Kirk Setser

NEW DISTRIBUTION RECORDS FOR THE QUERÉTERO DUSKY RATTLESNAKE CROTALUS AQUILUS (VIPERIDAE), WITH COMMENTS ON MORPHOLOGY AND HABITAT USE

Abstract We provide the 1st documented accounts of the Mexican endemic rattlesnake Crotalus aquilus from the state of México. The new records extend the known distribution of the species into a region where it may be sympatric with the superficially similar C. triseriatus. Because these taxa have previously been subject to some taxonomic confusion, we performed a preliminary morphological comparison using individuals of both species obtained from proximal localities. Our analyses support the supposition that these taxa are morphologically distinct. The new localities for C. aquilus are situated in high valleys that have been extensively modified by human settlement and agriculture.

Patterns of sexual dimorphism in Mexican alligator lizards, Barisia imbricata

We compare morphological characteristics of male and female Barisia imbricata, Mexican alligator lizards, and find that mass, head length, coloration, incidence of scars from conspecifics, tail loss, and frequency of bearing the color/pattern of the opposite sex are all sexually dimorphic traits. Overall size (measured as snout–vent length), on the other hand, is not different between the two sexes. We use data on bite scar frequency and fecundity to evaluate competing hypotheses regarding the selective forces driving these patterns. We contend that sexual selection, acting through male-male competition, may favor larger mass and head size in males, whereas large females are likely favored by natural selection for greater fecundity. In addition, the frequency of opposite-sex patterning in males versus females may indicate that the costs of agonistic interactions among males are severe enough to allow for an alternative mating strategy. Finally, we discuss how sexual and natural selective forces may interact to drive or mask the evolution of sexually dimorphic traits.

Female Reproduction in Thamnophis scaliger: The Significance of Parturition Timing

Abstract Reproduction involves costs and benefits to a female snake. Costs include increased predation risk as well as lower future survival and reproduction probability, whereas benefits are the current production of offspring. Reproduction also is highly energy demanding: at each reproductive event, a female must decide between the allocation of energy to female condition or to reproductive output. Moreover, energy allocation linked to reproduction may be influenced by environmental features, especially climate and food availability. During 2008 and 2009, we studied the female reproductive tactics of Thamnophis scaliger, a viviparous and earthworm-eating snake distributed at rather high altitudes in central México. Gravid females were collected in the field and maintained in captivity until giving birth. The low percentage of females that were gravid each year suggested they did not reproduce annually. This result probably was driven by the seasonal constrained availability and low-energy content of earthworms, which preclude females from acquiring sufficient energy to fuel more frequent reproduction. Females also produced rather large litter sizes, implying a significant investment in reproduction. Univariate and multivariate analyses (path analysis) found the expected relationships between maternal body size and litter size and mass, and there was no trade-off of energy allocation between postpartum females and their litters. We found, however, a noteworthy relationship between parturition date and postpartum female body condition. Females that reproduced later benefited most from foraging opportunities during the short rainy and high prey-availability period and remained in better body condition. Resumen La reproducción implica costes y beneficios para una hembra de ofidio. Los costes incluyen el aumento del riesgo a ser depredada, así como la disminución de la probabilidad de supervivencia y reproducción en el futuro, mientras que los beneficios son la producción de descendencia en el presente. La reproducción también demanda mucha energía, y cada vez que una hembra se reproduce, debe decidir entre la asignación de energía a su condición corporal postparto o al rendimiento reproductivo. Estas tácticas en la reproducción de los ofidios pueden estar condicionadas por factores medioambientales, especialmente el clima y la disponibilidad de alimento. En 2008 y 2009, estudiamos la táctica reproductiva de las hembras de Thamnophis scaliger, un ofidio vivíparo, especialista en el consumo de lombrices, distribuido a bastante altitud en el centro de México. Se colectaron hembras grávidas que se mantuvieron en cautividad hasta el parto. El bajo número de hembras grávidas cada año sugirió que no se reproducían anualmente. Este resultado estuvo probablemente condicionado por la estacionalmente limitada disponibilidad y el bajo contenido energético de las lombrices, que impiden a las hembras adquirir suficiente energía para una reproducción más frecuente. Además, las hembras produjeron camadas relativamente numerosas, lo que implica elevada inversión en reproducción. Análisis univariantes y multivariantes (análisis de vías) encontraron la esperada relación entre tamaño corporal materno, y el tamaño y peso de la camada, pero no hubo una compensación entre la energía asignada a las hembras postparto y sus camadas. Si encontramos una notable relación entre la fecha de parto y la condición postparto de las hembras. Las hembras que se reproducían más tarde se beneficiaban de mayor oportunidad de forrajeo durante el corto periodo lluvioso y de alta disponibilidad de presas, por lo que quedaban con mejor condición corporal.

