The effects of temperature and inter-individual variation on the locomotor performance of juvenile turtles

Environmental and Molecular Modulation of Motor Individuality in Larval Zebrafish

Innate behavioral biases such as human handedness are a ubiquitous form of inter-individual variation that are not strictly hardwired into the genome and are influenced by diverse internal and external cues. Yet, genetic and environmental factors modulating behavioral variation remain poorly understood, especially in vertebrates. To identify genetic and environmental factors that influence behavioral variation, we take advantage of larval zebrafish light-search behavior. During light-search, individuals preferentially turn in leftward or rightward loops, in which directional bias is sustained and non-heritable. Our previous work has shown that bias is maintained by a habenula-rostral PT circuit and genes associated with Notch signaling. Here we use a medium-throughput recording strategy and unbiased analysis to show that significant individual to individual variation exists in wildtype larval zebrafish turning preference. We classify stable left, right, and unbiased turning types, with most individuals exhibiting a directional preference. We show unbiased behavior is not due to a loss of photo-responsiveness but reduced persistence in same-direction turning. Raising larvae at elevated temperature selectively reduces the leftward turning type and impacts rostral PT neurons, specifically. Exposure to conspecifics, variable salinity, environmental enrichment, and physical disturbance does not significantly impact inter-individual turning bias. Pharmacological manipulation of Notch signaling disrupts habenula development and turn bias individuality in a dose dependent manner, establishing a direct role of Notch signaling. Last, a mutant allele of a known Notch pathway affecter gene, gsx2, disrupts turn bias individuality, implicating that brain regions independent of the previously established habenula-rostral PT likely contribute to inter-individual variation. These results establish that larval zebrafish is a powerful vertebrate model for inter-individual variation with established neural targets showing sensitivity to specific environmental and gene signaling disruptions. Our results provide new insight into how variation is generated in the vertebrate nervous system.

Measuring athletic performance in post-metamorphic fire salamanders

OBJECTIVE

Athletic performances are dynamic movements that are physically challenging and often predict individual success in ecological contexts. They stem from a complex integration of multiple phenotypic traits-e.g., morphological, physiological and behavioural-that dictate animal survival and individual fitness. However, directly quantifying athletic performances can be particularly challenging in cryptic, slow-moving species or not very reactive in attitude. Here we present and describe a rapid, simple, and low-cost method to measure athletic performance in post-metamorphic individuals of the fire salamander Salamandra salamandra. While extremely reactive during the larval stage, adult salamanders are, in fact, cryptic and relatively slow-moving.

RESULTS

Forcing terrestrial juveniles to swim under standard, albeit ecologically plausible, laboratory conditions, and using an automatic point-mass tracking tool, we were able to measure maximal and average performance indicators of post-metamorphic individuals. This method avoids inter-individual variation in motivation, as it forces individuals to perform at their best. Moreover, with this method, measures of athletic performance will be directly comparable between larval and terrestrial stages, allowing to study the contribution of carryover effects to the wide range of processes implicated in the eco-evo-devo of athletic performance in salamanders.

Influence of nest temperature on morphology of Leatherback Turtle (Dermochelys coriacea) hatchlings incubated in hatcheries in Oaxaca, Mexico

The influence of nest incubation temperatures on carapace shape and morphological traits of Leatherback Turtle (Dermochelys coriacea (Vandelli, 1761)) hatchlings incubated in two hatcheries in Oaxaca, Mexico, was evaluated. This study was carried out from October 2016 through May 2017. On each beach, there are community groups consisting of volunteers not affiliated with universities who protect and relocate the nests to increase hatching success. In each translocated nest, a data logger was placed in the centre of the egg mass. Hatchlings were collected as they emerged from each nest. The carapaces of the hatchlings were photographed and subjected to geometric morphometric analysis; also, hatchlings were weighed and their bodies measured. The mean temperature of 12 nests in each hatchery was recorded, with no significant differences between hatcheries. The principal component analysis revealed an overlapping of the carapace shape under different temperature durations. Temperature had a significant influence on hatchling morphology. Higher mean incubation temperatures produced hatchlings with low mass, smaller appendages, narrower carapace widths, and shorter flipper lengths. Lower mean incubation temperatures produced hatchlings with greater mass, wider appendage widths, wider carapace widths, and longer flipper lengths. Results indicate that the Leatherback hatchlings incubated in hatcheries demonstrate morphology that varies in relation to nest incubation temperature in a similar way to hatchlings produced in natural environments.

