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Atrazine and Amphibians: A Story of Profits, Controversy, and Animus

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Jason R Rohr at University of Notre Dame
Jason R Rohr
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Abstract

The herbicide atrazine is one of the most commonly used, well studied, and controversial pesticides on the planet. Much of the controversy surrounds the effects of atrazine on wildlife, particularly amphibians, and involves representatives from Syngenta Crop Protection, Inc., the company that produces atrazine, the US Environmental Protection Agency, and several academics with current, past, or no associations with Syngenta. Here, I briefly review the effects of atrazine on amphibians and provide a timeline of some of the most salient events in the history of the atrazine-amphibian controversy.
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... Despite the USEPA concluding in 2016 that atrazine poses risks to aquatic plants, fish, amphibians, mammals, birds, and reptiles (Farruggia et al. 2016), and in 2018, that it poses reproductive and developmental risks to humans, particularly children (US Environmental Protection Agency 2018), in 2020, the USEPA renewed the registration of atrazine and relaxed regulations, allowing 50% more atrazine to enter water bodies (Erickson 2019; US Environmental Protection Agency 2020). Although the story of atrazine has been partially told (Rohr 2018), these recent regulatory events, the Trump administration's regular dismissal of science (Lin 2019), and the change in the United States presidency, underscore the importance of drawing new attention to this remarkable story in the history of toxicology that is unfamiliar to so many. Hence, I provide a more complete and updated account of the most salient moments in the history of the atrazine controversy, emphasizing bent science and how it might have impacted decision making with potential consequences for ecosystem and human health and public trust in science. ...
... I focus predominantly on the effects of atrazine on amphibians because most of the controversy surrounding atrazine has centered on amphibian studies. Atrazine is documented to affect amphibian growth and timing of metamorphosis, (Larson et al. 1998;Karasov 2000, 2001;Boone and James 2003;Rohr et al. 2004;Storfer 2006a, 2006b) behaviors crucial for foraging and avoiding predators, (Rohr et al. 2003(Rohr et al. , 2004 and desiccation (Rohr andPalmer 2005, 2013). Moreover, delayed or persistent effects of atrazine on behavior and physiology can increase mortality risk (Storrs and Kiesecker 2004;Rohr and McCoy 2010b;Rohr andPalmer 2005, 2013). ...
... Atrazine is documented to affect amphibian growth and timing of metamorphosis, (Larson et al. 1998;Karasov 2000, 2001;Boone and James 2003;Rohr et al. 2004;Storfer 2006a, 2006b) behaviors crucial for foraging and avoiding predators, (Rohr et al. 2003(Rohr et al. , 2004 and desiccation (Rohr andPalmer 2005, 2013). Moreover, delayed or persistent effects of atrazine on behavior and physiology can increase mortality risk (Storrs and Kiesecker 2004;Rohr and McCoy 2010b;Rohr andPalmer 2005, 2013). There have been numerous studies on the effects of atrazine on physiology because of the role of physiology to vertebrate survival and conservation (Martin et al. 2010;Rohr et al. 2013b;Madliger et al. 2016). ...
The Atrazine Saga and its Importance to the Future of Toxicology, Science, and Environmental and Human Health
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The herbicide atrazine is one of the most commonly used, well studied, and controversial pesticides on the planet. Much of the controversy involves the effects of atrazine on wildlife, particularly amphibians, and the ethically questionable decision making of members of industry, government, the legal system, and institutions of higher education, in most cases in an effort to “bend science,” defined as manipulating research to advance economic, political, or ideological ends. In this Critical Perspective I provide a timeline of the most salient events in the history of the atrazine saga, which includes a multimillion‐dollar smear campaign, lawsuits, investigative reporting, accusation of impropriety against the US Environmental Protection Agency, and a multibillion‐dollar transaction. I argue that the atrazine controversy must be more than just a true story of cover‐ups, bias, and vengeance. It must be used as an example of how manufacturing uncertainty and bending science can be exploited to delay undesired regulatory decisions and how greed and conflicts of interest—situations where personal or organizational considerations have compromised or biased professional judgment and objectivity—can affect environmental and public health and erode trust in the discipline of toxicology, science in general, and the honorable functioning of societies. Most importantly, I offer several recommendations that should help to 1) prevent the history of atrazine from repeating itself, 2) enhance the credibility and integrity of science, and 3) enrich human and environmental health. Environ Toxicol Chem 2021;00:1–15.
