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Phenotypic plasticity may increase the performance and fitness and allow organisms to cope with variable environmental conditions. We studied within‐generation plasticity and transgenerational effects of thermal conditions on temperature tolerance and demographic parameters in Drosophila melanogaster. We employed a fully factorial design, in which both parental (P) and offspring generations (F1) were reared in a constant or a variable thermal environment. Thermal variability during ontogeny increased heat tolerance in P, but with demographic cost as this treatment resulted in substantially lower survival, fecundity, and net reproductive rate. The adverse effects of thermal variability (V) on demographic parameters were less drastic in flies from the F1, which exhibited higher net reproductive rates than their parents. These compensatory responses could not totally overcome the challenges of the thermally variable regime, contrasting with the offspring of flies raised in a constant temperature (C) that showed no reduction in fitness with thermal variation. Thus, the parental thermal environment had effects on thermal tolerance and demographic parameters in fruit fly. These results demonstrate how transgenerational effects of environmental conditions on heat tolerance, as well as their potential costs on other fitness components, can have a major impact on populations’ resilience to warming temperatures and more frequent thermal extremes. Within‐generation plasticity and transgenerational effects can ameliorate the impact of stressful thermal conditions on physiological and fitness‐related traits.

Global climate change poses one of the greatest threats to biodiversity. Most analyses of the potential biological impacts have focused on changes in mean temperature, but changes in thermal variance will also impact organisms and populations. We assessed the combined effects of the mean and variance of temperature on thermal tolerances, organismal survival, and population growth inDrosophila melanogaster. Because the performance of ectotherms relates nonlinearly to temperature, we predicted that responses to thermal variation (±0° or ±5°C) would depend on the mean temperature (17° or 24°C). Consistent with our prediction, thermal variation enhanced the rate of population growth (r max) at a low mean temperature but depressed this rate at a high mean temperature. The interactive effect on fitness occurred despite the fact that flies improved their heat and cold tolerances through acclimation to thermal conditions. Flies exposed to a high mean and a high variance of temperature recovered from heat coma faster and survived heat exposure better than did flies that developed at other conditions. Relatively high survival following heat exposure was associated with low survival following cold exposure. Recovery from chill coma was affected primarily by the mean temperature; flies acclimated to a low mean temperature recovered much faster than did flies acclimated to a high mean temperature. To develop more realistic predictions about the biological impacts of climate change, one must consider the interactions between the mean environmental temperature and the variance of environmental temperature.

Vector-borne diseases (VBD) are particularly susceptible to climate change because most of the diseases’ vectors are ectotherms, which themselves are susceptible to thermal changes. The Chagas disease is one neglected tropical disease caused by the protozoan parasite, Trypanosoma cruzi . One of the main vectors of the Chagas disease in South America is Triatoma infestans , a species traditionally considered to be restricted to domestic or peridomestic habitats, but sylvatic foci have also been described along its distribution. The infestation of wild individuals, together with the projections of environmental changes due to global warming, urge the need to understand the relationship between temperature and the vector’s performance. Here, we evaluated the impact of temperature variability on the thermal response of T . infestans . We acclimated individuals to six thermal treatments for five weeks to then estimate their thermal performance curves (TPCs) by measuring the walking speed of the individuals. We found that the TPCs varied with thermal acclimation and body mass. Individuals acclimated to a low and variable ambient temperature (18°C ± 5°C) exhibited lower performances than those individuals acclimated to an optimal temperature (27°C ± 0°C); while those individuals acclimated to a low but constant temperature (18°C ± 0°C) did not differ in their maximal performance from those at an optimal temperature. Additionally, thermal variability ( i . e ., ± 5°C) at a high temperature (30°C) increased performance. These results evidenced the plastic response of T . infestans to thermal acclimation. This plastic response and the non-linear effect of thermal variability on the performance of T . infestans posit challenges when predicting changes in the vector’s distribution range under climate change.

