Long bone histology of Chersina angulata : Interelement variation and life history data

The current study deduced the growth pattern and lifestyle habits of Chersina angulata based on bone histology and cross-sectional geometry of limb bones. Femora, humeri, and tibiae of seven different-sized individuals representing different ontogenetic stages were assessed to determine the interelement and intraskeletal histological variation within and among the tortoises. The bone histology of adult propodials consists of a highly vascularized, uninterrupted fibrolamellar bone tissue with a woven texture in the perimedullary and midcortical regions suggesting overall fast early growth. However, later in ontogeny, growth was slow and even ceased periodically as suggested by slowly formed parallel-fibered bone tissue and several growth marks in the pericortical region. In juvenile individuals, fibrolamellar bone tissue is restricted to the perimedullary regions of propodials as remnants of bone formed during the earliest stages of ontogeny. The epipodials are characterized by having parallel-fibered bone tissue present in their cortices; however, periodic arrests in growth are recorded at various times. Remnants of fibrolamellar bone tissue formed during early ontogeny occur in the epipodials of only a few individuals. Interelement variation is evident, in terms of variation in the orientation of vascular canals between individuals and within the same diaphyseal cross-sections. Different elements show varying cross-sectional geometry, which appear to be correlated with the fossorial behavior of the species. Our results show that of all the long bones, the tibia is least remodeled during ontogeny and it is therefore the best element for skeletochronology.     

Total Counts Versus Line Transects for Estimating Abundance of Small Gopher Tortoise Populations

Abstract: Status and trends of gopher tortoise (Gopherus polyphemus) populations are a critical information need for natural resource managers, researchers, and policy makers. Many tortoise populations are small and isolated, which can present challenges for deriving population estimates. Our objective was to compare abundance and density estimates for a small tortoise population derived using a total burrow count versus estimates obtained with line transect distance sampling (LTDS) using repeated surveys. We also compared results of the 2 survey methods using standard burrow-to-tortoise correction factors versus assessing occupancy of all burrows with a camera scope. In addition, we compared LTDS data obtained using a compass and measuring tape to define transects to those obtained using a Global Positioning System (GPS) and Personal Data Assistant (PDA) field computer to navigate transects. Line transect distance sampling with repeated surveys (both with a measuring tape and compass and with a GPS—PDA) yielded sufficient observations of tortoises to calculate population estimates. From 18% to 31% of burrows were occupied by tortoises as determined with the burrow camera. We found 25 burrows during the LTDS survey that we did not find in the total count survey, which demonstrated that the assumption of 100% detection for the total count was not met; hence, density or abundance measurements derived with this method were underestimates. We recommend using GPS—PDA technology, scoping all burrows detected, and using LTDS with repeated surveys to estimate abundance and density for small gopher tortoise populations.     

The limits of demographic buffering in coping with environmental variation

Animal populations have developed multiple strategies to deal with environmental change. Among them, the demographic buffering strategy consists in constraining the temporal variation of the vital rate(s) that most affect(s) the overall performance of the population. Tortoises are known to buffer their temporal variation in adult survival, which typically has the highest contribution to the population growth rate λ, at the expense of a high variability on reproductive rates, which contribute far less to λ. To identify the effects of projected increases in droughts in its natural habitat, we use field data collected across 15 locations of Testudo graeca in southeast Spain over a decade. We analyse the effects of environmental variables on reproduction rates. In addition, we couple the demographic and environmental data to parameterise an integral projection model to simulate the effects of different scenarios of drought recurrence on λ under different degrees of intensity in the survival–reproduction tradeoff. We find that droughts negatively affect the probability of laying eggs; however, the overall effects on λ under the current drought recurrence (one/decade) are negligible when survival is constant (independent of the reduction of reproduction by drought events) and when survival increased as a tradeoff with the reduction of reproduction rates, with a threshold to population viability at three or more droughts/decade. Additionally, we show that, although some species may buffer current environmental regimes by carefully orchestrating how their vital rates vary through time, a demographic buffering strategy is insufficient to ensure population viability in extreme regimes. Our findings support the hypothesis that the demographic buffering strategy has a limit of effectiveness when adverse conditions occur frequently. Our methodological approach provides a framework for ecologists to determine how effective the management of environmental drivers can be for demographically buffering populations, and which scenarios may not provide long-term population persistence.