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... Atrazine acts by inhibiting electron transport in Photosystem II, which disrupts the plant's ability to photosynthesize and causes starvation in broad-leaf plants and eventual death (Giddings et al 2004). It has been shown to have diverse effects on organisms such as amphibians and fish that develop and live in freshwater (Rohr 2018). Low concentrations of atrazine (1 μg/l) have been found to alter olfactory-mediated endocrine function in male Atlantic salmon (Salmo salar) (Moore and Lower, 2001). ...
... Glyphosate (Nphosphonomethyl glycine) is a non-selective herbicide used to control a wide variety of annual and perennial grasses and broadleaved weeds in commercial farmlands and household gardens. Glyphosate, known as Roundup®, is next to atrazine (Rohr 2018). These three herbicides are highly soluble in water. ...
Acute and sub-lethal toxicity in African mud catfish (Clarias gariepinus, Burchell, 1822) exposed to some pesticides
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Pesticides from agricultural run-off pose a severe threat to non-target organisms, such as fishes. This study was carried out to evaluate acute toxicity and histological and genotoxic effects (erythrocytic nuclear abnormalities) of lethal and sub-lethal concentrations of three commonly used pesticides: Atrazine, Butachlor and Glyphosate on the African mud catfish (Clarias gariepinus). Fishes were exposed to the pesticides for 96h periods to determine their LC50 and the sub-lethal effect at various concentrations (1/10th, 1/100th , 1/1000th 96h LC50) over 28 days. The 96h LC50 values were 7.63mg/l, 0.7mg/l and 15.97mg/l for atrazine, butachlor and glyphosate, respectively. Histological sections of the liver of C. gariepinus exposed to the three pesticides showed mild vascular congestion but no necrosis in the tissue. There was a significant (p<0.5) dose-dependent increase in micronuclei and nuclear abnormalities in the erythrocytes of exposed C. gariepinus compared to the control by 28 days. The study confirmed that C. gariepinus are at risk of adverse effects from exposure to pesticides. Discharge of agricultural run-off around water bodies should be prevented or prohibited to avoid adverse effects on aquatic life.
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... Despite the plethora of controversies surrounding the toxicity of atrazine based herbicides on non-target organisms such as amphibians [26], they have been proven to have non-target effects on animals [15,27]. For example, atrazine based herbicides are known to alter reproductive processes and development in insects, amphibians, fish, reptiles, birds, rodents and goats [15,16,[28][29][30] Vogel et al. [15] reported that exposure to atrazine had significant effects on males of Drosophila melanogaster Meigen (Diptera: Drosophilidae) mating ability and the number of eggs his partner laid when he was successful at mating. ...
Atrazine and 2, 4-D caused high mortality and reduced mobility in termites
Article
Recent empirical evidence suggests that herbicides have damaging effects on non-target organisms in both natural and semi-natural ecosystems. The African mound building termite, Macrotermes bellicosus, is an important beneficial insect that functions as an ecosystem engineer due to its role in the breakdown of dead and decaying materials. Here, we examined the effects of 2,4-D amine salt (2,4-D) and atrazine based herbicides viz. Vestamine ® and Ultrazine® on the survival and locomotion response of M. bellicosus. Worker termites were treated with a range of concentrations of Vestamine® (the recommended concentration: 6.25 ml per 500 ml of water, 0.25- and 0.5-fold below the recommended concentration and distilled water as control) and Ultrazine® (the recommended concentration: 3.75 ml per 500 ml of water, 0.25-, 0.5-, 2.0- and 4-fold of the recommended concentration and distilled water as control) for 24 hours for the mortality test, and allowed to run for 15 seconds for the locomotion trial. All concentrations of both Vestamine® and ltrazine® were highly toxic to worker termites and mortality increased as the concentration and time after treatment increased. For both herbicides, concentrations far less than the recommended rates caused 100% mortality. The speed of termites was significantly influenced by both Vestamine® and Ultrazine® as termites exposed to all tested concentrations of the herbicides exhibited reduced running speed than the control. These findings suggest that beneficial insects, especially M. bellicosus may experience high mortality (up to 100%) and reduced mobility if they are sprayed upon or come in contact with plant materials that have been freshly sprayed with (less or more than) the recommended concentrations of Vestamine ® and Ultrazine®. The findings of our study calls for the reassessment of the usage of 2,4-D and atrazine based herbicides in weed control in termite and other beneficial insect populated habitats.
