The Cavia intermedia cavy (Rodentia: Caviidae)a newly described species exclusively found in the Moleques do Sul Archipelago in the state of Santa Catarina, Brazilis one of the rarest mammals on the planet. This species is differentiated from other cavies on the basis of its genetic and morphologic characteristics, several of them resulting from its adaptation to an island environment. The objective of this research was to obtain behavioral data taken from within the required ecological matrix to shed light into the social organization, spatial distribution and resource use of the C. intermedia. Our approach used existing data for C. magna and C. aperea to frame results into a comparative context, highlighting possible effects from being cut off in an island environment. We have done 25 monthly trips to the island, of approximately 8 days each, between November 2007 and December 2009. We have used 49 traps for a capture-recapture program in which the animals where tagged, measured, weighted and checked for reproductive state. The behavioral observation took place in two-hour rounds (with intervals scans of one minute) in three areas of frequent cavy sightings (Clareira da barraca, Campinho I and Campinho II) totaling 636,7 hours. Out of the 132 individuals captured, 118 were tagged: 71 males and 47 females. This population was composed mostly of adults which had a

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Cavia guianae: sul da Venezuela, Guiana e provavelmente partes adjacentes ao norte do Brasil; Cavia anolaimae: arredores de Bogotá e Colômbia; Cavia porcellus Linnaeus, 1758 (anolaimae, cobaya, cutleri Bennet, 1836, leucopyga Cabanis 1848, longipillis): espécie domesticada provavelmente na região compreeendida entre o noroeste da Venezuela e o Chile central. Uma das revisões mais atuais sobre a taxonomia deste grupo no Brasil é de Ximenez (1980), no qual é descrita uma nova espécie, Cavia magna. As espécies selvagens descritas pelo autor são Cavia fulgida Wagler 1831, Cavia magna (depois denominada Cavia magna Ximenez, 1981) e Cavia aperea, esta com duas subespécies, C. aperea pamparum Thomas, 1917 e C. aperea aperea Erxleben, 1777. A subespécie C. aperea pamparum também foi citada por Massoia (1973). Segundo Ximenez (1980), Cavia aperea é a espécie de distribuição mais ampla do gênero, estendendo-se através do Brasil oriental e meridional, sul do Paraguai, Uruguai e

Graphical Abstract

Cavia intermedia, a rare species of cavy found exclusively on Moleques do Sul Island, Brazil, differs from other cavies in genetic and morphological characteristics and has been regarded as phylogenetically closest to Cavia magna. Here, I present morphological evidence for the likely origin of Cavia intermedia from an ancestor shared with Cavia magna: an external female genital organ similar in size and appearance to the male's penis. Masculinization of the female genitalia has been observed in Cavia magna but does not occur in Cavia aperea, which suggests a sister relationship between Cavia magna and Cavia intermedia.