Reproductive strategies and energetic adaptations of polar zooplankton

Summary

Key factors governing polar ocean ecosystems are low temperatures and a pronounced seasonal variability of ice cover, light regime and primary production. Depending on their ecological niche and trophic position, zooplankton species at high latitudes have developed a variety of reproductive strategies and energetic adaptations to cope with these extreme environmental conditions. Life-cycle strategies of the herbivorous copepods and euphausiids, which make up the major portion of polar zooplankton biomass, include seasonal vertical migration, dormancy (diapause, quiescence) and the accumulation of energy reserves. These lipid stores help to buffer the pulsed seasonal food supply, and they play an important role in fueling reproduction independent of phytoplankton. Only a smaller fraction of the lipid reserves accumulated during spring and summer are usually catabolized for metabolic maintenance during the food-limited dark season. These deposits are retained until the end of winter and allow early egg production and spawning prior to—or coinciding with—the onset of vernal primary production. It enables the new generation to make full use of the short productive season for growth and development to reach viable overwintering stages. The Antarctic krill Euphausia superba is an exception since it uses its depot lipids for metabolic maintenance during the dark season. It therefore relies on external resources (Primary production) for reproductive processes, resulting in a later spawning period as compared to the other euphausiids. Another important component of the herbivorous Antarctic zooplankton, the salps, have developed a very different reproductive strategy. They are able to switch from sexual reproduction to asexual budding (metagenesis), which allows extreme multiplication rates under favourable feeding conditions. Due to these successful adaptations, herbivores are able to build up huge stocks, in spite of the short productive period. Omnivorous and carnivorous zooplankton species, e.g., amphipods or chaetognaths, are not much constrained by the seasonality problem, but experience a more constant food supply. They show a tendency towards K strategies with a prolonged reproductive period, reduced egg numbers and increasing parental care. However, they do not exhibit such typical “polar adaptations” as developed by the herbivorous species.     

Acclimation responses to temperature vary with vertical stratification: implications for vulnerability of soil‐dwelling species to extreme temperature events

The occurrence of summer heat waves is predicted to increase in amplitude and frequency in the near future, but the consequences of such extreme events are largely unknown, especially for belowground organisms. Soil organisms usually exhibit strong vertical stratification, resulting in more frequent exposure to extreme temperatures for surface‐dwelling species than for soil‐dwelling species. Therefore soil‐dwelling species are expected to have poor acclimation responses to cope with temperature changes. We used five species of surface‐dwelling and four species of soil‐dwelling Collembola that habituate different depths in the soil. We tested for differences in tolerance to extreme temperatures after acclimation to warm and cold conditions. We also tested for differences in acclimation of the underlying physiology by looking at changes in membrane lipid composition. Chill coma recovery time, heat knockdown time and fatty acid profiles were determined after 1 week of acclimation to either 5 or 20 °C. Our results showed that surface‐dwelling Collembola better maintained increased heat tolerance across acclimation temperatures, but no such response was found for cold tolerance. Concordantly, four of the five surface‐dwelling Collembola showed up to fourfold changes in relative abundance of fatty acids after 1 week of acclimation, whereas none of the soil‐dwelling species showed a significant adjustment in fatty acid composition. Strong physiological responses to temperature fluctuations may have become redundant in soil‐dwelling species due to the relative thermal stability of their subterranean habitat. Based on the results of the four species studied, we expect that unless soil‐dwelling species can temporarily retreat to avoid extreme temperatures, the predicted increase in heat waves under climatic change renders these soil‐dwelling species more vulnerable to extinction than species with better physiological capabilities. Being able to act under a larger thermal range is probably costly and could reduce maximum performance at the optimal temperature.     

Reproductive strategies of sub-Antarctic lithodid crabs vary with habitat depth

We present the first fecundity and egg size data for three species of lithodid crab from Antarctic waters south of the Polar Front, caught in the fisheries operating around the island of South Georgia. In all species, fecundity was observed to increase with body size, but reproductive allocation was found to differ significantly between species. The highest relative fecundity (eggs^−g body weight) and smallest egg size was found in Paralomis spinosissima, which is found in shallower waters, whereas the lowest relative fecundity values and largest eggs were recorded in the deeper living species, Neolithodes diomedeae. Evidence is presented that closely related sympatric species may employ quite different reproductive strategies that suit their depth distribution whilst indicating the overriding evolutionary adaptation of reproductive traits to temperature and food availability.     

