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High-level inactivity despite favorable environmental conditions in the rock-dwelling dwarf tortoise Chersobius boulengeri
Abstract
In arid regions with summer rainfall, herbivorous reptiles are able to acquire water and fresh food in the presence of high environmental temperatures that can promote ectotherm activity. However, extremely high temperatures and below average rainfall may also limit foraging opportunities due to risks of overheating and predation while gathering scarce food. Karoo Dwarf Tortoises (Chersobius boulengeri) inhabit an arid region in South Africa where most rains fall around austral summer (October-May). We used focal-animal observations and instantaneous recording to assess their behavioral patterns. Despite relatively high rainfall and available plant growth, Karoo Dwarf Tortoises spent approximately 80-90% of their time in retreats. Whereas activity (behavior outside retreats) in the spring was unrelated to time of the day, possibly due to moderate ambient temperatures, activity in the summer was restricted to the afternoon and evening, when tortoises walked and scanned for food and retreats, and fed only 11 min/d on average. In summer, body temperature of tortoises within retreats was positively associated with retreat temperatures, but tortoises appeared to thermoregulate using bodily postures and possibly other means. We suggest that Karoo Dwarf Tortoises mitigate predation risks by maintaining a low level of activity and thermoregulating within retreats. The short feeding time of Karoo Dwarf Tortoises compared to other tortoise taxa may result in slow growth and reproductive rates, which might in turn affect population resilience and conservation needs of this endangered species.
... seen ( Figure 1) because of their small body size (max. length 110 mm; Boycott and Bourquin 2000), cryptic coloration (Boycott 1989), frequent inactivity (i.e., spending up to 90% of daytime in retreats; Loehr et al. 2021), and rarity. From 2005 until 2021, only 1 population was identified during surveys at 40 sites where the species had been located in the past (Hofmeyr et al. 2018b;J.-P. ...
... We estimated apparent survival (φ) and recapture probabilities (p) for the open tortoise population at the core of the study site based on our searches in February-March 2018, 2019, and 2020 using the Cormack-Jolly-Seber (CJS) formulation in Program MARK (version 9.0; White and Burnham 1999). We were unable to model time-dependence because of insufficient data, and assumed constant p because our standardized methodology (all field personnel received the same training and evaluation) did not rely on tortoise activity (Karoo dwarf tortoises spend most of daytime in retreats; Loehr et al. 2021), and sampling effort was similar among years. We expected apparent survival to depend on SV and sex, so we used φ(SV × sex) p(.) as our starting model. ...
Inconspicuous, secretive, or sparsely distributed species receive relatively little research attention, potentially leading to uncertainty about their status and lack of efforts to conserve them. Karoo dwarf tortoises (Chersobius boulengeri) are endemic to South Africa, spend most of the time in retreats at remote arid locations, and are seldom seen. We conducted a 3‐year (2018–2020) mark‐recapture study to investigate the size and structure of the only Karoo dwarf tortoise population currently known to exist. The population in the 16‐ha core of our study site consisted primarily of adult males and females, at a density of 3.3 individuals/ha. Many individuals had severely worn shells and appeared old. Small individuals (straight carapace length <65 mm) represented just 8% of the population and were mostly recent hatchlings. Overall, tortoises had high estimated survival rates (0.77–0.95; lower 95% confidence limit for the smallest tortoises was 0.16), despite a 15‐month drought. The lack of small individuals may reflect low levels of recruitment and population decline. Predation by corvids was an obvious threat to all size classes. We estimated that the local population across the 250‐ha study area was 800–900 males and females, and recommend precautionary conservation measures focused on reducing human‐subsidized avian predation. In South Africa, the only Karoo dwarf tortoise population currently known to exist comprised mostly adults, many of which appeared old, whereas juveniles were almost absent. To improve recruitment into adult stages and counter projected population decline, removal of potential nesting sites, food and water sources that are attractions for predatory corvids is recommended.
The predation of young tortoise is considered a major cause of mortality for many tortoise species. The predation by common ravens has been identified as being responsible for significant decreases in tortoise populations. Mediterranean spur-thighed tortoise hatchlings and juveniles in Maamora forest (Morocco) were studied in order to describe the size/age class predation of common ravens on young tortoises and infer the drivers of predation risk and population abundance. The results showed a high level of predation on young tortoises (<75 mm carapace length) attributed to ravens in areas with low vegetation cover, representing 100% of the cases of mortality (n = 147), but it was moderate in covered areas (n = 19), representing 12–27%. The population structure of living juveniles differed significantly between covered and uncovered areas, thus suggesting that raven predation might modify juvenile population structure. Finally, N-mixture models showed a positive relationship between (i) bare cover and tortoise detectability that is only evidenced when the plot is far from a perch and (ii) population abundance and shrub species-richness, being higher in uncovered areas. Our results improve the knowledge on predation and survival on this critical stage in life, which is crucial for the conservation of the Mediterranean spur-thighed tortoise.
The high level of phenotypic diversity in southern African tent tortoises (Psammobates tentorius complex) has for decades prevented systematists from developing a stable taxonomy for the group. Here, we used a comprehensive DNA sequence dataset (mtDNA: Cytb, ND4, ND4 adjacent tRNA-His, and tRNA-Ser, 12S, 16S; and nDNA: PRLR gene) of 455 specimens, and the latest phylogenetic and species delimitation analytical procedures, to unravel the long-standing P. tentorius complex systematic puzzle. Our results for mtDNA and nDNA were incongruent, with the poorly supported nDNA phylogeny differentiating the three recognized subspecies, and showing potential hybridization in some regions. In contrast, the concatenated mtDNA phylogeny identified seven operational taxonomic units, with strong support. Clades 1, 4, 5, and 7 corresponded to tortoises identified as P. t. tentorius, clade 3 to P. t. tri-meni, and clades 2 and 6 to P. t. verroxii. Our analyses showed conflicting topolo-gies for the placement of C6 (P. t. verroxii north of the Orange River), with stronger support for it being sister to C2 + C3 than to the other clades. Clades 1, 2, and 6 had significantly higher genetic diversity than clades 3, 4, 5, and 7, perhaps because these clades inhabit substantially larger areas. The potential for future cladogenic radiations seems high in C1 and C6, particularly in C6 for which the within-clade diversification level was highest. Further research involving microsatellite DNA, phy-logeographic evaluations, and morphological variation among clades is crucial for understanding the adaptive radiation of the P. tentorius complex and for modifying their taxonomy.
