Temperate species underfill their tropical thermal potentials on land.

Understanding how temperature determines the distribution of life is necessary to assess species' sensitivities to contemporary climate change. Here, we test the importance of temperature in limiting the geographic ranges of ectotherms by comparing the temperatures and areas that species occupy to the temperatures and areas species could potentially occupy on the basis of their physiological thermal tolerances. We find that marine species across all latitudes and terrestrial species from the tropics occupy temperatures that closely match their thermal tolerances. However, terrestrial species from temperate and polar latitudes are absent from warm, thermally tolerable areas that they could potentially occupy beyond their equatorward range limits, indicating that extreme temperature is often not the factor limiting their distributions at lower latitudes. This matches predictions from the hypothesis that adaptation to cold environments that facilitates survival in temperate and polar regions is associated with a performance trade-off that reduces species' abilities to contend in the tropics, possibly due to biotic exclusion. Our findings predict more direct responses to climate warming of marine ranges and cool range edges of terrestrial species.

Albatross movement suggests sensitivity to infrasound cues at sea.

The ways in which seabirds navigate over very large spatial scales remain poorly understood. While olfactory and visual information can provide guidance over short distances, their range is often limited to 100s km, far below the navigational capacity of wide-ranging animals such as albatrosses. Infrasound is a form of low-frequency sound that propagates for 1,000s km in the atmosphere. In marine habitats, its association with storms and ocean surface waves could in effect make it a useful cue for anticipating environmental conditions that favor or hinder flight or be associated with profitable foraging patches. However, behavioral responses of wild birds to infrasound remain untested. Here, we explored whether wandering albatrosses, Diomedea exulans, respond to microbarom infrasound at sea. We used Global Positioning System tracks of 89 free-ranging albatrosses in combination with acoustic modeling to investigate whether albatrosses preferentially orientate toward areas of 'loud' microbarom infrasound on their foraging trips. We found that in addition to responding to winds encountered in situ, albatrosses moved toward source regions associated with higher sound pressure levels. These findings suggest that albatrosses may be responding to long-range infrasonic cues. As albatrosses depend on winds and waves for soaring flight, infrasonic cues may help albatrosses to identify environmental conditions that allow them to energetically optimize flight over long distances. Our results shed light on one of the great unresolved mysteries in nature, navigation in seemingly featureless ocean environments.

Mismatches between phenotype and environment shape fitness at hyperlocal scales.

In the era of human-driven climate change, understanding whether behavioural buffering of temperature change is linked with organismal fitness is essential. According to the 'cost-benefit' model of thermoregulation, animals that live in environments with high frequencies of favourable thermal microclimates should incur lower thermoregulatory costs, thermoregulate more efficiently and shunt the associated savings in time and energy towards other vital tasks such as feeding, territory defence and mate acquisition, increasing fitness. Here, we explore how thermal landscapes at the scale of individual territories, physiological performance and behaviour interact and shape fitness in the southern rock agama lizard (Agama atra). We integrated laboratory assays of whole organism performance with behavioural observations in the field, fine-scale estimates of environmental temperature, and paternity assignment of offspring to test whether fitness is predicted by territory thermal quality (i.e. the number of hours that operative temperatures in a territory fall within an individual's performance breadth). Male lizards that occupied territories of low thermal quality spent more time behaviourally compensating for sub-optimal temperatures and displayed less. Further, display rate was positively associated with lizard fitness, suggesting that there is an opportunity cost to engaging in thermoregulatory behaviour that will change as climate change progresses.

Warmer and more acidic conditions enhance performance of an endemic low-shore gastropod.

