Writing Animals on Squirrel Appreciation Day

Who hasn't read books about, staring, or simply including animals? In a decades old tome on animal actors the author described the statistics of television commercials. The bottom line was that no matter how big a celebrity the company got the audience always remembered the product name better if there was a cute dog on the screen. Animals sell.
The question for an author then becomes how do you write an animal. Campbell, in his "Living Forest" series, simply wrote what he observed as a naturalist and freely and repeated admitted that any anthropomorphization was entirely his imagination. In fact one of the defining features of his writing was the mystery that animals presented. Other animal books simply turn animals into furry, feathered, and scaled people who speak and interact in a forest based sitcom. Jack London brought what many consider a more "realistic" approach, writing the animals as masses of instinct and primal need which even thousands of years of breeding and a lifetime of training couldn't overcome once Buck heard "The Call of the Wild". 
So what is your method for getting into the head of an animal you are writing? Something to think about on #SquirrelAppreciationDay