Thermal adaptation best explains Bergmann's and Allen's Rules across ecologically diverse shorebirds

Shorebirds Are Shrinking and Shape-Shifting: Declining Body Size and Lengthening Bills in the Past Half-Century

Animals are predicted to shrink and shape-shift as the climate warms, declining in size, while their appendages lengthen. Determining which types of species are undergoing these morphological changes, and why, is critical to understanding species responses to global change, including potential adaptation to climate warming. We examine body size and bill length changes in 25 shorebird species using extensive field data (> 200,000 observations) collected over 46 years (1975-2021) by community scientists. We show widespread body size declines over time, and after short-term exposure to warmer summers. Meanwhile, shorebird bills are lengthening over time but shorten after hot summers. Shrinking and shape-shifting patterns are consistent across ecologically diverse shorebirds from tropical and temperate Australia, are more pronounced in smaller species and vary according to migration behaviour. These widespread morphological changes could be explained by multiple drivers, including adaptive and maladaptive responses to nutritional stress, or by thermal adaptation to climate warming.

The underlying causes of differential migration: assumptions, hypotheses, and predictions

Mechanisms governing the migratory decisions of birds have long fascinated ecologists and sparked considerable debate. Identifying factors responsible for variation in migration distance, also known as differential migration, has been a popular approach to understanding the mechanisms underlying migratory behaviour more generally. However, research progress has been slowed by the continued testing of overlapping, non-mechanistic, and circular predictions among a small set of historically entrenched hypotheses. We highlight the body size hypothesis and suggest that the predictions commonly tested have impeded progress because body size relationships with migration distance are predictions made by several distinct hypotheses with contrasting mechanisms. The cost of migration itself has not been adequately accounted for in most hypotheses, and we propose two flight efficiency hypotheses with time- and energy-minimizing mechanisms that allow individuals to mitigate the risks inherent to longer migrations. We also advance two conceptual versions of the social dominance hypothesis based on two distinct underlying mechanisms related to distance minimization and food maximization that will help clarify the role of competition in driving migratory decisions. Overall, we describe and refine 12 mechanistic hypotheses proposed to explain differential migration (along with several other special-case hypotheses), seven of which have underlying mechanisms related to food limitation as past research has identified this to be an important driver of differential migration. We also thoroughly reviewed 145 publications to assess the amount of support for 10 critical assumptions underlying alternative hypotheses for differential migration in birds. Our review reveals that surprisingly few studies explicitly evaluate assumptions within a differential migration context. Generating and testing strong predictions and critical assumptions underlying mechanisms of alternative hypotheses will improve our ability to differentiate among these explanations of differential migration. Additionally, future intraspecific progress will be greatest if investigators continue to focus on mechanisms underlying variation in migration distance within rather than among demographic classes, as previous research has found differing mechanisms to be responsible for differential migration among demographic classes. Interspecifically, a thorough comparative analysis that seeks to explain variation in migration distance among species would broaden both our understanding of the mechanisms regulating current differential migration patterns and those that led to the evolution of migration more generally. Collectively, we provide a framework that, together with advances in animal-borne tracking and other technology, can be used to advance our understanding of the causes of differential migration distance, and migratory decisions more generally.

Optimal duration of whole-body cryostimulation exposure to achieve target skin temperature: influence of body mass index-a randomized cross-over controlled trial

BACKGROUND

The efficacy of whole-body cryostimulation (WBC) may be influenced by individual characteristics. The aim of this study is to determine the optimal exposure time required to reach the analgesic threshold of 13.6 °C, which has been proposed to be a target temperature to be reached at skin level. Our objective is also to follow the skin temperature changes during and after WBC considering the participants body mass index (BMI).

METHODS

Thirty healthy men were assigned into 2 groups based on their BMI [normal weight (n = 15; BMI = 21.53 ± 1.63 kg·m-2) and overweight (n = 15; BMI = 27.98 ± 1.16 kg·m-2)]. In a random order, each participant experienced a 4-min WBC exposure, as well as a control session with no cold exposure. Skin temperature was measured using a thermal imaging camera during and after cold exposure.

RESULTS

Normal weight participants reached the threshold in 4 min, whereas overweight participants reached it in 3 min 30 s. Following WBC, a rapid mean skin temperature (MsT°) increase was observed for both groups, immediately after exposure. However, after 30 min, MsT° remained significantly lower than at baseline.

