Billions of animals undertake migratory journeys every year, with powerful consequences for ecosystem dynamics. Key behaviours that enable successful migration are often guided by the visual system. The amount and quality of information that animals can extract from visual scenes are directly related to structural eye size-larger eyes can house larger pupils, enhancing light-gathering capacity and vision by improving visual acuity and contrast sensitivity. Migration should exert strong demands on individual visual performance, for example via foraging, antipredator benefits or navigational requirements. Yet, it remains elusive whether variations in eye morphology and corresponding visual capabilities are associated with migratory propensity. Here, we capitalize upon intra-population variation in migratory propensity (also known as partial migration) in roach, a common freshwater fish, to directly test for migration-associated variation in image-forming eyes within a species. In a multi-year field study tracking the migration decisions of over 2000 individuals in two different lake systems, we found that relative pupil size was positively associated with individual migration propensity. Computational simulations of the visual ecology associated with the observed differences in pupil size show that migrants have an extended visual detection range and that the performance gain is most pronounced for viewing small targets (e.g. planktonic prey) under low-light conditions. These results suggest that the larger pupils of migrants represent an adaptation for increased foraging efficiency to aid in the accumulation of critical pre-migratory energy reserves. Together, our anatomical and functional findings provide new perspectives on visual system design in relation to individual-level migratory decision-making.

Parasites often have a large impact on their hosts and can alter host phenotype to increase their own fitness, a phenomenon known as extended phenotype. Studies demonstrating extended phenotype for non-trophically transmitted parasites are scarce. Unionid mussels have a parasitic life stage adapted to parasitize fish which can affect host behavior, habitat use and growth rates, raising the question if parasitic freshwater mussels can also manipulate their host fish to compensate for downstream dispersal and to reach habitats favorable for newly excysted juvenile mussels. Wild-caught, parasite-naïve juvenile brown trout (Salmo trutta) were PIT-tagged, and half of the individuals were infested with parasitic larvae from the freshwater pearl mussel (Margaritifera margaritifera), all individuals were then returned to their home stream. During the following year, trout were tracked to investigate movement and habitat use, and also periodically recaptured to measure growth and body condition factor. The infested trout showed significantly higher upstream movement than non-infested trout and were more often recaptured in stream sections with slow-moving shallow water, particularly during the parasite excystment period (270 d post infestation). These data suggest that the juvenile mussels were successfully transported an average of 170 m upstream from the host trout release points to stream sections favorable for adult mussels. Infested trout survived as well as the non-infested, but had a significantly lower specific growth rate than non-infested trout. These results indicate a first example of extended phenotype in unionid mussels and highlight the importance of understanding glochidia-induced changes to host fish behavioral ecology.

The size structure of populations is crucial for predator–prey dynamics and ecosystem function. Anthropogenic pressures such as habitat alteration may affect the demography of many species. We investigated the size structure of European perch ( Perca fluviatilis ) populations in lakes before and after dam removal by using the planned removals of multiple dams in the outlets of boreal lakes in central Sweden as a natural experiment. In five lakes from which we had obtained data both before and after dam removal, removal resulted in a reduced abundance of small perch and an increase in body size of large, piscivorous perch, but there was no major effect on perch biomass. In a second comparison of 22 lakes where we only had access to data from either before or after removal, the effects were not as evident. We suggest that a plausible mechanism explaining a potential effect on perch demography was alterations in the predator–prey dynamics caused by changes in refuge habitat area for small fish. Specifically, dam removal caused a reduction of aquatic plant coverage. Large piscivorous fish are known to play a crucial regulatory role in controlling lake ecosystem function. Thus, we suggest that lake outlet dam removal may induce processes leading to positive effects on fish populations and ecosystem state. Moreover, our study emphasises the importance of before versus after studies to evaluate restoration measures.

