Beyond thermal limits: comprehensive metrics of performance identify key axes of thermal adaptation in ants

Urban abiotic stressors drive changes in the foraging activity and colony growth of the black garden ant Lasius niger

Changes in habitat characteristics are known to have profound effects on biotic communities and their functional traits. In the context of an urban-rural gradient, urbanisation drastically alters abiotic characteristics, e.g., by increasing environmental temperatures and through light pollution. These abiotic changes significantly impact the functional traits of organisms, particularly insects. Furthermore, changes in habitat characteristics also drive changes in the behavioural traits of animals, allowing them to adapt and thrive in new environments. In our study, we focused on the synanthropic ant species Lasius niger as a model organism. We conducted nocturnal field observations and complemented them with laboratory experiments to investigate the influence of night warming (NW) associated with Urban Heat Islands (UHI), light pollution (ALAN), and habitat type on ant foraging behaviour. In addition, we investigated the influence of elevated temperatures on brood development and worker mortality. Our findings revealed that urban populations of L. niger were generally more active during the night compared to their rural counterparts, although the magnitude of this difference varied with specific city characteristics. In laboratory settings, higher temperatures and continuous illumination were associated with increased activity level in ants, again differing between urban and rural populations. Rural ants exhibited more locomotion compared to their urban counterparts when maintained under identical conditions, which might enable them to forage more effectively in a potentially more challenging environment. High temperatures decreased the developmental time of brood from both habitat types and increased worker mortality, although rural colonies were more strongly affected. Overall, our study provides novel insights into the influence of urban environmental stressors on the foraging activity pattern and colony development of ants. Such stressors can be important for the establishment and spread of synanthropic ant species, including invasive ones, and the biotic homogenization of anthropogenic ecosystems.

Feeling the heat: Bumblebee workers show no acclimation capacity of upper thermal tolerance to simulated heatwaves

Climate change is our most significant challenge in the 21st century and among the main drivers of biodiversity loss. Recent distributional shifts and declines in crucial pollinators, such as bumblebees, seem to be associated to this phenomenon. However, despite future climate projections on climate warming, few studies have assessed the ability of temperate bumblebees to acclimate to extreme weather events, such as heatwaves. This study estimates the upper critical thermal limits (Critical Thermal Maximum (CTmax) and Heat Coma Temperature (HCT)), of the bumblebee subspecies Bombus terrestris audax, and assesses whether CTmax increases following exposure to a simulated heatwave. The critical thermal maximum occurred between 48.9 and 52.7 °C, while the heat coma temperature varied between 50.7 and 53.4 °C. After measurement of HCT, around 23% of bees survived 24 h or longer, but coordination was never recovered. There was no significant association between upper critical thermal limits and body mass, which highlights the need to investigate other factors to comprehend the mechanisms behind thermal tolerance limits. Furthermore, the heatwave treatments had no significant effect on the CTmax of bumblebee workers, indicating no acclimation capacity of upper thermal tolerance to simulated heatwaves. Our study provides insights into the upper thermal tolerance limits of Bombus terrestris audax and reveals that exposure to heatwave-like events does not change the upper thermal tolerance of bees, highlighting the need to develop effective strategies that might enable them to cope with extreme weather events.

Keep your cool: Overwintering physiology in response to urbanization in the acorn ant, Temnothorax curvispinosus

Winter presents a challenge for survival, yet temperate ectotherms have remarkable physiological adaptations to cope with low-temperature conditions. Under recent climate change, rather than strictly relaxing pressure on overwintering survival, warmer winters can instead disrupt these low-temperature trait-environment associations, with negative consequences for populations. While there is increasing evidence of physiological adaptation to contemporary warming during the growing season, the effects of winter warming on physiological traits are less clear. To address this knowledge gap, we performed a common garden experiment using relatively warm-adapted versus cold-adapted populations of the acorn ant, Temnothorax curvispinosus, sampled across an urban heat island gradient, to explore the effects of winter conditions on plasticity and evolution of physiological traits. We found no evidence of evolutionary divergence in chill coma recovery nor in metabolic rate at either of two test temperatures (4 and 10 °C). Although we found the expected plastic response of increased metabolic rate under the 10 °C acute test temperature as compared with the 4 °C test temperature, this plastic response, (i.e., the acute thermal sensitivity of metabolic rate), was not different across populations. Surprisingly, we found that winter-acclimated urban ant populations exhibited higher heat tolerance compared with rural ant populations, and that the magnitude of divergence was comparable to that observed among growing-season acclimated ants. Finally, we found no evidence of differences between populations with respect to changes in colony size from the beginning to the end of the overwintering experiment. Together, these findings indicate that despite the evolution of higher heat tolerance that is often accompanied by losses in low-temperature tolerance, urban acorn ants have retained several components of low-temperature physiological performance when assessed under ecologically relevant overwintering conditions. Our study suggests the importance of measuring physiological traits under seasonally-relevant conditions to understand the causes and consequences of evolutionary responses to contemporary warming.

