Interglacial Research Articles

Changes in the water-thermal regime of permafrost swamps in western siberia in response to climate warming

Changes in the water-thermal regime of swamps in the permafrost zone of Western Siberia are considered. Two types of swamps are prevalent in the study area: frost mound bogs (southern half of the area) and polygonal bogs (northern half of the area). The boundary between these swamps types is blurred and approximately matches the Polar Circle line. Detailed studies of the water-thermal regime of swamps in this area were carried out from 1971 to 1991 within the framework of the West Siberian expedition of the State Hydrological Institute. The present work uses a mathematical model of frost mound and polygonal bogs, which was developed based on the results of these studies. Daily meteorological data, specifically daily data on air temperature, precipitation totals and total and lower cloudiness from 23 stations are used as input parameters of the model. The duration of observations of meteorological stations ranges from 90 to 140 years. The mentioned period is divided into 2 parts, before and after 1978. All calculations were limited to the warm period, the boundaries of which are the transitions of average daily air temperature through 0 °C. The results of calculations indicate a widespread increase in the average warm period air temperature values, from 0.3 °C in the south to 1.6 °C in the north of the territory. In the warmest years (2.5% probability of exceeding) this increase in the south of the zone is 1.5 °C and in the extreme north is 3.5 °C. The greatest changes in the parameters of the water-thermal regime can be traced in the increase of the peat deposit thawing depth. In average, the maximum thawing increased by 4-5 cm, and in the warmest years by 7-10 cm. For the north of polygonal bogs, the difference in thawing depth in some years increases dramatically and reaches 16 cm. At the same time, it should be noted that the calculated thawing depth often exceeds the peat deposit thickness, especially in the northern part of polygonal bogs. In such cases, the peat deposit thawing values should be considered as potential. It is supposed that changes in the water-thermal regime of frost mound and polygonal bogs will contribute to the sequential evolution of the micro-landscape structure. The displacement of the boundaries of different types of swamps is complicated by orographic obstacles, primarily by the Siberian Uvals and requires a full-scale reorganization of the hydrographic network and structure of micro-landscapes.

Lithium enrichment in the Qarhan Salt Lake (China) was a long-term process driven by interglacial-glacial cycles

Abstract Qarhan Salt Lake, China’s largest brine-type potassium-lithium deposit, holds substantial economic value. However, the timing of lithium-rich brine formation, its origin, and the role of climate in its development remain unclear. This highlights the need for a deeper understanding of salt lake evolution, particularly regarding ore-forming brine composition and the palaeoclimate. Therefore, we conducted 230Th dating on halite deposits from borehole ZK6-7 (7.5–46.7 metres) in the northern Bieletan, revealing halite deposition at 237.79–30.29 thousand years before 2000 A.D. The maximum homogenisation temperature of fluid inclusions was 23.9–41.7 °C, indicating frequent palaeobrine temperature fluctuations, corresponding to palaeoclimatic events in the Qinghai–Tibet Plateau and Qaidam Basin. Lithium, potassium, magnesium, boron, and bromine concentrations were inversely correlated with the maximum homogenisation temperature. Lithium-rich brine formation in the Bieletan was a long-term process driven by interglacial-glacial cycles, where warm periods increased river flow and material supply, while dry-cold periods intensified evaporation, promoting mineral enrichment.

The living Barents Sea response to peak-warming and subsequent cooling

The Arctic warms nearly four times faster than the global average, with maximum warming in the Barents Sea. Concurrently, changes in species distribution in this productive and highly exploited sub-Arctic hotspot has been found. However, studies so far have mostly focused on the effect of gradual warming on single species or trophic groups. We assess changes in zooplankton, fish and zoobenthos assemblages (130 species in 23 groups) and found heterogeneous response to ongoing warming. Temporally constrained cluster analysis showed that the warming was not continuous over the study period 2005–2022 but occurred in three phases: an initial period (2005–2011) cooler than the average for the whole study period, followed by a very warm period (2012–2016) and finally a cooler period again (2017–2022). The biotic response was greatest in the areas of largest oceanographic changes: in the northwest, the biomass of biota increased in most groups, including Arctic fish species, whereas in the southeast, the biomass of several fish species declined, while that of jellyfish and invasive snow crab increased. New knowledge is useful for generating scenarios for ecosystem responses to climate change.