Blood biochemistry and haematology of migrating loggerhead turtles (Caretta caretta) in the Northwest Atlantic: reference intervals and intra-population comparisons

We documented blood biochemistry and haematology of healthy loggerhead turtles (Caretta caretta) in the Northwest (NW) Atlantic in order to establish clinical reference intervals (RIs) for this threatened population. Blood samples were analysed from migratory loggerheads captured off the Mid-Atlantic coast of the USA in 2011, 2012, 2013 and 2016 as part of a long-term research program. Blood variables were determined using a point-of-care analyser, and a veterinary diagnostic laboratory service. We calculated 95% RIs with associated 90% confidence intervals (CIs) for each blood variable. We compared results obtained from our study of migratory loggerheads with published data for similarly sized loggerheads resident at a seasonal temperate latitude foraging area. Significant differences in several blood variables between migratory and resident turtles provided insight on energetic and health status during different behavioural states. Temperature was significantly correlated with several blood variables: lactate, pCO₂, sodium, haemoglobin and lactate dehydrogenase. Our assessment of blood chemistry in healthy loggerhead turtles in the NW Atlantic provides a baseline for clinical comparisons with turtles impacted by anthropogenic and environmental threats, and highlights the importance of identifying unique aspects of biochemical and haematological profiles for sea turtles at the intra-population level.

Thermoregulatory performance and habitat selection of the eastern box turtle (Terrapene carolina carolina)

Environmental conditions may affect individual physiological processes that influence short-term performance and ultimately growth, survival and reproduction. As such, habitats selected by animals must provide suitable and adequate resources. Ectothermic species are highly dependent on climatic conditions and ambient temperatures that dictate body temperature regulation and in turn physiological processes. We investigated the thermoregulatory performance, habitat selection, and movements of an ectothermic vertebrate, the Eastern box turtle (Terrapene carolina carolina) to assess the importance of thermoregulatory physiology in habitat selection. We evaluated the relationship between habitat selection and thermoregulatory performance in Southwest Ohio over two active seasons from May until October. We found that T. carolina selected shaded habitats, including evergreen and deciduous forests, as well as herbaceous grasslands, conformed to the ambient temperatures throughout the active season, although these habitats had temperatures below those expected based on thermal optima of box turtles. Further, we found that movement was not correlated with internal body temperature. Our study shows that thermal conditions are not paramount in habitat selection of box turtles, but that cooler temperatures do not have an effect on the extent of their locomotion.

Thermal ecological physiology of native and invasive frog species: do invaders perform better?

Biological invasions are recognized as an important biotic component of global change that threatens the composition, structure and functioning of ecosystems, resulting in loss of biodiversity and displacement of native species. Although ecological characteristics facilitating the establishment and spread of non-native species are widely recognized, little is known about organismal attributes underlying invasion success. In this study, we tested the effect of thermal acclimation on thermal tolerance and locomotor performance in the invasive Xenopus laevis and the Chilean native Calyptocephalella gayi. In particular, the maximal righting performance (μMAX), optimal temperature (TO), lower (CTmin) and upper critical thermal limits (CTmax), thermal breadth (Tbr) and the area under the performance curve (AUC) were studied after 6 weeks acclimation to 10 and 20°C. We observed higher values of μmax and AUC in X. laevis in comparison to C. gayi. On the contrary, the invasive species showed lower values of CTmin in comparison to the native one. In contrast, CTmax, TO and Tbr showed no inter-specific differences. Moreover, we found that both species have the ability to acclimate their locomotor performance and lower thermal tolerance limit at low temperatures. Our results demonstrate that X. laevis is a better performer than C. gayi. Although there were differences in CTmin, the invasive and native frogs did not differ in their thermal tolerance. Interestingly, in both species the lower and upper critical thermal limits are beyond the minimal and maximal temperatures encountered in nature during the coldest and hottest month, respectively. Overall, our findings suggest that both X. laevis and C. gayi would be resilient to climate warming expectations in Chile.

Individual variation in thermal performance curves: swimming burst speed and jumping endurance in wild-caught tropical clawed frogs

The importance of studying individual variation in locomotor performance has long been recognized as it may determine the ability of an organism to escape from predators, catch prey or disperse. In ectotherms, locomotor performance is highly influenced by ambient temperature (Ta), yet several studies have showed that individual differences are usually retained across a Ta gradient. Less is known, however, about individual differences in thermal sensitivity of performance, despite the fact that it could represent adaptive sources of phenotypic variation and/or additional substrate for selection to act upon. We quantified swimming and jumping performance in 18 wild-caught tropical clawed frogs (Xenopus tropicalis) across a Ta gradient. Maximum swimming velocity and acceleration were not repeatable and individuals did not differ in how their swimming performance varied across Ta. By contrast, time and distance jumped until exhaustion were repeatable across the Ta gradient, indicating that individuals that perform best at a given Ta also perform best at another Ta. Moreover, thermal sensitivity of jumping endurance significantly differed among individuals, with individuals of high performance at low Ta displaying the highest sensitivity to Ta. Individual differences in terrestrial performance increased with decreasing Ta, which is opposite to results obtained in lizards at the inter-specific and among-individual levels. To verify the generality of these patterns, we need more studies on individual variation in thermal reaction norms for locomotor performance in lizards and frogs.