Coupling of canopy and understory food webs by ground‐dwelling predators

Understanding food‐web dynamics requires knowing whether species assemblages are compartmentalized into distinct energy channels, and, if so, how these channels are structured in space. We used isotopic analyses to reconstruct the food web of a Kenyan wooded grassland. Insect prey were relatively specialized consumers of either C₃ (trees and shrubs) or C₄ (grasses) plants. Arboreal predators (arthropods and geckos) were also specialized, deriving c. 90% of their diet from C₃‐feeding prey. In contrast, ground‐dwelling predators preyed considerably upon both C₃‐ and C₄‐feeding prey. This asymmetry suggests a gravity‐driven subsidy of the terrestrial predator community, whereby tree‐dwelling prey fall and are consumed by ground‐dwelling predators. Thus, predators in general couple the C₃ and C₄ components of this food web, but ground‐dwelling predators perform this ecosystem function more effectively than tree‐dwelling ones. Although prey subsidies in vertically structured terrestrial habitats have received little attention, they are likely to be common and important to food‐web organization.     

Stable isotopes, mesocosms and gut content analysis demonstrate trophic differences in two invasive decapod crustacea

1. Improving our understanding of dietary differences among omnivorous, benthic crustacea can help to define the scope of their trophic influence in benthic food webs. In this study, we examined the trophic ecology of two non‐native decapod crustaceans, the Chinese mitten crab (Eriocheir sinensis) (CMC) and the red swamp crayfish (Procambarus clarkii) (RSC), in the San Francisco Bay ecosystem to describe their food web impacts and explore whether these species are functionally equivalent in their impacts on aquatic benthic communities. 2. We used multiple methods to maximise resolution of the diet of these species, including N and C stable isotope analysis of field data, controlled feeding experiments to estimate isotopic fractionation, mesocosm experiments, and gut content analysis (GCA). 3. In experimental enclosures, both CMC and RSC caused significant declines in terrestrially derived plant detritus (P < 0.01) and algae (P < 0.02) relative to controls, and declines in densities of the caddisfly Gumaga nigricula by >50% relative to controls. 4. Plant material dominated gut contents of both species, but several sediment‐dwelling invertebrate taxa were also found. GCA and mesocosm results indicate that CMC feed predominantly on surface‐dwelling invertebrates, suggesting that trophic impacts of this species could include a shift in invertebrate community composition towards sediment‐dwelling taxa. 5. Stable isotope analysis supported a stronger relationship between CMC and both algae and algal‐associated invertebrates than with allochthonous plant materials, while RSC was more closely aligned with terrestrially derived detritus. 6. The trophic ecology and life histories of these two invasive species translate into important differences in potential impacts on aquatic food webs. Our results suggest that the CMC differs from the RSC in exerting new pressures on autochthonous food sources and shallow‐dwelling invertebrates. The crab's wide‐ranging foraging techniques, use of intertidal habitat, and migration out of freshwater at sexual maturity increases the distribution of the impacts of this important invasive species.     

Reduction in the metabolic levels due to phenotypic plasticity in the Pyrenean newt,Calotriton asper,during cave colonization

According to theories on cave adaptation, cave organisms are expected to develop a lower metabolic rate compared to surface organisms as an adaptation to food scarcity in the subterranean environments. To test this hypothesis, we compared the oxygen consumption rates of the surface and subterranean populations of a surface‐dwelling species, the newt Calotriton asper, occasionally found in caves. In this study, we designed a new experimental setup in which animals with free movement were monitored for several days in a respirometer. First, we measured the metabolic rates of individuals from the surface and subterranean populations, both maintained for eight years in captivity in a natural cave. We then tested individuals from these populations immediately after they were caught and one year later while being maintained in the cave. We found that the surface individuals that acclimated to the cave significantly reduced their oxygen consumption, whereas individuals from the subterranean population maintained in the cave under a light/dark cycle did not significantly modify their metabolic rates. Second, we compared these metabolic rates to those of an obligate subterranean salamander (Proteus anguinus), a surface aquatic Urodel (Ambystoma mexicanum), and a fish species (Gobio occitaniae) as references for surface organisms from different phyla. As predicted, we found differences between the subterranean and surface species, and the metabolic rates of surface and subterranean C. asper populations were between those of the obligate subterranean and surface species. These results suggest that the plasticity of the metabolism observed in surface C. asper was neither directly due to food availability in our experiments nor the light/dark conditions, but due to static temperatures. Moreover, we suggest that this adjustment of the metabolic level at a temperature close to the thermal optimum may further allow individual species to cope with the food limitations of the subterranean environment.     