Hunger Games: Supernatural Strategies Against Hunger in the Medieval North

However substantial the effect of the Medieval Warm Period may have been in northern Europe around 1000–1200, medieval Scandinavia was not exactly a place of plenty: the harshness of climatic conditions did still have a profound impact on food supplies, raising the need for alternative solutions. The sagas show that, in addition to practical responses, northern people could resort to “magic” and to their traditional pagan gods, but also to the powers of the new Christian faith to fight the ever-looming threat of hunger that had earlier shaped their idea of the afterlife and of the end of the world.

Assessment of leaf litter decomposition for biomonitoring in urban watercourses under contrasting thermal conditions.

Urbanization affects the structure and function of aquatic ecosystems, and its effect might depend on seasonal conditions. We aim to evaluate the applicability of in situ leaf litter decomposition experiments to assess the ecological integrity of urbanized streams in cool (autumn-winter) and warm (spring-summer) periods. Along these two periods, three reaches were selected in urban and three in reference segments in Pampean streams. In each reach at both periods, 25 bags of 450 μm (FM) and 25 bags of 20 mm (CM) mesh size were placed containing dry leaves of Populus nigra, and five bags of each type were periodically collected up to day 104. Decomposition rates were determined from mass loss by fitting to a negative exponential model against time (kd) and cumulate degree days (kdd). In both periods, kdd were lower in urban than in reference reaches (pcondition = 0.020, df = 1,137), but a larger difference occurred in the warm period. Even removing the effect of temperature, higher kdd were observed in warm than in cool waters (pperiod = 0.0004, df = 1, 137 for FM and pperiod = 0.002, df = 1,129 for CM). During the warm period experiment, the kdd reduction due to urbanization was 2.5 times higher than during the cool period. Invertebrates colonizing litter bags differed between stream conditions and between seasons. Tolerant insect larvae were more abundant in the warm period; Gastropods, nematodes, and crabs during the cool period. In conclusion, our experimental methodology was effective to assess the effects of urbanization on stream ecological integrity. As we predicted, season stood out as an important factor in the assessment.

Grain‐size evidence of Middle Pleistocene sedimentary environmental changes in the eastern Tai‐hang Mountains, North China

ABSTRACTGrain size is a proxy for climate and environmental change, and the results of grain size analyses can be used to infer the conditions and environment under which the sediments were formed. The eastern foothills of the Tai‐hang Mountains in northern China are on the edge of the East Asian Summer Monsoon and are extremely sensitive to climate and environmental changes. Understanding the ecological changes in this region is vital for coping with future climate change and solving related climate and environmental problems. This study uses parametric grain size end‐member analysis to discuss the depositional dynamics and ecological changes in the Middle Pleistocene of the Shijiazhuang JingXing YF profile. The results indicate that: (1) The OSL dates of the YF profile vary from 274 to 0.997 ka, and sedimentation began in the Middle Pleistocene. (2) The YF profile's sediments’ grain size is separated into four end elements: EM1 is the clay fraction produced by loam‐forming action; EM2 and EM3 are long‐distance transported fine silt and short‐distance transported silt at low altitudes, representing the variation in East Asian Summer Monsoon intensity; EM4 represents the near‐surface transported coarse silt fraction driven by East Asian Winter Monsoon power. (3) The grain size index reconstructed the paleoclimate of the Middle Pleistocene in the Shijiazhuang area, and it can be divided into four stages: 274–210 ka, the climate changed from cold to warm; 210–170 ka, was a warm period; 170–80 ka, the climate changed from warm to cold; and from 80–1 ka was a cold period. Multi‐indicator comparisons show that the monsoon evolution recorded in the Middle Pleistocene YF loess‐paleosol sequence is fundamentally consistent with changes in mid‐ to high‐latitude solar radiation, and changes in solar radiation modulate that monsoon changes in East Asia.