Critical thermal limits and temperature-dependent walking speed may mediate coexistence between the native winter ant (Prenolepis imparis) and the invasive Argentine ant (Linepithemahumile)

Comparing the thermal tolerance and performance of native and invasive species from varying climatic origins may explain why some native and invasive species can coexist. We compared the thermal niches of an invasive and native ant species. The Argentine ant (Linepithema humile) is an invasive species that has spread to Mediterranean climates worldwide, where it is associated with losses in native arthropod biodiversity. In northern California, long-term surveys of ant biodiversity have shown that the winter ant (Prenolepis imparis) is the native species best able to coexist with Argentine ants. Both species tend hemipteran scales for food, and previous research suggests that these species' coexistence may depend on seasonal partitioning: winter ants are active primarily in the colder winter months, while Argentine ants are active primarily in the warmer months in northern California. We investigated the physiological basis of seasonal partitioning in Argentine and winter ants by a) measuring critical thermal limits, and b) comparing how ant walking speed varies with temperature. While both species had similar CTmax values, we found differences between the two species' critical thermal minima that may allow winter ants to remain functional at ecologically relevant temperatures between 0 and 2.5 °C. We also found that winter ants' walking speeds are significantly less temperature-dependent than those of Argentine ants. Winter ants walk faster than Argentine ants at low temperatures, which may allow the winter ants to remain active and forage at lower winter temperatures. These results suggest that partitioning based on differences in temperature tolerance promotes the winter ant's continued occupation of areas invaded by the Argentine ant.

Ecological strategies of (pl)ants: Towards a world-wide worker economic spectrum for ants

Current global challenges call for a rigorously predictive ecology. Our understanding of ecological strategies, imputed through suites of measurable functional traits, comes from decades of work that largely focussed on plants. However, a key question is whether plant ecological strategies resemble those of other organisms.Among animals, ants have long been recognised to possess similarities with plants: as (largely) central place foragers. For example, individual ant workers play similar foraging roles to plant leaves and roots and are similarly expendable. Frameworks that aim to understand plant ecological strategies through key functional traits, such as the 'leaf economics spectrum', offer the potential for significant parallels with ant ecological strategies.Here, we explore these parallels across several proposed ecological strategy dimensions, including an 'economic spectrum', propagule size-number trade-offs, apparency-defence trade-offs, resource acquisition trade-offs and stress-tolerance trade-offs. We also highlight where ecological strategies may differ between plants and ants. Furthermore, we consider how these strategies play out among the different modules of eusocial organisms, where selective forces act on the worker and reproductive castes, as well as the colony.Finally, we suggest future directions for ecological strategy research, including highlighting the availability of data and traits that may be more difficult to measure, but should receive more attention in future to better understand the ecological strategies of ants. The unique biology of eusocial organisms provides an unrivalled opportunity to bridge the gap in our understanding of ecological strategies in plants and animals and we hope that this perspective will ignite further interest. Read the free Plain Language Summary for this article on the Journal blog.

Social Factors in Heat Survival: Multiqueen Desert Ant Colonies Have Higher and More Uniform Heat Tolerance

AbstractInvestigations of thermally adaptive behavioral phenotypes are critical for both understanding climate as a selective force and predicting global species distributions under climate change conditions. Cooperative nest founding is a common strategy in harsh environments for many species and can enhance growth and competitive advantage, but whether this social strategy has direct effects on thermal tolerance was previously unknown. We examined the effects of alternative social strategies on thermal tolerance in a facultatively polygynous (multiqueen) desert ant, Pogonomyrmex californicus, asking whether and how queen number affects worker thermal tolerances. We established and reared lab colonies with one to four queens, then quantified all colony member heat tolerances (maximum critical temperature [CT_max]). Workers from colonies with more queens had higher and less variant CT_max. Our findings resemble weak link patterns, in which colony group thermal performance is improved by reducing frequencies of the most temperature-vulnerable individuals. Using ambient temperatures from our collection site, we show that multiqueen colonies have thermal tolerance distributions that enable increased midday foraging in hot desert environments. Our results suggest advantages to polygyny under climate change scenarios and raise the question of whether improved thermal tolerance is a factor that has enabled the success of polygyne species in other climatically extreme environments.

The response of ants to climate change

Ants (Hymenoptera: Formicidae) are one of the most dominant terrestrial organisms worldwide. They are hugely abundant, both in terms of sheer numbers and biomass, on every continent except Antarctica and are deeply embedded within a diversity of ecological networks and processes. Ants are also eusocial and colonial organisms-their lifecycle is built on the labor of sterile worker ants who support a small number of reproductive individuals. Given the climatic changes that our planet faces, we need to understand how various important taxonomic groups will respond; this includes the ants. In this review, we synthesize the available literature to tackle this question. The answer is complicated. The ant literature has focused on temperature, and we broadly understand the ways in which thermal changes may affect ant colonies, populations, and communities. In general, we expect that species living in the Tropics, and in thermally variable microhabitats, such as the canopy and leaf litter environments, will be negatively impacted by rising temperatures. Species living in the temperate zones and those able to thermally buffer their nests in the soil or behaviorally avoid higher temperatures, however, are likely to be unaffected or may even benefit from a changed climate. How ants will respond to changes to other abiotic drivers associated with climate change is largely unknown, as is the detail on how altered ant populations and communities will ramify through their wider ecological networks. We discuss how eusociality may allow ants to adapt to, or tolerate, climate change in ways that solitary organisms cannot and we identify key geographic and phylogenetic hotspots of climate vulnerability and resistance. We finish by emphasizing the key research questions that we need to address moving forward so that we may fully appreciate how this critical insect group will respond to the ongoing climate crisis.