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... Despite the plethora of controversies surrounding the toxicity of atrazine based herbicides on non-target organisms such as amphibians [26], they have been proven to have non-target effects on animals [15,27]. For example, atrazine based herbicides are known to alter reproductive processes and development in insects, amphibians, fish, reptiles, birds, rodents and goats [15,16,[28][29][30] Vogel et al. [15] reported that exposure to atrazine had significant effects on males of Drosophila melanogaster Meigen (Diptera: Drosophilidae) mating ability and the number of eggs his partner laid when he was successful at mating. ...
Exposure of the African mound building termite, Macrotermes bellicosus workers to commercially formulated 2,4-D and atrazine caused high mortality and impaired locomotor response
Article
Full-text available
Recent empirical evidence suggests that herbicides have damaging effects on non-target organisms in both natural and semi-natural ecosystems. The African mound building termite, Macrotermes bellicosus, is an important beneficial insect that functions as an ecosystem engineer due to its role in the breakdown of dead and decaying materials. Here, we examined the effects of 2,4-D amine salt (2,4-D) and atrazine based herbicides viz. Vestamine® and Ultrazine® on the survival and locomotion response of M. bellicosus. Worker termites were treated with a range of concentrations of Vestamine® (the recommended concentration: 6.25 ml per 500 ml of water, 0.25- and 0.5-fold below the recommended concentration and distilled water as control) and Ultrazine® (the recommended concentration: 3.75 ml per 500 ml of water, 0.25-, 0.5-, 2.0- and 4-fold of the recommended concentration and distilled water as control) for 24 hours for the mortality test, and allowed to run for 15 seconds for the locomotion trial. All concentrations of both Vestamine® and Ultrazine® were highly toxic to worker termites and mortality increased as the concentration and time after treatment increased. For both herbicides, concentrations far less than the recommended rates caused 100% mortality. The speed of termites was significantly influenced by both Vestamine® and Ultrazine® as termites exposed to all tested concentrations of the herbicides exhibited reduced running speed than the control. These findings suggest that beneficial insects, especially M. bellicosus may experience high mortality (up to 100%) and reduced mobility if they are sprayed upon or come in contact with plant materials that have been freshly sprayed with (less or more than) the recommended concentrations of Vestamine® and Ultrazine®. The findings of our study calls for the reassessment of the usage of 2,4-D and atrazine based herbicides in weed control in termite and other beneficial insect populated habitats.
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... Atrazine has been reported to interfere with physiological and biochemical systems in the nontarget aquatic organisms, affecting functions such as development, reproduction, and survival of aquatic biota (Nwani et al., 2010;Rohr and McCoy, 2010). These effects corroborate the reports on extreme population declines in Piscean and amphibian communities (Hayes et al., 2006;Hayes et al., 2010;Rohr, 2013). Atrazine has also been reported to bio-accumulate in various organ tissues (Xing et al., 2012;Dornelles and Oliveira, 2014;Singh et al., 2017). ...