On the Vertical Distribution of the Metazoan Meiofauna in Shelf Break and Upper Slope Habitats of the NE Atlantic

The metazoan meiofauna of nine stations in shelf break and upper slope areas (70 to 1500 m water depth) of the N.E. Atlantic were investigated in order to assess which environmental factors are important in the control of densities and sediment profiles. Total meiofaunal densities (ranging between 368 and 1523 ind/10 cm²) were correlated with bacterial densities, an important food source for meiofauna. However, considering sediment vertical distribution profiles, the relative importance of both food and oxygen on the meiofauna became obvious. A combination of both bacterial densities and oxygen supply could explain about 95% of the variability in the vertical profiles of the meiofauna densities. Meiofauna numbers increase in proportion to food availability in the surface sediment layers, but this relationship breaks down in deeper sediment layers where the oxygen supply is often limiting, particularly in fine sediments.     

Anoxia tolerance in high Arctic terrestrial microarthropods

Abstract. 1. Ten Arctic species of Collembola and two species of cryptostigmatic mites survived anoxia at 5 °C over periods ranging from 1 to 36 days. 2. Highly active, surface‐dwelling collembolans such as Isotoma anglicana, Isotoma tschernovi, and Sminthurides malmgreni were the most susceptible to anoxia. Mites and Collembola living deeper in the soil or in wet habitats, such as Camisia anomia and Hypogastrura viatica, were most tolerant. Tolerance, however, appears more closely linked to taxonomic relatedness than to ecological groupings per se, although the two may coincide. 3. Implications for life‐history strategies, including metabolic cold adaptation in its broadest sense, are discussed.     

Food supply impacts sediment reworking by <Emphasis Type="Italic">Nereis diversicolor</Emphasis>

This study assessed the effects of food supply on sediment reworking by Nereis diversicolor. We hypothesized that food supply would enhance sediment reworking and that the frequency of food supply would affect the intensity of bioturbation. Mesocosm experiments consisted of four treatments: (1) without worms (control cores), (2) with worms and no food supply, (3) with worms and daily food supply, (4) with worms and weekly food supply. Fluorescent particles, used as tracers, were spread over the sediment surface. Sediment reworking was quantified after 28 days based on the tracer distribution profiles. Results showed that sediment reworking by N. diversicolor was exclusively due to non-local transport processes. Food supply greatly increased non-local transport coefficients (more than 3 times) in comparison with those measured in the absence of a food supply. However, the intensity of sediment bioturbation by these worms was unaffected by the frequency of food supply. This study showed that environmental conditions affecting the quantity of food supply at the water-sediment interface could strongly influence bioturbation process. Handling editor: P. Viaroli     

Environmental condition related reproductive strategies and sex ratio in hydras

Kaliszewicz, A. and Lipińska, A. 2011. Environmental condition related reproductive strategies and sex ratio in hydras. —Acta Zoologica (Stockholm) 00 :1–7. Temperature and food supply appeared to affect sex ratio, sex composition and percentage of sexual individuals in three Hydra species: Hydra vulgaris, Hydra circumcincta and Hydra viridissima. We found three sexes present: females, males and hermaphrodites depending on environmental conditions. Hydra vulgaris appeared to be a species with a temperature‐dependent sex determination (TSD). The males and hermaphrodites were present only under rising temperatures, whereas females were observed exclusively at lowering temperatures. Hydras reproduced asexually at constant room temperature. Unlimited food affected sex ratios and induced the presence of males in H. circumcincta at lowering temperatures. Thus, H. circumcincta may be recognised as another Hydra species in which sex is determined by environmental factors (ESD). Under rising temperatures, the number of hermaphroditic individuals was higher when food supply was unlimited in all three species, indicating that hermaphrodites may need more energy to produce both male and female gonads. Both temperature changes and food supply positively affected asexual reproductive strategies in hydras, especially budding rates. Hydra circumcincta appeared to be less agile than other hydras and able to self‐fertilise. It is likely that self‐fertilisation is an adaptation to the low probability of meeting a mate belonging to the other clone.     

