Research on the thermal ecology and physiology of free‐living organisms is accelerating as scientists and managers recognize the urgency of the global biodiversity crisis brought on by climate change. As ectotherms, temperature fundamentally affects most aspects of the lives of amphibians and reptiles, making them excellent models for studying how animals are impacted by changing temperatures. As research on this group of organisms accelerates, it is essential to maintain consistent and optimal methodology so that results can be compared across groups and over time. This review addresses the utility of reptiles and amphibians as model organisms for thermal studies by reviewing the best practices for research on their thermal ecology and physiology, and by highlighting key studies that have advanced the field with new and improved methods. We end by presenting several areas where reptiles and amphibians show great promise for further advancing our understanding of how temperature relations between organisms and their environments are impacted by global climate change.

Directional selection on size is common but often fails to result in microevolution in the wild. Similarly, macroevolutionary rates in size are low relative to the observed strength of selection in nature. We show that many estimates of selection on size have been measured on juveniles, not adults. Further, parents influence juvenile size by adjusting investment per offspring. In light of these observations, we help resolve this paradox by suggesting that the observed upward selection on size is balanced by selection against investment per offspring, resulting in little or no net selection gradient on size. We find that trade-offs between fecundity and juvenile size are common, consistent with the notion of selection against investment per offspring. We also find that median directional selection on size is positive for juveniles but no net directional selection exists for adult size. This is expected because parent-offspring conflict exists over size, and juvenile size is more strongly affected by investment per offspring than adult size. These findings provide qualitative support for the hypothesis that upward selection on size is balanced by selection against investment per offspring, where parent-offspring conflict over size is embodied in the opposing signs of the two selection gradients.

Summary1. Reproduction and immune function are critical processes, but organisms can rarely optimize both traits. Resultant reproduction-immunity trade-offs may be 'facultative', occurring only when resources are scarce, or they may be 'obligate', occurring regardless of resource availability.2. Previous research has tested for the 'facultative' or 'obligate' nature of reproduction-immunity trade-offs by measuring resource allocation (e.g. follicle size). However, measuring resource allocation alone may be insufficient when gauging the fitness consequences of reproduction-immunity trade-offs because the number and quality of eggs or offspring trade off with one another. 3. We used the Texas field cricket (Gryllus texensis) to provide the most direct test to date of whether a fitness trade-off between these two traits is 'facultative' or 'obligate'. We used a factorial design to manipulate food availability and immune status throughout adulthood. We then estimated lifetime fecundity, hatching success and their product (reproductive success), and we also measured several aspects of offspring quality (e.g. egg size and protein content, and hatchling size and energy stores). 4. A reproduction-immunity trade-off was 'obligate' in this species because immune challenge reduced reproductive success estimates regardless of food availability. Females with unlimited food were more fecund and produced more and larger hatchlings, but neither food availability nor immune status affected egg size, egg phenoloxidase activity, incubation duration, hatching success or hatchling energy stores. We observed a trade-off between offspring size and number -females favouring offspring size over fecundity produced fewer hatchlings, but their hatchlings were of higher quality (larger and more robust). 5. By demonstrating that not all eggs are created equal, we provide key insight into the role of reproductive allocation in the fitness trade-off between reproduction and immunity.

Parents can maximize their reproductive success by balancing the trade-off between investment per offspring and fecundity. According to theory, environmental quality influences the relationship between investment per offspring and offspring fitness, such that well-provisioned offspring fare better when environmental quality is lower. A major prediction of classic theory, then, is that optimal investment per offspring will increase as environmental quality decreases. To test this prediction, we release over 30,000 juvenile Atlantic salmon (Salmo salar) into eight wild stream environments, and we monitor subsequent growth and survival of juveniles. We estimate the shape of the relationship between investment per offspring (egg size) and offspring fitness in each stream. We find that optimal egg size is greater when the quality of the stream environment is lower (as estimated by a composite index of habitat quality). Across streams, the mean size of stream gravel and the mean amount of incident sunlight are the most important individual predictors of optimal egg size. Within streams, juveniles recaptured in stream subsections that featured larger gravels and greater levels of sunlight also grew relatively quickly, an association that complements our cross-stream analyses. This study provides the first empirical verification that environmental quality alters the relationship between investment per offspring and offspring fitness, such that optimal investment per offspring increases as environmental quality decreases.