CONCLUSION

Our findings suggest that appropriate WBC dosage may differ according to BMI. Understanding the impact of such variable on cold exposure outcomes can help to optimize WBC treatments and maximize potential benefits.

Adaptive Radiation Without Independent Stages of Trait Evolution in a Group of Caribbean Anoles

Adaptive radiation involves diversification along multiple trait axes, producing phenotypically diverse, species-rich lineages. Theory generally predicts that multi-trait evolution occurs via a "stages" model, with some traits saturating early in a lineage's history, and others diversifying later. Despite its multidimensional nature, however, we know surprisingly little about how different suites of traits evolve during adaptive radiation. Here, we investigated the rate, pattern, and timing of morphological and physiological evolution in the anole lizard adaptive radiation from the Caribbean island of Hispaniola. Rates and patterns of morphological and physiological diversity are largely unaligned, corresponding to independent selective pressures associated with structural and thermal niches. Cold tolerance evolution reflects parapatric divergence across elevation, rather than niche partitioning within communities. Heat tolerance evolution and the preferred temperature evolve more slowly than cold tolerance, reflecting behavioral buffering, particularly in edge-habitat species (a pattern associated with the Bogert effect). In contrast to the nearby island of Puerto Rico, closely related anoles on Hispaniola do not sympatrically partition thermal niche space. Instead, allopatric and parapatric separation across biogeographic and environmental boundaries serves to keep morphologically similar close relatives apart. The phenotypic diversity of this island's adaptive radiation accumulated largely as a by-product of time, with surprisingly few exceptional pulses of trait evolution. A better understanding of the processes that guide multidimensional trait evolution (and nuance therein) will prove key in determining whether the stages model should be considered a common theme of adaptive radiation.

Pattern and Process in a Rapidly Changing World: Ideas and Approaches

AbstractScience is as dynamic as the world around us. Our ideas continually change, as do the approaches we use to study science. Few things remain invariant in this changing landscape, but a fascination with pattern and process is one that has endured throughout the history of science. Paying homage to this long-held tradition, the 2023 Vice Presidential Symposium of the American Society of Naturalists focused on the role of pattern and process in ecology and evolution. It brought together a group of early-career researchers working on topics ranging from genetic diversity in microbes to changing patterns of species interactions in the geological record. Their work spanned the taxonomic spectrum from microbes to mammals, the temporal dimension from the Cenozoic to the present, and approaches ranging from manipulative experiments to comparative approaches. In this introductory article, I discuss how these diverse topics are linked by the common thread of elucidating processes underlying patterns and how they collectively generate novel insights into diversity maintenance at different levels of organization.

An Interspecific Assessment of Bergmann's Rule in Tenebrionid Beetles (Coleoptera, Tenebrionidae) along an Elevation Gradient

In endotherms, body size tends to increase with elevation and latitude (i.e., with decreasing temperatures) (Bergmann's rule). These patterns are explained in terms of heat balance since larger animals need to produce less heat relative to their size to maintain stable body temperatures. In ectotherms like most insects, where this mechanism cannot operate, a reverse pattern is frequently observed, as a higher surface area-to-volume ratio in colder climates may allow for more rapid heating and cooling. However, patterns of increasing body size with decreasing temperatures can also be observed in ectotherms if selection for more stable internal temperatures leads to smaller surface area-to-volume ratios. Data on tenebrionids from Latium (Central Italy) were used to model elevational variations in average values of body size (total length, mass and volume) and surface area-to-volume ratio. Analyses were performed by considering the whole fauna and two ecological groups separately: ground-dwelling species (geophilous) and arboreal (xylophilous) species. The surface area-to-volume ratios declined with increasing elevation in all cases, indicating that the need for heat conservation is more important than rapid heating and cooling. However, in xylophilous species (which typically live under bark), body size increased with increasing elevation, and in geophilous species, an opposite pattern was observed up to about 1000 m, followed by an increasing pattern. This suggests that a reduction in resource availability with elevation limits body size in geophilous species up to a certain elevation but not in xylophilopus species, which benefit from more climatically stable conditions and constant resources and need energy for overwintering.