Background Trypanosoma cruzi , the etiologic agent of Chagas disease, is transmitted via the dejections of triatomine insects such as Triatoma infestans. Parasite development inside the vector depends on temperature, which determines the extrinsic incubation period (EIP) and modulates the parasite load. As global warming is expected to increase mean temperatures and thermal variability, these shifts may influence vector competence. Methods Triatoma infestans individuals were experimentally infected with T. cruzi Dm28c strain and then exposed to four thermal regimes: two constant (18 °C and 27 °C) and two fluctuating (18 ± 5 °C and 27 ± 5 °C). Parasite load in the dejection samples was quantified by quantitative PCR over 42 days and the time to the first positive dejection determined to estimate the EIP. Results Higher temperatures significantly shortened the EIP, with mean values of 18.6 days at 18 ± 0 °C, 17.3 days at 18 ± 5 °C, 9.6 days at 27 ± 0 °C and 11.0 days at 27 ± 5 °C. Temperature variability did not affect the EIP but it did increase parasite load under cold conditions. Parasite load showed a bell-shaped curve, peaking earlier and at higher levels at warmer temperatures. A larger volume of ingested blood also reduced the EIP, especially under cold treatments. Conclusions Rising temperatures accelerate T. cruzi development within T. infestans , potentially enhancing vector competence under climate change scenarios. Although temperature variability did not affect the EIP, it increased parasite load, particularly under cold conditions, which is a relevant result considering that low temperatures have historically limited the vector and Chagas disease transmission. Temperature variability—not only mean warming—can modulate parasite development. Our results therefore provide novel and relevant insights into how climate change may alter vector-borne disease dynamics. Graphical Abstract

The relationship between ectotherm ecology and climatic conditions has been mainly evaluated in terms of average conditions. Average temperature is the more common climatic variable used in physiological and population studies, and its effect on individual and population-level processes is well understood. However, the intrinsic variability of thermal conditions calls attention to the potential effects that this variability could have in ecological systems. Regarding this point, two hypotheses are proposed. From the allocation principle, it may be inferred that if temperature variability is high enough to induce stress in the organisms, then this extra-cost should reduce the energetic budget for reproduction, which will be reflected in population parameters. Moreover, a mathematical property of non-linear functions, Jensen’s inequality, indicates that, in concave functions, like the temperature–reproduction performance function, variability reduces the expected value of the output variable, and again modifies population parameters. To test these hypotheses, experimental cultures of Tribolium confusum under two different thermal variability regimens were carried out. With these data, we fitted a simple population dynamics model to evaluate the predictions of our hypothesis. The results show that thermal variability reduces the maximum reproductive rate of the population but no other parameters such as carrying capacity or the nonlinear factor in a nonlinear version of the Ricker model, which confirms our hypotheses. This result has important consequences, such as the paradoxical increase in population variability under a decrease in thermal variability and the necessary incorporation of climatic variability to evaluate the net effect of climate change on the dynamics of natural populations.

Regulation of energy and water balance are primary components of homeostasis in all organisms. But the processes associated with such homeostasis can be costly and affect the fitness of individuals. As a result, individuals from variable environments are expected to invest more in compensatory mechanisms to maintain homeostasis than individuals from stable environments. Grammostola quirogai and Grammostola anthracina are two spiders of the Theraphosidae family that live in rocky-hill habitats in the Pampas ecoregion. Both species inhabit mesic environments with moderate rainy weather but different precipitation variability. G. quirogai only exists in rocky-hill habitats surrounded by prairies, far from oceanic and estuarine coasts. These habitats are exposed to higher and more variable precipitation rates due to a stronger influence of El Niño. In contrast, G. anthracina lives in areas with less heterogeneous precipitation rates, and its populations expand up to the Atlantic coast. In this study, we used these two species to explore the impacts of water deprivation on their metabolic rate and water loss. We did not detect changes in metabolic rate or differences in water loss as a result of a water restriction treatment in any of the species. However, the mean total values of evaporative water loss for our studied species were lower than that of xeric species. These results provide evidence that the total evaporative water loss in tarantulas may not be related to environmental characteristics, as it has been widely reported among insect species.