Behavioral response to heat stress of twig-nesting canopy ants

Tropical forests experience a relatively stable climate, but are not thermally uniform. The tropical forest canopy is hotter and thermally more variable than the understory. Heat stress in the canopy is expected to increase with global warming, potentially threatening its inhabitants. Here, we assess the impact of heating on the most abundant tropical canopy arthropods—ants. While foragers can escape hot branches, brood and workers inside twig nests might be unable to avoid heat stress. We examined nest choice and absconding behavior—nest evacuation in response to heat stress—of four common twig-nesting ant genera. We found that genera nesting almost exclusively in the canopy occupy smaller cavities compared to Camponotus and Crematogaster that nest across all forest strata. Crematogaster ants absconded at the lowest temperatures in heating experiments with both natural and artificial nests. Cephalotes workers were overall less likely to abscond from their nests. This is the first test of behavioral thermoregulation in tropical forest canopy ants, and it highlights different strategies and sensitivities to heat stress. Behavioral avoidance is the first line of defense against heat stress and will be crucial for small ectotherms facing increasing regional and local temperatures.

Critical thermal limits in ants and their implications under climate change

Critical thermal limits (CTLs) constrain the performance of organisms, shaping their abundance, current distributions, and future distributions. Consequently, CTLs may also determine the quality of ecosystem services as well as organismal and ecosystem vulnerability to climate change. As some of the most ubiquitous animals in terrestrial ecosystems, ants are important members of ecological communities. In recent years, an increasing body of research has explored ant physiological thermal limits. However, these CTL data tend to centre on a few species and biogeographical regions. To encourage an expansion of perspectives, we herein review the factors that determine ant CTLs and examine their effects on present and future species distributions and ecosystem processes. Special emphasis is placed on the implications of CTLs for safeguarding ant diversity and ant-mediated ecosystem services in the future. First, we compile, quantify, and categorise studies on ant CTLs based on study taxon, biogeographical region, methodology, and study question. Second, we use this comprehensive database to analyse the abiotic and biotic factors shaping ant CTLs. Our results highlight how CTLs may affect future distribution patterns and ecological performance in ants. Additionally, we identify the greatest remaining gaps in knowledge and create a research roadmap to promote rapid advances in this field of study.

Ant nest architecture is shaped by local adaptation and plastic response to temperature

Social insects are among the most abundant arthropods in terrestrial ecosystems, where they provide ecosystem services. The effect of subterranean activity of ants on soil is well-studied, yet little is known about nest architecture due to the difficulty of observing belowground patterns. Furthermore, many species’ ranges span environmental gradients, and their nest architecture is likely shaped by the climatic and landscape features of their specific habitats. We investigated the effects of two temperature treatments on the shape and size of nests built by Formica podzolica ants collected from high and low elevations in the Colorado Rocky Mountains in a full factorial experiment. Ants nested in experimental chambers with soil surface temperatures matching the local temperatures of sample sites. We observed a plastic response of nest architecture to conditions experienced during excavation; workers experiencing a high temperature excavated deeper nests than those experiencing a cooler temperature. Further, we found evidence of local adaptation to temperature, with a significant interaction effect of natal elevation and temperature treatment on nest size and complexity. Specifically, workers from high elevation sites built larger nests with more tunnels when placed in the cool surface temperature treatment, and workers from low elevation sites exhibited the opposite pattern. Our results suggest that subterranean ant nest architecture is shaped by a combination of plastic and locally adapted building behaviors; we suggest that the flexibility of this ‘extended phenotype’ likely contributes to the widespread success of ants.

Bumblebee resilience to climate change, through plastic and adaptive responses

Bumblebees are ubiquitous, cold-adapted eusocial bees found worldwide from subarctic to tropical regions of the world. They are key pollinators in most temperate and boreal ecosystems, and both wild and managed populations are significant contributors to agricultural pollination services. Despite their broad ecological niche at the genus level, bumblebee species are threatened by climate change, particularly by rising average temperatures, intensifying seasonality and the increasing frequency of extreme weather events. While some temperature extremes may be offset at the individual or colony level through temperature regulation, most bumblebees are expected to exhibit specific plastic responses, selection in various key traits, and/or range contractions under even the mildest climate change. In this review, we provide an in-depth and up-to-date review on the various ways by which bumblebees overcome the threats associated with current and future global change. We use examples relevant to the fields of bumblebee physiology, morphology, behaviour, phenology, and dispersal to illustrate and discuss the contours of this new theoretical framework. Furthermore, we speculate on the extent to which adaptive responses to climate change may be influenced by bumblebees' capacity to disperse and track suitable climate conditions. Closing the knowledge gap and improving our understanding of bumblebees' adaptability or avoidance behaviour to different climatic circumstances will be necessary to improve current species climate response models. These models are essential to make correct predictions of species vulnerability in the face of future climate change and human-induced environmental changes to unfold appropriate future conservation strategies.

The toughest animals of the Earth versus global warming: Effects of long-term experimental warming on tardigrade community structure of a temperate deciduous forest

Understanding how different taxa respond to global warming is essential for predicting future changes and elaborating strategies to buffer them. Tardigrades are well known for their ability to survive environmental stressors, such as drying and freezing, by undergoing cryptobiosis and rapidly recovering their metabolic function after stressors cease. Determining the extent to which animals that undergo cryptobiosis are affected by environmental warming will help to understand the real magnitude climate change will have on these organisms. Here, we report on the responses of tardigrades within a five-year-long, field-based artificial warming experiment, which consisted of 12 open-top chambers heated to simulate the projected effects of global warming (ranging from 0 to 5.5°C above ambient temperature) in a temperate deciduous forest of North Carolina (USA). To elucidate the effects of warming on the tardigrade community inhabiting the soil litter, three community diversity indices (abundance, species richness, and Shannon diversity) and the abundance of the three most abundant species (Diphascon pingue, Adropion scoticum, and Mesobiotus sp.) were determined. Their relationships with air temperature, soil moisture, and the interaction between air temperature and soil moisture were tested using Bayesian generalized linear mixed models. Despite observed negative effects of warming on other ground invertebrates in previous studies at this site, long-term warming did not affect the abundance, richness, or diversity of tardigrades in this experiment. These results are in line with previous experimental studies, indicating that tardigrades may not be directly affected by ongoing global warming, possibly due to their thermotolerance and cryptobiotic abilities to avoid negative effects of stressful temperatures, and the buffering effect on temperature of the soil litter substrate.