Adaptation to fluctuating temperatures across life stages in endotherms

Accelerating rates of climate change have intensified research on thermal adaptation. Increasing temperature fluctuations, a prominent feature of climate change, means that the persistence of many species depends on both heat and cold tolerance across the entire life cycle. In endotherms, research has focused on specific life stages, with changes in thermoregulation across life rarely being examined. Consequently, there is a need to (i) analyse how heat and cold tolerance mechanisms coevolve, and (ii) test whether antagonistic effects between heat and cold tolerance across different life stages limit thermal adaptation. Information on genes influencing heat and cold tolerance and how they are expressed through life will enable more accurate modelling of species vulnerabilities to future climatic volatility.

Viviparity imparts a macroevolutionary signature of ecological opportunity in the body size of female Liolaemus lizards

Viviparity evolved ~115 times across squamate reptiles, facilitating the colonization of cold habitats, where oviparous species are scarce or absent. Whether the ecological opportunity furnished by such colonization reconfigures phenotypic diversity and accelerates evolution is unclear. We investigated the association between viviparity and patterns and rates of body size evolution in female Liolaemus lizards, the most species-rich tetrapod genus from temperate regions. Here, we discover that viviparous species evolve ~20% larger optimal body sizes than their oviparous relatives, but exhibit similar rates of body size evolution. Through a causal modeling approach, we find that viviparity indirectly influences body size evolution through shifts in thermal environment. Accordingly, the colonization of cold habitats favors larger body sizes in viviparous species, reconfiguring body size diversity in Liolaemus. The catalyzing influence of viviparity on phenotypic evolution arises because it unlocks access to otherwise inaccessible sources of ecological opportunity, an outcome potentially repeated across the tree of life.

Age structure and body size of two Tibetan toad (Bufo tibetanus) populations from different elevations in China

Understanding how age and body size vary across elevations can provide insights into the evolution of life-history traits in animals. In the present study, we compared the demographic (using skeletochronology) and morphological traits of the Tibetan toad (Bufo tibetanus) between two populations from different elevational habitats (2650 vs. 3930 m). We found that (1) the mean age and body size of females were significantly greater than those of males in both populations; (2) both sexes of toads from the higher elevation tended to be significantly older in age and larger in body size; (3) there was a significant positive relationship between age and body size within each sex of the toad at both elevations; and (4) growth rates varied between the two populations, with the higher rate observed in the lower-elevation population. Our results suggested that factors other than age, such as elevation-associated temperature, influence the observed differences in body size between the two populations. Future research at a broader range of elevations should focus on these factors and evaluate their influence on animal growth patterns.

Body size shapes song in honeyeaters

Birdsongs are among the most distinctive animal signals. Their evolution is thought to be shaped simultaneously by habitat structure and by the constraints of morphology. Habitat structure affects song transmission and detectability, thus influencing song (the acoustic adaptation hypothesis), while body size and beak size and shape necessarily constrain song characteristics (the morphological constraint hypothesis). Yet, support for the acoustic adaptation and morphological constraint hypotheses remains equivocal, and their simultaneous examination is infrequent. Using a phenotypically diverse Australasian bird clade, the honeyeaters (Aves: Meliphagidae), we compile a dataset consisting of song, environmental, and morphological variables for 163 species and jointly examine predictions of these two hypotheses. Overall, we find that body size constrains song frequency and pace in honeyeaters. Although habitat type and environmental temperature influence aspects of song, that influence is indirect, likely via effects of environmental variation on body size, with some evidence that elevation constrains the evolution of song peak frequency. Our results demonstrate that morphology has an overwhelming influence on birdsong, in support of the morphological constraint hypothesis, with the environment playing a secondary role generally via body size rather than habitat structure. These results suggest that changing body size (a consequence of both global effects such as climate change and local effects such as habitat transformation) will substantially influence the nature of birdsong.

