MAT (J 200 j } ) PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON /J 114(1): 1-33. 2001. ^'^'^^^ three-toed sloth (Mammalia: Xenarthra) from Panama, with a review of the genus Bradypus Robert P. Anderson and Charles O. Handley, Jr. (RPA) Natural History Museum & Biodiversity Research Center, and Department of Ecology & Evolutionary Biology, 1345 Jayhawk Blvd., University of Kansas, Lawrence, Kansas 66045-7561, U.S.A.; (COH, Jr. — deceased) Division of Mammals, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, U.S.A. Abstract. — Morphological and morphometric analyses of three-toed sloths {Bradypus) from the islands of Bocas del Toro reveal rapid differentiation of several populations during the Holocene. These islands, lying off the Caribbean coast of western Panama, were separated from the adjacent mainland by rising sea levels during the past 10,000 years. The sequence of island formation and the approximate ages of the islands are known. In at least four independent events, sloths on five of the islands evolved smaller size following insulari-zation. Sloths on the younger islands remain conspecific with mainland pop-ulations of Bradypus variegatus. On Isla Escudo de Veraguas — the oldest and most remote island of the archipelago — however, the three-toed sloth has dif-ferentiated to the species level, and we here describe it as Bradypus pygmaeus. We provide updated diagnoses and distributions for the species of Bradypus, including a key to the genus. Resumen. — Se realizaron analisis morfologicos y morfometricos de los per-ezosos de tres dedos {Bradypus) de las islas de Bocas del Toro, que mostraron una diferenciacion rapida de varias de las poblaciones durante el Holoceno. Estas islas, que se ubican en la costa caribena del occidente de Panama, se separaron de tierra firme debido a aumentos en los niveles del mar durante los ultimos 10.000 afios. Se conoce la secuencia de formacion de las islas y sus edades aproximadas. Los perezosos de cinco de las islas evolucionaron hacia un tamaho corporal menor en por lo menos cuatro eventos independientes, siguiendo el proceso de insularizacion. Consideramos que tanto los perezosos de tierra firme como los de las islas jovenes son representantes de la especie Bradypus variegatus; sin embargo en la Isla Escudo de Veraguas, la mas vieja y mas remota del archipielago, el perezoso de tres dedos ha logrado el nivel de especie y lo describimos aca como una especie nueva, Bradypus pygmaeus. Presentamos caracteres diagnosticos y distribuciones para las especies de Bra-dypus, incluyendo una clave de las especies del genero. Together with armadillos and anteaters, ranges, one species of Choloepus and one sloths make up the Neotropical order Xe-species of Bradypus occur together in the narthra (Gardner 1993, or magnorder Xe-same habitat, exhibiting biotic sympatry ( = narthra sensu McKenna & Bell 1997). Two syntopy; Sunquist & Montgomery 1973, distantly related genera of sloths, Choloe-Wetzel 1985, Taube et al. 1999). The two pus (two-toed sloths) and Bradypus (three-genera are easily distinguished by the num-toed sloths) are extant. Over much of their ber of clawed digits on their forelimbs (two
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