Changing ocean temperatures are predicted to challenge marine organisms, especially when combined with other factors, such as ocean acidification. Acclimation, as a form of phenotypic plasticity, can moderate the consequences of changing environments for biota. Our understanding of how altered temperature and acidification together influence species' acclimation responses is, however, limited compared with that of responses to single stressors. This study investigated how temperature and acidification affect the thermal tolerance and righting speed of the girdled dogwhelk, Trochia cingulata. Whelks were acclimated for 2 weeks to combinations of three temperatures (11°C: cold, 13°C: moderate and 15°C: warm) and two pH regimes (8.0: moderate and 7.5: acidic). We measured the temperature sensitivity of the righting response by generating thermal performance curves from individual data collected at seven test temperatures and determined critical thermal minima (CTmin) and maxima (CTmax). We found that T. cingulata has a broad basal thermal tolerance range (∼38°C) and after acclimation to the warm temperature regime, both the optimal temperature for maximum righting speed and CTmax increased. Contrary to predictions, acidification did not narrow this population's thermal tolerance but increased CTmax. These plastic responses are likely driven by the predictable exposure to temperature extremes measured in the field which originate from the local tidal cycle and the periodic acidification associated with ocean upwelling in the region. This acclimation ability suggests that T. cingulata has at least some capacity to buffer the thermal changes and increased acidification predicted to occur with climate change.

Scaling of ear morphology across 127 bird species and its implications for hearing performance.

The dimensions of auditory structures among animals of varying body size can have implications for hearing performance. Larger animals often have a hearing range focused on lower frequencies than smaller animals, which may be explained by several anatomical mechanisms in the ear and their scaling relationships. While the effect of size on ear morphology and hearing performance has been explored in some mammals, anurans and lizards, much less is known about the scaling relationships for the single-ossicle, internally-coupled ears of birds. Using micro- and nano-CT scans of the tympanic middle and inner ears of 127 ecologically and phylogenetically diverse bird species, spanning more than 400-fold in head mass (2.3 to 950 g), we undertook phylogenetically-informed scaling analyses to test whether 12 morphological traits, of functional importance to hearing, maintain their relative proportions with increasing head mass. We then extended our analysis by regressing these morphological traits with measures of hearing sensitivity and range to better understand morphological underpinnings of hearing performance. We find that most auditory structures scale together in equal proportions, whereas columella length increases disproportionately. We also find that the size of several auditory structures is associated with increased hearing sensitivity and frequency hearing limits, while head mass did not explain these measures. Although both birds and mammals demonstrate proportional scaling between auditory structures, the consequences for hearing in each group may diverge due to unique morphological predictors of auditory performance.

No consistent effect of daytime versus night-time measurement of thermal tolerance in nocturnal and diurnal lizards.

While essential in understanding impacts of climate change for organisms, diel variation remains an understudied component of temporal variation in thermal tolerance limits [i.e. the critical thermal minimum (CTmin) and maximum (CTmax)]. For example, a higher Ctmax might be expected for an individual if the measurement is taken during the day (when heat stress is most likely to occur) instead of at night. We measured thermal tolerance (Ctmin and Ctmax) during both the daytime and night-time in 101 nocturnal and diurnal geckos and skinks in Hong Kong and in South Africa, representing six species and covering a range of habitats. We found that period of measurement (day vs. night) only affected Ctmin in South Africa (but not in Hong Kong) and that Ctmax was unaffected. Body size and species were important factors for determining Ctmax in Hong Kong and Ctmin in South Africa, respectively. Overall, however, we did not find consistent diel variation of thermal tolerance and suggest that measurements of critical thermal limits may be influenced by timing of measurement-but that such effects, when present, are likely to be context-dependent.

Aquatic birds have middle ears adapted to amphibious lifestyles.

Birds exhibit wide variation in their use of aquatic environments, on a spectrum from entirely terrestrial, through amphibious, to highly aquatic. Although there are limited empirical data on hearing sensitivity of birds underwater, mounting evidence indicates that diving birds detect and respond to sound underwater, suggesting that some modifications of the ear may assist foraging or other behaviors below the surface. In air, the tympanic middle ear acts as an impedance matcher that increases sound pressure and decreases sound vibration velocity between the outside air and the inner ear. Underwater, the impedance-matching task is reversed and the ear is exposed to high hydrostatic pressures. Using micro- and nano-CT (computerized tomography) scans of bird ears in 127 species across 26 taxonomic orders, we measured a suite of morphological traits of importance to aerial and aquatic hearing to test predictions relating to impedance-matching in birds with distinct aquatic lifestyles, while accounting for allometry and phylogeny. Birds that engage in underwater pursuit and deep diving showed the greatest differences in ear structure relative to terrestrial species. In these heavily modified ears, the size of the input areas of both the tympanic membrane and the columella footplate of the middle ear were reduced. Underwater pursuit and diving birds also typically had a shorter extrastapedius, a reduced cranial air volume and connectivity and several modifications in line with reversals of low-to-high impedance-matching. The results confirm adaptations of the middle ear to aquatic lifestyles in multiple independent bird lineages, likely facilitating hearing underwater and baroprotection, while potentially constraining the sensitivity of aerial hearing.