Evaluation of Passive Strategies for Achieving Hygrothermal Comfort in Social Housing Buildings in the Dominican Republic

In a building, the thermal satisfaction an individual may experience generally influences their health, well-being, productivity, and energy consumption. The concept of thermal comfort and its importance in buildings has been known for some time. However, in the Dominican Republic, discussing thermal comfort in social housing is a challenge since there have not been many studies applied to this context, especially to social housing. For this reason, this research analyzed the thermal behavior of a typical social housing building through energy simulation, aiming to highlight the importance of passive strategies to improve comfort in a warm and humid climate without using air conditioning. The simulation was conducted using OpenStudio v3.9, which utilizes the EnergyPlus v9.4 calculation engine. Three case studies were analyzed, implementing passive measures and seeking to achieve temperatures within the comfort ranges of the housing prototype. The results show that combining different passive strategies for warm-humid climates significantly reduces temperature, achieving reductions of up to 2.8 °C in the colder period and up to 3.2 °C in the warmer period.

New Insight into the Demography History, Evolution, and Phylogeography of Horseshoe Crabs with Special Emphasis on American Species

Xiphosurids (Merostomata, Xiphosura) are a group of chelicerates with a rich and complex evolutionary history that is constantly being updated through new discoveries. In this study, we re-estimated the divergence time of the extant horseshoe crab species with new fossil calibration points and addressed the inter- and intraspecific relationships of the American horseshoe crab through a phylogeographic perspective. In order to achieve our objectives, three datasets were compiled from fragments of different lengths of the COI gene that include sequences from 154 individuals, representing the Mexican populations. In addition to these, the datasets also included previously published sequences corresponding to individuals from different US populations and Asian horseshoe crab species. Firstly, we estimated the divergence times of extant horseshoe crab species by Bayesian methods using multiple fossil calibration points. Subsequently, we investigated the phylogeographic relationships and demographic history of Limulus polyphemus in the Americas utilizing various datasets. The time of divergence of the two Asian species clades was estimated to be approximately 127 million years ago (Ma). Phylogeographic relationships between the Asian and American species are linked through a minimum of 86 mutational steps. In America, phylogeographic relationships reflect differentiation between US and Mexican populations of L. polyphemus. We detect signs of demographic expansion for the Mexican population during the last 75,000 years, as well as an absence of phylogeographic structuring. The evolutionary history of horseshoe crabs is older than previously believed; however, the current distribution and demographic changes have probably been influenced by environmental events of the recent past, such as the glacial–interglacial periods that occurred during the Pleistocene.

A warming pulse in the Antarctic continent changed the landscape during the Middle Ages

The Antarctic landscape is one of the most stable environments on the Earth, at least since approximately 14 million years ago when most glaciers in continental Antarctica changed from temperate to cold-based, and previous extensive fluvial activity disappeared. Here, we detected a large landscape change on a coastal glacier in continental Antarctica (Boulder Clay Glacier) that occurred in the Medieval Warm Period. Such change consists in a glacial unconformity marked by a continuous sediment layer and an erosion channel on the past glacier surface. This channel, more than 4 kilometers long, represents a local deepening of a glacial unconformity that cuts the underlying glacial strata and was clearly imaged and mapped by Ground Penetrating Radar data. Four boreholes were allowed to calibrate the sediment layer so identified because it was observed in all boreholes at depths between 1.85 and 3.07 m. Moreover, the occurrence at a depth of 11.11 meters of mosses suitable for the dating through radiocarbon dating provided the age of 1050 calibrated years before the present, implying that the erosion event occurred during the Medieval Warm Period between 900 and 989 before the present.