The evolution of investment per offspring (I) is often viewed through the lens of the classic theory, in which variation among individuals in a population is not expected. A substantial departure from this prediction arises in the form of correlations between maternal body size and I, which are observed within populations in virtually all taxonomic groups. Based on the generality of this observation, we suggest it is caused by a common underlying mechanism. We pursue a unifying explanation for this pattern by reviewing all theoretical models that attempt to explain it. We assess the generality of the mechanism upon which each model is based, and the extent to which data support its predictions. Two classes of adaptive models are identified: models that assume that the correlation arises from maternal influences on the relationship between I and offspring fitness [w(I)], and those that assume that maternal size influences the relationship between I and maternal fitness [W(I)]. The weight of evidence suggests that maternal influences on w(I) are probably not very general, and even for taxa where maternal influences on w(I) are likely, experiments fail to support model predictions. Models that assume that W(I) varies with maternal size appear to offer more generality, but the current challenge is to identify a specific and general mechanism upon which W(I) varies predictably with maternal size. Recent theory suggests the exciting possibility that a yet unknown mechanism modifies the offspring size-number trade-off function in a manner that is predictable with respect to maternal size, such that W(I) varies with size. We identify two promising avenues of inquiry. First, the trade-off might be modified by energetic costs that are associated with the initiation of reproduction ('overhead costs') and that scale with I, and future work could investigate what specific overhead costs are generally associated with reproduction and whether these costs scale with I. Second, the trade-off might be modified by virtue of condition-dependent offspring provisioning coupled with metabolic factors, and future work could investigate the proximate cause of, and generality of, condition-dependent offspring provisioning. Finally, drawing on the existing literature, we suggest that maternal size per se is not causatively related to variation in I, and the mechanism involved in the correlation is instead linked to maternal nutritional status or maternal condition, which is usually correlated with maternal size. Using manipulative experiments to elucidate why females with high nutritional status typically produce large offspring might help explain what specific mechanism underlies the maternal-size correlation.

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How parents divide the energy available for reproduction between size and number of offspring has a profound effect on parental reproductive success. Theory indicates that the relationship between offspring size and offspring fitness is of fundamental importance to the evolution of parental reproductive strategies: this relationship predicts the optimal division of resources between size and number of offspring, it describes the fitness consequences for parents that deviate from optimality, and its shape can predict the most viable type of investment strategy in a given environment (e.g., conservative vs. diversified bet-hedging). Many previous attempts to estimate this relationship and the corresponding value of optimal offspring size have been frustrated by a lack of integration between theory and empiricism. In the present study, we draw from C. Smith and S. Fretwell's classic model to explain how a sound estimate of the offspring size--fitness relationship can be derived with empirical data. We evaluate what measures of fitness can be used to model the offspring size--fitness curve and optimal size, as well as which statistical models should and should not be used to estimate offspring size--fitness relationships. To construct the fitness curve, we recommend that offspring fitness be measured as survival up to the age at which the instantaneous rate of offspring mortality becomes random with respect to initial investment. Parental fitness is then expressed in ecologically meaningful, theoretically defensible, and broadly comparable units: the number of offspring surviving to independence. Although logistic and asymptotic regression have been widely used to estimate offspring size-fitness relationships, the former provides relatively unreliable estimates of optimal size when offspring survival and sample sizes are low, and the latter is unreliable under all conditions. We recommend that the Weibull-1 model be used to estimate this curve because it provides modest improvements in prediction accuracy under experimentally relevant conditions.

Classic egg size theory predicts that, in a given environment, there is a level of maternal investment per offspring that will maximize maternal fitness. However, positive correlations among egg size and female body size are observed within populations in diverse animal taxa. A popular explanation for this phenomenon is that, in some populations, morphological constraints on egg size, such as ovipositor size (insects) or pelvic aperture width (lizards and turtles), limit egg size. Egg size may therefore increase with female body size due to body size-specific constraints on investment per offspring, coupled with selection towards an optimal egg size. We use 17 years of data from a population of painted turtles Chrysemys picta to evaluate this hypothesis. In accordance with our predictions, we find that (1) morphological constraints on egg size are apparent only in relatively small females, similarly (2) egg mass exhibits a strong asymptotic relationship with female body size, suggesting egg mass is optimized only at large body sizes, (3) clutch size, not egg mass, varies with female condition, and (4) clutch size varies more than egg mass across years. Contrary to our predictions, we observe that (5) the egg mass-clutch size tradeoff is not less pronounced at large body sizes. Our data do not fully support the traditional hypothesis, and recent models suggest that this hypothesis is indeed overly simplistic. When the selective environment of a female's offspring is influenced by her phenotype, optimal egg size may vary among maternal phenotypes. This concept can explain correlations among egg size and body size in many taxa, as well as the patterns observed in the present study. In this paradigm, a tight coupling of aperture width (or other 'constraints') and egg size may occur in small females, even when such morphological features are not causally related to variation in egg size. In this spirit, we question validity of invoking morphological constraints to explain covariation among egg size and female body size.