Diet of New Mexico Ridge-nosed Rattlesnake (Crotalus willardi obscurus) in the Sierra San Luis and Sierra Pan Duro, México

Abstract We examined the diet of New Mexico Ridge-nosed Rattlesnakes (Crotalus willardi obscurus) from the Sierra San Luis and the Sierra Pan Duro in the northern Sierra Madre Occidental, México. All snakes included in this study were encountered during August, September, and October in 2003 and 2004. Including recaptures of previously captured individuals, 107 of 199 snakes contained at least one prey item. We examined 103 prey items from 92 of these snakes; some snakes contained multiple prey items. Approximately two-thirds of prey items were ectotherms (54.4% lizards and 13.6% scolopendromorph centipedes); birds (21.4%) and mammals (10.7%) were also taken. The diet of juvenile snakes (n = 32) consisted primarily of lizards (62.5%) and centipedes (25.8%), although large juveniles also consumed mammals (6.3%) and passerine birds (6.3%). Adult snakes (n = 71) fed primarily on lizards (50.7%) and passerine birds (28.2%) but also consumed mammals (12.7%) and centipedes (8.4%). Crotalus willardi in the Sierra San Luis and Sierra Pan Duro consumed more birds than has been reported from C. willardi in nearby populations and continued to consume centipedes as adults. Resumen Examinamos la dieta de la New Mexico Ridge-nosed Rattlesnake (Crotalus willardi obscurus) en la Sierra San Luis y la Sierra Pan Duro en el norte de la Sierra Madre Occidental, México, de serpientes colectadas en los meses de agosto, septiembre y octubre en 2003 y 2004. Capturamos 199 serpientes contando capturas y recapturas. Y de ellas 107 contenían restos de por lo menos una presa. Examinamos 103 restos de presas provenientes de 92 serpientes; algunas serpientes presentaban restos de múltiples presas. Aproximadamente dos tercios de las presas fueron ectotermos (54.4% lagartijas y 13.6% ciempiés); también fueron consumidas aves (21.4%) y mamíferos (10.7%). La dieta de las serpientes juveniles (n = 32) consistió principalmente de lagartijas (62.5%) y ciempiés (25.8%), aunque los juveniles grandes también consumieron mamíferos (6.3%) y aves paseriformes (6.3%). Las serpientes adultas (n = 71) se alimentan principalmente de lagartijas (50.7%) y aves paseriformes (28.2%), aunque también consumieron mamíferos (12.7%) y ciempiés (8.4%). Las serpientes en la Sierra San Luis y la Sierra Pan Duro consumieron más aves de lo que ha sido reportado para las poblaciones cercanas y los adultos continúan alimentándose de ciempiés.

Venom Ontogeny in the Mexican Lance-Headed Rattlesnake (Crotalus polystictus)

As trophic adaptations, rattlesnake venoms can vary in composition depending on several intrinsic and extrinsic factors. Ontogenetic changes in venom composition have been documented for numerous species, but little is known of the potential age-related changes in many rattlesnake species found in México. In the current study, venom samples collected from adult and neonate Crotalus polystictus from Estado de México were subjected to enzymatic and electrophoretic analyses, toxicity assays (LD50), and MALDI-TOF mass spectrometry, and a pooled sample of adult venom was analyzed by shotgun proteomics. Electrophoretic profiles of adult males and females were quite similar, and only minor sex-based variation was noted. However, distinct differences were observed between venoms from adult females and their neonate offspring. Several prominent bands, including P-I and P-III snake venom metalloproteinases (SVMPs) and disintegrins (confirmed by MS/MS) were present in adult venoms and absent/greatly reduced in neonate venoms. Age-dependent differences in SVMP, kallikrein-like, phospholipase A2 (PLA2), and L-amino acid oxidase (LAAO) activity levels were confirmed by enzymatic activity assays, and like many other rattlesnake species, venoms from adult snakes have higher SVMP activity than neonate venoms. Conversely, PLA2 activity was approximately 2.5 × greater in venoms from neonates, likely contributing to the increased toxicity (neonate venom LD50 = 4.5 μg/g) towards non-Swiss albino mice when compared to adult venoms (LD50 = 5.5 μg/g). Thrombin-like (TLE) and phosphodiesterase activities did not vary significantly with age. A significant effect of sex (between adult male and adult female venoms) was also observed for SVMP, TLE, and LAAO activities. Analysis of pooled adult venom by LC-MS/MS identified 14 toxin protein families, dominated by bradykinin-inhibitory peptides, SVMPs (P-I, P-II and P-III), disintegrins, PLA2s, C-type-lectins, CRiSPs, serine proteinases, and LAAOs (96% of total venom proteins). Neonate and adult C. polystictus in this population consume almost exclusively mammals, suggesting that age-based differences in composition are related to physical differences in prey (e.g., surface-to-volume ratio differences) rather than taxonomic differences between prey. Venoms from adult C. polystictus fit a Type I pattern (high SVMP activity, lower toxicity), which is characteristic of many larger-bodied rattlesnakes of North America.

Rattle Button Loss in Juvenile Ridge-nosed Rattlesnakes (Crotalus willardi): A Novel Mechanism for the Developmental Delay of the Rattle

Abstract Although a keratinous rattle structure at the end of the tail is the defining trait of rattlesnakes, there is variation in the ability of rattles to produce sound. In previously reported cases of reduced rattle functionality, near complete loss of sound production occurs in adult snakes that do not retain loose interlocking rattle segments. Here we report for the first time a distinct phenomenon: the delay in development of a functional rattle among neonate and juvenile Ridge-nosed Rattlesnakes (Crotalus willardi) resulting from universal loss of the rattle button, normally the first loose segment retained in functional rattles. Loose segments added during subsequent sheds are retained at rates similar to two other rattlesnake species examined (based on data from over 1,000 wild-caught rattlesnakes) and presumably other rattlesnakes with sound-producing rattles. This pattern of delayed rattle development results in effectively silent cohorts for approximately the first year of life and provides further insight into the ecological context of rattle function and evolution.