The discursive practices of individual academic disciplines differ in many ways, which is why numerous studies of academic discourse adopt cross-disciplinary perspectives to explore the character and extent of those differences. Less attention has, however, been given to interdisciplinary discourses which incorporate the findings and/or research methods from a number of disciplines. This paper focuses on the discourse of one of the new critical interdisciplinarities: posthumanism. More specifically, it examines how posthumanist discourse integrates knowledge produced by the soft and hard sciences (as well as other sources) to build its perspective on animals and their relations with humans. Using Martin and White’s (2005) appraisal framework to study knowledge claims collected from selected scholarly monographs adopting a posthumanist perspective, this study demonstrates that posthumanist claims referring to biological knowledge and experiential evidence tend to contain neutral, positive and endorsing formulations, while the knowledge from the soft sciences is reported in more critical ways, which is consistent with the aims of critical interdisciplinarities, i.e. questioning and transforming the dominant knowledge structure within different disciplines. Additionally, this paper provides evidence of the importance of popular science within interdisciplinary research in the humanities. It also sheds some light on the rhetorical practices within the scholarly monograph as a genre, particularly concerning the relative flexibility of its discursive conventions in comparison with those expected from a research article.