How useful are thermal vulnerability indices?

To forecast climate change impacts across habitats or taxa, thermal vulnerability indices (e.g., safety margins and warming tolerances) are growing in popularity. Here, we present their history, context, formulation, and current applications. We highlight discrepancies in terminology and usage, and we draw attention to key assumptions underpinning the main indices and to their ecological and evolutionary relevance. In the process, we flag biases influencing these indices that are not always evaluated. These biases affect both components of index formulations, namely: (i) the characterisation of the thermal environment; and (ii) an organism's physiological and behavioural responses to more frequent and severe warming. Presently, many outstanding questions weaken a thermal vulnerability index approach. We describe ways to validate vulnerability index applications and outline issues to be considered in further developing these indices.

Untangling the structural and molecular mechanisms underlying colour and rapid colour change in a lizard, Agama atra.

With functions as diverse as communication, protection and thermoregulation, coloration is one of the most important traits in lizards. The ability to change colour as a function of varying social and environmental conditions is thus an important innovation. While colour change is present in animals ranging from squids, to fish and reptiles, not much is known about the mechanisms behind it. Traditionally, colour change was attributed to migration of pigments, in particular melanin. More recent work has shown that the changes in nanostructural configuration inside iridophores are able to produce a wide palette of colours. However, the genetic mechanisms underlying colour, and colour change in particular, remain unstudied. Here we use a combination of transcriptomic and microscopic data to show that melanin, iridophores and pteridines are the main colour-producing mechanisms in Agama atra, and provide molecular and structural data suggesting that rapid colour change is achieved via melanin dispersal in combination with iridophore organization. This work demonstrates the power of combining genotypic (gene expression) and phenotypic (microscopy) information for addressing physiological questions, providing a basis for future studies of colour change.

The evolution of critical thermal limits of life on Earth.

Understanding how species' thermal limits have evolved across the tree of life is central to predicting species' responses to climate change. Here, using experimentally-derived estimates of thermal tolerance limits for over 2000 terrestrial and aquatic species, we show that most of the variation in thermal tolerance can be attributed to a combination of adaptation to current climatic extremes, and the existence of evolutionary 'attractors' that reflect either boundaries or optima in thermal tolerance limits. Our results also reveal deep-time climate legacies in ectotherms, whereby orders that originated in cold paleoclimates have presently lower cold tolerance limits than those with warm thermal ancestry. Conversely, heat tolerance appears unrelated to climate ancestry. Cold tolerance has evolved more quickly than heat tolerance in endotherms and ectotherms. If the past tempo of evolution for upper thermal limits continues, adaptive responses in thermal limits will have limited potential to rescue the large majority of species given the unprecedented rate of contemporary climate change.

The evolution of insect body coloration under changing climates.

Insects have been influential models in research on color variation, its evolutionary drivers and the mechanistic basis of such variation. More recently, several studies have indicated that insect color is responding to rapid climate change. However, it remains challenging to ascertain drivers of color variation among populations and species, and across space and time, as multiple biotic and abiotic factors can interact and mediate color change. Here, we describe some of the challenges and recent advances made in this field. First, we outline the main alternative hypotheses that exist for insect color variation in relation to climatic factors. Second, we review the existing evidence for contemporary adaptive evolution of insect color in response to climate change and then discuss factors that can promote or hinder the evolution of color in response to climate change. Finally, we propose future directions and highlight gaps in this research field. Pigments and structures producing insect color can vary concurrently or independently, and may evolve at different rates, with poorly understood effects on gene frequencies and fitness. Disentangling multiple competing hypotheses explaining insect coloration should be key to assign color variation as an evolutionary response to climate change.

