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Why don’t we see more larger brained species in our ecosystem?

Species with larger brains seem to have a higher general intelligence. So why haven’t all species evolved larger brains? Possibly because general intelligence relies on a capability for social learning, so larger brains are only useful for species that rely more on social learning. – Kaj Sotala

Larger brains cost require more fuel, larger brains require larger heads, larger heads are harder to scaffold, scaffolding costs nutrients, there are so many trade-offs… scaffolding+big brains are often a the expense of other important morphologies … locomotion abilities, large craniums at the expense of stronger and bigger mouths, and in many species larger heads correlate with more complications during birth.

Perhaps I should add – (and this seems similar to some AI singleton scenarios) – larger brains (as mentioned, correlates with greater intelligence), could give distinct first mover advantages when coupled with other morphologies that enable the organism to generally & efficiently manipulate its environment (opposable thumbs etc). The first mover advantage may be so powerful as to diminish dependencies on environmental factors – or the rest of the ecological web. We, as arguably the most intelligent species on the planet, seem to be moving through an era where we as a species decreasingly depend on aspects of the ecosystem or aspects of our evolutionary endowed morphology – we synthesize better alternatives – and its happening so fast that evolution by natural selection can’t keep up – it’s production of ‘new better models’ of organisms is crowded out by the rapid progress of civilization. So perhaps one reason we don’t see more larger brained species, is for anthropic reasons – ruffly that the higher the occurrence of large brained morphologies, the higher the likelihood of one single species taking the first mover advantages related to high intelligence – and as a result quickly becoming technologically advanced enough for either subduing and manipulating the ecosystem – which may crowd out (and essentially retard) the power of evolution by natural selection to evolve larger brains in other species. The species may go on to inadvertently destroy the ecosystem, intentionally phase out the current ecosystem (involving blind natural selection) perhaps for ethical or instrumental reasons, or leave the ecosystem and natural selection to its own devices and go interstellar – in which case there would be at least one less large brained species in the ecosystem, and perhaps re-opening a niche for another species to fill.

This was inspired by Kaj Sotala’s query about why we don’t see more larger brained species than we currently do.