Geology topics
Phil A. Medica
Publications and source records attributed to Phil A. Medica.
Crotalus oreganus lutosus (Great Basin Rattlesnake). Defensive behavior/head hiding and tongue display
A variety of snake species employ head hiding as defensive behavior (Greene 1973. J. Herpetol. 7:143–161), but such behavior seems to be rarely observed in rattlesnakes. A recent report (Medica 2009. Herpetol. Rev. 40:95–97) presented observations on Crotalus scutulatus scutulatus and cited cases of similar behavior involving C. atrox , C. ruber , and C. viridis viridis . Here we report an encounter on 28 May 2016 with a C. oreganus lutosus (total length ca. 900 mm) observed 16 km S of Fallon, Churchill County, Nevada, USA. At 0925 h this snake was spotted by AKJ near the edge of the pavement on U.S. Hwy. 95. When approached, the snake presented typical active defensive behavior, with the rattle elevated (although not rattling), striking occasionally, and frequently flicking its conspicuous lavender and black tongue. As AKJ continued to take pictures the snake coiled more tightly (Fig. 1A), maintaining its rattle concealed and finally hiding its head beneath its coils (Fig. 1B). After photographing this pose AKJ made his first physical contact with the snake by scooping it up with a small shovel and placing it off the road. At this point the snake recovered its feisty attitude and would not retreat despite attempts to move it further away from the road. The snake did not rattle during the entire encounter (which lasted less than 5 min). Head hiding as a “last resort” in response to a persistent unavoidable threat may be more widespread within the family Crotalidae than previously documented.
Direct and indirect effects of environmental variability on growth and survivorship of pre-reproductive Joshua trees, Yucca brevifolia Engelm (Agavaceae)
• Premise of study: Accurate demographic information about long-lived plant species is important for understanding responses to large-scale disturbances, including climate change. It is challenging to obtain these data from desert perennial plants because seedling establishment is exceptionally rare, and estimates of survival are lacking for their vulnerable early stages. Desert wildfires, urbanization, and climate change influence the persistence of the long-lived Yucca brevifolia . Quantitative demographic attributes are crucial for understanding how populations will respond to disturbances and where populations will recede or advance under future climate scenarios. • Methods: We measured survival in a cohort of 53 pre-reproductive Y. brevifolia at Yucca Flat, Nevada, USA, for 22 yr and recorded their growth, nurse-plant relationships, and herbivory. • Key results: Herbivory by black-tailed jackrabbits ( Lepus californicus ) caused severe losses of plants during the first and second years (45% and 31%, respectively). Surviving plants experienced <2.5% annual mortality. Survival for the population was 19% over 22 yr. Plants <25 cm in height had lower life expectancy. Average growth rate (± SD) for plants that survived to the last census was 3.12 ± 1.96 cm yr −1 , and growth rates were positively associated with precipitation. Thirty-year-old Y. brevifolia had not yet reproduced. • Conclusions: A rare establishment event for Y. brevifolia during 1983–1984, triggered by above-average summer rainfall, provided a unique opportunity to track early survival and growth. Infrequent but acute episodes of herbivory during drought influenced demography for decades. Variability in survival among young Y. brevifolia indicates that size-dependent demographic variables will improve forecasts for this long-lived desert species under predicted regional climate change.
Gopherus agassizii (Agassiz’s desert tortoise). scute dysecdysis/scute sloughing
Desert tortoises with scute injuries due to fire or disease related processes can result in loss of the scute. These animals appear to function normally, and can replace the scute material with a keratinized layer that covers the bone. This paper describes a tortoise with severe scute loss from a wildfire in 2005, and an animal that lost its scute for unknown reasons. Both animals appeared to be healthy in all other aspects, and lived for many years afterward.
Coluber (= Masticophis) flagellum piceus (Red Racer). Arboreal/nocturnal behavior
Many species of snakes display arboreal behavior and are often found in vegetation many meters above ground. Mojave Desert snake species rarely get very far above the ground surface and are perhaps limited by predominantly low growing vegetation. Coluber flagellum piceus is considered strictly diurnal and although may ascend vegetation while active, is thought to retreat to subterranean refugia at night. Werler and Dixon (2000. Texas Snakes: Identification, Distribution, and Natural History. University of Texas Press, Austin. 437 pp.) stated “The western coachwhip is active above ground only during daylight hours. It almost invariably retires to depths of an animal burrow before sundown, where it spends the night.” The normal morning activity period for C. flagellum appears to be from approximately 0700–1000 h from June through September (Jones and Whitford 1989. Southwest. Nat. 34:460–467). Secor and Nagy (1994. Ecology 75:1600–1614), and Secor (1995. Herpetol. Monogr. 9:169–186) determined that the average body temperature for active above ground activity of C. flagellum was (33.1°C + 0.1°C, range of 24.0°C–40.8°C, N = 502), stating that coachwhip snakes were seldom active on the surface with body temperatures below 28°C. Here, I report a C. f. piceus that apparently spent the night draped in a Creosote Bush ( Larrea tridentata ).
Effects of subsidized predators, resource variability, and human population density on desert tortoise populations in the Mojave Desert, USA
Understanding predator–prey relationships can be pivotal in the conservation of species. For 2 decades, desert tortoise Gopherus agassizii populations have declined, yet quantitative evidence regarding the causes of declines is scarce. In 2005, Ft. Irwin National Training Center, California, USA, implemented a translocation project including 2 yr of baseline monitoring of desert tortoises. Unusually high predation on tortoises was observed after translocation occurred. We conducted a retrospective analysis of predation and found that translocation did not affect the probability of predation: translocated, resident, and control tortoises all had similar levels of predation. However, predation rates were higher near human population concentrations, at lower elevation sites, and for smaller tortoises and females. Furthermore, high mortality rates were not limited to the National Training Center. In 2008, elevated mortality (as high as 43%) occurred throughout the listed range of the desert tortoise. Although no temporal prey base data are available for analysis from any of the study sites, we hypothesize that low population levels of typical coyote Canis latrans prey (i.e. jackrabbits Lepus californicus and other small animals) due to drought conditions influenced high predation rates in previous years. Predation may have been exacerbated in areas with high levels of subsidized predators. Many historical reports of increased predation, and our observation of a range-wide pattern, may indicate that high predation rates are more common than generally considered and may impact recovery of the desert tortoise throughout its range.