I don’t think organisms end up with 40 billion cortical neurons without either some strong selection for at least some sub-dimensions of intelligence, or being as big as Godzilla.
One could naively expect that the neuron count (especially touch and motor) sensory processing modules are proportional to the surface area of an organism. However I think this is unrealistic: Bears don’t need nearly as fine precision on what square centimeter of skin was touched (or what millimeter the paw moves) than mice, and generally this is because precision gets less relevant given body size.
So let’s say the precision an organism needs is proportional to the square root of the 1-dimensional-size (aka sqrt(surface_area)) of the organism. Aka if a mice is 5cm tall and a bear 2m, the spacing between sensors on the mouse skin vs on the bear skin would be sqrt(0.05) vs sqrt(2). The number of sensors on the skin surface is proportional to the square of the distancing between sensors, so the overall number of sensors is proportional to the 1-dimensional-size (aka sqrt(surface_area)).
A brown bear has 250million neorons in the neocortex and is maybe 2m tall. So to get just by scaling size to 40billion neorons an organism would have to be 40⁄0.25 * 2m = 320m tall. So actually bigger than godzilla.
Justification for this:
One could naively expect that the neuron count (especially touch and motor) sensory processing modules are proportional to the surface area of an organism. However I think this is unrealistic: Bears don’t need nearly as fine precision on what square centimeter of skin was touched (or what millimeter the paw moves) than mice, and generally this is because precision gets less relevant given body size.
So let’s say the precision an organism needs is proportional to the square root of the 1-dimensional-size (aka sqrt(surface_area)) of the organism. Aka if a mice is 5cm tall and a bear 2m, the spacing between sensors on the mouse skin vs on the bear skin would be sqrt(0.05) vs sqrt(2). The number of sensors on the skin surface is proportional to the square of the distancing between sensors, so the overall number of sensors is proportional to the 1-dimensional-size (aka sqrt(surface_area)).
A brown bear has 250million neorons in the neocortex and is maybe 2m tall. So to get just by scaling size to 40billion neorons an organism would have to be 40⁄0.25 * 2m = 320m tall. So actually bigger than godzilla.