The reproductive biology of Bothrops erythromelas, a small pit viper from the Caatinga, a semiarid biome in Brazil, is described based on analysis of individuals deposited in zoological collections. Males are smaller and also attain sexual maturity at a smaller size than females. Female reproductive cycle is seasonal with an extended period of secondary vitellogenesis and births occurring in a restricted period from late spring to early summer. Sperm storage in females may probably occur in infundibular tubular glands and uterine muscular twisting (UMT), which is a polymorphic condition within B. erythromelas. Seasonal spermatogenesis in males is variable with some intraspecific variation regarding the male reproductive stage per season. Most males are reproductively active during spring/summer and reproductively quiescent during autumn/winter, although some individuals vary (e.g., show testicular spermatogenesis and active sexual segment of the kidneys (SSK) during winter). The SSK could be identified in every male. Most males showed highly hypertrophied SSK in spring/summer and moderately hypertrophied SSK in autumn/winter. The ampulla ductus deferentis was observed and histochemical reactions were conducted. We discuss the probable influence of the unique environmental conditions of the Caatinga region and phylogenetic inertia in the reproductive patterns of this snake species. 1. Introduction The Caatinga domain, a mosaic of thorny bushes and seasonal dry forest, is a semiarid biome exclusive to Brazil. It is characterized by an extremely irregular rainfall pattern with severe dry seasons in some years . Reptiles that live in arid environments may show physiological, behavioral, ecological, and morphological adaptations related to water scarcity in the environment . Recently, a long-term study in the field showed that timing of parturition is influenced by rainfall seasonality in Crotalus atrox, a pit viper species from the Sonoran Desert . To our knowledge, few studies addressed questions related to the influence of rainfall in the determination of reproductive strategies in snakes [3, 4]. Pit viper species from the Caatinga region are good models to explore questions on the influence of environmental variables in the reproductive cycles because this region has unique characteristics regarding patterns of temperature and rainfall , and some data on reproduction of pit viper species from the Neotropical region including species from the Caatinga region are available for comparisons [4–9]. Bothrops erythromelas is a terrestrial and
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