Sex-specific effects of wind on the flight decisions of a sexually dimorphic soaring bird.

In a highly dynamic airspace, flying animals are predicted to adjust foraging behaviour to variable wind conditions to minimize movement costs. Sexual size dimorphism is widespread in wild animal populations, and for large soaring birds which rely on favourable winds for energy-efficient flight, differences in morphology, wing loading and associated flight capabilities may lead males and females to respond differently to wind. However, the interaction between wind and sex has not been comprehensively tested. We investigated, in a large sexually dimorphic seabird which predominantly uses dynamic soaring flight, whether flight decisions are modulated to variation in winds over extended foraging trips, and whether males and females differ. Using GPS loggers we tracked 385 incubation foraging trips of wandering albatrosses Diomedea exulans, for which males are c. 20% larger than females, from two major populations (Crozet and South Georgia). Hidden Markov models were used to characterize behavioural states-directed flight, area-restricted search (ARS) and resting-and model the probability of transitioning between states in response to wind speed and relative direction, and sex. Wind speed and relative direction were important predictors of state transitioning. Birds were much more likely to take off (i.e. switch from rest to flight) in stronger headwinds, and as wind speeds increased, to be in directed flight rather than ARS. Males from Crozet but not South Georgia experienced stronger winds than females, and males from both populations were more likely to take-off in windier conditions. Albatrosses appear to deploy an energy-saving strategy by modulating taking-off, their most energetically expensive behaviour, to favourable wind conditions. The behaviour of males, which have higher wing loading requiring faster speeds for gliding flight, was influenced to a greater degree by wind than females. As such, our results indicate that variation in flight performance drives sex differences in time-activity budgets and may lead the sexes to exploit regions with different wind regimes.

Infrasonic hearing in birds: a review of audiometry and hypothesized structure-function relationships.

The perception of airborne infrasound (sounds below 20 Hz, inaudible to humans except at very high levels) has been documented in a handful of mammals and birds. While animals that produce vocalizations with infrasonic components (e.g. elephants) present conspicuous examples of potential use of infrasound in the context of communication, the extent to which airborne infrasound perception exists among terrestrial animals is unclear. Given that most infrasound in the environment arises from geophysical sources, many of which could be ecologically relevant, communication might not be the only use of infrasound by animals. Therefore, infrasound perception could be more common than currently realized. At least three bird species, each of which do not communicate using infrasound, are capable of detecting infrasound, but the associated auditory mechanisms are not well understood. Here we combine an evaluation of hearing measurements with anatomical observations to propose and evaluate hypotheses supporting avian infrasound detection. Environmental infrasound is mixed with non-acoustic pressure fluctuations that also occur at infrasonic frequencies. The ear can detect such non-acoustic pressure perturbations and therefore, distinguishing responses to infrasound from responses to non-acoustic perturbations presents a great challenge. Our review shows that infrasound could stimulate the ear through the middle ear (tympanic) route and by extratympanic routes bypassing the middle ear. While vibration velocities of the middle ear decline towards infrasonic frequencies, whole-body vibrations - which are normally much lower amplitude than that those of the middle ear in the 'audible' range (i.e. >20 Hz) - do not exhibit a similar decline and therefore may reach vibration magnitudes comparable to the middle ear at infrasonic frequencies. Low stiffness in the middle and inner ear is expected to aid infrasound transmission. In the middle ear, this could be achieved by large air cavities in the skull connected to the middle ear and low stiffness of middle ear structures; in the inner ear, the stiffness of round windows and cochlear partitions are key factors. Within the inner ear, the sizes of the helicotrema and cochlear aqueduct are expected to play important roles in shunting low-frequency vibrations away from low-frequency hair-cell sensors in the cochlea. The basilar papilla, the auditory organ in birds, responds to infrasound in some species, and in pigeons, infrasonic-sensitive neurons were traced back to the apical, abneural end of the basilar papilla. Vestibular organs and the paratympanic organ, a hair cell organ outside of the inner ear, are additional untested candidates for infrasound detection in birds. In summary, this review brings together evidence to create a hypothetical framework for infrasonic hearing mechanisms in birds and other animals.

