Are intelligent avians, fish or reptilians possible?

[Mayabird]

One thing to remember is that a large brain is a resource hog that doesn't pay off in the short term, only in the long term. For a species to develop one they need the long term pay off to make it worth while. Gorillas, chimpanzees, and elephants are all social animals with manipulative appendages that are quite useful to an intelligent animal. Our own ancestors didn't start having dramatic increases in brain size (compared to other primates) until the time when they started using stone tools. While I am reluctant to say that being a carnivore is a necessary attribute of attaining sapience, the increased hunting prowess offered by stone tools seems to have been a factor.

A vegetarian sapient animal may not be impossible, but carnivores and omnivores are at an advantage because meat and fat are much richer sources of energy than plants. Just the bone marrow of one kill can provide someone's daily caloric needs. It also means that less energy need go into digestion and more into running the brain. Our brains use around 20% of our body's energy. It's a lot when living on the margins of survival.

[Comrade Tortoise]

Indeed. Animals, pound for pound, are much easier to digest than plants of equal mass. Caloric measurements can sometimes be a bit deceptive because we dont actually process foods by burning them in a bomb calorimeter. Such measurements are usefull for measuring productivity of an eco-system, and energy transfer between trophic levels, but they dont accuratly reflect the processing our bodies do.

To put it simply, we cannot digest cellulose, so the vast majority of any given plant that we eat is just going to clean our system like a glorified pipe snake. We may be able to get some nutrients out of it, after we break cells open while chewing, but the vast majority of the plant is just wasted space in our stomachs, that keeps us regular. And that is before inefficiencies in the digestion process.

We CAN however, break down most animal foods. Bone, meat, fat, blood, marrow. All of it is broken down into it's chemical components for use in energy or protien building very easily. It is much more efficient energetically speaking, to eat meats.

Sure, second law of thermodynamics means that less energy gets transfered between trophic levels, so predators cant exist in the high numbers that herbivors can. But it IS more efficient for the individual and their energy needs to consume animal protien

[Cynical Cat]

Just to throw out an interesting number, the brain is responsible for 85% of a human's energy consumption when at rest.

[Shark Bait]

Cephalopods are not very social, octopi are rather territorial squid sometimes school to feed and mate though they are also known to become cannibalistic if over stimulated (humbolt squid feeding behaviors are more tenacious than sharks, lions, and wolves combined). Cuddle fish will tolerate each other and don’t get territorial but they also do not have much in the way of social structure. The problem however becomes one of can they make tools much beyond shaped stone tools? How would an aquatic animal fashion metallic tools, aside from that consider octopi have not evolved in millions upon millions of years, probably a reason for that.