Abstract The regulation of body temperature (thermoregulation) and of water balance (defined here as hydroregulation) are key processes underlying ecological and evolutionary responses to climate fluctuations in wild animal populations. In terrestrial (or semiterrestrial) ectotherms, thermoregulation and hydroregulation closely interact and combined temperature and water constraints should directly influence individual performances. Although comparative physiologists traditionally investigate jointly water and temperature regulation, the ecological and evolutionary implications of these coupled processes have so far mostly been studied independently. Here, we revisit the concept of thermo‐hydroregulation to address the functional integration of body temperature and water balance regulation in terrestrial ectotherms. We demonstrate how thermo‐hydroregulation provides a framework to investigate functional adaptations to joint environmental variation in temperature and water availability, and potential physiological and/or behavioral conflicts between thermoregulation and hydroregulation. We extend the classical cost–benefit model of thermoregulation in ectotherms to highlight the adaptive evolution of optimal thermo‐hydroregulation strategies. Critical gaps in the parameterization of this conceptual optimality model and guidelines for future empirical research are discussed. We show that studies of thermo‐hydroregulation refine our mechanistic understanding of physiological and behavioral plasticity, and of the fundamental niche of the species. This is illustrated with relevant and recent examples of space use and dispersal, resource‐based trade‐offs, and life‐history tactics in insects, amphibians, and nonavian reptiles.
Of the 356 species of turtles worldwide, approximately 61% are threatened or already extinct. Turtles are among the most threatened of the major groups of vertebrates, in general, more so than birds, mammals, fishes or even the much besieged amphibians. Reasons for the dire situation of turtles worldwide include the familiar list of impacts to other species including habitat destruction, unsustainable overexploitation for pets and food, and climate change (many turtles have environmental sex determination). Two notable characteristics of pre-Anthropocene turtles were their massive population sizes and correspondingly high biomasses, the latter among the highest values (over 855 kilograms per hectare) ever reported for animals. As a result of their numerical dominance, turtles have played important roles as significant bioturbators of soils, infaunal miners of sea floors, dispersers and germination enhancers of seeds, nutrient cyclers, and consumers. The collapse of turtle populations on a global scale has greatly diminished their ecological roles.
We used ultrasonography and radiography to evaluate whether body size (BS), body condition (BC) and environmental factors influenced reproductive investments by Chersobius signatus in September 2000 and from September 2003 to November 2004. Follicle volume increased with increasing BS, BC and rainfall but decreased with increasing temperature. Egg volume averaged 8.8% of body volume, increased allometrically with BS, but was not affected by BC, rainfall or temperature and did not differ between first and second clutches. Most females produced at least one single‐egg clutch per year, even when BC was low. Many females produced two, possibly more, clutches but some forfeited reproduction in some years. Females often retained large follicles from one season to the next with 18%–71% carrying pre‐ovulatory sized follicles in different times of the year. Increasing temperatures likely influenced timing of first ovulations in August while decreasing water and food availability, linked with high temperatures, possibly terminated ovulations by November. The gravid season was short compared to other winter‐rainfall tortoises in South Africa. We propose that the females' small bodies may constrain investments in eggs, resulting in low fecundity, which is countered by producing large hatchlings with high survivorship in an arid habitat.
We studied the temperature relations of wild and zoo Aldabra giant tortoises (Aldabrachelys gigantea) focusing on: 1) the relationship between environmental temperature and tortoise activity patterns (n=8 wild individuals), and 2) on tortoise body temperature fluctuations, including how their core and external body temperatures vary in relation to different environmental temperature ranges (seasons; n=4 wild, and n=5 zoo individuals). In addition, we surveyed the literature to review the effect of body mass on core body temperature range in relation to environmental temperature in the Testudinidae. Diurnal activity of tortoises was bimodally distributed, and influenced by environmental temperature and season. The mean air temperature at which activity is maximised was 27.9˚C, with a range of 25.8–31.7˚C. Furthermore, air temperature predicted changes in the core body temperature better than did mass, and only during the coldest trial did tortoises with higher mass show more stable temperatures. Our results, together with the overall Testudinidae overview, suggest that, once variation in environmental temperature has been taken into account, there is little effect of mass on the temperature stability of tortoises. Moreover, the presence of thermal inertia in an individual tortoise depends on the environmental temperatures, and we found no evidence for inertial homeothermy. Finally, patterns of core and external body temperatures in comparison to environmental temperatures suggest that Aldabra giant tortoises act as mixed conformer-regulators. Our study provides a baseline to manage the thermal environment of wild and rewilded populations of an important island ecosystem engineer species in an era of climate change.
Ectotherms frequently thermoregulate behaviorally to improve physiological processes such as digestion and growth, but basking and other thermoregulatory activities can also increase predation risk. Organismal and environmental characteristics can, in some species, influence predation risk associated with thermoregulation and thereby relax or tighten constraints on thermoregulatory behavior, physiological performance , and, ultimately, life history traits. Providing one of the first such investigations in turtles, we examine whether behavior and thermal environment modulate a thermoregulation-predation risk trade-off in juvenile gopher tortoises (Gopherus polyphemus). Young gopher tortoises experience very high predation pressure, and their declining species faces many challenges, including human-induced increased shading of its environment. We hypothesized that in response to simulated predator approach, basking hatchling and juvenile tortoises would: (i) hide inside burrows; (ii) hide for shorter durations in cooler burrows presumably due to greater constraints on physiological performance; and (iii) spend greater time at the surface following disturbance in cool environments because individuals would need to bask more to maintain preferred body temperatures. Basking tortoises always hid inside burrows when approached and exhibited very long flight initiation distances (x = 45 m) that increased with age/size. Individuals fled into burrows even when it was not possible for them to see the approaching researcher, suggesting the use of vibrations (aerial/ground) to detect potential predators and a possible antipredator function for exceptionally large otoliths characteristic of the species. Tortoises hid for short durations (x = 18.3 min), especially in cool burrows, suggesting that they optimize hiding responses to balance physiological costs and antipredator benefits. Additionally, surface activity following disturbance consisted primarily of basking and correlated negatively with burrow temperature. These findings suggest that thermal environment influences predation risk in ectotherms whose surface activity is driven primarily by thermoregulatory requirements and highlight potential benefits of warm, well-insolated habitats, such as endangered longleaf pine (Pinus palustris) ecosystems, for juvenile gopher tortoises. K E Y W O R D S basking, flight initiation distance, Gopherus polyphemus, otolith, thermal ecology, turtle
This is our 8th edition of an annotated checklist of all recognized and named taxa of the world’s modern chelonian fauna, documenting recent changes and controversies in nomenclature through early 2017, and including all primary synonyms, updated from 7 previous checklists (Turtle Taxonomy Working Group 2007b, 2009, 2010, 2011, 2012, 2014; Rhodin et al. 2008). We provide an updated comprehensive listing of taxonomy, names, and conservation status of all turtles and tortoises of the world, including detailed distribution maps. We strive to record the most recent justified taxonomic assignment of taxa in a hierarchical framework, providing annotations, including alternative possible arrangements, for some proposed changes. We provide common English names and detailed distributional data for all taxa, listing occurrence by countries and many smaller political or geographic subunits (states or regions), including indications of native, extirpated, and introduced (modern or prehistoric) populations. We include current published and draft IUCN Red List status assessments for all turtles, as well as CITES listings. The diversity of turtles and tortoises in the world that has existed in modern times (since 1500 AD) and currently generally recognized as distinct and included in this checklist, now consists of 356 species. Of these, 60 are polytypic, representing 122 additional recognized subspecies, or 478 total taxa of modern turtles and tortoises. Of these, 7 species and 3 subspecies, or 10 taxa (2.1%), have gone extinct. As of the current IUCN 2017 Red List, 148 turtle species (60.4% of 245 species listed, 41.6% of all 356 recognized modern species) are officially regarded as globally Threatened (Critically Endangered [CR], Endangered [EN], or Vulnerable [VU]). We record additional draft Red List assessments by the IUCN Tortoise and Freshwater Turtle Specialist Group (TFTSG) of previously “unevaluated” species, and updated draft re-assessments of previously listed species, allowing us to evaluate the overall current threat levels for all turtles and tortoises. Of the 356 total species of turtles and tortoises, 114 (32.0%) are CR or EN, 179 (50.3%) are Threatened (CR, EN, or VU), and 186 (52.2%) are Threatened or Extinct. If we provisionally adjust for predicted threat rates of Data Deficient and Not Evaluated species, then ca. 59% of all extant turtles are Threatened. These numbers and percentages of Threatened species have increased since our last checklist. Turtles are among the most threatened of the major groups of vertebrates, in general more than birds, mammals, cartilaginous or bony fishes, or amphibians.