Navigating through the r packages for movement.

The advent of miniaturized biologging devices has provided ecologists with unprecedented opportunities to record animal movement across scales, and led to the collection of ever-increasing quantities of tracking data. In parallel, sophisticated tools have been developed to process, visualize and analyse tracking data; however, many of these tools have proliferated in isolation, making it challenging for users to select the most appropriate method for the question in hand. Indeed, within the r software alone, we listed 58 packages created to deal with tracking data or 'tracking packages'. Here, we reviewed and described each tracking package based on a workflow centred around tracking data (i.e. spatio-temporal locations (x, y, t)), broken down into three stages: pre-processing, post-processing and analysis, the latter consisting of data visualization, track description, path reconstruction, behavioural pattern identification, space use characterization, trajectory simulation and others. Supporting documentation is key to render a package accessible for users. Based on a user survey, we reviewed the quality of packages' documentation and identified 11 packages with good or excellent documentation. Links between packages were assessed through a network graph analysis. Although a large group of packages showed some degree of connectivity (either depending on functions or suggesting the use of another tracking package), one third of the packages worked in isolation, reflecting a fragmentation in the r movement-ecology programming community. Finally, we provide recommendations for users when choosing packages, and for developers to maximize the usefulness of their contribution and strengthen the links within the programming community.

The evolutionary potential of an insect invader under climate change.

Although the impacts of climate change and invasive species are typically studied in isolation, they likely interact to reduce the viability of plant and animal populations. Indeed, invasive species, by definition, have succeeded in areas outside of their native range and may therefore have higher adaptive capacity relative to native species. Nevertheless, the genetic architecture of the thermal niche, which sets a limit to the potential for populations to evolve rapidly under climate change, has never been measured in an invasive species in its introduced range. Here, we estimate the genetic architecture of thermal performance in the harlequin beetle (Harmonia axyridis), a Central Asian species that has invaded four continents. We measured thermal performance curves in more than 400 third-generation offspring from a paternal half-sib breeding experiment and analyzed the genetic variance-covariance matrix. We show that while the critical thermal limits in this species have an additive genetic basis, most components of the thermal performance curve have low heritability. Moreover, we found evidence that genetic correlations may constrain the evolution of beetles under climate change. Our results suggest that some invasive species may have limited evolutionary capacity under climate change, despite their initial success in colonizing novel environments.

The Bogert Effect and environmental heterogeneity.

A classic question in evolutionary biology is whether behavioral flexibility hastens or hinders evolutionary change. The latter idea, that behavior reduces the number of environmental states experienced by an organism and buffers that organism against selection, has been dubbed the "Bogert Effect" after Charles Bogert, the biologist who first popularized the phenomenon using data from lizards. The Bogert Effect is pervasive when traits like body temperature, which tend to be invariant across space in species that behaviorally thermoregulate, are considered. Nevertheless, behavioral thermoregulation decreases or stops when spatial variation in operative temperature is low. We compared environmental temperatures, thermoregulatory behavior, and a suite of physiological and morphological traits between two populations of the southern rock agama (Agama atra) in South Africa that experience different climatic regimes. Individuals from both populations thermoregulated efficiently, maintaining body temperatures within their preferred temperature range throughout most of their activity cycle. Nevertheless, they differed in the thermal sensitivity of resting metabolic rate at cooler body temperatures and in morphology. Our results support the common assertion that thermoregulatory behavior may prevent divergence in traits like field-active body temperature, which are measured during periods of high environmental heterogeneity. Nevertheless, we show that other traits may be free to diverge if they are under selection during times when environments are homogenous. We argue that the importance of the Bogert Effect is critically dependent on the nature of environmental heterogeneity and will therefore be relevant to some traits and irrelevant to others in many populations.

