He said, “Pee-er review.”
Explanation: This joke is a pun on the term “peer review,” which refers to the evaluation of work by others in the same field, commonly used in academic or professional settings. The punchline, “Pee-er review,” humorously replaces “peer” with “pee,” suggesting that dogs pee where other dogs have already done so as a way of “reviewing” each other’s markings. The humor comes from the clever word substitution and the idea that dogs follow a similar system to human peer review, but for marking territory.