This dissertation offers an interdisciplinary perspective on later medieval views of animals, focusing on the Latin, French, and English texts circulating in England. Reading across scientific and poetic genres, it considers medieval zoology in relation to its textual and experiential sources, its impacts on social hierarchies, and its continuing relevance for ecological activism. I draw on methods from literary studies, the history of science, and the environmental humanities to explore both the uses of literary devices in philosophical texts and the uses of zoological knowledge in imaginative writing. Chapter 1 examines descriptions of reason, canine intelligence, and nonhuman souls in Adelard of Bath’s Questiones naturales (Natural Questions). I show that the text’s dialogic form reflects its emphasis on reason and dissent as scientific practices, while its uses of figurative language promote cross-species comparisons and identifications. In Chapter 2, I explore observation and experience in Albertus Magnus’s De animalibus (On Animals). I address Albert’s “polyphonic” commentary style; the relationship between his experiential anecdotes and medieval exempla; and his view of animal cognition, including the limits of human understanding and the potential for multispecies knowledge production. Chapter 3 applies trans theory to Marie de France’s Yonec and the Roman de Silence, both of which depict gender and species as unstable, intertwined, and more mutable than social class. Chapter 4 surveys medieval knowledge of cetaceans in a range of scientific and literary texts, including the Navigatio sancti Brendani abbatis (Voyage of Saint Brendan the Abbot) and Patience. I demonstrate that whales put pressure on the human/animal boundary and consider how medieval knowledge might benefit modern cetology. Finally, Chapter 5 analyzes poetic accounts of ecological crisis and mourning, focusing on how grief could deprive humans of their characteristic capacities for reason and verbal expression. After considering the emphasis on human responsibility toward nonhuman nature in Cleanness, I examine Geoffrey Chaucer’s depictions of inarticulable grief and interspecies empathy in his Franklin’s Tale, Knight’s Tale, and Book of the Duchess.