Worker Size Diversity Has No Effect on Overwintering Success under Natural Conditions in the Ant Temnothorax nylanderi

Simple Summary

Winter is a harsh season for organisms living in temperate zones. Winter is often associated with starvation and cold temperatures, and these pressures can strongly affect organism survival. Living in groups can help these animals to cope with winter pressures. Social groups contain individuals which can vary in different ways: physiology, behavior, morphology, etc. In social insects such as ants, worker size leads to different responses to starvation and cold temperature in the laboratory. In this study, we investigated whether worker size affects colony and individual survival under natural conditions. We manipulated both worker size diversity and mean worker size within colonies of the ant Temnothorax nylanderi, reintroduced them in the field, and measured colony survival after overwintering. We found similar colony and individual (both adults and young) survival during winter between treatment colonies with reduced size diversity and/or manipulated mean worker size compared to control colonies with unmanipulated worker size. This result highlights that worker size diversity has no influence on colony performance in this species and more broadly questions the interest of worker size in social insect species with moderate worker size diversity. We discuss the potential sources of worker size diversity, including social context and selfish behavior.

Abstract

Winter is a difficult period for animals that live in temperate zones. It can inflict high mortality or induce weight loss with potential consequences on performance during the growing season. Social groups include individuals of various ages and sizes. This diversity may improve the ability of groups to buffer winter disturbances such as starvation or cold temperature. Studies focusing on the buffering role of social traits such as mean size and diversity of group members under winter conditions are mainly performed in the laboratory and investigate the effect of starvation or cold separately. Here, we experimentally decreased worker size diversity and manipulated worker mean size within colonies in order to study the effect on overwintering survival in the ant Temnothorax nylanderi. Colonies were placed under natural conditions during winter. Colony survival was high during winter and similar in all treatments with no effect of worker size diversity and mean worker size. Higher brood survival was positively correlated with colony size (i.e., the number of workers). Our results show that the higher resistance of larger individuals against cold or starvation stresses observed in the laboratory does not directly translate into higher colony survival in the field. We discuss our results in the light of mechanisms that could explain the possible non-adaptive size diversity in social species.

In a nutshell, a reciprocal transplant experiment reveals local adaptation and fitness trade-offs in response to urban evolution in an acorn-dwelling ant

Urban-driven evolution is widely evident, but whether these changes confer fitness benefits and thus represent adaptive urban evolution is less clear. We performed a multiyear field reciprocal transplant experiment of acorn-dwelling ants across urban and rural environments. Fitness responses were consistent with local adaptation: we found a survival advantage of the "home" and "local" treatments compared to "away" and "foreign" treatments. Seasonal bias in survival was consistent with evolutionary patterns of gains and losses in thermal tolerance traits across the urbanization gradient. Rural ants in the urban environment were more vulnerable in the summer, putatively due to low heat tolerance, and urban ants in the rural environment were more vulnerable in winter, putatively due to an evolved loss of cold tolerance. The results for fitness via fecundity were also generally consistent with local adaptation, if somewhat more complex. Urban-origin ants produced more alates in their home versus away environment, and rural-origin ants had a local advantage in the rural environment. Overall, the magnitude of local adaptation was lower for urban ants in the novel urban environment compared with rural ants adapted to the ancestral rural environment, adding further evidence that species might not keep pace with anthropogenic change.

Physiological adaptation to cities as a proxy to forecast global-scale responses to climate change

Cities are emerging as a new venue to overcome the challenges of obtaining data on compensatory responses to climatic warming through phenotypic plasticity and evolutionary change. In this Review, we highlight how cities can be used to explore physiological trait responses to experimental warming, and also how cities can be used as human-made space-for-time substitutions. We assessed the current literature and found evidence for significant plasticity and evolution in thermal tolerance trait responses to urban heat islands. For those studies that reported both plastic and evolved components of thermal tolerance, we found evidence that both mechanisms contributed to phenotypic shifts in thermal tolerance, rather than plastic responses precluding or limiting evolved responses. Interestingly though, for a broader range of studies, we found that the magnitude of evolved shifts in thermal tolerance was not significantly different from the magnitude of shift in those studies that only reported phenotypic results, which could be a product of evolution, plasticity, or both. Regardless, the magnitude of shifts in urban thermal tolerance phenotypes was comparable to more traditional space-for-time substitutions across latitudinal and altitudinal clines in environmental temperature. We conclude by considering how urban-derived estimates of plasticity and evolution of thermal tolerance traits can be used to improve forecasting methods, including macrophysiological models and species distribution modelling approaches. Finally, we consider areas for further exploration including sub-lethal performance traits and thermal performance curves, assessing the adaptive nature of trait shifts, and taking full advantage of the environmental thermal variation that cities generate.