Differential energy allocation as an adaptation to different habitats in the parasitic wasp <Emphasis Type="Italic">Venturia canescens</Emphasis>

Environmental pressures are expected to favour organisms that optimally allocate metabolic resources to reproduction and survival. We studied the resource allocation strategies and the associated tradeoffs in the parasitoid wasp Venturia canescens, and their adaptation to the characteristics of the environment. In this species, individuals of two reproductive modes coexist in the same geographical locations, but they mainly occur in distinct habitats. Thelytokous (asexual) wasps are mostly found in anthropogenic habitats, where hosts tend to aggregate and food is absent. Arrhenotokous (sexual) wasps are exclusively found in natural habitats, where hosts are scattered and food is present. We analysed (1) the quantity of energy stored during ontogeny, (2) the tradeoff between reproduction and survival, by measuring egg load and longevity and (3) the host patch exploitation behaviour of the wasps at emergence. Arrhenotokous wasps emerged with more metabolic resources than thelytokous ones, especially glycogen, a nutrient that could be used for flying in search of hosts and/or food. Thelytokous wasps allocated more energy than arrhenotokous wasps to egg production: this would allow them to parasitize more hosts. The tradeoff between egg production and longevity was not revealed within reproductive modes, but when comparing them. At emergence, arrhenotokous wasps tended to exploit host patches less thoroughly than thelytokous wasps, suggesting that by leaving the host patch, they search for food. The results clearly showed adaptations to the characteristics of habitats preferentially inhabited by the two reproductive modes, and suggested a mechanism that facilitates their coexistence in natural conditions.     

Adaptive mechanisms during food restriction in <Emphasis Type="Italic">Acomys russatus</Emphasis>: the use of torpor for desert survival

The golden spiny mouse (Acomys russatus) is an omnivorous desert rodent that does not store food, but can store large amounts of body fat. Thus, it provides a good animal model to study physiological and behavioural adaptations to changes in food availability. The aim of this study was to investigate the time course of metabolic and behavioural responses to prolonged food restriction. Spiny mice were kept at an ambient temperature of 27°C and for 3 weeks their food was reduced individually to 30% of their previous ad libitum food intake. When fed ad libitum, their average metabolic rate was 82.77±3.72 ml O₂ h^–1 during the photophase and 111.19±4.30 ml O₂ h^–1 during the scotophase. During food restriction they displayed episodes of daily torpor when the minimal metabolic rate gradually decreased to 16.07±1.07 ml O₂ h^–1, i.e. a metabolic rate depression of approximately 83%. During the hypometabolic bouts the minimum average body temperature T_b, decreased gradually from 32.6±0.1°C to 29.0±0.4°C, with increasing duration of consecutive bouts. In parallel, the animals increased their activity during the remaining daytime. Torpor as well as hyperactivity was suppressed immediately by refeeding. Thus golden spiny mice used two simultaneous strategies to adapt to shortened food supply, namely energysaving torpor during their resting period and an increase in locomotor activity pattern during their activity period.     

Oxygen and carbon isotope studies in recent foraminifera from the southwest Indian ocean

Eighteen species of planktonic foraminifera have been analyzed for their oxygen and carbon isotopic composition in five Recent samples of deep-sea sediment from the southwest Indian Ocean; one sample of glacial age and one mid-Holocene sample were also studied. On the basis of oxygen isotopic composition three groups are recognized. Species in the first group (Globigerinoides ruber, G. sacculifer and G. conglobatus; G. Globigerina rubescens and Globigerinita glutinata) calcity in the near-surface Tropical Water, so that the oxygen isotopic composition of their test carbonate may be used to indicate surface temperature. Species in the second group (Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, Orbulina universa, Globigerinella siphonifera and Sphaeroidinella dehiscens) are associated with the sub-surface high-salinity Subtropical Water, so that their oxygen isotope composition indicates trends in the temperature of this water mass. The third group (the species of Globorotalia) calcity in the deeper Central Water. The average oxygen isotopic composition of each Globorotalia species is more or less constant over the range studied and does not reflect the surface temperature trend.The carbon isotopic composition of three species (Globigerina rubescens, Globigerinoides ruber and Globigerinita glutinata indicate departure from isotopic equilibrium by at least 3%₀. Among the remaining species the variation of carbon isotopic composition with depth (where depth is inferred from the temperature estimated from oxygen isotopic composition) implies that N. dutertrei, P. obliquiloculata and G. siphonifera occupy the shallow subsurface oxygen minimum, while the deeper-dwelling globorotaliids approach the deeper oxygen minimum. Hence it is possible, despite scatter among the data, to discern the pattern of oxygen content with depth in the overlying water masses from an examination of oxygen and carbon isotopic composition among foraminiferal species in the sediment. This promises to be an exciting new tool for palaeo-oceanographic investigations.     

