in the late 1970s he developed charcot-marie-tooth syndrome, which ultimately left him a quadriplegic. as a result, he shifted his research to squirrels. he carried out field research in north america, india, and southeast asia. as his mobility deteriorated he shifted his focus to museum studies of systematics and anatomy, using the smithsonian's collection of over 30,000 specimens of squirrels. in addition to numerous technical publications, he co-authored the book "squirrels of the world", a profusely illustrated, encyclopedic treatment of all 285 known species of squirrel.