Pedal to the metal: Cities power evolutionary divergence by accelerating metabolic rate and locomotor performance

Metabolic rates of ectotherms are expected to increase with global trends of climatic warming. But the potential for rapid, compensatory evolution of lower metabolic rate in response to rising temperatures is only starting to be explored. Here, we explored rapid evolution of metabolic rate and locomotor performance in acorn-dwelling ants (Temnothorax curvispinosus) in response to urban heat island effects. We reared ant colonies within a laboratory common garden (25°C) to generate a laboratory-born cohort of workers and tested their acute plastic responses to temperature. Contrary to expectations, urban ants exhibited a higher metabolic rate compared with rural ants when tested at 25°C, suggesting a potentially maladaptive evolutionary response to urbanization. Urban and rural ants had similar metabolic rates when tested at 38°C, as a consequence of a diminished plastic response of the urban ants. Locomotor performance also evolved such that the running speed of urban ants was faster than rural ants under warmer test temperatures (32°C and 42°C) but slower under a cooler test temperature (22°C). The resulting specialist-generalist trade-off and higher thermal optimum for locomotor performance might compensate for evolved increases in metabolic rate by allowing workers to more quickly scout and retrieve resources.

Realised rather than fundamental thermal niches predict site occupancy: Implications for climate change forecasting

Thermal performance traits are regularly used to make forecasts of the responses of ectotherms to anthropogenic environmental change, but such forecasts do not always differentiate between fundamental and realised thermal niches. Here we determine the relative extents to which variation in the fundamental and realised thermal niches accounts for current variation in species abundance and occupancy and assess the effects of niche-choice on future-climate response estimations. We investigated microclimate and macroclimate temperatures alongside abundance, occupancy, critical thermal limits and foraging activity of 52 ant species (accounting for >95% individuals collected) from a regional assemblage from across the Western Cape Province, South Africa, between 2003 and 2014. Capability of a species to occupy sites experiencing the most extreme temperatures, coupled with breadth of realised niche, explained most deviance in occupancy (up to 75%), while foraging temperature range and body mass explained up to 50.5% of observed variation in mean species abundance. When realised niches are used to forecast responses to climate change, large positive and negative effects among species are predicted under future conditions, in contrast to the forecasts of minimal impacts on all species that are indicated by fundamental niche predictions.

Adaptations to thermal stress in social insects: recent advances and future directions

Thermal stress is a major driver of population declines and extinctions. Shifts in thermal regimes create new environmental conditions, leading to trait adaptation, population migration, and/or species extinction. Extensive research has examined thermal adaptations in terrestrial arthropods. However, little is known about social insects, despite their major role in ecosystems. It is only within the last few years that the adaptations of social insects to thermal stress have received attention. Herein, we discuss what is currently known about thermal tolerance and thermal adaptation in social insects - namely ants, termites, social bees, and social wasps. We describe the behavioural, morphological, physiological, and molecular adaptations that social insects have evolved to cope with thermal stress. We examine individual and collective responses to both temporary and persistent changes in thermal conditions and explore the extent to which individuals can exploit genetic variability to acclimatise. Finally, we consider the costs and benefits of sociality in the face of thermal stress, and we propose some future research directions that should advance our knowledge of individual and collective thermal adaptations in social insects.

Temperature drives caste-specific morphological clines in ants

The morphology of organisms relates to most aspects of their life history and autecology. As such, elucidating the drivers of morphological variation along environmental gradients might give insight into processes limiting species distributions. In eusocial organisms, the concept of morphology is more complex than in solitary organisms. Eusocial insects such as ants exhibit drastic morphological differences between reproductive and worker castes. How environmental selection operates on the morphology of each caste, and whether caste-specific selection has fitness consequences is largely unknown, but is potentially crucial to understand what limits ant species' distributions. Here we aimed to examine whether ant shape and body size covaries with climate at the scale of an entire continent, and whether such relationship might be caste specific. We used 26,472 georeferenced morphometric measurements from 2,206 individual ants belonging to 32 closely related North American species in the genus Formica to assess how ant morphology relates to geographic variation in the abiotic environment. Although precipitation and seasonality explained some of the geographic variation in morphology, temperature was the best predictor. Specifically, geographic variation in body size was positively related to temperature, meaning that ants are smaller in cold than in warm environments. Moreover, the strength of the relationship between size and temperature was stronger for the reproductive castes (i.e. queens and males) than for the worker caste. The shape of workers and males also varied along these large-scale abiotic gradients. Specifically, the relative length of workers' legs, thoraxes and antennae positively related to temperature, meaning that they had shorter appendages in cold environments. In contrast, males had smaller heads, but larger thoraxes in more seasonal environments. Overall, our results suggest that geographic variation in ambient temperature influences the morphology of ants, but that the strength of this effect is caste specific. In conclusion, whereas ant ecology has traditionally focused on workers, our study shows that considering the ecology of the reproductive castes is imperative to move forward in this field.

Seasonal plasticity of thermal tolerance in ants

Analyses of heat tolerance in insects often suggest that this trait is relatively invariant, leading to the use of fixed thermal maxima in models predicting future distribution of species in a warming world. Seasonal environments expose populations to a wide annual temperature variation. To evaluate the simplifying assumption of invariant thermal maxima, we quantified heat tolerance of 26 ant species across three seasons that vary two-fold in mean temperature. Our ultimate goal was to test the hypothesis that heat tolerance tracks monthly temperature. Ant foragers tested at the end of the summer, in September, had higher average critical thermal maximum (CT_max ) compared to those in March and December. Four out of five seasonal generalists, species actively foraging in all three focal months, had, on average, 6°C higher CT_max in September. The invasive fire ant, Solenopsis invicta, was among the thermally plastic species, but the native thermal specialists still maintained higher CT_max than S. invicta. Our study shows that heat tolerance can be plastic, and this should be considered when examining species-level adaptations. Moreover, the plasticity of thermal traits, while potentially costly, may also generate a competitive advantage over species with fixed traits and promote resilience to climate change.

