Field observations of the mating behaviour of snakes are scarce, probably because of the secretive nature and low encounter rates of many species (Sasa and Curtis, 2006). In the Neotropical rattlesnake, Crotalus durissus Linnaeus, 1758, mating has been reported only in captive individuals (Almeida-Santos et al., 1999). Here we describe the first record of the mating behaviour of the Neotropical rattlesnake, Crotalus durissus, in nature (Fig. 1 A). Observations were made on 9 March 2017, at 14:54 h, a warm and sunny day (temperature = 27.1 o C; relative humidity = 66%), in an ecotone between dry forest and Cerrado (Brazilian savannah) in Prudente de Morais, Minas Gerais, Brazil (-19.2841 °S,-44.0628 °W; datum WGS 84). A male and a female C. durissus were found copulating beneath a small pile of leaf litter in a densely vegetated area near a pasture. The snakes were accidentally disturbed when the observer approached the place where they were coiled. Consequently, the female attempted to flee, dragging the male for about 5 minutes (Fig. 1 B-D). Copulation lasted about 20 minutes, but it had already begun when the snakes were found. After copulation, the male took about 20 minutes to retract the hemipenis back into the base of the tail. Retraction of the hemipenis began by the calyces, which are pulled back into the tail by a contraction of the retractor muscle (Dowling and Savage, 1960; Porto et al., 2013). Retraction of the calyces lasted about 15 minutes, and the base of the hemipenis remained unchanged for about 5 minutes until retracted into the tail (Fig. 1 E-F). The male remained stretched out for about 20 minutes and showed no defensive posture even with the presence of the observer. We then noticed drops of blood on the vegetation and the hemipenis (Fig. 1 E-F). We could not determine the origin of the blood, but we suggest two nonexclusive hypotheses. The hemipenis spicules may have hurt the female’s vagina while she was dragging the male over a long distance. Alternatively, the male may have suffered an injury to the hemipenis while being dragged quickly by the female. The slow hemipenis retraction and the male’s fatigue after copulation may better support the second hypothesis. Potential costs for male C. durissus during mating season include increased activity and energy expenditure in searching for oestrous females (Almeida-Santos et al., 2004; Aldridge and Duvall, 1992) and male-male combat (Almeida-Santos et al., 1999). For female C. durissus, which assume a passive posture in copulation, energy expenditure may be lower, thus allowing them to flee as soon as the copulation ends. The copulation reported here occurred in late summer, which coincides with the peak of spermiogenesis and sperm storage in the ductus deferentia (Almeida-Santos et al., 2004). At that time, male C. durissus are actively searching for oestrous females (Almeida-Santos et al., 2004). The observed copulation coincides with the timing of mating in North American Crotalus in late summer and autumn (Aldridge and Duvall, 2002), which corroborates a phylogenetic conservatism of mating season in the genus. Acknowledgments. We thank Henrique Braz for suggestions and language editing. References Aldridge, R.D., Duvall, D. (2002): Evolution of the mating season in the pitvipers of North America. Herpetol. Monographs 16: 1–25. Almeida-Santos, S.M., Laporta-Ferreira, I.L., Antoniazzi, M.M., Jared, C. (2004): Sperm storage in males of the snake Crotalus durissus terrificus (Crotalinae: Viperidae) in southeastern Herpetology Notes, volume 14: 225-227 (2021) (published online on 01 February 2021) Mating in free-ranging Neotropical rattlesnakes, Crotalus durissus: Is it risky for males? Selma Maria Almeida-Santos 1,* , Thiago Santos 2 , and Luis Miguel Lobo 1 1 Instituto Butantan, Laboratório de Ecologia e Evolução, Avenida Vital Brasil, 1500, Butantã, São Paulo, SP, 05503- 900, Brazil. 2 Pontifícia Universidade Católica de Minas Gerais, Laboratório de Ecologia Evolutiva de Anfíbios e Répteis, Belo Horizonte, MG, 30535-610, Brazil. * Corresponding author. E-mail: selma.santos@butantan.gov.br © 2021 by Herpetology Notes. Open Access by CC BY-NC-ND 4.0.