Many species of reptiles maintain their body temperature behaviorally in a narrow range, even in the presence of considerable environmental temperature variation, by choosing microhabitats with different temperatures. In freshwater turtles, thermoregulation is generally achieved by aerial basking, even though they perform all other vital activities such as food consumption and reproduction in the water. Therefore, time budgets related to basking should be constrained and individuals should maximize the energy per unit time during basking, potentially by increasing basking frequency at noon during colder months and increasing use of basking when water temperature decreases. We analyzed basking behavior during the austral summer to study the effects of season, water temperature, and time of day in 2 South American freshwater turtles: Trachemys dorbigni (black-bellied slider) and Phrynops hilarii (Hilaire's side-necked turtle). We found that water temperature negatively affected basking frequency in both species differently; basking by T. dorbigni occurred on a diel cycle while basking by P. hilarii occurred on a seasonal level. Both species showed a bell-shaped basking frequency during the day, with more individuals basking at noon than in the morning and afternoon. However, only P. hilarii showed a significant seasonal effect on basking, with basking frequency decreasing in summer. These results suggest the thermoregulatory role of basking behavior in 2 austral turtle species and its trade-off with other vital activities.

The transmission and incidence of vector-borne diseases rely on vector distribution and life history traits such as survival, fecundity, and feeding. Since arthropod disease vectors are ectotherms, these vital rates are strongly influenced by temperature. Chagas disease is a neglected tropical disease caused by the protozoan parasite, Trypanosoma cruzi. This parasite is transmitted when the feces of the infected triatomine enter the bloodstream of the host. One of the most important vector-species of this disease in the Southern Cone region of South America is Triatoma infestans. In this study, we evaluated the role of constant and variable environmental temperature on the feeding behavior of T. infestans. Fifth-instar nymphs were acclimatized to 4 thermal treatments comprising 2 temperatures (27 °C and 18 °C) with and without diurnal thermal variability (27 ± 5 °C and 18 ± 5 °C). Individuals were fed weekly for 7 wk to quantify their feeding. Our results showed lower feeding frequency in nymphs acclimatized to cold temperature compared to those from warmer temperature treatments. However, treatments with thermal variability presented a nonlinear effect on feeding, with an increased feeding rate in the cold, variable treatment and a decreased feeding rate in the warm, variable treatment. Individuals maintained under cold treatments, the variable temperature exhibited a higher feeding rate and the lowest amount of ingested blood among all treatments. Thus, natural diurnal temperature variation cannot be ignored if we are to make more accurate T. cruzi transmission risk predictions now and in the future.

The origin of endothermy is a puzzling phenomenon in the evolution of vertebrates. To address this issue several explicative models have been proposed. The main models proposed for the origin of endothermy are the aerobic capacity, the thermoregulatory and the parental care models. Our main proposal is that to compare the alternative models, a critical aspect is to determine how strongly natural selection was influenced by body temperature, and basal and maximum metabolic rates during the evolution of endothermy. We evaluate these relationships in the context of three main hypotheses aimed at explaining the evolution of endothermy, namely the parental care hypothesis and two hypotheses related to the thermoregulatory model (thermogenic capacity and higher body temperature models). We used data on basal and maximum metabolic rates and body temperature from 17 rodent populations, and used intrinsic population growth rate (R(max)) as a global proxy of fitness. We found greater support for the thermogenic capacity model of the thermoregulatory model. In other words, greater thermogenic capacity is associated with increased fitness in rodent populations. To our knowledge, this is the first test of the fitness consequences of the thermoregulatory and parental care models for the origin of endothermy.

Temperature is a major factor affecting population abundance and individual performance. Net reproductive rate ( R 0 ) and intrinsic rate of increase ( r ) differ in their response to different temperature regimes and much of the difference is mediated by generation time ( Tg ). Here, we evaluate the effects of thermal mean and variability on R 0 , r and Tg , at four population densities in Drosophila melanogaster . The results show that R 0 , r and Tg present differential responses to thermal variation. Although temperature effects on R 0 and Tg are non-linear, r response was negligible. R 0 and Tg comprise a generational time scale, while r is at a chronological time scale. Thus, we argue that individuals growing under different thermal environments perform similarly on a chronological scale, but differently on a generational scale.