This thesis firstly aims at discussing the early works of American poet Marianne Moore (1887-1972) through the bio-philosophical perspectives developed since the investigations of Estonian Jacob von Uexküll (1864-1944). The study elucidates Uexküll’s research on the web-like forms of life that is the Umwelt of animals and Moore’s creation of poetic environments. Such investigations provide a basis for the analysis of Moore’s animals and environments in dialogue with Martin Heidegger’s (1889-1976) concepts of “poverty in world”, and “animal captivation”. Uexküll’s and Heidegger’s concepts are revised by Italian Giorgio Agamben (1942- ), who proposes that there is an openness in the state of being ontologically captivated, caused by interactional processes occurring within the environment. Subsequently, taking into account these same perspectives, this thesis offers a comparative study of Marianne Moore and Brazilian poet João Cabral de Melo Neto (1920-1999), engaging, respectively, her early poems with his book O Cão Sem Plumas [The Dog without Feathers], written in 1950. From the bio-philosophical perspectives previously discussed, this study focuses on moral and ethical stances addressed towards interpretations of the onto-ethological (Buchanan, 2008) nature of animals. The study analyses how both Moore and Melo Neto convey their ethical reflections and specific moral issues through expressions of nature and animal life, especially when they emphasise contexts of violence, misery and deprivation, either in material or conceptual respects, involved with the ontological and world-forming conditions of both animals and human beings. Therefore, the research will focus on their use of literary devices, such as allegories, and literary genres, such as fables, in order to develop both explicit and implicit dimensions of their poetry, thus providing a deeper understanding of the ontological status of animals and human beings.