We studied nest-site fidelity (1995–2000) and nest predation (1997–2000), and nesting movements (1999–2000) in a population of Chrysemys picta marginata at Miller's Marsh on Beaver Island, Michigan. On average, turtles nested farther (mean = 122.3 m) from the marsh than turtles in previously studied populations, possibly because of the relatively large nesting areas (n = 5) of our study. Nest predation was 17.4% over all years and was independent of nest location (distance from water, road vs. field nests, nesting field). Many turtles showed nest-site fidelity between years in that they favored one or two nesting areas and had annual inter-nest distances (ID) that were positively skewed and significantly less (mean = 88.7 m) than IDs of randomly paired nests of different individuals. Radiotelemetry indicated that most turtles emerged during the mid-afternoon and nested during the evening hours. Nesting excursion duration averaged 11.6 h and 75% remained on land overnight after nesting. The use of terrestrial refugia before and after the nesting process presumably reflected thermal limitations on movements. When compared to turtles that nested relatively close to the marsh, turtles that nested relatively far from the marsh initiated nesting movements earlier in the day, had circuitous routes to and from the nest site and had relatively long nesting excursion duration. The location of an aquatic home range apparently influenced nest-site selection and fidelity in some individuals although other individuals nested relatively far from the aquatic home range.
The persistence of a population of ectothermic vertebrates can be closely tied to variations in weather patterns that influence diel or seasonal cycling of temperature and moisture levels. We studied the effects of drought and weather patterns on the summer activity and movements of Spotted Turtles (Clemmys guttata) in a small wetland in southwestern Michigan over 2 y. During the periods of time with standing water, turtles aquatically active in depressions or terrestrial in grass-sedge-rush and Sphagnum hummocks but their movements were unaffected by daily weather patterns. Turtles tended to occupy multiple core areas within a relatively contiguous home range. When the wetland dried, turtles estivated beneath vegetation, or estivated or maintained limited activity in small forest ponds immediately adjacent to the wetland. In 2006, turtles resumed activity following heavy rains during late Jul. and early Aug. In 2007, however, late summer rains were not sufficient to restore substantial standing water in the wetland and so turtle movements were relatively low. Home range and core area size were significantly smaller in 2007 than in 2006, apparently because of the relatively short summer hydroperiod that occurred during 2007. Unlike previously studied C. guttata populations, the turtles of our population did not travel among multiple upland and lowland habitats, perhaps because such environments were not of higher quality than our wetland, or because the risks of traveling to them were too great.
Understanding the interface between spatial and thermal ecology is integral to understanding energy acquisition and the life histories of ectotherms. Snapping turtles (Chelydra serpentina) occupy a wide range of habitats that vary greatly in their thermal properties. We studied activity, movements, and thermoregulation of C. serpentina in a small, land-locked lake in central Michigan, USA, using radiotelemetry. Consistent with our a priori predictions, turtles were active within core areas along the lake's edge, showed both diurnal and nocturnal activity, and did not make extensive interwetland movements. Turtles left the lake only for nesting or to hibernate in Sphagnum peat or in the banks of a nearby stream. Home range and core area size estimates of C. serpentina were small compared with other previously studied populations, perhaps in part because of the small dimensions of our lake. Contrary to our prediction of a broad Tset (thermoregulatory set-point) range for a large-bodied, habitat generalist turtle, we found a comparatively low and narrow laboratory-determined Tset range (22°C–26°C). Turtle body temperatures (Tb) cycled between May and August and attained maximal values during the evening hours, a pattern that likely results from thermal inertia, the selection of aquatic thermal patches, or both, as Te (operative temperatures) declined. Turtles most effectively maintained Tb within Tset during July and August when thermal conditions were most favorable. Throughout most of the active season the highly aquatic habits of C. serpentina apparently negated the effects of variations in daily weather conditions and their effects on incoming levels of solar radiation on variation in Tb. However, turtles maintained higher average Tb on sunny days compared with overcast days in April and May, the coolest months of the study. Diel Tb cycling ceased during September and October and average Tb declined despite favorable thermal conditions, at least during September, a pattern that could reflect a downward shift in the Tset range. Comparatively, C. serpentina is less effective at thermoregulating than is a small-bodied species at a similar latitude (Chrysemys picta marginata). Apparently, thermal inertia and lack of atmospheric basking proclivities influenced the thermoregulatory precision in our turtles.