Parasites often have a large impact on their hosts and can alter host phenotype to increase their own fitness, a phenomenon known as extended phenotype. Studies demonstrating extended phenotype for non-trophically transmitted parasites are scarce. Unionid mussels have a parasitic life stage adapted to parasitize fish which can affect host behavior, habitat use and growth rates, raising the question if parasitic freshwater mussels can also manipulate their host fish to compensate for downstream dispersal and to reach habitats favorable for newly excysted juvenile mussels. Wild-caught, parasite-naïve juvenile brown trout (Salmo trutta) were PIT-tagged, and half of the individuals were infested with parasitic larvae from the freshwater pearl mussel (Margaritifera margaritifera), all individuals were then returned to their home stream. During the following year, trout were tracked to investigate movement and habitat use, and also periodically recaptured to measure growth and body condition factor. The infested trout showed significantly higher upstream movement than non-infested trout and were more often recaptured in stream sections with slow-moving shallow water, particularly during the parasite excystment period (270 d post infestation). These data suggest that the juvenile mussels were successfully transported an average of 170 m upstream from the host trout release points to stream sections favorable for adult mussels. Infested trout survived as well as the non-infested, but had a significantly lower specific growth rate than non-infested trout. These results indicate a first example of extended phenotype in unionid mussels and highlight the importance of understanding glochidia-induced changes to host fish behavioral ecology.

Migration is critical for life-cycle completion in diadromous fish species. River connectivity is vital in facilitating these large-scale movement events, but the extent of present-day river fragmentation can interfere with these migrations. Fish passage solutions (FPSs) are commonly implemented with the aim of improving river connectivity. In our study, we investigated the performance of two types of FPSs, spill regimes and complete dam removal, on Atlantic salmon (Salmo salar) smolt migrations. We used acoustic telemetry to monitor migration behavior and passage success of 120 wild smolts released in three different groups/sites: one group with two dams to pass to reach the river mouth, a second group with one dam to pass, and a control group without any barriers to pass (upstream of a recently removed hydroelectric dam). Smolt passage probabilities were similar for the two studied dams (87% and 86%) but showed variation in path choice, delay times, and loss rates. Passage success was influenced by several factors, such as body size, diel period, and water temperature, but not flow. Cumulative passage success to the river mouth was 61%, with most individuals being lost within lentic river stretches, either in the forebays of hydroelectric power stations or in naturally wide river stretches. Within the recently rehabilitated river sections (post dam removal), passage speeds were significantly faster than all other sections of the river (post-rehabilitation x¯ = 56.1 km/day) with significantly faster speeds compared to pre-rehabilitation (pre-x¯ = 28.0 km/day). Our findings provide valuable information on the benefits of dam removal and highlight the need for further rehabilitation measures in upriver reaches where barriers still affect downstream passage.

Fish fins are remarkable devices of propulsion. Fin morphology is intimately linked to locomotor performance, and hence to behaviours that influence fitness, such as foraging and predator avoidance. This foreshadows a connection between fin morphology and variation in predation risk. Yet, whether prey can adjust fin morphology according to changes in perceived risk within their lifetime (a.k.a. predator-induced plasticity) remains elusive. Here, we quantify the structural size of five focal fins in crucian carp (Carassius carassius) following controlled manipulations to perceived predation risk (presence/absence of pike Esox lucius). We also assess if crucian carp respond to increased predation risk by shifts in dorsal fin colouration, and test for differences in how fish actively use their dorsal fins by quantifying the area of the fin displayed in behavioural trials. We find that crucian carp show phenotypic plasticity with regards to fin size as predator-exposed fish consistently have larger fins. Individuals exposed to perceived predation risk also increased dorsal fin darkness and actively displayed a larger area of the fin to potential predators. Our results thus provide compelling evidence for predator-induced fin enlargement, which should result in enhanced escape swimming performance. Moreover, fin-size plasticity may evolve synergistically with fin colouration and display behaviour, and we suggest that the adaptive value of this synergy is to enhance the silhouette of deep-bodied and hard-to-capture prey to deter gape-limited predators prior to attack. Together, our results provide new perspectives on the role of predation risk in development and evolution of fins. Fish fin morphology is intimately linked to locomotor performance, and hence to behaviours that influence fitness, such as foraging and predator avoidance. We provide evidence for predator-induced fin enlargement. The expression of larger fins likely results in increaseed escape swimming performance and enhances the silhouette of a deep-bodied and hard-to-capture prey.image

Inducible defences allow prey to increase survival chances when predators are present while avoiding unnecessary costs in their absence. Many studies report considerable inter-individual variation in inducible defence expression, yet what underlies this variation is poorly understood. A classic vertebrate example of a predator-induced morphological defence is the increased body depth in crucian carp (Carassius carassius), which reduces the risk of predation from gape-size limited predators. Here, we report that among-individual variation in morphological defence expression can be linked to sex. We documented sexual dimorphism in lakes in which crucian carp coexisted with predators, where females showed shallower relative body depths than males, but not in a predator-free lake. When exposing crucian carp from a population without predators to perceived predation risk in a laboratory environment (presence/absence of pike, Esox lucius), we found that males expressed significantly greater morphological defence than females, causing sexual dimorphism only in the presence of predators. We uncovered a correlative link between the sex-specific inducible phenotypic response and gene expression patterns in major stress-related genes (POMC, MC3R, and MC4R). Together, our results highlight that sex-specific responses may be an important, yet underappreciated, component underlying inter-individual differences in the expression of inducible defences, even in species without pronounced sexual dimorphism.