Repeatability of escape response performance in the queen scallop (Aequipecten opercularis)

In order for natural selection to operate, physiological and behavioural traits must exhibit both inter-individual variability and intra-individual consistency (i.e. repeatability) in performance. In this study, we describe individual variation and temporal repeatability in the escape responses of the queen scallop Aequipecten opercularis and determine whether individuals exhibited consistently high or low rankings in different aspects of the escape response. Five measures of individual performance were recorded on 4 occasions (days 0, 2, 7 and 28) providing proxies for sensory acuity (response latency), immediate and sustained swimming performance (burst and average clap-rates) and swimming endurance (total number of claps and total time spent clapping). All components of the escape response exhibited significant inter-individual variability (all P<0.0001). Escape response latency, burst clap-rate, total number of claps and total duration spent clapping maintained significant repeatability over 28 days (all P<0.016). Average clap-rate was repeatable in the short term (2 days, P<0.0001) but repeatability declined by 28 days (P=0.097). Concordance analysis indicated that individuals maintained the same performance rankings over time for each component of the escape response (all P<0.001). In addition, some individuals ranked as consistently high or low performers across response latency, burst and average clap-rate and total number of claps. An individual's ability to evade predators through the provision of an escape response of an appropriate magnitude, subject to physiological, behavioural and organismal constraints, will have clear fitness-related consequences.

Effects of body temperature on righting performance of native and invasive freshwater turtles: consequences for competition

Righting behavior of aquatic turtles might be subject to coadaptation pressures between preferred basking temperature and locomotion, given that it is mainly performed on land and may critically determine the survival of turtles. We analyzed the effect of body temperature (T(b)) on righting performance of two species of freshwater turtles, the endangered native Spanish terrapin (Mauremys leprosa), and the red-eared slider (Trachemys scripta elegans), an introduced invasive species that is displacing native turtles in the Iberian Peninsula. Interspecific differences in morphology, basking requirements and behavioral responses have been found between Spanish terrapins and introduced sliders. Therefore, we hypothesized that T(b) might differentially affect righting behavior of these two turtle species. We found a clear effect of T(b) on righting response of both M. leprosa and T. scripta, with the performance enhanced at the preferred basking temperature of each turtle species. These results suggest that righting might be coadapted to preferred basking temperature in freshwater turtles. Also, M. leprosa required longer times to right on average than T. scripta, which denotes a higher efficiency of introduced sliders at righting performance. These interspecific behavioral asymmetries in righting performance between native and exotic turtles might contribute to the greater competitive ability of introduced T. scripta, favoring the expansion of exotic sliders in the new environments in which they are introduced, in detriment to native Spanish terrapins.

Feeding status and basking requirements of freshwater turtles in an invasion context

Thermoregulatory behavior and feeding status are strongly related in ectotherms. A trade-off between maintenance of energy balance and digestion efficiency has been recently proposed to affect thermoregulation in these animals. On the other hand, competition for basking sites has been described between Iberian turtles and the introduced red-eared slider (Trachemys scripta elegans). T. scripta negatively interferes with basking behavior of native turtles and benefits from a greater capacity to retain body heat, which may likely result in thermoregulatory advantages for the introduced sliders. Consequently, complex effects and alterations in metabolic rates of native turtles might derive from a deficient basking behavior. We compared the basking requirements of the endangered native Spanish terrapin (Mauremys leprosa) and those of the introduced red-eared slider, analyzing the upper set point temperature (USP) (defined as the body temperature at which basking ceased) of both native and introduced turtles, under feeding and fasting conditions. We found higher values of USP in the native species, and a reduction of this temperature associated with food deprivation in the two turtle species. This adjustment of thermoregulatory behavior to the nutritional status found in freshwater turtles suggests that ectotherms benefit from metabolic depression as an adaptive mechanism to preserve energy during periods of fasting. However, a reduction in metabolic rates induced by competition with sliders might lead M. leprosa to a prolonged deficiency of their physiological functions, thus incurring increased predation risk and health costs, and ultimately favoring the recession of this native species in Mediterranean habitats.

