Supernumerary rattle growth in a Mexican lance-headed rattlesnake Crotalus polystictus

C of teratological conditions and other developmental abnormalities are important to document in reptiles and amphibians because they can bring to light new emergent pathogens, diseases or conditions that can have ecological and conservation implications (Barr et al., 2020; Kaiser, 1997; Schmidt, 1997; Bishop et al., 1991; 1998; Bell et al., 2006) or highlight understudied physiological, developmental or healing processes that are in need of more focused research (Barr et al., 2019; Alibardi & Meyer-Rochow, 2021). In rattlesnakes (Crotalus and Sistrurus), a well-studied group of New World pit vipers (Crotalinae), a wide variety of developmental aberrations have been recorded (for example Klauber, 1956; Pendlebury, 1976; Wallach, 2007; Sant’Anna et al., 2013; Murphy, 2018), but with regard to the specialised crotaline rattle, abnormalities have been rare; they include a case of accelerated growth in the structure which resulted in the accumulation of an abnormal number of rattle segments over a brief period (Walker et al., 2008), and the loss of the rattle style and matrix (Rice et al., 2016). Here, we report on an unusual aberration in an adult Mexican lance-headed rattlesnake Crotalus polystictus that resulted in the growth of secondary rattle tissue. As part of an Association of Zoos and Aquariums-managed Species Survival Plan (SSP) for the species, C. polystictus has been maintained by Audubon Zoo since 2002. In August 2019, a 13-year-old captive-bred male (AZA regional studbook #192) that had been maintained at Audubon Zoo since 2009, underwent a veterinary physical examination due to concerns over an uncharacteristic decrease in its body mass (30.6 %) over the previous winter cooling period, and apparent difficulties with regaining its lost weight despite feeding regularly. The examination was performed by staff veterinarians while the animal was conscious and restrained in a tube, and its demeanor was bright, alert, and responsive. Full body orthogonal radiographs and coelomic ultrasonography showed no abnormalities. Blood was collected from the caudal coccygeal vein and submitted for a complete blood count, biochemistry panel, and protein electrophoresis; no parameters were significantly abnormal. Upon close visual inspection, a small (ca. 1 mm), firm, subcutaneous swelling was noticed on the right lateral aspect of the tail at the base of the rattle, which, based on its size and general appearance, was presumed to be a caseous abscess. The mass was aspirated using a 22-gauge hypodermic needle, and a small amount of yellowish liquid was collected and submitted for cytological examination. No microbial organisms were observed; occasional white blood cells were noted. The mass was left untreated to be closely monitored for changes in size and potential impacts on the animal’s activity and behaviour. A follow-up examination in September 2020 revealed that the mass had increased in size to ca. 2 mm in diameter and had begun to protrude from the skin. With the specimen restrained in a tube, the exposed emergent tissue was closely examined and revealed by palpation not to be an abscess, but rather the same keratinised tissue as the rattle. This was later reaffirmed during a study on pit viper biofluorescence (Paul & Mendyk, 2021) in which the emergent tissue fluoresced the same colour and intensity as the primary rattle when illuminated with a 365 nm ultraviolet LED torch in a darkened room (Fig. 1A). Since the snake had begun to regain its body weight, was eating well and was otherwise behaving normally, it was decided not to intervene surgically and to continue monitoring the mass. In December 2021, the specimen was re-examined, measured (69.2 cm, SVL 74.9 cm, TL 212 g), and radiographed again. Dorsoventral radiographs showed a soft tissue opacity growth lateral to the rattle on the right side (Fig. 1B), and there was no evidence of infection or other disease process to the underlying bone. The continued outward growth of the mass appeared to be displacing the primary rattle, causing it The Herpetological Bulletin 165, 2023: 32–33