Behavioural frequencies and time budgets for male and female Chersina angulata were recorded in spring, September 2004. The daily activity of the population was 10.51 ± 0.42 h (mean ± CI), but individual males and females were in the open for 2.57 ± 1.12 h and 1.58 ± 1.44 h, respectively. Both sexes spent nearly 3.5 h per day basking with 90% of the basking time in the cover of sparse vegetation. Feeding and walking were the most frequent active behaviours. The activity pattern of the sexes differed between 10:00 and 12:00 in the morning when males spent more time out of cover on active behaviours, particularly feeding, than females did. The daily activity pattern of the population peaked between 10:00 and 12:00, reflecting the activity pattern of the more active sex, males. Females were out of cover, active, and feeding at higher temperatures than were males. The high temperatures probably allowed females to complete activities more efficiently, reducing their exposure time and predation risk. Although fighting and mating represented small fractions of male time budgets, the extended physical presence through walking and feeding may help males to establish dominance hierarchies and enhance mating success.
We subjected neonate Desert Tortoises (Gopherus agassizii) that hatched inside fenced, predator-resistant field enclosures containing natural vegetation to either a natural rainfall regime or a regime of natural rainfall plus irrigation (supplemental precipitation) over a five-year period, to test the hypothesis that mimicking an above-average rainfall regime in years of average or low natural rainfall will improve rates of survival and growth. We also tested the hypothesis that survivorship of released 1-yr olds will be high, due to a decline in predation susceptibility once the vulnerable nesting and hatchling phases are completed. Survivorship inside the enclosures during the first year of life was high (averaging 90%) in both groups, even during a record low rainfall year, but growth rates were always substantially higher (2 to 16-[1] greater) in rain-supplemented juveniles. Body condition index (CI) measurements indicated that first-year juveniles without added rain were able to maintain body conditions similar to rain-supplemented juveniles during two average rainfall years, but not during a drought year. Older juveniles without added rain died during the latter part of the 16 mo drought, suggesting that the high drought survivorship of first-year non-supplemented juveniles may be related to the yolk they carried after hatching, along with possible behavioral and physiological diffrerences. Nearly all yearlings that were set free (in autumn) were dead within 6 mo, regardless of whether they had supplemental rain or not during their first year inside enclosures, and regardless of whether they were released near the head-start enclosures or a kilometer away. The main cause of mortality was predation, primarily by ravens. The poor survival of released yearling tortoises and the drought-induced death of nearly all older captive juveniles raised without added precipitation lead us to recommend that rain supplementation and delayed release be incorporated in the protocol for head-starting Desert Tortoises.
Documented retreat-site selection during summer by gravid Thamnophis elegans at Eagle Lake, NE California. Snakes usually retreated under rocks of intermediate thickness (20-30 cm). Rock thickness had the dominant effect on potential Tb available to snakes and in turn on thermal physiology. Snakes selecting thin rocks (<20 cm thick) or very shallow burrows would die of heat stress in midafternoon and would have strongly impaired physiological capacity from cold at night. Snakes selecting very thick rocks (>40 cm thick) or remaining at the bottom of deep burrows would not warm to Tb in their preferred range. Snakes selecting burrows or intermediate-thickness rocks may be able to have either the highest energy gain or the lowest overall metabolic rate, depending on the particular Tb they select. -from Authors
We released and monitored 53 juvenile Agassiz's Desert Tortoises (Gopherus agassizii), aged two to 15 y, that were hatched and head-started inside predator-resistant field enclosures. We set free these tortoises under a variety of conditions to evaluate effects of release distance, season of release, and age/body size on homing behavior and survivorship. Some juveniles moved large distances following release, but homing itself was undetectable. The lack of homing behaviors was likely due to release distance and selection of release sites out of the line-of-sight of natal enclosures. The use of small halfway-house enclosures to accustom some relocated juveniles to release sites for four months before release (i.e. " soft release ") had no effect on subsequent movements or survivorship during the first year following release. Survivorship was not affected by distance of release from natal enclosures, which ranged from 546 m to 1.4 km. Survivorship through one year was similar for juveniles released in spring or autumn. After two years, most small juveniles had been killed by predators, but survivorship increased with body size and age. Juveniles over approximately 100 mm MCL (midline carapace length) and nine years of age when released exhibited high survivorship. However, following a long drought the previous two years, predation by Coyotes (Canis latrans) was heavy on these larger juveniles in the third year after release. Thus, survivorship after three years was relatively low (34%) with the youngest, smallest cohort (two years old when released) exhibiting the lowest survivorship (4%). We recommend releasing head-started tortoises after they attain a body size of larger than 100 mm MCL and selecting release sites at least 546 m from enclosures.
We examined the activity patterns of angulate tortoises, Chersina angulata, on Dassen Island, South Africa, during spring, summer and winter. During typical spring and summer days, C. angulata exhibited a bimodal pattern of activity, with activity being suspended around midday. However, during winter, and on cool and wet summer days, activity was unimodal, peaking around midday. Temporal patterns of activity appeared strongly influenced by environmental factors, particularly temperature. Chersina angulata were never observed to be active at air temperatures below 14.0°C or above 28.7°C. Activity levels were lowest during summer, when animals were active only 1.75% of the time. Brief appearances of available water (usually from condensing fog) during summer, however, resulted in a dramatic increase in activity levels. Males were significantly more active than females during summer and spring, but no significant difference was found between the sexes in winter. Results of this study are principally contrasted with those from a study conducted near the easternmost range of C. angulata, an area that experiences a markedly different climate.