Food availability as a major driver in the evolution of life‐history strategies of sibling species

Life‐history theory predicts trade‐offs between reproductive and survival traits such that different strategies or environmental constraints may yield comparable lifetime reproductive success among conspecifics. Food availability is one of the most important environmental factors shaping developmental processes. It notably affects key life‐history components such as reproduction and survival prospect. We investigated whether food resource availability could also operate as an ultimate driver of life‐history strategy variation between species. During 13 years, we marked and recaptured young and adult sibling mouse‐eared bats (Myotis myotis and Myotis blythii) at sympatric colonial sites. We tested whether distinct, species‐specific trophic niches and food availability patterns may drive interspecific differences in key life‐history components such as age at first reproduction and survival. We took advantage of a quasi‐experimental setting in which prey availability for the two species varies between years (pulse vs. nonpulse resource years), modeling mark‐recapture data for demographic comparisons. Prey availability dictated both adult survival and age at first reproduction. The bat species facing a more abundant and predictable food supply early in the season started its reproductive life earlier and showed a lower adult survival probability than the species subjected to more limited and less predictable food supply, while lifetime reproductive success was comparable in both species. The observed life‐history trade‐off indicates that temporal patterns in food availability can drive evolutionary divergence in life‐history strategies among sympatric sibling species.     

Palökologie und Biostratigraphie der Pelecypoden des Unteren Mitteldevon (Eifelium) der Eifel

During Eifelian times pelecypods were rare but steadily occurring elements among the invertebrate fauna. As can be concluded from shell shape, form, and inflation most of them lived infaunal, only few occupied the sediment surface. Their habitats were the deeper subtidal domains where silt- and marlstones predominated. Despite their conspicuous longevity they may regionally be used for stratigraphic purposes especially by considering two or more species at a time. Furthermore they aid in attaining a higher level of correlation by comprising high-energy facies into stratigraphic considerations. But since longevity favored shifting of their habitable area diachronousy with facies they are no suitable fossils for international correlation.

     

Effects of burrow condition and seed handling time on hoarding strategies of Edward's long-tailed rat (Leopoldamys edwardsi)

Many hoarding rodents use burrows not only for dwelling and protection from natural enemies, but also for food storage. However, little is known how burrows used by scatter-hoarding animals influence their foraging behaviors. In addition, handling time for a given food item has a fundamental impact on hoarding strategies of these hoarding animals: food items with longer handling time are more likely to be hoarded due to increasing predation risk because the animals spend more time outside their burrows if they consumed such food. By providing with two types of artificial burrows (aboveground vs. underground) and two types of food items (i.e. seeds) with contrasting handling times, we investigated how burrow condition and handling time co-influence hoarding strategies of a key scatter-hoarding rodent, Edward's long-tailed rat (Leopoldamys edwardsi) in large enclosures in southwest China. We found that only a few animals larder-hoarded fewer seeds when only aboveground burrows were available, while over 80% of the animals preferred to use the underground burrows and hoard significantly more seeds in the burrows when both aboveground and underground burrows were provided simultaneously. We also found that seed handling time significantly affected hoarding strategies of the animals: they consumed and/or scatter-hoarded more Camellia oleifera seeds with shorter handling time outside the burrow, but consumed and larder-hoarded more Lithocarpus harlandii seeds with longer handling time in underground burrows. Our study indicates that both burrow types and seed handling time have important impacts on hoarding strategies of scatter-hoarding animals.     

Rapid Redox Signal Transmission by “Cable Bacteria” beneath a Photosynthetic Biofilm

Recently, long filamentous bacteria, belonging to the family Desulfobulbaceae, were shown to induce electrical currents over long distances in the surface layer of marine sediments. These “cable bacteria” are capable of harvesting electrons from free sulfide in deeper sediment horizons and transferring these electrons along their longitudinal axes to oxygen present near the sediment-water interface. In the present work, we investigated the relationship between cable bacteria and a photosynthetic algal biofilm. In a first experiment, we investigated sediment that hosted both cable bacteria and a photosynthetic biofilm and tested the effect of an imposed diel light-dark cycle by continuously monitoring sulfide at depth. Changes in photosynthesis at the sediment surface had an immediate and repeatable effect on sulfide concentrations at depth, indicating that cable bacteria can rapidly transmit a geochemical effect to centimeters of depth in response to changing conditions at the sediment surface. We also observed a secondary response of the free sulfide at depth manifest on the time scale of hours, suggesting that cable bacteria adjust to a moving oxygen front with a regulatory or a behavioral response, such as motility. Finally, we show that on the time scale of days, the presence of an oxygenic biofilm results in a deeper and more acidic suboxic zone, indicating that a greater oxygen supply can enable cable bacteria to harvest a greater quantity of electrons from marine sediments. Rapid acclimation strategies and highly efficient electron harvesting are likely key advantages of cable bacteria, enabling their success in high sulfide generating coastal sediments.     

