Brain size, ecology and sociality: a reptilian perspective. Biological Journal of the Linnean Society, bly206.
https://doi.org/10.1093/biolinnean/bly206
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Nick Pharris
> Unexpectedly, there was a strong positive
> relationship between degree of sociality and
> brain size, such that solitary species had the
> largest brains.
Wouldn’t that be a strong *negative* relationship between degree of sociality and brain size?
James T. Stroud
Sorry, I have been in the field and away from the internet for a few days. Yes, you’re absolutely right
Jon Richfield
Interesting article.
This is not at all my field, so my remarks are both anecdotal and unorganised; I certainly have undertaken NO brain-size measurements. I write from South Africa, where most of my experience is with Bradypodion chameleons. They have repeatedly startled me with their awareness of watchers (put one in a bush and watch it, and it does its best to look inconspicuous; leave and hide behind a suitable screen, and it scoots down to the ground and charges off for freedom). Let one notice a rival at a distance of several metres with no direct access, and it goes through the threat routines and charges off through the most promising indirect route along bushes and twigs.
It is reminiscent of the behaviour of the Salticidae genus Portia, of which at lest some species can follow an indirect route to observed prey, even if it loses sight of the target en route.
And snakes, local handlers agree: catch ten cobras, expect ten personalities, including differences in attempts to escape from handling equipment. Mambas vary wildly in their aggression and they sometimes are disturbingly aware of human presence; standing still does not always fool them. Same with some other arboreal snakes such as boomslang and twig snakes.
Bottom line: I have become deeply reserved in trusting either the type or depth of any correlation between brain size and apparent intelligence.