Eggs of the turtle Chrysemys picta bellii from four western Nebraska populations were incubated under laboratory conditions to compare egg size and hatchling size as measures of offspring size among populations and in the context of maternal body size. Generally, hatchling size (mass and carapace length) was linearly related to egg size (mass, width, and length) and did not vary between the two years of the study or among populations after adjusting for egg size. However, hatchling carapace length adjusted for egg width was greatest in a population with the most elongate eggs indicating that linear egg measurements may not be useful for interpopulation comparisons. The smaller range of variation, but greater variability in hatchling size relative to egg size and a narrow range of maternal body sizes (carapace length and mass), seemed to preclude a correlation between maternal body size and hatchling size within two populations. In two other populations, hatchling carapace length, egg width, and egg length increased with maternal carapace length by a common slope, although egg wet mass had a steeper slope than did hatchling wet mass. It is probable that under natural incubation conditions (e.g., warmer, drier, and more variable), hatchling mass may increase only slightly or not at all with maternal body size. My results suggest that both hatchling and egg size should be considered in turtle life history studies, particularly for models that predict delayed sexual maturation when offspring size and survival increase with maternal body size and age.
Discriminating between conspecific and heterospecific communication signals has important implications for evolution. The benefits of such discriminations are clear for sympatric congeners, but if conspecific display recognition (CDR) has a cost, it should be relaxed in species that have evolved in isolation. Of the nine lava lizard species (Microlophus spp.) endemic to the Galápagos Islands, all are thought to have evolved in allopatry and none overlap in geographic distribution. Although prior research failed to reveal CDR in male M. grayii on Floreana Island, male M. indefatigabilis on Santa Cruz Island showed a response bias toward conspecific displays under the same experimental conditions. In the present study we tested for CDR in two additional species: M. albemarlensis on Isabella Island and M. bivittatus on San Cristóbal Island. Subjects were presented with computer-controlled robots that either performed the conspecific display, a reversed-inverted conspecific display, or a display from an Anolis lizard species. Results revealed evidence of CDR in male M. albemarlensis, but no such evidence in male M. bivittatus. Interestingly, bathymetric data provide a potential explanation for CDR in M. indefatigabilis and M. albemarlensis: a land bridge between Santa Cruz and Isabella might have resulted in secondary contact and reinforcement in these two species during Pleistocene glacial maxima. Another possibility is that intraspecific selection for discrimination among males has caused heterospecific displays to be viewed as low-fidelity conspecific displays. Nevertheless, our findings are consistent with CDR being lost early in Galápagos Microlophus evolution, as predicted for allopatric speciation, and re-emerging later only where selection favored the discrimination of male displays.
During terrestrial nesting forays, north-temperate freshwater turtles may experience a range of environmental temperatures that could cause overheating or constrain movement if temperatures are too high or low. Yet, we have very little understanding of how body temperature (Tb) varies in freshwater turtles during various nesting foray activities. We studied Tb variation in Chrysemys picta marginata from a small marsh in northern Michigan, 2003–2004. Three individuals left the marsh, nested, and returned to the marsh in a single day; whereas, 5 others remained on land for up to 3 days, mostly immobile under leaf litter. While on land, Tb of turtles and air temperature (Ta) showed parallel, diel oscillations. Mean Tb of mobile turtles exceeded prevailing Ta values but was similar to Tb recorded for those same individuals while aquatic 1 week before and after the nesting foray. Therefore, active nesting turtles on forays may have maintained suitable Tb values by use of sun or shade. During the nesting process, Tb decreased from 23.5°C to 20.0°C. Turtles that entered terrestrial refugia after evening nesting had Tb values similar to those while mobile, often at times when the evening sky illuminated the nesting areas. Therefore, low light levels, which could limit navigation, and Tb values, which could impede movements, did not directly cause turtles to seek terrestrial refugia after nesting. Instead, we suggest that impending low environmental temperatures, waning light levels that would eventually impair navigation abilities, and, ultimately, the risk of predation while returning to the marsh caused turtles to seek terrestrial refugia.
Females of some lizard species exhibit conspicuous coloration during the breeding cycle (“nuptial coloration”) that elicits male courtship. We conducted two field experiments with San Cristóbal Lava Lizards (Microlophus bivittatus) in the Galápagos Islands to determine how the presence and extent of nuptial coloration on lizard robots affected responses of adult males and females. Robots programmed to perform conspecific bobbing displays had a morphological appearance that mimicked (1) a conspecific female without nuptial coloration (non-red control stimulus); (2) a conspecific female with nuptial coloration (normal red stimulus); or (3) a female with an extent of nuptial coloration beyond the range of conspecific variation (super-normal red stimulus). In Experiment 1, subjects witnessed two stimuli in sequence, being presented first with the side of a robot that exhibited conspecific nuptial coloration or with the opposite side of the same robot that lacked nuptial coloration. Results showed no effect of subject sex or stimulus order, but subjects exhibited more display and shorter display latencies in response to the normal red stimulus than to the non-red stimulus. In Experiment 2, new subjects were shown either non-red, normal red, or super-normal red stimulus. In contrast to Experiment 1, results of this experiment revealed sex differences in the amount of display elicited from subjects. Among the findings, males exhibited less display to the super-normal stimulus than to the non-red and normal red stimuli whereas the quantity of display elicited from females by the super-normal stimulus was similar to that evoked by the other two stimuli. We discuss our results in the context of prior studies and offer suggestions for future research.
