Agradecimentos: We want to thank Thaynara M. Machado, MSc. Julia R. Ernetti, MSc. Mariana R. Pontes, MSc. Joelma Prado, Ana F. Barbosa, MSc. Simone Dena, Ana Lúcia Rodrigues, MSc. João Bovolon, Natalia Aranha, and Dr. Joice Ruggeri for their help in collecting the individuals of the different species, for providing videos and photographs. We also thank Dr. Antônia Cecília Z. Amaral, Alan R. Batistão, Dr. Valéria H. A. C. Quitet, and Felipe R. Santos for helping with laboratory equipment. We also want to thank Dr. Olívia G. S. Araújo, Dr. Manuella F. G. Andrade, Dr. André R. S. Garraffoni, and Dr. Iuri R. Dias for reading previous versions of the manuscript. Grants and fellowships were provided by the Consejo Zacatecano de Ciencia Tecnología e Innovación (COZCyT), the National Center for Scientific and Technologic Development (CNPq #130108/2022-7, #302834/2020-6), the São Paulo Research Foundation (FAPESP #2016/25358-3, #2022/09659-4), and Coordination of Superior Level Staff Improvement (CAPES #001) Ver menos

Abstract: Anurans of the genus Brachycephalus are among the smallest vertebrates in the world, due to an extreme process of miniaturization. As an example of this process, Brachycephalus species show loss of fingers, loss of the eardrum and middle ear, bone fusions, and the presence of paravertebral plates and parotic plaque. However, no studies addressing the consequences of miniaturization on internal organs, such as the lungs and heart, are currently available. Thus, this study aimed to investigate if overall small body size has affected the cardiorespiratory system. We investigated, via dissections, individuals of four Brachycephaloidea species: Brachycephalus rotenbergae, B. pitanga, Eleutherodactylus johnstonei, and Ischnocnema parva. We observed that B. rotenbergae and B. pitanga present a reduction of the atrial septum and absence of the carotid body. On the other hand, despite being a member of the sister genus to Brachycephalus (both genera belong to the Brachycephalidae), individuals of Ischnocnema present a heart with a complete septum and carotid body; this is also observed in E. johnstonei (Eleutherodactylidae). We observed that B. rotenbergae and B. pitanga have thin skin with a one to two cell thick germ layer, and their lungs likely exhibit lower blood supply when compared to individuals of the E. johnstonei and I. parva species. Based on the observed structures, we suggest that in species of Brachycephalus, respiration is performed mainly through the skin, and their lungs may have a reduced respiratory function Ver menos