Dawn S. Wilson

ESTIMATING AGE OF TURTLES FROM GROWTH RINGS: A CRITICAL EVALUATION OF THE TECHNIQUE

The technique of counting growth rings to estimate age of turtles is widespread in the scientific literature. Review articles to date have provided primarily lists of authors who have found the technique useful or not, but these reviews have failed to evaluate properly how well the technique actually works. In an attempt to examine how well the published literature supports a biologically meaningful relationship between age and number of growth rings, we surveyed 145 scientific papers that have used counts of rings on scutes to estimate age of individual turtles. Of the 145 papers surveyed, the authors of 44 papers, which comprised 49 case studies, presented data testing the use of growth ring counts for a population of turtles. Of these 49 case studies, 6 reported that the use of the technique was reliable for aging their turtle species past sexual maturity, 15 reported its use to be reliable for aging turtles to young adult (i.e., sexual maturity), 8 reported its use to be reliable for aging juvenile turtles, 2 found it to be reliable with no age limit given, and 8 reported its use unreliable as a method for aging their turtles. The remainder of the case studies presented data that were difficult for us to interpret as reliable or unreliable. Although 22 papers addressed the pattern of growth ring deposition, only four case studies had sufficient data to indicate that a consistent number of rings was added each year. In this paper, we illustrate how the widespread use of this technique ultimately has led to its acceptance without the rigor of tests of its validity or accuracy. We conclude that (a) studies attempting to calibrate the relationship between growth rings and age are few, (b) a majority of the papers that we surveyed referenced other papers that did not themselves include a test justifying growth ring counts as an estimate of turtle age, (c) aging turtles from counts of growth rings might be feasible in some types of studies, for some species at some locations, but only after calibrating the relationship between ring counts and age for each circumstance, and (d) there is currently no justification for generalizing the use of growth rings to estimate turtle age for many species.

Beyond Optimal Escape Theory: Microhabitats as Well as Predation Risk Affect Escape and Refuge Use by the Phrynosomatid Lizard Sceloporus virgatus.

Escape studies often focus on one variable, but tactics and refuge use vary with microhabitats, exposure, distance to refuge, and temperature. We studied these effects and effects of microhabitats and risk factors (distance from refuge, temperature) on flight initiation distance (FID, distance between predator and prey when escape begins) and distance fled (DF) in the lizard Sceloporus virgatus . FID increased as distance to refuge increased and temperature decreased. DF increased as FID increased, supporting the hypothesis that individual differences in boldness are consistent among encounter phases. Refuges were rock crevices, trees, logs, and grass clumps. Interhabitat differences in FID and DF matched those in distance to refuge. FID was longer for lizards on rocks and ground than trees due to proximity to and use of refuge. Lizards on trees rarely changed microhabitats, moving to the far side and unpredictably up or down. Lizards on slopes fled long distances up slopes. Most lizards on rocks entered crevices or switched microhabitats. Lizards on ground usually changed microhabitats. Optimal escape theory accurately predicted effects of risk on FID, but initial microhabitats and final microhabitats and refuges affected tactics, FID and DF. DF was affected by risk, being longer when lizards remained visible.

Nesting Behavior of the Striped Mud Turtle, Kinosternon baurii (Testudines: Kinosternidae)

The nesting ecology of Kinosternon baurii was studied on a sandhill in central Florida from September 1991 through February 1995. Gravid females were marked and tracked to their nest sites using thread bobbins attached to the carapace. Peak nesting season was September through November with a second minor peak in June. Females laid 1-3 clutches per year. When left unprotected from predators, egg predation was 100%; however, when nests were protected from predators, 88% of the nests had full or partial embryo survival to hatching. Nesting females moved an average of 137 m from the wetland to their nest sites and showed fidelity to a particular nesting area among years. Movements to and from nest sites coincided with rainfall. As documented for other kinosternid species, females of K baurii prolonged their stays on the sandhill after nesting by burying underground near their nest sites. Deterrence of egg predators from the nest site as a result of the females presence has been suggested as a possible explanation for why females do not return to the wetland immediately after nest completion; however, physiological limitations caused by energy expenditure and/or evaporative water loss also may aid in explaining this behavior.