Carbon dioxide-induced ocean acidification is producing a range of new selection pressures on marine calcifying organisms that show phenotypic plasticity in their shell morphology in response to predators. Although there are numerous studies on the effects of ocean acidification and predation risk on marine bivalves in isolation, information concerning their combined effects is still lacking. To bridge this gap, we conducted a long-term mesocosm experiment using mussel populations with different histories of predator exposure: crab-experienced and crab-naïve. Mussels were exposed to either lower pH or crab cues and the combination of both of these treatments for 4 mo. We demonstrate that both crab-experienced and crab-naïve mussels have heavier, thicker, rounder and, thus, stronger shells in response to crab cues, whereas low pH significantly decreased shell mass, thickness and strength. Mussels with previous crab experience showed greater plasticity in response to crab cues than crab-naïve mussels. However, the differences in plasticity between naïve and crab-experienced mussels to crab cues disappeared in the acidification treatment. Exposure to low pH and crab cues resulted in antagonistic interactions for all traits, except for shell length, where the combined effect was additive. However, there was no difference between populations in the interaction type for any of the traits. Our study provides increased understanding of potential implications for mussel populations under climate change.

Habitat heterogeneity is a crucial driver for species distribution across scales. Harbour porpoise Phocoena phocoena basin-wide distribution is linked to prey availability, and small-scale (kilometres to tens of kilometres) differences in distribution are prevalent. However, information on porpoise distribution and foraging-behaviour variations on a micro-scale (~100 m to kilometres) is limited. To monitor harbour porpoise distribution and foraging activity on a micro-scale, we deployed passive acoustic dataloggers, logging porpoise acoustic activity at 6 sites in a small, high porpoise-density area in southern Sweden. Data were collected for almost a year, giving detailed time series on porpoise activity. The time series were analysed using dynamic time warping to compare activity patterns between sites. Large differences were found between sites separated by only a few hundred meters, indicating micro-scale spatial preference. Spectral analysis for temporal cyclicity in activity revealed a dominant peak for 24 h cycles with higher activity at night for all sites. All sites also had a second peak for 29.5 d, linked to the lunar cycle with higher activity during full moon. Activity was overall highest during autumn and winter (September-December). Spatial and temporal patterns were linked to foraging, showing a positive correlation between porpoise presence and the percent of time present with detected foraging. The study demonstrates that harbour porpoise spatial distribution on a micro-scale should be considered in e.g. behavioural, management and conservation studies and actions. In addition, we show that time series statistical methodology is informative and appropriate for analysis of acoustic temporal data.

Piscivorous fish are important predators in aquatic systems and as such they can have far-reaching effects on ecosystem composition and function. These effects depend on piscivore predation rates and behaviour, and recruitment of young-of-the-year fish into piscivory can hereby govern ecosystem properties. Growth and recruitment can differ between water bodies due to e.g. general productivity, but information on variation in juvenile growth and body condition between habitats within water bodies is scant. We here evaluate growth, body condition, food occurrence and stomach contents of an important piscivore, pike (Esox lucius), over the first growth season in two contrasting and spatially separated homogenous habitat types (emergent and submerged vegetation separated by 50 m of open sand) within the same lake. Individual size and body condition in pike were higher in the submerged vegetation early in the season, whereas by the end of their first summer pike were larger and in higher body condition in the emergent vegetation, in spite of occurrence of zooplankton, macroinvertebrates and fish prey being overall higher in the submerged vegetation. Pike showed habitat-specific patterns of macroinvertebrate consumption (higher in the submerged vegetation) and date-specific patterns of zooplankton (higher early in the season), macroinvertebrate (lower late in the season) and fish (higher later in the season) consumption that were not a result of occurrence of food types, as occurrence and consumption patterns did not match. We conclude that pike that hatched in the emergent vegetation habitat were larger towards the end of the season and, hence, these pike should have a higher survival probability and possibly contribute more to pike population density and predation at older ages, but also that submerged vegetation provides an alternative and added recruitment environment for pike in shallow lake ecosystems.

