The text seems to be a play on the idiomatic expression 'to be long in the tooth', meaning to be old or aged, but it does not clearly exploit different meanings of a word or similar-sounding words for a humorous effect. The phrase 'falls short in the head' could imply a lack of intelligence, and 'long in the heels' might suggest the need for extra effort or to make up for the deficiency in intelligence with hard work. However, without a clear play on words or homophones, it does not constitute a pun.
Who falls short in the head must be long in the heels .