We describe a new species of Colostethus that is unusual in that males have blue digits on the hands and blue toe discs; females have blue discs on the fingers and toes but lack blue digits. The new species occurs in lowland tropical rainforest south of Manaus, Amazonas, Brazil, in the municipality of Castanho, at km 12 on the road to Autazes. Six species of Colostethus occur in the vicinity of Manaus; two, C. stepheni and C. marchesianus, occur north of the Amazon River, and four, including the new species and three undescribed species, occur south of the river. Four of these six species have small body size; i.e., adult male SVLs are ≤18.0 mm. Two of the species, both undescribed, from south of the Amazon have adult male SVLs ≥19.5 mm. Calls of all six species are unique; calls are produced either continuously, as inthe case of the new species, C. marchesianus, and one of the large undescribed species south of the Amazon river, or in bouts of varying lengths. The function of the blue digits in males of the new species is unknown but may be associated with visual communication during reproductive behavior. Blue toes or discs were present only in adults during the breeding season.
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