The leopard tortoise (Stigmochelys pardalis) is the largest of southern Africa's 13 tortoise species, and occurs in a variety of habitats from arid and semi-arid areas to mesic grassveld, savanna and bushveld. Seasonal activity patterns of S. pardalis were investigated as a function of rainfall, sex, time of day, temperature and time after sunrise on farmland in the semi-arid Nama-Karoo, South Africa. We predicted that because of seasonal rainfall, and subsequent increase in the food available, activity patterns of leopard tortoises would vary greatly among seasons, but that the primary constraint on activity levels within a season would be ambient temperature. Type of activity, time of day that the activity was performed, and the frequency that each activity was performed, differed among seasons. There was no overall seasonal difference in the level of activity with sex, but in certain seasons and with regard to specific activities, there were significant differences between the sexes. Diel activity was primarily bimodal in summer and autumn, and unimodal in winter and spring, with nonthermoregulatory activities being performed primarily in the afternoon. There was a positive correlation between number of tortoises caught and rainfall per season, but activity levels and number of tortoises walking and feeding was not correlated with seasonal rainfall. Leopard tortoise activity behaviours responded to ambient temperature, but results indicate that activity is also initiated by the time since sunrise.
In some herbivores, home range size is correlated negatively with the primary productivity of their environments. Although the South African Succulent Karoo is arid, it receives relatively predictable winter rainfall and therefore relatively predictable primary production toward the following spring. Lush springs may alternate with occasional drought years. Over five spring seasons with variable rainfall patterns, I studied home range size in 21 male and 31 female Namaqualand Speckled Tortoises (Homopus signatus) in the Succulent Karoo using opportunistic sightings, thread-trailing, and radio telemetry. In addition, daily movement distances were determined for 8 males and 28 females with the use of thread-trailing over three spring seasons. The average home range size for H. signatus was 3,470 m2, much smaller than home ranges for tortoises in other arid regions. Home range size did not differ between the sexes, but was negatively correlated with rainfall. Daily movement distances were also similar between sexes, but tortoises moved greater distances in a relatively warm spring. It appears that temporary mesic conditions in spring enable male and female H. signatus to meet their nutritional needs in a relatively small area, and the flexible response to variable environmental conditions may enhance tortoise survival during drought years.
Abstract From 2011-2012, we studied a gopher tortoise (Gopherus polyphemus) population with a historically high prevalence of antibodies to Mycoplasma agassizii to assess long-term effects of upper-respiratory-tract disease (URTD) on tortoise behavior. We radiotracked 30 adult tortoises (16 males, 14 females) from a long-term study site with the use of mark-recapture methods to determine site fidelity and to compare home-range size to that of a study in 1997. An additional 10 tortoises (six males, four females) with severe clinical signs of URTD from elsewhere in the study area were radiotracked and compared to tortoises that were asymptomatic or had only mild clinical signs. We also monitored thermoregulatory behavior of tortoises with the use of data loggers affixed to the carapace. There was no significant difference in home-range size between the asymptomatic tortoises and those with mild symptoms (mean 1.38 ha). Home ranges of tortoises with severe URTD were significantly larger than asymptomatic or mildly affected tortoises. Tortoises with severe clinical signs moved long distances over short periods, contradicting a hypothesis that chronically infected tortoises are less likely to emigrate. Prevalence of M. agassizii antibodies was similar among the three groups (98% overall), but prevalence of antibody to a second pathogen associated with URTD, Mycobacterium testudineum, was lower in the asymptomatic (n = 14, 7%) and mild-symptoms (n = 7, 14%) groups than the severe-symptoms group (n = 8, 50%). Variation in the average carapacial temperatures of tortoises with severe URTD was significantly different from carapacial temperatures of mild and asymptomatic tortoises, suggesting differences in thermoregulatory behavior of severely ill tortoises. Our 15-yr recapture data suggest that, despite high prevalence of M. agassizii, population density has not decreased over time. However, emigration, especially of tortoises with severe clinical disease, may play an important role in dispersal and persistence of pathogens.
Abstract Upper respiratory tract disease (URTD) caused by Mycoplasma agassizii is considered a threat to desert tortoise populations that should be mitigated as part of the recovery of the species. Clinical signs can be intermittent and include serous or mucoid nasal discharge and respiratory difficulty when nares are occluded. This nasal congestion may result in a loss of the olfactory sense. Turtles are known to use olfaction to identify food items, predators, and conspecifics; therefore, it is likely that URTD affects not only their physical well-being but also their behavior and ability to perform necessary functions in the wild. To determine more specifically the impact nasal discharge might have on free-ranging tortoises (Gopherus agassizii), we compared the responses of tortoises with and without nasal discharge and both positive and negative for M. agassizii antibodies to a visually hidden olfactory food stimulus and an empty control. We found that nasal discharge did reduce sense of smell and hence the ability to locate food. Our study also showed that moderate chronic nasal discharge in the absence of other clinical signs did not affect appetite in desert tortoises.
While demographic changes in short-lived species may be observed relatively quickly in response to climate changes, measuring population responses of long-lived species requires long-term studies that are not always available. We analyzed data from a population of threatened Agassiz’s desert tortoises (Gopherus agassizii) at a 2.59 km2 study plot in the Sonoran Desert ecosystem of Joshua Tree National Park, California, USA from 1978 to 2012 to examine variation in apparent survival and demography in this long-lived species. Transect-based, mark-recapture surveys were conducted in 10 of those years to locate living and dead tortoises. Previous modeling suggested that this area would become unsuitable as tortoise habitat under a warming and drying climate scenario. Estimated adult population size declined greatly from 1996 to 2012. The population appeared to have high apparent survival from 1978 to 1996 but apparent survival decreased from 1997 to 2002, concurrent with persistent drought. The best model relating apparent survivorship of tortoises ⩾18 cm over time was based on a three year moving average of estimated winter precipitation. The postures and positions of a majority of dead tortoises found in 2012 were consistent with death by dehydration and starvation. Some live and many dead tortoises found in 2012 showed signs of predation or scavenging by mammalian carnivores. Coyote (Canis latrans) scats and other evidence from the site confirmed their role as tortoise predators and scavengers. Predation rates may be exacerbated by drought if carnivores switch from preferred mammalian prey to tortoises during dry years. Climate modeling suggests that the region will be subjected to even longer duration droughts in the future and that the plot may become unsuitable for continued tortoise survival. Our results showing wide fluctuations in apparent survival and decreasing tortoise density over time may be early signals of that possible outcome.
Common Ravens (Corvus corax) in the Mojave Desert of California, USA are subsidized by anthropogenic resources. Large numbers of nonbreeding ravens are at- tracted to human developments and thus are spatially restricted, whereas breeding ravens are distributed more evenly throughout the area. We investigated whether the spatial dis- tribution of risk of predation by ravens to juveniles of the threatened desert tortoise ( Go- pherus agassizii) was determined by the spatial distribution of (1) nonbreeding ravens at human developments (leading to ''spillover'' predation) or (2) breeding individuals through- out developed and undeveloped areas (leading to ''hyperpredation''). Predation risk, mea- sured using styrofoam models of juvenile desert tortoises, was high near places attracting large numbers of nonbreeding ravens, near successful nests, and far from successful nests when large numbers of nonbreeding ravens were present. Patterns consistent with both ''spillover'' predation and ''hyperpredation'' were thus observed, attributed to the non- breeding and breeding segments of the population, respectively. Furthermore, because lo- cations of successful nests changed almost annually, consistent low-predation refugia for juvenile desert tortoises were nearly nonexistent. Consequently, anthropogenic resources for ravens could indirectly lead to the suppression, decline, or even extinction of desert tortoise populations.