Variable phenology but consistent loss of ice cover on 1213 Minnesota lakes

AbstractLake ice cover is declining globally with important implications for lake ecosystems. Ice loss studies often rely on small numbers of lakes with long‐term data. We analyzed variation and trends in ice cover phenology from 1213 lakes over 74 yr (1949–2022) in Minnesota (USA), during which ice cover duration declined at a rate of 2 d per decade (14 d total) and became more variable. Despite variation in phenology, just 10–20% of lakes differed from statewide phenological trends. Accounting for synchronous annual variation and estimating trends over long time periods (e.g., > 40 yr) were critical for obtaining robust estimates of ice loss. The constant rates estimated here were consistent with recent global estimates (1.7–1.9 d per decade) and suggest that, even if present, accelerating rates of ice loss could be difficult to detect in the midst of shorter‐term periods of warming and increasing variability.

Long- and short-term variability of Arctic sea-ice cover during the Last Interglacial and Marine Isotope Stage 11c

Sea-ice-free Arctic summers are expected within the next decades due to anthropogenic warming. Studying past warm interglacials offers valuable insights into the Arctic climate system under similar conditions. Here we present a series of simulations that represent a chronological sequence of interglacial climate states over Marine Isotope Stages 5e and 11c using the Community Earth System Model. While previous studies attempted to explain the Arctic sea-ice evolution through changes in summer solstice insolation, we demonstrate that summer sea-ice area during both interglacial periods is primarily driven by an inverse relationship with integrated summer energy. Although proxy data yield conflicting results on Last Interglacial Arctic sea-ice cover, our simulations suggest seasonally ice-free conditions across the Arctic during the early interglacial phase. In both interglacials, climate states with intermediate levels of summer sea-ice area are characterized by maximum interannual variability in sea-ice area.

‘Chimes of resilience’: what makes forest trees genetically resilient?

SummaryForest trees are foundation species of many ecosystems and are challenged by global environmental changes. We assemble genetic facts and arguments supporting or undermining resilient responses of forest trees to those changes. Genetic resilience is understood here as the capacity of a species to restore its adaptive potential following environmental changes and disturbances. Importantly, the data come primarily from European temperate tree species with large distributions and consider only marginally species with small distributions. We first examine historical trajectories of trees during repeated climatic changes. Species that survived the Pliocene–Pleistocene transition and underwent the oscillations of glacial and interglacial periods were equipped with life history traits enhancing persistence and resilience. Evidence of their resilience also comes from the maintenance of large effective population sizes across time and rapid microevolutionary responses to recent climatic events. We then review genetic mechanisms and attributes shaping resilient responses. Usually, invoked constraints to resilience, such as genetic load or generation time and overlap, have limited consequences or are offset by positive impacts. Conversely, genetic plasticity, gene flow, introgression, genetic architecture of fitness‐related traits and demographic dynamics strengthen resilience by accelerating adaptive responses. Finally, we address the limitations of this review and highlight critical research gaps.

Population genomics reveals population structure and local adaptation of the Helicoverpa armigera lineage in Xinjiang, China