Variation in Shell Morphology of the Western Pond Turtle (Actinemys Marmorata Baird and Girard) from Three Aquatic Habitats in Northern California

Abstract Intraspecific variation in turtle life-history traits has been well documented for many turtle species; however, comparative studies of such traits for the Western Pond Turtle (Actinemys marmorata) are lacking. Here we compare three years of morphometric data on A. marmorata from three study sites in northern California. Our study sites represent various habitat types where A. marmorata can be found and include foothill creeks (BCCR), valley sloughs (SR), and human-made canals (HS). We used maximum carapace length (MCL) as a measure of overall body size and found that A. marmorata differed in body size among sites. For all sites combined, males were significantly larger in mean MCL than females. Among the three sites, both mean male and female body sizes were significantly different (HS > SR > BCCR). Within sites, males were larger than females at SR and HS, but no difference was found between males and females at BCCR. Controlling for body size (MCL), A. marmorata in the foothill creek habitat were flatter and narrower than those in both the valley slough and human-made canal habitats. The observed variation in body size among sites could possibly be caused by differences in prey availability, water temperature, or microhabitat characteristics. Although we have no hard evidence to explain our observed size differences in A. marmorata, we suggest that phenotypic plasticity is responsible for determining maximum size at maturity. Additionally, it is unclear at this time whether fusiform, or compressed shell shape, in A. marmorata offers any adaptive significance in lotic environments.

How to stay alive after losing your tail

Prey that autotomize appendages to escape predation may suffer reduced escape speed and be unable to autotomize lost parts, rendering them more vulnerable. Little is known about behavioural compensation for increased risk by altering decisions about fleeing and entering refuges. Flight initiation distance (FID) is distance between prey and an approaching predator when the prey begins to flee. Escape theory predicts that FID increases with predation risk, but FID decreased after autotomy in the lizard Scincella lateralis, which compensated by becoming more cryptic via reduced mobility. We tested the hypotheses that in a lizard that does not decrease surface activity, stay closer to refuge, or shift to crypsis after autotomy associated with reduced speed, behavioural compensation occurs by increasing FID, distance fled and probability of entering refuge. Running speed in the striped plateau lizard Sceloporus virgatus decreased by 43%, suggesting that selection should favor compensation. Lizards altered escape behaviour after autotomy by increasing FID and refuge use, but did not increase distance fled. To predict effects of autotomy on FID and distance fled, the nature of qualitative compensatory changes must be known. Factors likely to affect which compensatory mechanisms are used include foraging mode and refuge availability.

Thermal Cost of Refuge Use Affects Refuge Entry and Hiding Time by Striped Plateau Lizards Sceloporus virgatus

Abstract Escaping prey decide whether to enter and how long to stay in refuges. According to refuge-use theory, hiding time increases as costs of emerging increase and costs of staying in refuge decrease. We studied effects of air temperature on probability of entering refuges and the effects of thermal cost on hiding time (duration in refuge). Few striped plateau lizards (Sceloporus virgatus) used refuges at intermediate air temperatures, but most readily entered cool refuges at the lowest air temperatures and relatively warm refuges at higher temperatures. Because running speed in lizards decreases as body temperature decreases, S. virgatus that are cool upon morning emergence presumably reduced their probability of being captured by entering refuges. As air and presumably body temperatures increase, refuges are initially cooler than outside, contributing to infrequent use. At higher, but not thermally stressful, temperatures, a greater probability of using refuges may reflect lower thermal costs in refuges that have warmed. Hiding time decreased as temperatures became cooler in refuges than outside. Our results confirm previous work on actively foraging lacertid lizards showing that hiding time decreases as thermal costs of refuge use increases, and extend this finding to a very distantly related phrynosomatid species that is an ambush forager. Thus, the thermal cost of refuge use affects hiding time even in lizards that do not require high body temperature for prolonged foraging movements. A growing body of knowledge supports the hypothesis that tradeoffs between costs of emerging and remaining in refuges guide decisions about hiding time.