The way of life of amphibians and reptiles, in contrast to that of birds and mammals, is based on low energy flow. Many of the morphological and physiological characteristics of ectothermal tetrapods that are normally considered to be primitive are in fact adaptations that facilitate a life of low energy demand. Their modest energy requirements allow amphibians and reptiles to exploit various adaptive zones unavailable to birds and mammals. Small body size is the most important of these; 80% of all lizard species and 90% of salamanders have adult body masses less than those of small birds and mammals. An elongate body form, a widespread and successful morphotype among amphibians and reptiles, is energetically unfeasible for endotherms. Amphibians and reptiles also are better suited than birds and mammals to ecological situations characterized by periodic shortages of food, water, or oxygen. At the ecosystem level, the most important consequence of the low energy requirements of amphibians and reptiles is their efficiency of biomass production, which greatly exceeds that of birds and mammals. Their secondary production makes amphibians and reptiles as important as birds or mammals in terrestrial ecosystems.
In arid environments, limited resources may compromise allocations to reproduction, while unpredictable rainfall may confound the timing of reproductive events. Here we evaluate how female tent tortoises (Psammobates tentorius tentorius) allocate resources to reproduction while coping with low and unpredictable rainfall. Vitellogenesis in tent tortoises started in autumn, when rainfall was highest and most predictable. Tent tortoises required a high body condition and large follicles to ovulate their first clutches, but a temperature threshold for ovulation may have prevented breeding during the winter months. Females produced small clutches (1.78±0.63; 1–3 eggs) but produced several clutches (3.7±1.6; 1–6 clutches) from spring to late autumn. Clutch frequency (CF) was correlated to body condition, and CF rather than clutch size (CS) determined annual fecundity. Although body size influenced egg size, body size had no effect on CS, CF or fecundity. There was no trade-off between relative egg size and CS. The small clutches of tent tortoises limit the risks of producing too many offspring at one time, a scenario that may leave females without resources for their own needs. When clutches are small, an extended breeding season enhances reproductive output by providing females more time and opportunities to leave offspring.
We studied the ecology of Psammobates oculifer over 13 months near Kimberley, South Africa, to ascertain if the population's life history traits conform to chelonian patterns in arid environments. Capture rates were highest in spring and lowest in winter when environmental conditions were respectively most and least favourable for tortoise activity. Body condition did not change from autumn to spring, but reached lower values during the summer drought. Capture effort averaged 5 hours/tortoise, which corresponds closely to that of species with low population densities in arid regions. Population size structure was skewed towards adults, indicative of low recruitment and/or low juvenile survivorship. Females were larger and heavier than males, confirming sexual dimorphism in this species. Body size of cohorts scaled to annuli counts, indicating a close correspondence between body size and age. Telemetered adults deposited one or no growth ring in the year of study; consequently, annuli counts could underestimate adult age. Regression analyses showed that male and female growth rates did not differ, but males matured at a smaller size and younger age than females. The smallest male showing reproductive behaviour had 12 annuli and a shell volume of 157 cm3, while similar measures for females were 14 annuli and 185 cm3. The sex ratio of the population did not differ from 1:1 but the bias towards males in spring, and towards females in autumn, indicates that studies limited to particular seasons can misrepresent life history traits of populations. We concluded that the life history of P. oculifer conforms to chelonian patterns in arid regions.
Psammobates geometricus has a limited distribution in the southwestern Cape, South Africa, where it occurs in small habitat fragments separated by agricultural and urban developments. Space use and its determining factors thus represent critical information for the effective conservation of this species. We used radiotelemetry and thread-trailing to study the movement patterns and space use of geometric tortoises over an annual cycle, and estimated home ranges with minimum convex polygons and fixed-kernel methods. Home range size of geometric tortoises showed large inter-individual variation, and for females, a positive relationship to body size. Females, the larger sex, had larger home ranges and mean daily displacements than males had. Female space use was high through most of the year, except in the wet season, when food was abundant, temperatures relatively low, and soft soils allowed easy nesting. Males used more space and displaced further in the non-nesting than nesting season, perhaps to optimise mating opportunities in the non-nesting season before females ovulate. Home ranges were more elongate and overlapped more in fallow fields than in natural vegetation, suggesting that habitat degradation alters home range structure. The space geometric tortoises used for their annual activities ranged from 1.02 to 44.85 ha. The large home ranges and effects of habitat degradation should influence the size of reserves, and the destiny of geometric tortoises in small habitat fragments.
Five thread-trailed hingeback tortoises (Kinixys spekii) were observed continuously for a total of 260 hr on four hot days (at intervals of 1-2 weeks) in the rainy season. Activity occurred in all hours from 06.00-19.00 hr; the population daily activity period was 13 hr. The daily duration of surface activity of individuals was on average 8.2 hr day-1 between first and last daily movement. The daily time budget included 1.95 hr locomotion, 0.86 hr feeding, and 5.2 hr stationary above ground (including long periods in indistinct surface refuges). There were no significant differences in total activity between individual tortoises or study days. Combining the daily activity period of the population and data from single sightings would greatly overestimate the amount of time spent active; observations over complete days are necessary for a true time budget. Increasing the number of days of observation of each individual decreased the variability of the data only slightly.
Homopus s. signatus inhabits an arid region with unpredictable winter rainfall. To help understand the species' response to variation in rainfall, we measured spring body condition (BC) over five years and seasonal BC over 15 months. Total rainfall influenced BC but the timing and frequency of rain were important. In two successive years with low rainfall, BC was significantly lower in the year when most rain fell in late winter, than in the year with more frequent showers. Females tended to have a higher spring BC than males, perhaps because females nest in spring. The low spring BC of juveniles, relative to adults, may be ascribed to the juvenile shell that is less ossified than that of adults, giving juveniles a lower body mass to shell volume ratio. Seasonal fluctuations in BC were substantial; female body mass declined by 18% from summer to mid-winter but increased after the first rains to high values in spring. We compared two BC indices, the ratio of body mass to 1) carapace length and 2) shell volume. The latter index varied more among years and provides a ratio that closely approximates the body density of tortoises.
