Nanotechnologies that have been linked to the possibility of enhancing cognitive capabilities of human beings might also be deployed to reduce or eliminate such capabilities in non-human vertebrate animals. A surprisingly large literature on the ethics of such disenhancement has been developed in response to the suggestion that it would be an ethically defensible response to animal suffering both in medical experimentation and in industrial livestock production. However, review of this literature illustrates the difficulty of formulating a coherent ethical debate. (...) Well structured arguments for disenhancement can be made on the basis of mainstream views on the basis of ethical obligations to animals, but these arguments have not been persuasive against the moral intuition that disenhancements are unethical. At the same time, attempts to ground these intuitions in a coherent philosophical doctrine have been plagued by logical fallacies and question begging assertions. As such, the debate over animal disenhancement forecasts an enduring conundrum with respect to the core question of transforming the nature of sentient beings, and this conundrum is logically independent of claims that relate either to the distinctive of human beings or to issues deriving from the emphasis on enhancement. (shrink)