Can Cities Activate Sleeper Species and Predict Future Forest Pests? A Case Study of Scale Insects

Sleeper species are innocuous native or naturalized species that exhibit invasive characteristics and become pests in response to environmental change. Climate warming is expected to increase arthropod damage in forests, in part, by transforming innocuous herbivores into severe pests: awakening sleeper species. Urban areas are warmer than natural areas due to the urban heat island effect and so the trees and pests in cities already experience temperatures predicted to occur in 50-100 years. We posit that arthropod species that become pests of urban trees are those that benefit from warming and thus should be monitored as potential sleeper species in forests. We illustrate this with two case studies of scale insects that are important pests of urban trees in parts of the US. Melanaspis tenebricosa and Parthenolecanium quercifex are geographically native to the US but take on invasive characteristics such as higher survival and reproduction and become disconnected from natural enemies on urban trees due to the urban heat island effect. This allows them to reach high densities and damage their host trees. Parthenolecanium quercifex density increases up to 12 times on urban willow oaks with just 2 °C of warming due to higher survival and adaptation to warmer temperatures. The urban heat island effect also creates a phenological mismatch between P. quercifex and its parasitoid complex, and so egg production is higher. Melanaspis tenebricosa density can increase 300 times on urban red maples with 2.5 °C of warming. This too is due to direct effects of warmer temperatures on survival and fecundity but M. tenebricosa also benefits from the drought stress incurred by warmer urban trees. These effects combine to increase M. tenebricosa density in forests as well as on urban trees at latitudes higher than its native range. We illustrate how cities provide a unique opportunity to study the complex effects of warming on insect herbivores. Studying pestilent urban species could be a pragmatic approach for identifying and preparing for sleeper species.

Trade-Offs in Cold Resistance at the Northern Range Edge of the Common Woodland Ant Aphaenogaster picea (Formicidae)

Geographic variation in low temperatures at poleward range margins of terrestrial species often mirrors population variation in cold resistance, suggesting that range boundaries may be set by evolutionary constraints on cold physiology. The northeastern woodland ant Aphaenogaster picea occurs up to approximately 45°N in central Maine. We combined presence/absence surveys with classification tree analysis to characterize its northern range limit and assayed two measures of cold resistance operating on different timescales to determine whether and how marginal populations adapt to environmental extremes. The range boundary of A. picea was predicted primarily by temperature, but low winter temperatures did not emerge as the primary correlate of species occurrence. Low summer temperatures and high seasonal variability predicted absence above the boundary, whereas high mean annual temperature (MAT) predicted presence in southern Maine. In contrast, assays of cold resistance across multiple sites were consistent with the hypothesis of local cold adaptation at the range edge: among populations, there was a 4-min reduction in chill coma recovery time across a 2° reduction in MAT. Baseline resistance and capacity for additional plastic cold hardening shifted in opposite directions, with hardening capacity approaching zero at the coldest sites. This trade-off between baseline resistance and cold-hardening capacity suggests that populations at range edges may adapt to colder temperatures through genetic assimilation of plastic responses, potentially constraining further adaptation and range expansion.

Remarkable insensitivity of acorn ant morphology to temperature decouples the evolution of physiological tolerance from body size under urban heat islands

Environmental temperature can alter body size and thermal tolerance, yet the effects of temperature rise on the size-tolerance relationship remain unclear. Terrestrial ectotherms with larger body sizes typically exhibit greater tolerance of high (and low) temperatures. However, while warming tends to increase tolerance of high temperatures through phenotypic plasticity and evolutionary change, warming tends to decrease body size through these mechanisms and thus might indirectly contribute to worse tolerance of high temperatures. These contrasting effects of warming on body size, thermal tolerance, and their relationship are increasingly important in light of global climate change. Here, we used replicated urban heat islands to explore the size-tolerance relationship in response to warming. We performed a common garden experiment with a small acorn-dwelling ant species collected from urban and rural populations across three different cities and reared under five laboratory rearing temperatures from 21 to 29 °C. We found that acorn ant body size was remarkably insensitive to laboratory rearing temperature (ant workers exhibited no phenotypic plasticity in body size across rearing temperature) and among populations experiencing cooler rural versus warmer urban environmental temperatures (no evolved differences in body size between urban and rural populations). Further, this insensitivity of body size to temperature was highly consistent across each of the three cities we examined. Because body size was robust to temperature variation, previously described plastic and evolved shifts in heat (and cold) tolerance in acorn ant responses to urbanization were shown to be independent of shifts in body size. Indeed, genetic (colony-level) correlations between heat and cold tolerance traits and body size revealed no significant association between size and tolerance. Our results show how typical trait correlations, such as between size and thermal tolerance, might be decoupled as populations respond to contemporary environmental change.