AbstractBACKGROUNDThe cotton bollworm, Helicoverpa armigera, is a global pest of many crops, particularly cotton. Populations in Xinjiang (north‐western China) are distinct from other major lineages in eastern China and elsewhere. Cotton is grown in Xinjiang in a wide range of climatic conditions but little is known about H. armigera population structure and gene exchange, either within Xinjiang or between it and the eastern China populations.RESULTSPopulation structure analyses showed significant gene flow throughout Xinjiang but the western‐most collections, Kashi and Shawan, were the most divergent, both from each other and from the other collections. Demographic history dynamics revealed a peak in ancestral population size for Xinjiang populations during a relatively warm and humid period about 5000–10 000 years ago. Selective sweep analyses found local adaptation of genes in Kashi including several CYP9A genes and two trehalose transport 1 (Tret1) genes, and genes in Shawan including several CYP6AE genes plus the basic juvenile hormone‐suppressible protein 1 gene (BJSP‐1) and two Methuselah genes. Spikes of introgression including the cyc, tim and per genes related to circadian rhythm were also detected between Kashi and/or Shawan and other Xinjiang populations.CONCLUSIONHelicoverpa armigera from Shawan and Kashi are genetically differentiated from each other and from other Xinjiang populations at the whole‐genome level. Despite some evidence of introgression, they also show evidence for local adaptation in genes related to detoxification, various stress tolerances and development. Management strategies against H. armigera in Xinjiang should consider these regional differences. © 2025 Society of Chemical Industry.

Divergence of Two Diospyros Species Promoted by Geo‐Climatic Processes Across Dry Valleys and Lowlands in Subtropical China

ABSTRACTAimExploring the divergence and speciation processes is central to understanding the origin and maintenance of biodiversity. The dry valley mosaic in the Hengduan Mountains and its southern extension range is one of the special landscapes globally and harbours a high level of endemic plants of tropical margin nature. The dry valleys and lowlands‐adapted plants constitute a unique system to explore how the geological and ecological effects have shaped the speciation processes here.LocationSubtropical China.TaxonDiospyros dumetorum and Diospyros saxatilis.MethodsWe surveyed patterns of genetic variation from 33 populations spanning their distribution ranges using three chloroplast DNA fragments and eight nuclear microsatellite loci. Approximate Bayesian computation was conducted to compare population demographic models considering gene flow and to estimate key parameters. We used Ecological Niche Models (ENMs) to reconstruct changes in range distribution during glacial and interglacial periods and evaluate their niche differentiation.ResultsPlastid and nuclear data strongly support divergence between the two species occurring during the Pliocene/Pleistocene period followed by secondary contact in response to climatic oscillations. ENMs predicted drastic distribution range changes during the Quaternary and identified significant differences in climatic conditions at the set of locations occupied by these two species. Correlation analyses were conducted, and the results suggest that geographical isolation rather than environmental heterogeneity shapes the current population genetic structure within the two species.Main ConclusionPhylogeographical patterns revealed in our results document that geographic vicariance associated with tectonic uplifts initiated allopatric species divergence between the two dry valleys and lowlands‐adapted Diospyros species, and niche differentiation perhaps further accelerated the speciation processes with limited subsequent gene flow. Our findings could contribute to the knowledge of forces that drive speciation processes in the unique landscape of dry valleys, as well as the evolution of the flora in subtropical China.

Dietary ecology of Smilodon across time and space: Additional perspectives from Smilodon gracilis and Smilodon fatalis in Florida

AbstractSmilodon, the iconic saber‐toothed cat, was a Pleistocene apex predator comprised of three morphologically distinct species: Smilodon fatalis, Smilodon gracilis, and Smilodon populator. While at Rancho La Brea (RLB) the paleobiology of S. fatalis is analogous to African lions in terms of carcass utilization, far less is known about S. fatalis outside of southern California. Further, Smilodon gracilis specimens are rare and less studied throughout most of its former range. This study expands our understanding of Smilodon's dietary ecology in North America by using dental microwear texture analysis (DMTA) to quantify carcass utilization and feeding behavior via complexity (Asfc) and anisotropy (epLsar) in populations of S. gracilis and S. fatalis from Florida, USA. Results reveal that S. fatalis populations from Florida and RLB have indistinguishable diets across climatically disparate regions during the Pleistocene. Additionally, S. fatalis and S. gracilis in Florida appear indistinguishable based on dental microwear textures, suggesting both species exhibited a generalist feeding strategy despite their temporal and morphological differences. Lastly, microwear in S. gracilis demonstrates a shift away from a tougher and potentially harder diet (epLsar and Asfc) during the glacial Inglis 1A site, as compared to the interglacial Leisey Shell Pit 1A site—similar to previously documented dietary shifts in glacial and interglacial populations of S. fatalis at RLB. Collectively, our findings are consistent with the hypothesis that the dietary behavior of S. fatalis and S. gracilis was generally consistent across space and time, with more subtle dietary differences occurring during glacial versus interglacial periods.