Opportunities for behavioral rescue under rapid environmental change.

Laboratory measurements of physiological and demographic tolerances are important in understanding the impact of climate change on species diversity; however, it has been recognized that forecasts based solely on these laboratory estimates overestimate risk by omitting the capacity for species to utilize microclimatic variation via behavioral adjustments in activity patterns or habitat choice. The complex, and often context-dependent nature, of microclimate utilization has been an impediment to the advancement of general predictive models. Here, we overcome this impediment and estimate the potential impact of warming on the fitness of ectotherms using a benefit/cost trade-off derived from the simple and broadly documented thermal performance curve and a generalized cost function. Our framework reveals that, for certain environments, the cost of behavioral thermoregulation can be reduced as warming occurs, enabling behavioral buffering (e.g., the capacity for behavior to ameliorate detrimental impacts) and "behavioral rescue" from extinction in extreme cases. By applying our framework to operative temperature and physiological data collected at an extremely fine spatial scale in an African lizard, we show that new behavioral opportunities may emerge. Finally, we explore large-scale geographic differences in the impact of behavior on climate-impact projections using a global dataset of 38 insect species. These multiple lines of inference indicate that understanding the existing relationship between thermal characteristics (e.g., spatial configuration, spatial heterogeneity, and modal temperature) is essential for improving estimates of extinction risk.

Thermal tolerance patterns across latitude and elevation.

Linking variation in species' traits to large-scale environmental gradients can lend insight into the evolutionary processes that have shaped functional diversity and future responses to environmental change. Here, we ask how heat and cold tolerance vary as a function of latitude, elevation and climate extremes, using an extensive global dataset of ectotherm and endotherm thermal tolerance limits, while accounting for methodological variation in acclimation temperature, ramping rate and duration of exposure among studies. We show that previously reported relationships between thermal limits and latitude in ectotherms are robust to variation in methods. Heat tolerance of terrestrial ectotherms declined marginally towards higher latitudes and did not vary with elevation, whereas heat tolerance of freshwater and marine ectotherms declined more steeply with latitude. By contrast, cold tolerance limits declined steeply with latitude in marine, intertidal, freshwater and terrestrial ectotherms, and towards higher elevations on land. In all realms, both upper and lower thermal tolerance limits increased with extreme daily temperature, suggesting that different experienced climate extremes across realms explain the patterns, as predicted under the Climate Extremes Hypothesis. Statistically accounting for methodological variation in acclimation temperature, ramping rate and exposure duration improved model fits, and increased slopes with extreme ambient temperature. Our results suggest that fundamentally different patterns of thermal limits found among the earth's realms may be largely explained by differences in episodic thermal extremes among realms, updating global macrophysiological 'rules'. This article is part of the theme issue 'Physiological diversity, biodiversity patterns and global climate change: testing key hypotheses involving temperature and oxygen'.

Thermal landscape change as a driver of ectotherm responses to plant invasions.

A growing body of research demonstrates the impacts of invasive alien plants on native animals, but few studies consider thermal effects as a driver of the responses of native organisms. As invasive alien plants establish and alter the composition and arrangement of plant communities, the thermal landscapes available to ectotherms also change. Our study reviews the research undertaken to date on the thermal effects of alien plant invasions on native reptiles, amphibians, insects and arachnids. The 37 studies published between 1970 and early 2019 portray an overall detrimental effect of invasive plants on thermal landscapes, ectothermic individuals' performance and species abundance, diversity and composition. With a case study of a lizard species, we illustrate the use of thermal ecology tools in plant invasion research and test the generality of alien plant effects: changes in thermoregulation behaviour in invaded landscapes varied depending on the level of invasion and lizard traits. Together, the literature review and case study show that thermal effects of alien plants on ectotherms can be substantial albeit context-dependent. Further research should cover multiple combinations of native/invasive plant growth forms, invasion stages and ectotherm traits. More attention is also needed to test causality along the chain of effects from thermal landscapes to individuals, populations and communities.