Evolution, not transgenerational plasticity, explains the adaptive divergence of acorn ant thermal tolerance across an urban-rural temperature cline

Although studies increasingly disentangle phenotypic plasticity from evolutionary responses to environmental change, few test for transgenerational plasticity in this context. Here, we evaluate whether phenotypic divergence of acorn ants in response to urbanization is driven by transgenerational plasticity rather than evolution. F2 generation worker ants (offspring of laboratory-born queens) exhibited similar divergence among urban and rural populations as field-born worker ants, suggesting that evolutionary divergence rather than transgenerational plasticity was primarily responsible for shifts toward higher heat tolerance and diminished cold tolerance in urban acorn ants. Hybrid offspring from matings between urban and rural populations also indicated that evolutionary divergence was likely the primary mechanism underlying population differences in thermal tolerance. Specifically, thermal tolerance traits were not inherited either maternally or paternally in the hybrid pairings as would be expected for strong parental or grandparental effects mediated through a single sex. Urban-rural hybrid offspring provided further insight into the genetic architecture of thermal adaptation. Heat tolerance of hybrids more resembled the urban-urban pure type, whereas cold tolerance of hybrids more resembled the rural-rural pure type. As a consequence, thermal tolerance traits in this system appear to be influenced by dominance rather than being purely additive traits, and heat and cold tolerance might be determined by separate genes. Though transgenerational plasticity does not appear to explain divergence of acorn ant thermal tolerance, its role in divergence of other traits and across other urbanization gradients merits further study.

Habitat, latitude and body mass influence the temperature dependence of metabolic rate

The sensitivity of metabolic rate to temperature constrains the climate in which ectotherms can function, yet the temperature dependence of metabolic rate may evolve in response to biotic and abiotic factors. We compiled a dataset on the temperature dependence of metabolic rate for heterotrophic ectotherms from studies that show a peak in metabolic rate at an optimal temperature (i.e. that describe the thermal performance curve for metabolic rate). We found that peak metabolic rates were lower in aquatic than terrestrial habitats and increased with body mass, latitude and the optimal temperature. In addition, the optimal temperature decreased with latitude. These results support competing hypotheses about metabolic rate adaptation, with hotter being better in the tropics but colder being better towards the poles. Moreover, our results suggest that the temperature dependence of metabolic rate is more complex than previously suggested.

Draft Aphaenogaster genomes expand our view of ant genome size variation across climate gradients

Given the abundance, broad distribution, and diversity of roles that ants play in many ecosystems, they are an ideal group to serve as ecosystem indicators of climatic change. At present, only a few whole-genome sequences of ants are available (19 of >16,000 species), mostly from tropical and sub-tropical species. To address this limited sampling, we sequenced genomes of temperate-latitude species from the genus Aphaenogaster, a genus with important seed dispersers. In total, we sampled seven colonies of six species: Aphaenogaster ashmeadi, Aphaenogaster floridana, Aphaenogaster fulva, Aphaenogaster miamiana, Aphaenogaster picea, and Aphaenogaster rudis. The geographic ranges of these species collectively span eastern North America from southern Florida to southern Canada, which encompasses a latitudinal gradient in which many climatic variables are changing rapidly. For the six genomes, we assembled an average of 271,039 contigs into 47,337 scaffolds. The Aphaenogaster genomes displayed high levels of completeness with 96.1% to 97.6% of Hymenoptera BUSCOs completely represented, relative to currently sequenced ant genomes which ranged from 88.2% to 98.5%. Additionally, the mean genome size was 370.5 Mb, ranging from 310.3 to 429.7, which is comparable to that of other sequenced ant genomes (212.8-396.0 Mb) and flow cytometry estimates (210.7-690.4 Mb). In an analysis of currently sequenced ant genomes and the new Aphaenogaster sequences, we found that after controlling for both spatial autocorrelation and phylogenetics ant genome size was marginally correlated with sample site climate similarity. Of all examined climate variables, minimum temperature, and annual precipitation had the strongest correlations with genome size, with ants from locations with colder minimum temperatures and higher levels of precipitation having larger genomes. These results suggest that climate extremes could be a selective force acting on ant genomes and point to the need for more extensive sequencing of ant genomes.

Urban heat islands advance the timing of reproduction in a social insect

For many species, the timing of life cycle events is advancing under contemporary global climate change. However, much less is known regarding phenological shifts as a result of other sources of anthropogenic change, such as urban warming. In both cases, progress has been hampered by a focus on phenological traits such as the timing of emergence, rather than the phenology of more directly related fitness traits such as the timing of reproduction. Here we explore how urban heat island effects shape the timing of reproduction in an acorn-dwelling ant. We used a common garden experiment with acorn ants collected from three cities in the eastern United States along a latitudinal gradient and reared long-term in the laboratory under five temperature treatments. This allowed us to quantify the effects of temperature on reproductive phenology across three scales-a biogeographic temperature cline, three urban vs. rural temperature comparisons, and five laboratory rearing temperatures. At our northernmost and southernmost cities (spanning 6° of latitude), we found both urbanization and warmer laboratory rearing temperature significantly advanced reproductive phenology; ants from the lowest latitude city also had earlier reproductive phenology compared with the higher latitude cities. In the field, the differences in urban versus rural acorn ant reproductive phenology translate to approximately one month earlier reproduction in the urban populations. For insects with synchronous mating events, such as ants, shifts in the already short window of time to reproduce could limit mating across environments, potentially leading to reproductive isolation between urban and rural populations.