Measurement report: Sources and meteorology influencing highly time-resolved PM2.5 trace elements at three urban sites in the extremely polluted Indo-Gangetic Plain in India

Abstract. High-time-resolution aerosol measurements across various Indo-Gangetic Plain (IGP) regions are critical due to these regions' dense populations, intense industrial activities, recurring pollution episodes, and agricultural practices, as well as the associated health impacts and broader climate implications of aerosol. However, studies (mostly offline) have been predominantly concentrated in the upper IGP (U-IGP), with limited spatial coverage across the central IGP (C-IGP). This study aims to bridge the gap by examining both U-IGP and C-IGP regions, offering an in-depth analysis of elemental concentrations, source apportionment, inter-regional comparisons, seasonal variations, meteorological influences, and health risks through high-resolution Xact measurements conducted at three urban locations. During the colder seasons, elements such as sulfur (S), chlorine (Cl), and potassium (K) were found to dominate elemental PM2.5 concentrations, while the warm seasons showed notable changes in aluminum (Al), silica (Si), strontium (Sr), and barium (Ba) concentrations. These variations underscore the influence of meteorological conditions and different emission sources on pollution levels. From a health perspective, average levels of carcinogenic elements (lead (Pb), nickel (Ni), arsenic (As), and chromium (Cr)) were usually kept below reference levels. However, Pb concentrations exceeded these thresholds by 40 %–50 % during both periods in the U-IGP, largely attributed to coal combustion and lead smelting activities. Using the Multilinear Engine 2 (ME-2) solver, positive matrix factorization (PMF) deconvolved Cl-rich, coal combustion, Cu-rich, dust, solid fuel combustion 1 (SFC1), SFC2, and S-rich components. In both IGP regions, Cl-rich sources, S-rich sources, and SFC1 dominated the elemental sources, with differences observed in their relative contributions, indicating the influence of regional emissions. The role of meteorology in elemental PM2.5 variations during both clean and polluted episodes in the IGP regions was observed. During pollution episodes, the relative contribution of Cl-rich sources increased when PM2.5 concentrations showed a sharp increase as compared to clean periods, especially during cold periods, indicating the role of emissions from trash burning (plastic and PVC) and steel industries along with favorable meteorological conditions in the formation and accumulation of pollutants. During the warm period, the relative contribution of Cl-rich sources in the C-IGP is less, but during the cold period, both the U-IGP and the C-IGP have significant Cl-rich source contributions (44 %), indicating the role of chlorine in extreme haze air pollution in the whole IGP region. During warm periods, high-pollution events occurred across the IGP with major contributions from SFC1 (crop residue burning), indicating that severe pollution events also depend on seasonal emission sources despite less favorable meteorological conditions. These findings provide valuable insights into the spatial and temporal dynamics of elemental pollution across the IGP, offering a robust foundation for devising targeted mitigation strategies and regulatory policies to safeguard public health in this densely populated region.