The ecology,behaviour and physiology of fishes on coral reef flats,and the potential impacts of climate change

Reef flats, typically a low‐relief carbonate and sand habitat in shallow water leeward of the reef crest, are one of the most extensive zones on Pacific coral reefs. This shallow zone often supports an abundant and diverse fish assemblage that is exposed to more significant variations in physical factors, such as water depth and movement, temperature and ultraviolet (UV) radiation levels, than most other reef fishes. This review examines the characteristics of reef flat fish assemblages, and then investigates what is known about how they respond to their biophysical environment. Because of the challenges of living in shallow, wave‐exposed water, reef flats typically support a distinct fish assemblage compared to other reef habitats. This assemblage clearly changes across tidal cycles as some larger species migrate to deeper water at low tide and other species modify their behaviour, but quantitative data are generally lacking. At least some reef flat fish species are well‐adapted to high temperatures, low oxygen concentrations and high levels of UV radiation. These behavioural and physiological adaptations suggest that there may be differences in the demographic processes between reef flat assemblages and those in deeper water. Indeed, there is some evidence that reef flats may act as nurseries for some species, but more research is required. Further studies are also required to predict the effects of climate change, which is likely to have multifaceted impacts on reef flats by increasing temperature, water motion and sediment load. Sea‐level rise may also affect reef flat fish assemblages and food webs by increasing the amount of time that larger species are able to forage in this zone. The lack of data on reef flats is surprising given their size and relative ease of access, and a better understanding of their functional role within tropical marine seascapes is urgently required.     

The effects of temperature and food on copepodite development,growth and reproduction in three species of Boeckella (Copepoda; Calanoida)

Three species of Boeckella (B. triarticulata, B. dilatata and B. hamata) were reared from copepodite I to adult at three naturally fluctuating food levels and three temperatures in a 3 × 3 × 3 factorial design. Development times, lengths, times to produce the first clutch, clutch sizes, egg voumes, interclutch times, infertile clutches, total egg production, total reproductive time and mortality were measured. Development times for the three species were surprisingly similar but mortality differed among the treatments. The three species differed in their reproductive strategies. B. triarticulata, the largest species, produced a few large clutches. B. dilatata, the smallest species, produced many small clutches and invested the most energy per egg. B. hamata produced larger clutches than B. dilatata but not as many. B. hamata was the most tolerant of low, food levels and exhibited the most plasticity across the treatments. Thus, greater plasticity in life history characters is associated with survival in a wider-range of conditions.     

Water deprivation drives intraspecific variability in lizard heat tolerance

Quantifying intraspecific variation in heat tolerance is critical to understand how species respond to climate change. In a previous study, we recorded variability in critical thermal maxima (CT_max) by 3 °C among populations of small Iberian lizard species, which could substantially influence predictions of climate-driven activity restriction. Here, we undertake experiments to examine whether we could reproduce similar levels of heat-tolerance variability in response to water deficit. We hypothesized that deprivation of drinking water should increase variability in CT_max between populations more than deprivation of food under the theoretical expectation that the variation of the more limiting resource must trigger stronger variation in physiological performance. We measured CT_max after manipulating availability of live prey and drinking water in two populations of an arid and a mesic lizard species from the Iberian Peninsula. We quantified a mean CT_max across all studied lizards of 44.2 °C ± 0.2 SE for the arid species and 41.7 °C ± 0.3 SE for the mesic species. Using multimodel inference, we found that water deprivation (combined with food supply) caused population differences in CT_max by 3 to 4 °C which were two to three times wider than population differences due to food deprivation (combined with water supply) or to food and water provision. To highlight the need for more thermo-hydroregulatory research, we examined bias in research effort towards thermal versus hydric environmental effects on heat tolerance through a systematic literature review. We show that environmental temperature has been used five times more frequently than precipitation in ecological studies of heat tolerance of terrestrial species. Studies linking thermal tolerance of ectotherms to the interplay of air temperature and water availability are needed in the face of projected increases in aridity and drought in the 21st century, because the balance of body temperature and water resources are functionally interlinked.