Is thermal sensitivity affected by predation risk? A case study in tadpoles from ephemeral environments.

Changes in environmental temperature may induce variations in thermal tolerance and sensitivity in ectotherm organisms. These variations generate plastic responses that can be analyzed by examining their Thermal Performance Curves (TPCs). Additionally, some performance traits, like locomotion, could be affected by other factors such as biological interactions (e.g., predator-prey interaction). Here, we evaluate if the risk of predation modifies TPCs in Mendoza four-eyed frog (Pleurodema nebulosum, Burmeister, 1861) and Guayapa's four-eyed frog (Pleurodema guayapae, Barrio, 1964), two amphibian species that occur in ephemeral ponds in arid environments. We measured thermal tolerances and maximum swimming velocity at six different temperatures in tadpoles under three situations: control, exposure to predator chemical cues, and exposure to conspecific alarm cues. TPCs were fitted using General Additive Mixed Models. We found that curves of tadpoles at risk of predation differed from those of control mainly in thermal sensitivity parameters. Our work confirms the importance of biotic interactions have in thermal physiology.

Does temperature at local scale explain thermal biology patterns of temperate tadpoles?

Most of the literature on temperature-organism interactions rely on mean temperature (mostly air), disregarding the real complexity of this variable. There is a growing consensus about the importance of considering the temperature fluctuations as a mechanism improving organism's performance. Tadpoles are small body size ectotherm organisms that behave isothermally with their environment. As such, are good models for studying their thermal biology relative to their immediate environment. We studied six anuran tadpole species in North Patagonia, Alsodes gargola, Hylorina sylvatica, Batrachyla taeniata, Pleurodema thaul, P. bufoninum and Rhinella spinulosa, distributed in a West-East altitudinal cline with different environments and thermal conditions. We evaluated the relationship between thermal descriptors at a local scale and the thermal biology patterns of these temperate tadpoles. We estimated thermal tolerance limits and thermal sensitivity of locomotion of each species. The different aquatic environments showed important differences in local thermal conditions, associated with observed differences in the thermal traits in these tadpoles. Species exposed to lower temperature fluctuations and lower environmental mean temperatures showed lower swimming optimal temperatures and narrower thermal tolerance ranges. We found greater variability in the upper than in the lower critical limits in these Patagonian anuran tadpoles. Minimum critical temperatures were close to freezing temperature, possibly in detriment of their tolerance to high temperatures. Overall, our results suggest that these species are adapted to low temperatures. Finally, warming tolerances and predicted thermal safety margins, show that none of the studied species appear to be under thermal stress that may compromise their survival at the present time or in the near future, under a moderate climate change scenario.

How and when melanic coloration is an advantage for lizards: the case of three closely-related species of Liolaemus.

Body temperature affects various aspects of ectotherm biology. Reptiles, as ectotherms, gain and control their temperature mainly through behavioural adjustments, although some body traits may also be advantageous. According to the thermal melanism hypothesis (TMH) dark colour may be thermally advantageous in cold environments. Additionally, differences in thermoregulatory capacity may also affect performance. We analysed the role of melanism in the thermoregulation and sprint speed performance of three species of Liolaemus lizards from Argentinean Patagonia. Liolaemus shitan, L. elongatus and L. gununakuna are phylogenetically close, with similar body sizes and life history traits, but differ in their melanic colouration, L. shitan being the darkest and L. gununakuna the lightest species. We estimated sprint speed performance curves and heating rates, and recorded final body temperature and sprint speed achieved after a fixed heating time, from two different initial body temperatures, and with and without movement restriction. Performance curves were similar for all the species, but for L. gununakuna the curve was more flattened. Darker species showed faster heating rates, ran faster after fixed heating trials at the lowest temperature, and reached higher body temperatures than L. gununakuna, but this was compensated for by behavioural adjustments of the lighter lizards. Similarity of sprint speed performance may be due to the conservative nature of this character in these species, while variation in heating ability, particularly when starting from low temperatures, may reflect plasticity in this trait. The latter provides support for the TMH in these lizards, as melanism helps them increase their body temperature. This may be especially advantageous at the beginning of the day or on cloudy days, when temperatures are lower.