Although many tortoise species inhabit drought-prone regions with potentially limiting resources, these species have long, iteroparous lives. To assess reproductive responses to variation in rainfall, and to understand interactions among egg size, body size, body condition, and rainfall, we studied egg production in the Namaqualand Speckled Padloper, Homopus signatus signatus, in Springbok, South Africa, in five consecutive spring seasons. Annual rainfall was low and varied substantially (131-226 mm). The percentage of females that were gravid (36-75%) differed among years and correlated with the amount of rain in the months prior to nesting. Gravid females had a higher body condition than that of non-gravid females, presumably because individual variation in resource acquisition caused some females to forfeit reproduction. The body condition of gravid and non-gravid females differed among years and was lowest in the year of lowest rainfall. In most years, egg size correlated to female size, but neither female size nor egg size differed among years. Egg size did not significantly correlate to maternal body size in dry years, when other determinants, such as body condition, seemed to outweigh the effect of body size. Egg volume represented up to 11.9% of female shell volume. The adult shell is somewhat flexible dorso-ventrally, which may help females accommodate the large egg, as indicated by the larger shell height and volume of gravid compared to non-gravid females. Large eggs may be advantageous for H. s. signatus, as larger hatchlings may survive better in arid environments. Since the range of H. s. signatus is threatened with aridification, the effects of drought on egg production may seriously challenge the long-term survival of populations
This paper provides the first account of chacma baboons (Papio cynocephalus ursinus) feeding on the angulate tortoise (Chersina angulata) at De Hoop Nature Reserve, Western Cape Province, South Africa. Six cases of actual predation and in cases of attempted predation were observed during the 13-month study. Predated tortoises were smaller than those subjected to attempted predation, with both of these groups being smaller than the population mean. Predation by baboons may be an important factor limiting tortoise population sizes at De Hoop Nature Reserve.
There is little quantitative information regarding the two subspecies of the world's smallest tortoise, Homopus signatus. To help characterise the northern subspecies H. s. signatus and evaluate the development of shell size and colour patterns, we measured shell characteristics of wild juvenile, male and female H. s. signatus. When scaling shell dimensions on carapace length, male shell size represented an extension of juvenile shell size for shell height, width and volume, but not plastron length. The slope of plastron length scaled on carapace length was smaller for males than for juveniles or females, suggesting differential growth of the plastron. The smaller male plastron translates to large shell openings, perhaps to improve locomotion and to facilitate tail movement during copulation. Conversely, the slope of female shell height, width and volume (scaled) was larger than for juveniles or males, presumably providing more space to accommodate follicles and the large egg. Serration of the marginal scutes and shell colour changed with body size, and shell colour pattern differed between sexes. The carapace was darker at intermediate body sizes (large juveniles and small adults); the increase in dark appearance resulted primarily from widening of the dark pigment band around scute margins as growth laminae were added. Both the lighter shell colour and reduced serration of large adults may be due to shell wear; large adults may produce less dark pigment, and older laminae disappear due to flaking or peeling. Females had a darker overall colour, more rays and fewer speckles than did males. This dimorphism may relate to thermoregulation, gamete protection or intraspecific communication. Both male and female patterns may confer crypsis if the sexes use microhabitats differently.
Several climate models predict that the western Succulent Karoo in South Africa will aridi-fy. This region includes the range of the smallest tortoise, Homopus signatus signatus. Although the effects of rainfall on the physiology and ecology of H. s. signatus received attention in recent years, the results of these studies have not been integrated to facilitate conservation planning. Here we evaluate the importance of body size and the responses of H. s. signatus to rainfall variation to make recommendations for the taxon's conservation. The small body of H. s. signatus offers one solution to its habitat of low primary productivity and rocky slopes. Nevertheless, female fecundity and egg size increase with female size, and large eggs result in large hatchlings capable of surviving their harsh environment. Females accumulate nutrients in the rainfall season, winter, but also in the dry season, to enable the production of large eggs. Egg production decreases during drought, although some females continue to channel resources to reproduction, apparently at the cost of their own growth. Reduced fecundity and growth, a result of aridification, would likely lower the production of large eggs and hatchlings. Therefore, conservation measures that reduce the mortality of large females may aid population sus-tainability. Because egg and hatchling size might drop below a minimum viable size in an aridified environment, H. s. signatus conservation would benefit from the development of suitable habitat corridors to enable tortoise movements to regions that will receive sufficient rainfall in the future.
Homopus signatus signatus is a small tortoise that is endemic to arid northwestern South Africa, where its range is threatened by aridification. It is not clear how the taxon will respond to an aridifying habitat, emphasizing the need for population monitoring. I conducted a mark–recapture study over 5 years with above- and below-average rainfall to analyze population structure, abundance, apparent survival, and effects of rainfall. The population had a continuous size-class distribution and contained similar frequencies of males, females, and juveniles, indicative of a viable population with reproduction and recruitment into adulthood. It consisted of transient and resident individuals, with an abundance of 16–21 resident tortoises/ha. High spring activity levels appeared to result in high (0.35–0.94) recapture rates, which differed among years as a result of different search efforts. Apparent annual survival rates of residents depended on group and increased with shell volume, ranging from 0.74 to 0.99. Although females were the larger sex, their apparent survival tended to be slightly lower than male survival, possibly because of high female activity to accumulate nutrients for the production of large eggs. Rainfall had little effect on apparent survival, suggesting that physiological responses to drought exploited by H. s. signatus are effective in helping them survive. Consequently, mortality during current, periodic droughts does not seem to oppose recommended conservation measures to improve adult survival and delay detrimental effects of aridification on growth and reproduction.
Survival of adult Desert Tortoises (Gopherus agassizii) appears related to site-specific variation in precipitation and productivity of annual plants. We studied adult tortoise survival rates at two closely situated, but physiographically different, sites in the eastern Mojave Desert over a nine-year period (spring 1992 to spring 2001). Survival rates were initially derived from population surveys conducted over a three-year period and by radio-telemetry monitoring over a seven-year period beginning in 1994. After a period of initial stability, survival rates on the two sites diverged over the study period, and seven-year survival rates estimated from radio-telemetry monitoring were 0.900 and 0.269, respectively. A die-off in 1996 on the latter site appears to have been triggered by a period of drought, which began in the summer of 1995, coupled with a failure of annual vegetation production in 1996. Depressed survival rates on this site were associated with drought conditions during three of four years. Although the decline had the appearance of an epizootic, there were no clinical signs of disease. Relatively short-term drought, combined with little or no annual biomass, appears to have caused severe reductions in tortoise survival. If periods of drought-induced low survival are common over relatively small areas, then source-sink population dynamics may be an important factor determining tortoise population densities.
Early spring population parameters and activity of wild Namaqualand speckled padlopers (Homopus signatus signatus) were monitored using mark-recapture and thread-trailing methods. Eighty-three tortoises were studied between 22 August and 22 September 2000. The maximum number of scute rings counted was 14, with females appearing to have longer lives than males. Tortoises were encountered on rock slabs, on a rocky hill slope, and frequented adjacent, gently sloping fields with many flowering plants. The frequent use of shallow concealed crevices as hiding places may be, in part, caused by greater availability of such refuges. Single specimens were usually found in refugia, but occasionally male-female pairs were observed. The shelter types identified (rock and spaces under shrubs) were rarely used for more than one night. Activity was unimodal. Activity area and daily movements were lower in H. s. signatus than in other tortoise species, possibly because of their smaller body size, cooler ambient spring temperatures during this study, a short measurement period, smaller habitat requirement of H. s. signatus, or some combination of the four.