Climate change and its effects on body size and shape: the role of endocrine mechanisms

In many organisms, rapidly changing environmental conditions are inducing dramatic shifts in diverse phenotypic traits with consequences for fitness and population viability. However, the mechanisms that underlie these responses remain poorly understood. Endocrine signalling systems often influence suites of traits and are sensitive to changes in environmental conditions; they are thus ideal candidates for uncovering both plastic and evolved consequences of climate change. Here, we use body size and shape, a set of integrated traits predicted to shift in response to rising temperatures with effects on fitness, and insulin-like growth factor-1 as a case study to explore these ideas. We review what is known about changes in body size and shape in response to rising temperatures and then illustrate why endocrine signalling systems are likely to be critical in mediating these effects. Lastly, we discuss research approaches that will advance understanding of the processes that underlie rapid responses to climate change and the role endocrine systems will have. Knowledge of the mechanisms involved in phenotypic responses to climate change will be essential for predicting both the ecological and the long-term evolutionary consequences of a warming climate. This article is part of the theme issue 'Endocrine responses to environmental variation: conceptual approaches and recent developments'.

Birds are better at regulating heat loss through their legs than their bills: implications for body shape evolution in response to climate

Endotherms use their appendages-such as legs, tails, ears and bills-for thermoregulation by controlling blood flow to near-surface blood vessels, conserving heat when it is cold, and dissipating heat in hot conditions. Larger appendages allow greater heat dissipation, and appendage sizes vary latitudinally according to Allen's rule. However, little is known about the relative importance of different appendages for thermoregulation. We investigate physiological control of heat loss via bird bills and legs using infrared thermography of wild birds. Our results demonstrate that birds are less able to regulate heat loss via their bills than their legs. In cold conditions, birds lower their leg surface temperature to below that of their plumage surface, retaining heat at their core. In warm conditions, birds increase their leg surface temperature to above that of their plumage surface, expelling heat. By contrast, bill surface temperature remains approximately 2°C warmer than the plumage surface, indicating consistent heat loss under almost all conditions. Poorer physiological control of heat loss via bird bills likely entails stronger selection for shorter bills in cold climates. This could explain why bird bills show stronger latitudinal size clines than bird legs, with implications for predicting shape-shifting responses to climate change.

Temperature-dependent Developmental Plasticity and Its Effects on Allen's and Bergmann's Rules in Endotherms

Ecogeographical rules, describing common trends in animal form across space and time, have provided key insights into the primary factors driving species diversity on our planet. Among the most well-known ecogeographical rules are Bergmann's rule and Allen's rule, with each correlating ambient temperature to the size and shape of endotherms within a species. In recent years, these two rules have attracted renewed research attention, largely with the goal of understanding how they emerge (e.g., via natural selection or phenotypic plasticity) and, thus, whether they may emerge quickly enough to aid adaptations to a warming world. Yet despite this attention, the precise proximate and ultimate drivers of Bergmann's and Allen's rules remain unresolved. In this conceptual paper, we articulate novel and classic hypotheses for understanding whether and how plastic responses to developmental temperatures might contributed to each rule. Next, we compare over a century of empirical literature surrounding Bergmann's and Allen's rules against our hypotheses to uncover likely avenues by which developmental plasticity might drive temperature-phenotype correlations. Across birds and mammals, studies strongly support developmental plasticity as a driver of Bergmann's and Allen's rules, particularly with regards to Allen's rule. However, plastic contributions toward each rule appear largely non-linear and dependent upon: (1) efficiency of energy use (Bergmann's rule) and (2) thermal advantages (Allen's rule) at given ambient temperatures. These findings suggest that, among endotherms, rapid changes in body shape and size will continue to co-occur with our changing climate, but generalizing the direction of responses across populations is likely naive.

Variation in bill surface area is associated with local climatic factors across populations of the plain laughingthrush

Recent studies have found that avian bill and tarsus morphology may have evolved in response to climatic conditions, and these organs play important roles in thermoregulation and water retention in extreme environments. Here, we examined whether bill surface area and tarsus length were associated with climatic conditions in the plain laughingthrush, Garrulax davidi, which mainly occurs in north China and occupies several climatic zones from east to west. We measured bill surface area and tarsus length in 321 adults from 11 populations, almost encompassing all habitat types of the species. We analyzed the relationships among these morphological traits and local climatic factors. Bill surface area was positively correlated with maximum temperature, indicating that bill surface area tended to be larger in hotter environments. Furthermore, we found a negative relationship among bill surface area and winter precipitation, indicating that bill surface area tended to be larger in arid areas. However, we did not find any relationships between tarsus length and climatic factors. These results suggest that local climates may shape the evolution of bill morphology divergence, and summer seems to be the critical season for thermoregulation in this temperate zone passerine.