Pattern in the Compass Orientation of Gopher Tortoise Burrows at Different Spatial Scales

Individuals of the gopher tortoise (Gopherus polyphemus (Daudin)) seem not to orient their burrows in particular directions, within populations, except where topographic relief influences them to do so. This result suggests that burrow orientation pattern might be expressed on a scale larger than that of the individual population, one that encompasses regularities in topographic relief. We determined the orientations of more than 3,500 gopher tortoise burrows throughout Florida. These burrows tend to be oriented more often in the primary (E, S, W, N) compass directions than in the secondary (NE, SE, SW, NW) directions. We suggest that the best explanation for this finding is that the man-made and natural topographic features in Florida that influence orientation are themselves often oriented in the primary compass directions. Burrow orientation pattern in the gopher tortoise appears to be expressed on a relatively-large scale.

Effect of stress on female-specific ornamentation

SUMMARY Signal honesty is theorized to be maintained by condition-dependent trait expression. However, the mechanisms mediating the condition dependence of sexually selected traits are often unknown. New work suggests that elevated glucocorticoid levels during physiological stress may play a role in maintaining signal honesty. Here, we experimentally examine the effect of both chronic and acute stress on the expression of the condition-dependent ornamentation of female striped plateau lizards, Sceloporus virgatus. Females were stressed either chronically via corticosterone implants or relatively acutely via autotomy, were sham manipulated or were left unmanipulated. Both stressors resulted in elevations in corticosterone within physiologically relevant levels, though the implants resulted in significantly higher levels than did autotomy. Corticosterone-implanted females were less likely to produce a clutch of eggs, but those individuals that did reproduce had reproductive output similar to that of females from other treatment groups. Compared with females in other groups, the corticosterone-implanted females tended to develop smaller ornaments that had less UV and orange-to-red wavelength reflectance relative to medium wavelength reflectance. The sex steroid hormones testosterone and estradiol were correlated to corticosterone levels, but did not appear to underlie the effect on ornament expression; of the steroids measured, only corticosterone levels were negatively related to ornament size and coloration. Thus, the condition-dependent ornamentation of female lizards is sensitive to chronic elevations in stress hormones, supporting their importance in the maintenance of signal honesty.

LONGER HIDING TIME IN REFUGE IMPLIES GREATER ASSESSED RISK AFTER CAPTURE AND AUTOTOMY IN STRIPED PLATEAU LIZARDS (SCELOPORUS VIRGATUS)

Abstract During encounters with predators, prey that flee into refuges decide how long to hide. Refuge use theory predicts that hiding time increases as the risk of emerging increases. Lizards that escape by autotomizing their tails incur costs, including temporary decrease in speed and loss of ability to use autotomy. Thus, risk of being captured upon emergence is greater after autotomy. Also, a predator that captures a prey may later be assessed as posing greater threat. We predicted that hiding time increases after a lizard has been captured or has undergone autotomy. A previous study of striped plateau lizards (Sceloporus virgatus) showed that the proportion of lizards that entered refuges increased after autotomy but not after earlier capture. To examine unstudied effects of autotomy and capture on hiding time, we conducted a 2 × 2 factorial field experiment with handling and autotomy as factors. The four groups were (1) unhandled intact controls; (2) unhandled autotomized; (3) captured (and handled) intact, but not autotomized; and (4) captured, handled and autotomized. Because entering cool refuges entails costly decrease in body temperature, hiding times are shorter in cooler refuges. We controlled this effect statistically by conducting analyses of covariance incorporating difference in air temperature inside and outside refuges as the covariate. As predicted, autotomy and handling led to longer hiding times. However, handling affected hiding time only in intact lizards. Our results add autotomy and capture to risk factors known to affect hiding time, augmenting a growing body of knowledge supporting the hypothesis that trade-offs between costs of emerging and remaining in a refuge guide decisions about hiding time.