Although many species may be vulnerable to changes in climate, forecasting species-level responses can be challenging given the array of physiological, behavioral, and demographic attributes that might be affected. One strategy to improve forecasts is to evaluate how species responded to climatic variation in the past. We used 22 years of capture-recapture data for Sonoran desert tortoises (Gopherus morafkai) collected from 15 locations across their geographic range in Arizona to evaluate how environmental factors affected spatial and temporal variation in survival. Although rates of annual survival were generally high ([Formula: see text] = 0.92), survival of adults decreased with drought severity, especially in portions of their range that were most arid and nearest to cities. In three locations where large numbers of carcasses from marked tortoises were recovered, survival of adults was markedly lower during periods of severe drought ([Formula: see text] = 0.77-0.81) compared to all other periods ([Formula: see text] = 0.93-0.98). Assuming continued levels of dependency of humans on fossil fuels, survival of adult tortoises is predicted to decrease by an average of 3 % during 2035-2060 relative to survival during 1987-2008 in 14 of the 15 populations we studied. This decrease could reduce persistence of tortoise populations, especially in arid portions of their range. Temporal and spatial variation in drought conditions are important determinants of survival in adult desert tortoises.
Turtles and tortoises (chelonians) have been integral components of global ecosystems for about 220 million years and have played important roles in human culture for at least 400,000 years. The chelonian shell is a remarkable evolutionary adaptation, facilitating success in terrestrial, freshwater and marine ecosystems. Today, more than half of the 360 living species and 482 total taxa (species and subspecies combined) are threatened with extinction. This places chelonians among the groups with the highest extinction risk of any sizeable vertebrate group. Turtle populations are declining rapidly due to habitat loss, consumption by humans for food and traditional medicines and collection for the international pet trade. Many taxa could become extinct in this century. Here, we examine survival threats to turtles and tortoises and discuss the interventions that will be needed to prevent widespread extinction in this group in coming decades.
Ectotherms have various means of dealing with low environmental temperatures, but relatively few species have been rigorously investigated. Consequently, we have little information to predict how ectotherm populations might respond to global temperature changes. Tortoises from temperate and subtropical regions often overcome periodically cool conditions by hibernation, but speckled dwarf tortoises (Chersobius signatus) need to remain active to exploit ephemeral resources in their arid winter-rainfall habitat. This study investigated how dwarf tortoises cope with low temperatures in winter and spring, by measuring thermal habitat quality and thermoregulation based on differently-sized operative temperature models in sun, shade, and in deep crevices. Investigations continued in summer and autumn to obtain a year-round picture of thermoregulatory challenges. Although large models (i.e., larger than dwarf tortoises) were expected to have lower operative temperatures than smaller models, due to the former's larger thermal inertia, all model sizes had similar temperatures. Hence, the species' small body size does not appear constrained by obtainable body temperatures in cool seasons. Nevertheless, low operative temperatures in winter posed a challenge for the tortoises, which reached their field-preferred body temperature for an average of only 0.8-0.9. h per day. Moreover, a low thermoregulation effectiveness suggested that tortoises traded-off physiological benefits of favourable body temperatures against predation risk. Spring and autumn provided higher temperatures, but summer caused the greatest thermoregulatory challenge. Although summer body temperatures were closer to field-preferred body temperature than in any other season, tortoises required rock crevices to avoid overheating. The small size of dwarf tortoises might help them utilise crevices. In summer, maximum operative temperatures in crevices were similar to field-preferred body temperature, indicating that an increase in environmental temperatures might be detrimental to dwarf tortoises. In light of projected temperature rises, future studies should assess if dwarf tortoises can cope with higher environmental temperatures in summer.
Long-lived, iteroparous species exhibit various mechanisms to overcome temporary unfavorable conditions and promote adult survival and population persistence. Consequently, populations may appear relatively stable while subtle and slow-progressing threats cause their decline. The speckled tortoise (Homopus signatus) is a long-lived, iteroparous species in South Africa that is thought to be secure in areas that meet the species’ known requirements. To verify its stability, I monitored a dense speckled tortoise population, integrating 2 mark-recapture studies (2000–2004 and 2012–2015) in long-term joint live and dead encounters and POPAN population models. From 2000 to 2015, the study site remained fenced, ungrazed, and was not modified in any way, yet the size-class frequency distribution, sex ratio, and abundance of speckled tortoises drastically changed. Population numbers decreased 66%, mainly because of the disappearance of juveniles and males. Modeling results identified a reduction in the number of entrants into the population (via births and immigration) as the likely cause of population changes, whereas apparent survival remained high. Observations, in part anecdotal, of increasing numbers of pied crows (Corvus albus) on the study site suggest that increased predation on speckled tortoise hatchlings contributed to the population decline. Examination of rainfall data for 1990–2015 did not reveal evidence of aridification in the study area, a documented cause of reduced reproduction in speckled tortoises. The results of this study caution against the assumption that areas in the range of the speckled tortoise that meet its known requirements can maintain healthy populations.
The greater padloper, Homopus femoralis, is largely endemic to South Africa. Its ecology remains unstudied, yet the efficient planning of field research is complicated by lack of data on its activity patterns. I studied a population in spring, summer and autumn 2008–2011, and found that H. femoralis was active (i.e. basking, drinking, feeding or walking) only during brief intervals following rainfall or imminent rain, perhaps to avoid avian predators or physiological costs of water and food shortages. Future studies might locate active tortoises in the highest rainfall months, and use telemetry to identify activity patterns throughout the year.
Feeding in the wild of the tortoise Homopus signatus signatus in South Africa was recorded and the diet compared to previously reported fecal analysis. Several new plant species were identified in the diet, and analyses differed by ca. 24% in terms of species present. Tortoises did not feed on all plant species or parts to the same extent, indicating selective feeding.
This chapter focuses on thermoregulation and dormancy of reptiles and amphibians. Amphibians and reptiles are ectothermic and rely on environmental sources to gain heat, their options for activity are more limited than those for endothermic tetrapods that maintain elevated body temperature from metabolic heat. All physiological processes in ectotherms are temperature dependent. All amphibians and reptiles produce metabolic heat but at a level far below that of mammals and birds, and few have the necessary insulation to prevent its rapid loss. Among amphibians, behavioral thermoregulation is difficult to separate from behavioral mechanisms for water conservation because they lose water readily through the skin. . For reptiles, the most apparent process affected by temperature is behavior; a cold reptile is not as active as a warm one. Dormancy behaviors are commonly segregated into hibernation for avoidance of winter cold and aestivation for all others, including acyclic drought-caused dormancy. In addition, physiological studies of amphibian and reptilian dormancy indicate that many species alter cardiovascular function and suppress metabolic activities to conserve energy and ensure adequate oxygen to vital organs during extended periods of inactivity.
The Nama-Karoo is a semi-arid inland biome in South Africa dominated by dwarf shrubs with grasses, shrubs, geophytes and herbs at varying levels of abundance. The position of the Nama-Karoo/grassland boundary is determined in part by rainfall amount, and in recent years there has been an increase in grassiness, correlated with good rains. This has allowed wildfires, an unusual occurrence, to burn at several sites in the central and eastern regions of the biome. The general effect of fire has been to convert dwarf shrublands to grassland with the extirpation of several nonsprouters species. A collection of photographs describes this effect. It is anticipated that these nonsprouters will recolonise by seed over time, but could be eliminated if fire frequency is high enough to eliminate their seedbank. It is predicted that if grassy conditions persist in the Nama-Karoo, then fire will be an important factor that shapes the Nama-Karoo/grassland boundary.
