There's actually a lot of evidence in primates and other animals that they return favors.

De Waal is pointing to a well-established finding in ethology and primatology: many social animals engage in reciprocal behavior—helping, sharing, grooming, or supporting others in ways that are later repaid. The remark pushes back against the idea that reciprocity is uniquely human or requires explicit moral rules or contracts. Instead, it suggests that the roots of cooperation and “fairness” lie in evolved social strategies that stabilize group living. In de Waal’s broader work, such evidence is used to argue for continuity between human morality and animal social instincts, grounding ethics in biology as well as culture.