Wild animals have parasites that can compromise their physiological and/or behavioural performance. Yet, the extent to which parasite load is related to intraspecific variation in performance traits within wild populations remains relatively unexplored. We used pumpkinseed sunfish (Lepomis gibbosus) and their endoparasites as a model system to explore the effects of infection load on host aerobic metabolism and escape performance. Metabolic traits (standard and maximum metabolic rates, aerobic scope) and fast-start escape responses following a simulated aerial attack by a predator (responsiveness, response latency, and escape distance) were measured in fish from across a gradient of visible (i.e. trematodes causing black spot disease counted on fish surfaces) and non-visible (i.e. cestodes in fish abdominal cavity counted post-mortem) endoparasite infection. We found that a higher infection load of non-visible endoparasites was related to lower standard and maximum metabolic rates, but not aerobic scope in fish. Non-visible endoparasite infection load was also related to decreased responsiveness of the host to a simulated aerial attack. Visible endoparasites were not related to changes in metabolic traits nor fast-start escape responses. Our results suggest that infection with parasites that are inconspicuous to researchers can result in intraspecific variation in physiological and behavioral performance in wild populations, highlighting the need to more explicitly acknowledge and account for the role played by natural infections in studies of wild animal performance.

Aquatic ectotherms are vulnerable to thermal stress, with embryos predicted to be more sensitive than juveniles and adults. When examining the vulnerability of species and life stages to warming, comparable methodology must be used to obtain robust conclusions. Critical thermal methodology is commonly used to characterize acute thermal tolerances in fishes, with critical thermal maximum (CTmax) referring to the acute upper thermal tolerance limit. At this temperature, fish exhibit loss of controlled locomotion due to a temperature-induced collapse of vital physiological functions. While it is relatively easy to monitor behavioural responses and measure CTmax in larval and adult fish, this is more challenging in embryos, leading to a lack of data on this life stage, or that studies rely on potentially incomparable metrics. Here, we present a novel method for measuring CTmax in fish embryos, defined by the temperature at which embryos stop moving. Additionally, we compare this measurement with the temperature of the embryos’ last heartbeat, which has previously been proposed as a method for measuring embryonic CTmax. We found that, like other life stages, late-stage embryos exhibited a period of increased activity, peaking approximately 2–3°C before CTmax. Measurements of CTmax based on last movement are more conservative and easier to record in later developmental stages than measurements based on last heartbeat, and they also work well with large and small embryos. Importantly, CTmax measurements based on last movement in embryos are similar to measurements from larvae and adults based on loss of locomotory control. Using last heartbeat as CTmax in embryos likely overestimates acute thermal tolerance, as the heart is still beating when loss of response/equilibrium is reached in larvae/adults. The last movement technique described here allows for comparisons of acute thermal tolerance of embryos between species and across life stages, and as a response variable to treatments.

Reservoirs are known to accelerate the mobilization and cycling of mercury and carbon as a result of flooding of terrestrial organic matter, which can lead to environmental concerns at local and broader spatial scales. We explored the covariation of mercury (Hg) and carbon (C) functional pools in natural and recently dammed portions of the aquatic network of the Romaine River watershed in Northern Quebec, Canada, to understand how the fate of these elements varies across systems with contrasting hydrology and environmental conditions. We found that total Hg (THg) concentrations in surface waters were relatively constant along the network, whereas both the concentrations and proportions of MeHg tended to increase in reservoirs compared to surrounding nonflooded systems, and along the cascade of reservoirs. Whereas THg was related to total and terrestrial pools of dissolved organic carbon (DOC), MeHg was weakly related to DOC but strongly linked to surface concentrations of CO 2 , as well as to concentrations of iron and manganese. The latter are proxies of cumulative organic matter processing within the network, presumably in anoxic portions of shallow bays, deep reservoir waters, and river sediments, as well as in prior seasons (e.g., under ice). Our results suggest that these deep boreal reservoirs acted more as transformation sites for Hg that was already present than as mobilizers of new Hg, and that under ice metabolism plays a role in MeHg production in these systems as we found strong dichotomies in MeHg patterns between spring and summer.

1. Wild animals have parasites. This inconvenient truth has far-reaching implications for biologists measuring animal performance traits: infection with parasites can alter host behaviour and physiology in profound and sometimes counterintuitive ways. Yet, to what extent do studies on wild animals take individual infection status into account? 2. We performed a systematic review across eight scientific journals primarily publishing studies in animal behaviour and physiology over a 5-year period to assess the proportion of studies which acknowledge, treat or control for parasite infection in their study design and/or analyses.3. We explored whether parasite inclusion differed between studies that are experimental versus observational, conducted in the field vs the laboratory and measured behavioural vs physiological traits. We also investigated the importance of other factors such as the journal, the trait category (e.g. locomotion, reproduction) measured, the vertebrate taxonomic group investigated and the host climatic zone of origin. 4. Our results show that parasite inclusion was generally lacking across recent studies on wild vertebrates. In over 680 filtered papers, we found that only 21.9% acknowledged the potential effects of infections on animal performance in the text, and only 5.1% of studies treated animals for infection (i.e. parasite control) or considered infection status in the statistical analyses (i.e. parasite analysis). Parasite inclusion, control and analysis were higher in laboratory compared to field studies and higher for physiological studies compared to behavioural studies but did not differ among journals, performance trait categories and taxonomic groups. Among climatic zones, parasite inclusion, control and analysis were higher in tropical, subtropical and temperate zones than in boreal and polar zones.