Tortoises that live in regions where food plants grow in winter may have to cope with relatively low environmental temperatures to obtain resources. The speckled tortoise, Homopus signatus, inhabits an arid winter rainfall range where it is active in winter and spring at environmental temperatures well below its preferred body temperature. Although H. signatus is a threatened species, we have no information how it deals with low environmental temperatures. Therefore, we made continuous recordings of behaviour in nine female H. signatus on 29 days in the early spring. The group of females as a whole showed activity (i.e., behaviours other than hiding) throughout the day in a unimodal pattern. However, individual tortoises were active only for approximately 4.5 h per day and spent as much as 73% of their active time basking, mostly under the protective cover of shrubs. In addition, a negative relationship between the percentage of active time spent in sun and environmental temperature indicated that H. signatus used active behaviours other than basking to absorb heat, particularly on cold days. Tortoises completed all active behaviours other than basking in 1.2 h per day, including a mere 24 min of feeding, probably facilitated by the abundant availability of food plants in the early spring. We predict that a reduced availability of food plants for H. signatus might lead to increased active time and possibly increased predation pressure, or to a decreased proportion of active time spent basking and reduced body temperatures.
Chuckwallas living on Black Mountain in the Mojave Desert experienced poor conditions for growth during 1970. After annual plants died and perennials dehydrated in May, the diet of these strictly herbivorous lizards shifted from primarily annuals to perennials only. Chuckwallas stopped eating in late August or September, and entered winter burrows in October after having lost 35% of their body weight during the year. These lizards did not reproduce in 1970, and male home range sizes and aggressive behavior were considerably reduced. Home ranges overlapped extensively, and did not change in size from spring to summer. Chuckwallas were abroad for about eight hours per day in spring, but in summer activity was restricted to about an hour around sunset. Chuckwallas apparently abandon costly social behavior and reproduction in years when succulent food resources are scarce. This may enhance survival of the population during relatively dry years.
In many parts of the world the boundaries between grassland and shrubland biomes have changed substantially over the course of the last century. Many are projected to shift further from being grass-dominated to shrub-dominated by 2050 under global climate change and land use change projections. This paper used long-term surveys and repeat photography to assess vegetation change at the shrubland-grassland ecotone in semi-arid, South Africa. Changes in several climate variables as well as in the cover of grasses and dwarf shrubs over three time periods (1962, 1989 and 2009) were investigated at eight localities within a broad 500 km ecotone between the Grassland and Nama-karoo biomes. Results showed that for most sites grass cover has increased and that dwarf shrub cover has decreased over time. This contradicts earlier views which warned against the expansion of dwarf shrublands in response to over-grazing as well as more recent views which suggest that more mesic biomes in the Karoo Midlands will contract in response to climate-induced aridification. The decline in stocking densities and more conservation-friendly land management practices together with an increase in large wet events in the Nama-karoo biome may have contributed to the increase in grass cover.
The effects of thermoregulatory postures and orientation to the sun on steady-state body temperatures in the zebra-tailed lizard (Callisaurus draconoides) were determined using computer simulation techniques. The parameters evaluated for each posture included: (a) silhouette area (A/sub S/) as a function of solar zenith angle (Z) and relative aximuth angle (RAZ) and (b) the convective heat transfer coefficient (h/sub C/). Four simulations, incorporating micrometeorological data from the literature, were used to evaluate relationships between postures, orientation to the sun, micrometeorological variables and the mechanisms of heat transfer. The simulations show that for a given posture: RAZ orientation to the sun may account for a 1 C range in body temperature; (2) conductive heat gain is not an important mechanism for elevating the steady-state body temperature in the morning but may increase the rate of heat gain under certain conditions; (3) the posture of a lizard and its convective environment are more important in determining a lizard's body temperature than is its orientation to the sun.
(1) I studied seasonal patterns of diurnal retreat-site selection by a nocturnal, rock-dwelling lizard Christinus marmoratus, as well as the way physical characteristics of potential retreat-sites affect the thermal conditions within them. (2) Lizards were selective of the diameter and degree of shading of rocky retreat-sites, and these two variables significantly influenced the thermal conditions beneath rocks. Rock thickness also exerted a strong influence on thermal conditions but lizards were not strongly selective of this characteristic. (3) Seasonal changes in the pattern of retreat-site selection, and in thermal conditions within retreat-sites, strongly suggest that temperature affected quantitative changes in the types of rocky retreat-sites used by the lizards. A significant seasonal shift in the qualitative nature of retreat-sites used by lizards, from rocks during spring to deep crevices in the bedrock during summer, also appears to be thermally driven.
Movements of five Kinixys spekii were measured continuously during six weeks of their activity season, by thread-trailing. The mean daily movement distance was negatively correlated with maximum shade air temperature, but not significantly related to minimum temperature, rainfall, or humidity. There was a midday period of inactivity of increasing length as maximum air temperature exceeded 29° C. Temperatures of models suggest that lower activity on hot days was due to the risk of overheating in this small (mean mass 617 g) tortoise. Kinixys spekii had a mean field body temperature (Tb) of 27° C, which is low compared to other tortoises (including sympatric Geochelone pardulis, mean Tb= 32.5° C), but salivated at a similar Tb (38.4° C). This pattern also occurs in other small species; mean and maximum field Tbs of tortoises are positively correlated with body mass, but salivation and critical maximum Tbs do not vary with body mass. Low field Tb provides a wide safety margin before evaporation of water becomes necessary. Kinixys sprkii had a short annual activity period, with about 95% of sightings in four months (December to March). Monthly sightings of K. spekii were correlated with rainfall, but not significantly related to temperature or humidity. Seasonal activity is therefore related to rainfall, but within the rainy season the level and pattern of daily activity is related to temperature.
Limited evidence in the literature suggests that the assumption of equal catchability in mark-recapture studies of turtles may be invalid. These papers suggest that turtles may alter their behavior to avoid recapture following initial handling. If turtles are less likely to be recaptured following initial capture and handling, then population models estimating demographic characteristics and population sizes from mark-recapture data may be invalid (e.g., overestimating population size). Because conservation and management decisions are largely based on population sizes and trends, it is essential to investigate whether marking animals has any effect on behavior. We tested the effects of handling and permanently marking gopher tortoises (Gopherus polyphemus) at the Kennedy Space Center, Florida from 20-22 July 2003 to determine if short-term activity and recapture rates were affected. We used two distinct groups of tortoises: one group was handled (captured, measured, marked) and the other was not handled, but was marked with paint from a distance of 2.2 m. Although the behavior of handled and non-handled animals was distinct, we found no differences in recapture rates or time to recapture between the groups. We suggest that handled gopher tortoises are as likely to be recaptured as tortoises that have not been handled, although more subtle or long-term effects cannot be ruled out.