Animal movement is a multifaceted process that occurs for multiple reasons with powerful consequences for food web and ecosystem dynamics. New paradigms and technical innovations have recently pervaded the field, providing increasingly powerful means to deliver fine-scale movement data, attracting renewed interest. Specifically in the aquatic environment, tracking with acoustic telemetry now provides integral spatiotemporal information to follow individual movements in the wild. Yet, this technology also holds great promise for experimental studies, enhancing our ability to truly establish cause-and-effect relationships. Here, we argue that ponds with well-defined borders (i.e. “islands in a sea of land”) are particularly well suited for this purpose. To support our argument, we also discuss recent experiences from studies conducted in an innovative experimental infrastructure, composed of replicated ponds equipped with modern aquatic telemetry systems that allow for unparalleled insights into the movement patterns of individual animals.

Animal migration is one of the most spectacular and visible behavioural phenomena in nature with profound implications for a range of ecological and evolutionary processes. Successful migration hinges on the ability to exploit temporary resources (e.g. food) and evade threats (e.g. predators) as they arise, and thus the timing of migration is often regarded as a dominant predictor of individual migratory success. However, with the exception of intensively studied taxa (mainly birds), relatively few studies have investigated inter-individual annual and seasonal variation in migratory timing and performance, or tested predictions on how migration across high and low predation-risk habitats may exert selection on migratory timing. In particular, studies that assess the survival consequences of variation in migratory timing remain rare, which is most likely due to the logistical challenges associated with monitoring survival success and population-level characteristics simultaneously. Here, we address the above-mentioned questions using roach Rutilus rutilus, a fish that migrates from lakes characterised by high predation risk into low-risk streams during winter. Specifically, we used individual-based tracking of roach in two European lake systems over multiple migration periods (9 and 7 years respectively), to obtain highly detailed (year-round scheduling, repeat journeys and the fate of individuals) data on the variability/synchrony of migratory timing in spring and autumn respectively. We report seasonal differences in the variability of migratory timing, with lower variance and higher migration synchrony in spring lake arrival timing as compared to autumn lake departure timing. Furthermore, the timing of autumn migration is more variable across years than the timing of spring migration. Second, we find that later arrival to the lake habitat is positively associated with apparent survival from 1 year to the next, whereas we found no effect of lake departure timing on survival probability. These findings represent rare evidence showing how intraspecific variation in timing in migratory fish differs across years and seasons, and how variation in timing can translate into survival consequences for prey in systems characterised by high predation risk.

In ecological sciences, animal diets are often simplified to ‘resources' or ‘caloric quantities'. However, in the present study, we investigated the optimal foraging strategy of Atlantic codGadus morhuawhen both macro‐ and micro‐nutritional requirements are accounted for. Proteins cannot be synthesized from fatty acids, so the proteins for gonad development must come from other dietary sources. In addition, micronutrients are required in smaller quantities. For example, for cod, arachidonic acid (ARA) acts as a micronutrient precursor for prostaglandins, which is important for reproduction. We formulated a dynamic state‐dependent model to make predictions about optimal diet choice and foraging behavior. We applied the model to a case study in the strait between Denmark and Sweden. The model predicted that energy acquired from dietary protein should be twice that acquired from lipids, with a small increase in the lipid requirements when gonads are growing. The model also predicted that the ‘energy sparing effect of lipids' made it beneficial to engage in risky foraging activity to supplement a lean diet with a little bit of fat. When we constructed the model to also optimize ARA uptake, the cod consumed relatively more ARA‐rich crabs in the months prior to spawning, despite the otherwise poor energetic value of this prey. In support of the model predictions, field observations indicated that lipid stores reached a peak shortly after the arrival of the lipid‐rich migrating herring and the fatty acid signal of these herring were evident in the liver of nearly all cod. Three month later, only half of the cod contained the herring‐derived fatty acid signal, supporting the predicted shift in prey type prior to spawning. From these model predictions and field observations, we conclude that, also in the wild, nutritional requirements can be at least as important as pure energy acquisition.