How sensitive are temperate tadpoles to climate change? The use of thermal physiology and niche model tools to assess vulnerability.

Ectotherms are vulnerable to climate change, given their dependence on temperature, and amphibians are particularly interesting because of their complex life cycle. Tadpoles may regulate their body temperature by using suitable thermal microhabitats. Thus, their physiological responses are the result of adjustment to the local thermal limits experienced in their ponds. We studied three anuran tadpole species present in Argentina and Chile: Pleurodema thaul and Pleurodema bufoninum that are seasonal and have broad geographic ranges, and Batrachyla taeniata, a geographically restricted species with overwintering tadpoles. Species with restricted distribution are more susceptible to climate change than species with broader distribution that may cope with potential climatic changes in the environments in which they occur. We aim to test whether these species can buffer the potential effects of climate warming. We used ecological niche models and the outcomes of their thermal attributes (critical thermal limits, optimal temperature, and locomotor performance breadth) as empirical evidence of their capacity. We found that Pleurodema species show broader performance curves, related to their occurrence, while the geographically restricted B. taeniata shows a narrower thermal breadth, but is faster in warmer conditions. The modeled distributions and empirical physiological results suggest no severe threats for these three anurans. However, the risk level is increasing and a retraction of their distribution range might be possible for Pleurodema species, and some local population extinctions may happen, particularly for the narrowly distributed B. taeniata.

Anuran tadpoles learn to recognize injury cues from members of the same prey guild.

Recognition of predation risk from cues released from injured heterospecific could be beneficial when prey belongs to the same prey guild. Here, we performed three experiments. Experiment 1 showed that P. thaul tadpoles reduced their activity levels when exposed to conspecific injury cues, but not when exposed to amphipod injury cues. Experiment 2 tested whether P. thaul tadpoles can learn to recognize predation risk from chemical cues released from injured heterospecifics from the same prey guild (amphipod, Hyalella patagonica). A group of tadpoles were conditioned by exposing them to a specific concentration of amphipod injury cues paired with conspecific injury cues. Two days later, we evaluated changes in the activity of tadpoles when they were exposed to amphipod cues. As a control of learning, we used an unpaired group. Additionally, we used more control groups to fully investigate the learning mechanism. Our results showed that tadpoles can learn to recognize predation risk from injured amphipods and that the mechanism underlying the observed learned response could be associative. Experiment 3 replicated Experiment 2 and also showed that a low concentration of amphipod cues did not sustain that learning.

Effect of body mass and melanism on heat balance in Liolaemus lizards of the goetschi clade.

The body temperature of ectotherms depends on the environmental temperatures and behavioral adjustments, but morphology may also have an effect. For example, in colder environments, animals tend to be larger and to show higher thermal inertia, as proposed by Bergmann's rule and the heat balance hypothesis (HBH). Additionally, dark coloration increases solar radiation absorption and should accelerate heat gain (thermal melanism hypothesis, TMH). We tested Bergmann's rule, the HBH and the TMH within the ITALIC! Liolaemus goetschilizard clade, which shows variability in body size and melanic coloration. We measured heating and cooling rates of live and euthanized animals, and tested how morphology and color affect these rates. Live organisms show less variable and faster heating rates compared with cooling rates, suggesting behavioral and/or physiological adjustments. Our results support Bergmann's rule and the HBH, as larger species show slower heating and cooling rates. However, we did not find a clear pattern to support the TMH. The influence of dorsal melanism on heating by radiation was masked by the body size effect in live animals, and results from euthanized individuals also showed no clear effects of melanism on heating rates. Comparison among three groups of live individuals with different degrees of melanism did not clarify the influence of melanism on heating rates. However, when euthanized animals from the same three groups were compared, we observed that darker euthanized animals actually heat faster than lighter ones, favoring the TMH. Although unresolved aspects remain, body size and coloration influenced heat exchange, suggesting complex thermoregulatory strategies in these lizards, probably regulated through physiology and behavior, which may allow these small lizards to inhabit harsh weather environments.

