On the topic of how to measure intelligence in different species, I found this passage on pp. 256-257 of Georg Streidter's textbook:
Over the last 50 years or so, it has become apparent that some nonmammals perform just as well as mammals in various learning and "intelligence" tests, as long as the tests are designed with the animal's "special needs" in mind. Davidson (1966), for example, showed that alligators fail to learn a simple discrimination task if the reward is food, but readily master the same task, in the same apparatus, if they are offered the opportunity to escape from excessive heat. Such a finding might have surprised Tinklepaugh or Edinger, but it makes perfect sense once you realize that alligators (as ectothermic creatures with low metabolic rates) can go without food for long periods of time but must frequently move out of the sun to prevent heatstroke. In other words, comparative psychologists have realized that it is blatantly unfair to run reptiles or other nonmammals through intelligence tests that were designed by mammals for mammals (Streidter 2005:256-257).
This is near the beginning of a chapter on mammalian brain evolution, the introduction of which ends: "After all, the subject of the book is the evolution of brains, not intelligence" (258). The focus on the functional and the adaptive is refreshing -- since the evolutionary utility of the brain is for solving adaptive problems.
Nevertheless, he later links expanding brain size on the mammal lineage with metabolic rate -- which may be the most straightforward of possible connections, since the sensory evolution of early mammals involved both complex gains (e.g., olfactory) and losses (e.g. chromatic vision) of function.
I'll probably be quoting more as I get into this.
Davidson RS. 1966. Operate stimulus control applied to maze behavior: heat escape conditioning and discrimination reversal in Alligator mississippiensis. J Exp Anal Behav 9:671-676.
Streidter GF. 2005. Principles of Brain Evolution. Sinauer, Sunderland MA.