Morphological characterization and staging of bumble bee pupae

Bumble bees (Hymenoptera: Apidae, Bombus) are important pollinators and models for studying mechanisms underlying developmental plasticity, such as factors influencing size, immunity, and social behaviors. Research on such processes, as well as expanding use of gene-manipulation and gene expression technologies, requires a detailed understanding of how these bees develop. Developmental research often uses time-staging of pupae, however dramatic size differences in these bees can generate variation in developmental timing. To study developmental mechanisms in bumble bees, appropriate staging of developing bees using morphology is necessary. In this study, we describe morphological changes across development in several bumble bee species and use this to establish morphology-based staging criteria, establishing 20 distinct illustrated stages. These criteria, defined largely by eye and cuticle pigmentation patterns, are generalizable across members of the subgenus Pyrobombus, and can be used as a framework for study of other bumble bee subgenera. We examine the effects of temperature, caste, size, and species on pupal development, revealing that pupal duration shifts with each of these factors, confirming the importance of staging pupae based on morphology rather than age and the need for standardizing sampling.

Evolution of thermal tolerance and its fitness consequences: parallel and non-parallel responses to urban heat islands across three cities

The question of parallel evolution-what causes it, and how common it is-has long captured the interest of evolutionary biologists. Widespread urban development over the last century has driven rapid evolutionary responses on contemporary time scales, presenting a unique opportunity to test the predictability and parallelism of evolutionary change. Here we examine urban evolution in an acorn-dwelling ant species, focusing on the urban heat island signal and the ant's tolerance of these altered urban temperature regimes. Using a common-garden experimental design with acorn ant colonies collected from urban and rural populations in three cities and reared under five temperature treatments in the laboratory, we assessed plastic and evolutionary shifts in the heat and cold tolerance of F1 offspring worker ants. In two of three cities, we found evolved losses of cold tolerance, and compression of thermal tolerance breadth. Results for heat tolerance were more complex: in one city, we found evidence of simple evolved shifts in heat tolerance in urban populations, though in another, the difference in urban and rural population heat tolerance depended on laboratory rearing temperature, and only became weakly apparent at the warmest rearing temperatures. The shifts in tolerance appeared to be adaptive, as our analysis of the fitness consequences of warming revealed that while urban populations produced more sexual reproductives under warmer laboratory rearing temperatures, rural populations produced fewer. Patterns of natural selection on thermal tolerances supported our findings of fitness trade-offs and local adaptation across urban and rural acorn ant populations, as selection on thermal tolerance acted in opposite directions between the warmest and coldest rearing temperatures. Our study provides mixed support for parallel evolution of thermal tolerance under urban temperature rise, and, importantly, suggests the promising use of cities to examine parallel and non-parallel evolution on contemporary time scales.

Evolution of plasticity in the city: urban acorn ants can better tolerate more rapid increases in environmental temperature

Because cities contain high levels of impervious surfaces and diminished buffering effects of vegetation cover, urbanized environments can warm faster over the day and exhibit more rapid warming over space due to greater thermal heterogeneity in these environments. Whether organismal physiologies can adapt to these more rapid spatio-temporal changes in temperature rise within cities is unknown, and exploring these responses can inform not only how plastic and evolutionary mechanisms shape organismal physiologies, but also the potential for organisms to cope with urban development. Here, we examined how plasticity in thermal tolerance under faster and slower rates of temperature change might evolve in response to the more rapid spatio-temporal temperature rise in cities. We focused on acorn ants, a temperature-sensitive, ground-dwelling ant species that makes its home inside hollowed out acorns. We reared acorn ant colonies from urban and rural populations under a common garden design in the laboratory and assessed the thermal tolerances of F1 offspring workers using both fast (1°C min^-1) and slow (0.2°C min^-1) rates of temperature change. Relative to the rural population, the urban population exhibited higher heat tolerance when the temperature was increased quickly, providing evidence that temperature ramp-rate plasticity evolved in the urban population. This result was correlated with both faster rates of diurnal warming in urban acorn ant nest sites and greater spatial heterogeneity in environmental temperature across urban foraging areas. By contrast, rates of diurnal cooling in acorn ant nest sites were similar across urban and rural habitats, and correspondingly, we found that urban and rural populations responded similarly to variation in the rate of temperature decrease when we assessed cold tolerance. Our study highlights the importance of considering not only evolutionary differentiation in trait means across urbanization gradients, but also how trait plasticity might or might not evolve.

Experimental winter warming modifies thermal performance and primes acorn ants for warm weather

The frequency of warm winter days is increasing under global climate change, but how organisms respond to warmer winters is not well understood. Most studies focus on growing season responses to warming. Locomotor performance is often highly sensitive to temperature, and can determine fitness outcomes through a variety of mechanisms including resource acquisition and predator escape. As a consequence, locomotor performance, and its impacts on fitness, may be strongly affected by winter warming in winter-active species. Here we use the acorn ant, Temnothorax curvispinosus, to explore how thermal performance (temperature-driven plasticity) in running speed is influenced by experimental winter warming of 3-5°C above ambient in a field setting. We used running speed as a measure of performance as it is a common locomotor trait that influences acquisition of nest sites and food in acorn ants. Experimental winter warming significantly altered thermal performance for running speed at high (26 and 36°C) but not low test temperatures (6 and 16°C). Although we saw little differentiation in thermal performance at cooler test temperatures, we saw a marked increase in running speed at the hotter test temperatures for ants that experienced warmer winters compared with those that experienced cooler winters. Our results provide evidence that overwintering temperatures can substantially influence organismal performance, and suggest that we cannot ignore overwintering effects when forecasting organismal responses to environmental changes in temperature.