Lack of functional link in the tadpole morphology induced by predators.

Most studies of predator-induced plasticity have focused on documenting how prey species respond to predators by modifying phenotypic traits and how traits correlate with fitness. We have previously shown that Pleurodema thaul tadpoles exposed to the dragonfly Rhionaeschna variegata responded strongly by showing morphological changes, less activity, and better survival than non-exposed tadpoles. Here, we tested whether there is a functional link between morphological plasticity and increased survival in the presence of predators. Tadpoles that experienced predation risk were smaller, less developed, and much less active than tadpoles without this experience. Burst speed did not correlate significantly with morphological changes and predator-induced deeper tails did not act as a lure to divert predator strikes away from the head. Although we have previously found that tadpoles with predator-induced morphology survive better under a direct predator threat, our results on the functional link between morphology and fitness are not conclusive. Our results suggest that in P. thaul tadpoles (1) burst speed is not important to evade predators, (2) those exposed to predators reduce their activity, and (3) morphological changes do not divert predator attacks away from areas that compromise tadpole survivalEE. Our results show that morphological changes in P. thaul tadpoles do not explain burst speed or lure attraction, although there was a clear reduction of activity, which itself reduces predation. We propose that changes in tadpole activity could be further analyzed from another perspective, with morphological change as an indirect product of behavior mediated by physiological mechanisms.

Running in cold weather: morphology, thermal biology, and performance in the southernmost lizard clade in the world (Liolaemus lineomaculatus section: Liolaemini: Iguania).

The integration or coadaptation of morphological, physiological, and behavioral traits is represented by whole-organism performance traits such as locomotion or bite force. Additionally, maximum sprint speed is a good indicator of whole-organism performance capacity as variation in sprinting ability can affect survival. We studied thermal biology, morphology, and locomotor performance in a clade of Liolaemus lizards that occurs in the Patagonian steppe and plateaus, a type of habitat characterized by its harsh cold climate. Liolaemus of the lineomaculatus section display a complex mixture of conservative and flexible traits. The phylogenetically informed analyses of these ten Liolaemus species show little coevolution of their thermal traits (only preferred and optimum temperatures were correlated). With regard to performance, maximum speed was positively correlated with optimum temperature. Body size and morphology influenced locomotor performance. Hindlimbs are key for maximal speed, but forelimb length was a better predictor for sustained speed (i.e. average speed over a total distance of 1.2 m). Finally, sustained speed differed among species with different diets, with herbivores running on average faster over a long distance than omnivores.

Effect of UV-B exposure on eggs and embryos of patagonian anurans and evidence of photoprotection.

In Patagonia, enhanced levels of ultraviolet radiation (UVR) are affecting not only the southernmost territories but the northern as well, and there is a growing body of evidence indicating effects on terrestrial and aquatic communities. Anurans occur along a wide geographical gradient in Patagonia and are common in shallow waters, usually breeding during spring when seasonal UV-B radiation is at its highest level. In this paper we analyse the effective irradiances (I) experienced in nature by eggs and embryos of the patagonian anurans Pleurodema bufoninum, Pleurodema thaul and Bufo spinulosus papillosus, considering water and egg jelly optical properties. Besides, we determine photoprotective pigments in eggs and embryos of the different species, and ontogenetic shifts in pigment concentration. Finally, we studied the effects of natural and enhanced levels of UV-B on eggs of P. bufoninum aiming to evaluate the role of the jelly envelope in screening the damaging radiation. The evidence gathered in this work shows that natural and enhanced UV-B levels did not induce significant changes on the survivorship but incremented the occurrence of malformations. Also, the species experiencing high levels of exposure presented high concentrations of melanin suggesting that exposure to sunlight triggers photoprotection by pigments. Collectively our results suggest that the studied species bear certain levels of adaptation to cope with high ambient UVR conferred by environmental and biological factors.