Freshwater mussels in the order Unionida are highly adapted to parasitize fish for the primary purpose of dispersal. The parasitic larval stage affixes itself to the gills or fins of the host where it becomes encysted in the tissue, eventually excysting to develop into a free-living adult. Research on the parasitic interactions between unionids and their host fishes has garnered attention recently due to the increase in worldwide preservation efforts surrounding this highly endangered and ecologically significant order. With the exception of heavy infestation events, these mussels cause minor effects to their hosts, typically only observable effect in combination with other stressors. Moreover, the range of effect intensities on the host varies greatly with the species involved in the interaction, an effect that may arise from different evolutionary strategies between long- and short-infesting mussels; a distinction not typically made in conservation practices. Lower growth and reduced osmotic potential in infested hosts are commonly observed and correlated with infestation load. These effects are typically also associated with increases in metabolic rate and behaviour indicative of stress. Host fish seem to compensate for this through a combination of rapid wound healing in the parasitized areas and higher ventilation rates. The findings are heavily biased towards Margaritifera margaritifera, a unique mussel not well suited for cross-species generalizations. Furthermore, the small body of molecular and genetic studies should be expanded as many conclusions are drawn from studies on the ultimate effects of glochidiosis rather than proximate studies on the underlying mechanisms.

The propensity to kill and consume conspecifics (cannibalism) varies greatly between and within species, but the underlying mechanisms behind this variation remain poorly understood. A rich literature has documented that consistent behavioural variation is ubiquitous across the animal kingdom. Such inter-individual behavioural differences, sometimes referred to as personality traits, may have far-reaching ecological consequences. However, the link between predator personality traits and the propensity to engage in cannibalistic interactions remains understudied. Here, we first quantified personality in Eurasian perch (Perca fluviatilis), measured as activity (time spent moving) and sociability (time spent near conspecifics). We then gave perch of contrasting behavioural types the option to consume either conspecific or heterospecific (roach, Rutilus rutilus) prey. Individual perch characterized by a social-active behavioural phenotype (n = 5) selected roach before being cannibalistic, while asocial-inactive perch (n = 17) consumed conspecific and heterospecific prey evenly. Thus, asocial-inactive perch expressed significantly higher rates of cannibalism as compared to social-active individuals. Individual variation in cannibalism, linked to behavioural type, adds important mechanistic understanding to complex population and community dynamics, and also provides insight into the diversity and maintenance of animal personality.

Abstract Predator‐inducible defenses constitute a widespread form of adaptive phenotypic plasticity, and such defenses have recently been suggested linked with the neuroendocrine system. The neuroendocrine system is a target of endocrine disruptors, such as psychoactive pharmaceuticals, which are common aquatic contaminants. We hypothesized that exposure to an antidepressant pollutant, fluoxetine, influences the physiological stress response in our model species, crucian carp, affecting its behavioral and morphological responses to predation threat. We examined short‐ and long‐term effects of fluoxetine and predator exposure on behavior and morphology in crucian carp. Seventeen days of exposure to a high dose of fluoxetine (100 µg/L) resulted in a shyer phenotype, regardless of the presence/absence of a pike predator, but this effect disappeared after long‐term exposure. Fluoxetine effects on morphological plasticity were context‐dependent as a low dose (1 µg/L) only influenced crucian carp body shape in pike presence. A high dose of fluoxetine strongly influenced body shape regardless of predator treatment. Our results highlight that environmental pollution by pharmaceuticals could disrupt physiological regulation of ecologically important inducible defenses.

The European eel population is critically endangered due to a multitude of human-induced factors such as habitat fragmentation, parasites, fishing, and climate change. In freshwater, downstream migrating silver eels encountering hydroelectric plants often suffer substantial delays and increased mortality from trash-rack impingement and turbine-induced mortality. Downstream passage problems can be ameliorated by implementing different types of downstream passage solutions that show variable but promising results for salmonids, but their performance for silver eels remains largely unknown. To address these knowledge gaps, radio telemetry was used to monitor the downstream migration of silver eels during 2 years past a hydroelectric plant recently equipped with two new fish passage solutions, consisting of an angled bar rack with a full-depth bypass, and a nature-like fishway. No tagged eels passed through the turbines, but bypassed the dam evenly between the two passage solutions, resulting in a 95% impediment passage efficiency and a median passage time of 1 h. Movement patterns and route selection were associated with variation in discharge and most individuals approached both passage solutions before passing, resulting in route-specific efficiencies of 69% for the angled rack and bypass, and 46% for the nature-like fishway. We conclude that the combination of a new bypass, paired with an angled rack, and a large nature-like fishway provided downstream migrating silver eels with a highly effective combination of passage solutions, with high impediment passage success and relatively low passage times.

