Biology and Conservation of North American Tortoises (original) (raw)
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Research on North American Tortoises: A Critique With Suggestions for the Future
1994
Research and conservation of North American tortoises (genus Gopherus) increased greatly in the past 20 years, but the quantity and quality ofthe studies of each of the four species vary widely. Only work on the gopher tortoise (G. polyphemus) was sufficiently broad in geographic scope and in coverage of topics to begin answering basic biological questions. More rigorous studies of the biology of all four species are needed for comprehensive information, including better definitions or evaluations of distribution, range limits, use of habitats, life histories, juvenile ecology, and physiology. An obstacle to past and current research is the preponderance of unpublished literature and lack of scientific hypotheses, especially of studies on desert tortoises (G. agassizii). Important management decisions have been made without adequate knowledge about the biology of the affected species. We suggest that future studies of all four species should be comparable (e.g., analyze growth rates between species), test hypotheses, and be designed for publication in peer-reviewed outlets. These efforts will improve the research and conservation of North American tortoises.
Chapter 7. Ecological Observations on Desert Tortoises in Mexico
We sampled Gopherus morafkai, Morafka’s, or Sonoran Desert tortoise) across its geographic distribution at 43 localities, at 37 of which we found tortoises, in northwestern Mexico during 2005- 2013. Observed relative abundance did not differ among desertscrub, thornscrub, and tropical deciduous forest, although sampling was more difficult in tropical deciduous forest and we suspect that tortoise abundances are highest there, at least based on work in the Alamos region. Based on growth rings, desert tortoises we observed in desertscrub averaged older than those in thornscrub and tropical deciduous forest, with the difference occurring in pre-adult age classes. These data suggest that desert tortoises in tropical deciduous forest, and possibly in thornscrub, grow more rapidly to adult size than those in desertscrub.
ZooKeys, 2016
Desert tortoises (Testudines; Testudinidae; Gopherus agassizii group) have an extensive distribution throughout the Mojave, Colorado, and Sonoran desert regions. Not surprisingly, they exhibit a tremendous amount of ecological, behavioral, morphological and genetic variation. Gopherus agassizii was considered a single species for almost 150 years but recently the species was split into the nominate form and Morafka's desert tortoise, G. morafkai, the latter occurring south and east of the Colorado River. Whereas a large body of literature focuses on tortoises in the United States, a dearth of investigations exists for Mexican animals. Notwithstanding, Mexican populations of desert tortoises in the southern part of the range of G. morafkai are distinct, particularly where the tortoises occur in tropical thornscrub and tropical deciduous forest. Recent studies have shed light on the ecology, morphology and genetics of these southern 'desert' tortoises. All evidence warrants recognition of this clade as a distinctive taxon and herein we describe it as Gopherus evgoodei sp. n. The description of the new species significantly reduces and limits the distribution of G. morafkai to desertscrub habitat only. By contrast, G. evgoodei sp. n. occurs in thornscrub and tropical deciduous forests only and this leaves it with the smallest range of the three sister species. We present conservation implications for the newly described Gopherus evgoodei, which already faces impending threats.
Physiological ecology of desert tortoises in southern Nevada
Herpetologica
Seasonal changes in water balance, energetics, food consumption, daily behavior, diet, osmoregulation and body mass were measured using doubly labeled water, radiotelemeters, and field behavior observations of tortoises (Gopherus agassizii) through a year (1976)(1977) in Rock Valley, Nevada. Aboveground activity extended over the warm seasons (between March and November), and tortoises hibernated in burrows through winter. In spring, tortoises were active for about 3 h every fourth day. Some drank rainwater and they all gained weight while eating succulent annual plants, which provided excess water and potassium that were stored in the urinary bladder, but energy intake was less than required to meet energy expenses via respiration, and body solids declined. As food plants dried in late spring, tortoises eventually achieved positive energy balance while eating grasses, but body masses declined due to negative water balance. Bladder urine became isosmotic with blood plasma. and both became more concentrated. As the summer drought continued, feeding diminished and tortoises spent progressively more time estivating in burrows. Thundershowers in July triggered emergence from estivation, and tortoises drank much rainwater, voided concentrated urine, accumulated dilute urine, restored normal plasma osmotic concentration, resumed feeding on the still-dry grasses and forbs, and accumulated much surplus energy. However, body masses declined during this time due to negative water balance, and osmotic concentrations in bladder urine increased. More rain in September again relieved this osmotic stress as tortoises drank, urinated, and stored dilute urine. Energy balance remained strongly positive until tortoises began feeding on the succulent new sprouts of annual plants that germinated in late September, and the tortoises returned to a spring-like physiological situation through mid-November when hibernation began. Tortoises apparently relinquish maintenance of internal homeostasis on a daily basis during most of the year, while tolerating large imbalances in their water, energy and salt budgets. This ability enables them to exploit resources that are only available periodically, while balancing their water and salt budgets on an annual basis and showing an energetic profit. The environmental properties of rainfall (beneficial to tortoises) and dietary potassium content (deleterious) appear to have major effects on the health of these impressive reptiles.
Endangered Species Research, 2011
Radio telemetry of 29 desert tortoises Gopherus agassizii in 2000 and 60 in 2001 at 4 sites in the Mojave Desert of California, USA, revealed that males had larger home ranges and moved greater distances than females, but that tortoises at sites with more rainfall did not always have larger home ranges or move greater distances. The Ivanpah Valley received more rainfall than the Fort Irwin area, Superior Valley, and Fremont, but differences between home range size and rainfall were not consistent. Within the Ivanpah Valley, the plot at the highest elevation received more rainfall than the lower plots. However, within the Ivanpah Valley, tortoises at the 3 locations did not differ in home range and movement. In the wetter year (2001), female tortoises moved greater distances each day at Ivanpah than female tortoises at Superior and Fremont, which were drier sites. Males moved farther per day than females in both years. In 2001, female tortoises in the Ivanpah Valley used more burrows than tortoises at Fremont and Superior, but similar numbers as tortoises at Fort Irwin. There was no mortality of tortoises in 2000 and a 5% mortality in 2001. KEY WORDS: Desert tortoise • Radio telemetry • Rainfall • Ivanpah Valley • Fort Irwin • Minimum convex polygon • Climate change Resale or republication not permitted without written consent of the publisher