Ecological predictors of interspecific variation in bird bill and leg lengths on a global scale

Bills and legs are two vital appendages for birds, and they exhibit huge interspecific variation in form and function, yet no study has examined the global predictors of this variation. This study examined global gradients in the relative lengths of bird bills and tarsi (i.e. exposed leg parts) to body size across non-migratory birds, while accounting for phylogeny. We found that relative bill length and tarsus length were related to diet, habitat density, latitude, annual mean temperature, temperature variability and hand-wing index (HWI), a proxy for birds' flight efficiency. Among these factors, diet played a primary role in predicting bill length, with nectar-feeding pollinators, vertivores, invertivores and omnivores having longer bills; HWI emerged as the predominant predictor of tarsus length, wherein species with higher HWI had shorter tarsi. However, the effects of these factors differed between passerines and non-passerines, with some temperature-related effects exhibiting opposite trends between these two groups. Our findings highlight the compromise in adaptations for feeding, thermoregulation and flight performance between the two distinct appendages.

Egg characteristics vary longitudinally in Arctic shorebirds

Arctic environments are changing rapidly and if we are to understand the resilience of species to future changes, we need to investigate alterations in their life histories. Egg size and egg shape are key life-history traits, reflecting parental investment as well as influencing future reproductive success. Here we focus on egg characteristics in two Arctic shorebirds, the Dunlin (Calidris alpina) and the Temminck's stint (Calidris temminckii). Using egg photos that encompass their full breeding ranges, we show that egg characteristics exhibit significant longitudinal variations, and the variation in the monogamous species (Dunlin) is significantly greater than the polygamous species (Temminck's stint). Our finding is consistent with the recent "disperse-to-mate" hypothesis which asserts that polygamous species disperse further to find mates than monogamous species, and by doing so they create panmictic populations. Taken together, Arctic shorebirds offer excellent opportunities to understand evolutionary patterns in life history traits.

Energetic costs of bill heat exchange demonstrate contributions to thermoregulation at high temperatures in toco toucans (Ramphastos toco)

Body temperature regulation under changes in ambient temperature involves adjustments in heat production and heat exchange rates between the animal and the environment. One mechanism involves the modulation of the surface temperature of specific areas of the body through vasomotor adjustment. In homeotherms, this thermoregulatory adjustment is essential for the maintenance of body temperature over a moderate temperature range, known as the thermal neutral zone (TNZ). The bill of the toco toucan (Ramphastos toco) has been described as a highly efficient thermal window and hypothesized to assist in the thermal homeostasis of this bird. Herein, we directly evaluated the contribution of heat exchange through the bill of the toco toucan and role of the bill in the delimitation of the TNZ. To do this, we measured metabolic rate (MR), via oxygen consumption, over a range of ambient temperatures from 0 to 35°C. MR measurements were made in birds with the bill intact and with the bill insulated. The limits of the TNZ did not differ between treatments, ranging from 10.8 to 25.0°C. The MR differed among treatments only at elevated temperatures (30 and 35°C), reaching 0.92±0.11 ml O2 g-1 h-1 (mean±s.d.) for the intact group and 1.13±0.13 ml O2 g-1 h-1 for the insulated group. These results indicate that although heat dissipation through the bill does not contribute significantly to widening of the TNZ, it may well be critically important in assisting body temperature regulation at higher temperatures extending above the upper limit of the TNZ.

Allometry reveals trade-offs between Bergmann's and Allen's rules, and different avian adaptive strategies for thermoregulation

Animals tend to decrease in body size (Bergmann's rule) and elongate appendages (Allen's rule) in warm climates. However, it is unknown whether these patterns depend on each other or constitute independent responses to the thermal environment. Here, based on a global phylogenetic comparative analysis across 99.7% of the world's bird species, we show that the way in which the relative length of unfeathered appendages co-varies with temperature depends on body size and vice versa. First, the larger the body, the greater the increase in beak length with temperature. Second, the temperature-based increase in tarsus length is apparent only in larger birds, whereas in smaller birds, tarsus length decreases with temperature. Third, body size and the length of beak and tarsus interact with each other to predict the species' environmental temperature. These findings suggest that the animals' body size and shape are products of an evolutionary compromise that reflects distinct alternative thermoregulatory adaptations.