Impacts of increasing compound hot-dry events on vegetation under the warming-wetting trend in Northwest China

Impacts of increasing compound hot-dry events on vegetation under the warming-wetting trend in Northwest China

Contraction of the Western Pacific Tropical Rain Belt and Weakening of the Walker Circulation: Future Lessons From the Past Two Interglacials

AbstractThe western Pacific warm pool (WPWP) is the heat engine of the global climate system delivering vast amounts of heat and moisture to the atmosphere. Controls on regional convection, however, are numerous, making it difficult to simulate past and future changes in WPWP hydroclimate with confidence. Here, we synthesize new and previously available precipitation sensitive records from the WPWP spanning the last and present interglacial periods. We find two primary modes of rainfall variability, both driven by precession forcing, that are common to both interglacial periods: (a) a contraction of the tropical rain band across the interglacial and (b) a mid‐interglacial strengthening of the Pacific Walker Circulation (PWC). We further demonstrate that while the amplitude of the change in seasonal insolation across the Holocene is far lower than during the LIG due to the low eccentricity state of Earth's orbit, the response of regional rainfall is comparable during both interglacials, indicating a nonlinear response to the insolation forcing. Finally, we suggest an enhanced sensitivity of the PWC to non‐insolation climate forcing, including greenhouse gases and sea level change, under strongly reduced boreal fall insolation as observed during the late Holocene and late LIG.

Changes in Planktic Foraminiferal Distribution, Productivity, and Preservation in the Barents Sea During the Last Three Millennia

AbstractPlanktic foraminifers are ubiquitous marine calcifiers sensitive to ocean biogeochemical and physical changes. Their fossil remains have thus been widely used for the reconstruction of past oceanographic and climatic changes. Here, we have investigated the distribution patterns of planktic foraminiferal species, their abundance, shell size, and preservation state in two sediment cores from the northern and southern Barents Sea during the last three millennia and compared them with a living assemblage from the water column. The study area covers the cold Arctic zone in the north and the warm boreal zone in the south. In the north, we found very low to almost zero abundances of fossil planktic foraminifers, mainly since c. 1300 Common Era (C.E.), and dominance by the polar species Neogloboquadrina pachyderma. In the south, higher abundances were found, where the taxa was dominated by the subpolar species Turborotalita quinqueloba, Globigerinita uvula and Neogloboquadrina incompta. The highest foraminiferal concentrations occurred in the transition phases between cold and warm periods, with a rapid decrease in abundance during the last two hundred years coinciding with the Industrial Revolution. The lack of planktic foraminifers since c. 1300 C.E. in the north and the consistently low %CaCO3 are attributed to CaCO3 dissolution in the sediment, possibly driven by the decomposition of total organic matter as suggested by high %TOC, high surface productivity, and the presence of solely agglutinated benthic foraminifers. Dissolution of CaCO3 muted the signals of paleoproductivity and paleoclimate in the north, while the southern site showed clear variability throughout the investigated time interval.

Temperature‐Related Differences in Hair Cortisol Among Outdoor‐Housed Rhesus Macaques (Macaca mulatta)

ABSTRACTTemperature has a known potential to influence glucocorticoid concentrations obtained from fecal samples in nonhuman primates. Studies reliant on hair cortisol estimates obtained using samples from outdoor subjects, however, may not control for temperature. This omission is despite the general utility of hair as a sample matrix with relatively longer periods of accrual time. We examined these dynamics in rhesus macaques (Macaca mulatta); importantly, this population of rhesus macaques exhibits covariation between season and breeding behavior. Thus, we also examined temperature relative to contributions of social behavior associated with breeding seasons, which may coincide with climatic shifts. We utilized 1921 hair samples from 580 subjects across six large outdoor‐housed mixed‐sex groups at the California National Primate Research Center to quantify the capacity for warmer or cooler outdoor temperatures to influence hair cortisol concentrations. We found that colder maximum temperature estimates over the days preceding hair sampling were associated with elevated hair cortisol concentrations, relative to warmer periods. Temperature contributed similarly in a model with a reduced data set (1418 samples) which included breeding‐associated social behaviors. Consortship behavior was associated with hair cortisol without temperature, but was not associated with temperature included. Aggression was associated with cortisol, with or without the inclusion of temperature. Outdoor temperature is an important confound or covariate to account for statistically or via careful study design. Inclusion is especially important among research projects reliant on hair cortisol from outdoor‐housed primates and spanning multiple seasons.