Temperature effects on snapping performance in the common snapper Chelydra serpentina (Reptilia, Testudines)

Studies on the effect of temperature on whole-animal performance traits other than locomotion are rare. Here we investigate the effects of temperature on the performance of the turtle feeding apparatus in a defensive context. We measured bite force and the kinematics of snapping in the Common Snapping Turtle (Chelydra serpentina) over a wide range of body temperatures. Bite force performance was thermally insensitive over the broad range of temperatures typically experienced by these turtles in nature. In contrast, neck extension (velocity, acceleration, and deceleration) and jaw movements (velocity, acceleration, and deceleration) showed clear temperature dependence with peak acceleration and deceleration capacity increasing with increasing temperatures. Our results regarding the temperature dependence of defensive behavior are reflected by the ecology and overall behavior of this species. These data illustrate the necessity for carefully controlling T(b) when carrying out behavioral and functional studies on turtles as temperature affects the velocity, acceleration, and deceleration of jaw and neck extension movements. More generally, these data add to the limited but increasing number of studies showing that temperature may have important effects on feeding and defensive performance in ectotherms.

Influence of incubation temperature on morphology and locomotion performance of Leatherback (Dermochelys coriacea) hatchlings

The journey of Leatherback ( Dermochelys coriacea (Vandelli, 1761)) hatchlings from nest to the sea is a vulnerable life-history stage. Studies have shown that nest incubation temperatures influence hatchling morphology and locomotor performance, which may affect hatchling fitness. We obtained incubation temperature profiles from 16 Leatherback nests in Tobago, West Indies, during the 2008 nesting season (March–June). There was significant variation among mean nest incubation temperatures, which had a significant influence on hatchling morphology. Using principal components analysis, we determined the morphological traits that explained the most variation among hatchlings, which allowed investigation of the relationship between hatchling morphology and terrestrial locomotion speed. Hatchlings with a narrower carapace width and longer flipper reach (produced at lower incubation temperatures) had significantly faster terrestrial speed and total run time than those with opposite characteristics (produced at higher incubation temperatures). Our results demonstrate that lower incubation temperatures produce hatchlings with traits that are significantly advantageous to terrestrial locomotion. These findings suggest that nest incubation temperature is important in determining hatchling fitness, as nest incubation temperature significantly influences hatchling morphology and locomotor capabilities. This study supplements related findings in Green Turtles ( Chelonia mydas (L., 1758)), but also illustrates some unique features in Leatherbacks.

Estimating maximum performance: effects of intraindividual variation

Researchers often estimate the performance capabilities of animals using a small number of trials per individual. This procedure inevitably underestimates maximum performance, but few studies have examined the magnitude of this effect. In this study we explored the effects of intraindividual variation and individual sample size on the estimation of locomotor performance parameters. We measured sprint speed of the lizard Sceloporus occidentalis at two temperatures (20°C and 35°C),obtaining 20 measurements per individual. Speed did not vary temporally,indicating no training or fatigue effects. About 50% of the overall variation in speed at each temperature was due to intraindividual variation. While speed was repeatable, repeatability decreased slightly with increasing separation between trials. Speeds at 20°C and 35°C were positively correlated,indicating repeatability across temperatures as well. We performed statistical sampling experiments in which we randomly drew a subset of each individual's full set of 20 trials. As expected, the sample's maximum speed increased with the number of trials per individual; for example, five trials yielded an estimate averaging 89% of the true maximum. The number of trials also influenced the sample correlation between mean speeds at 20°C and 35°C; for example, five trials yielded a correlation coefficient averaging 90% of the true correlation. Therefore, intraindividual variation caused underestimation of maximal speed and the correlation between speeds across temperatures. These biases declined as the number of trials per individual increased, and depended on the magnitude of intraindividual variation, as illustrated by running sampling experiments that used modified data sets.

Geometry and self-righting of turtles

Terrestrial animals with rigid shells face imminent danger when turned upside down. A rich variety of righting strategies of beetle and turtle species have been described, but the exact role of the shell's geometry in righting is so far unknown. These strategies are often based on active mechanisms, e.g. most beetles self-right via motion of their legs or wings; flat, aquatic turtles use their muscular neck to flip back. On the other hand, highly domed, terrestrial turtles with short limbs and necks have virtually no active control: here shape itself may serve as a fundamental tool. Based on field data gathered on a broad spectrum of aquatic and terrestrial turtle species we develop a geometric model of the shell. Inspired by recent mathematical results, we demonstrate that a simple mechanical classification of the model is closely linked to the animals' righting strategy. Specifically, we show that the exact geometry of highly domed terrestrial species is close to optimal for self-righting, and the shell's shape is the predominant factor of their ability to flip back. Our study illustrates how evolution solved a far-from-trivial geometrical problem and equipped some turtles with monostatic shells: beautiful forms, which rarely appear in nature otherwise.