Predation is a well‐studied driver of ecological selection on prey traits, which frequently drives divergence in anti‐predator performance across environments that vary in predation risk. However, predation also alters prey mortality regimes, where low predation risk often results in higher prey densities and consequently higher intensities of intraspecific resource competition. In addition, predation risk alters the foraging context, as acquiring food can be risky in the presence of predators. Thus, different predation regimes can drive divergent selection on traits associated with resource competition, such as foraging behaviours. Moreover, because sexes often differ in susceptibility to predation and limitations to their reproductive output, the intensity of the tradeoff between predator avoidance and resource competition may depend on sex. We used a laboratory experiment to assess key aspects of foraging performance in a predator‐free context in Bahamas mosquitofish Gambusia hubbsi wild‐caught from multiple populations that experience either high or low levels of predation risk. When competing for limited food resources at a common density, females from low‐predation regimes showed higher foraging and food consumption rates than females from high‐predation regimes. Males showed fewer differences between predation regimes, and an opposite pattern from females. We suggest these sex‐specific effects result from females facing a greater tradeoff between predation risk and resource competition, combined with males from high‐predation environments elevating foraging behaviours in the absence of nearby predators and females. Females of this species are larger than males, bear live young and show higher foraging rates in the wild than males. On the other hand, males spend more time pursuing females in the wild, and may exhibit greater flexibility in foraging behaviours based on the immediate context. Our results show that varying levels of predation risk can lead to differences in behaviours associated with resource competition, but these effects can strongly differ between sexes.

Dams, weirs, and hydropower facilities are often cited as migratory barriers which impart significant reductions in fitness among migratory fish species. Even where upstream and downstream passage options are available, barrier passage can still often result in energetic or physical costs which compound delays or cause mortality. Past studies have identified variables associated with such fitness reductions, though few examine their effects in the context of the whole river scale. To this end, we assessed the migratory rates and downstream passage of radio‐tagged Atlantic salmon (Salmo salar) smolts through nine river sections (including two reservoir sections and one dammed section) along a 20 km stretch of river. Migration stoppages were not found to be elevated in reservoir or dammed sections, while migration rates were best described by physical river properties (width), biological traits (smolt total length), and seasonal variables (diel period) rather than anthropogenic factors. These results suggest the negative effect of reservoirs may primarily be due to their influence on river width and may be negligible when width is largely unaffected by an impoundment. Similarly, spilling water during fish migrations as a mitigative measure appears to make delays negligible. These conditions and actions may not completely marginalize the effect of dams, however, as a negative trend was still observed resulting from passage effects at the dam.

Predation risk is often invoked to explain variation in stress responses. Yet, the answers to several key questions remain elusive, including the following: (1) how predation risk influences the evolution of stress phenotypes, (2) the relative importance of environmental versus genetic factors in stress reactivity and (3) sexual dimorphism in stress physiology. To address these questions, we explored variation in stress reactivity (ventilation frequency) in a post-Pleistocene radiation of live-bearing fish, where Bahamas mosquitofish (Gambusia hubbsi) inhabit isolated blue holes that differ in predation risk. Individuals of populations coexisting with predators exhibited similar, relatively low stress reactivity as compared to low-predation populations. We suggest that this dampened stress reactivity has evolved to reduce energy expenditure in environments with frequent and intense stressors, such as piscivorous fish. Importantly, the magnitude of stress responses exhibited by fish from high-predation sites in the wild changed very little after two generations of laboratory rearing in the absence of predators. By comparison, low-predation populations exhibited greater among-population variation and larger changes subsequent to laboratory rearing. These low-predation populations appear to have evolved more dampened stress responses in blue holes with lower food availability. Moreover, females showed a lower ventilation frequency, and this sexual dimorphism was stronger in high-predation populations. This may reflect a greater premium placed on energy efficiency in live-bearing females, especially under high-predation risk where females show higher fecundities. Altogether, by demonstrating parallel adaptive divergence in stress reactivity, we highlight how energetic trade-offs may mould the evolution of the vertebrate stress response under varying predation risk and resource availability.