Cardiotoxicity in African clawed frog (Xenopus Laevis) sub-chronically exposed to environmentally relevant atrazine concentrations: Implications for species survival
Article
  • Jun 2019
  • AQUAT TOXICOL
The toxic effects of different atrazine concentrations on tadpoles and adult male African clawed frogs (Xenopus laevis) were assessed in a controlled laboratory environment following 90 days' exposure. The aim was to elucidate the danger of atrazine exposure on the cardiac tissue relative to its critical function of rhythmic contractility, fundamental for optimal blood circulation and homeostasis. Tadpoles and adult frogs were exposed to 0 μg/L (control), 0.01 μg L-1, 200 μg L-1 and 500 μg L-1 concentrations of atrazine for 90 days. Mortality was concenration-dependent and significantly increased in juvenile group (77%, 43%, 23% and 0 respectively for 500 μg L-1, 200 μg L-1, 0.01 μg L-1, and control group). While the mean juvenile heart area decreased concentration-dependently, adult frog mean heart area significantly increased in the 200 μg L-1 group only and mean heart weight change was variable across all exposure levels. Light microscopy of hematoxylin and eosin (H&E) and Mallory-Heidenhain rapid one-step staining techniques on cardiac tissue sections of the juvenile and adult frogs revealed shrinkage of cardiac muscle cells into thin wavy myocytes. Additionally, disorganized branching of muscle fibres with reduced striations were observed in 0.01 μg L-1 and 200 μg L-1 but hypertrophied myocytes, thickened intensely staining myofibrils in the 500 μg L-1 group in juvenile and adult frogs. Significant increase in the mean percentage area of connective tissue in all the treated groups (p < 0.036) were also recorded. Immunohistochemistry analysis showed decreased eNOS localization in cardiac tissue in 200 μg L-1 and 500 μg L-1 of both juvenile and adult group, suggestive of decreased cardiac contractility due to atrazine exposure. The results indicate that environmentally relevant atrazine concentrations cause significant mortality in tadpoles while concentrations ≥200 μg L-1 adversely affect cardiac muscle morphology and may induce functional perturbations in cardiac tissue contractility and consequent dysfunction which generally may have an adverse impact on their survival and longevity.
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Chemicals: pesticides
Chapter
  • Jan 2020
The fourth edition of this compendium of information resources detailed the explosion of the literature regarding pesticide toxicology since publication of the third edition in 2000. Indeed a search using GOOGLE Scholar, an engine that gathers primarily papers from research journals (i.e., the primary research literature), on just the word “pesticide” today returns over two million “hits.” The fourth edition focused more on the mammalian toxicology of pesticide technology. Thus the information highlighted was primarily focused on human exposure and epidemiology of pesticides with minor attention to environmental chemistry and ecotoxicology.The focus of this updated chapter is greatly expanded to cover more conventional questions in the realm of environmental toxicology since 2009. In particular, key references regarding the major controversies about pesticides covers effects on frogs (amphibians) and bees (mainly the honey bee, Apis mellifera). Literature addressing human health hazards and risk assessment of organophosphorus and pyrethroid insecticides was updated with information about neonicotinoid insecticides and glyphosate herbicide. Additionally, the literature about transgenerational epigenetic effects was listed.
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Early-life exposure to a herbicide has enduring effects on pathogen-induced mortality
Article
Full-text available
Proc. R. Soc. B 280, 20131502. (7 December 2013; Published online 23 October 2013) (doi:10.1098/rspb.2013.1502). In the second paragraph of the ‘Material and methods’ section, the concentration of atrazine is incorrectly listed as 65.9 ± 3.48 mg l⁻¹, mean ± s.e. The concentration should be listed as 65.9 ± 3.48 µg l⁻¹, mean ± s.e. • © 2014 The Author(s) Published by the Royal Society. All rights reserved.
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Exposure to the Herbicide Atrazine Nonlinearly Affects Tadpole Corticosterone Levels
Article
Full-text available
  • Jun 2017
To determine whether the herbicide, atrazine, affects the stress hormone corticosterone, we exposed Osteopilus septentrionalis (Cuban Treefrog) tadpoles to four concentrations of atrazine and two controls (water and acetone) for three time durations (4, 28, and 100 h). Atrazine concentration, but not exposure duration, had significant nonlinear effects on whole-body corticosterone. Relative to controls, intermediate concentrations of atrazine (10.2 and 50.6 μg/L) tended to lower corticosterone, whereas the lowest (0.1 μg/L) and highest atrazine concentrations (102 μg/L) elevated corticosterone. These results indicate that atrazine exposure might dysregulate corticosterone, a hormone integral to vertebrate immunity, neurogenesis, and health.
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The pros and cons of ecological risk assessment based on data from different levels of biological organization
Article
Full-text available
Ecological risk assessment (ERA) is the process used to evaluate the safety of manufactured chemicals to the environment. Here we review the pros and cons of ERA across levels of biological organization, including suborganismal (e.g., biomarkers), individual, population, community, ecosystem and landscapes levels. Our review revealed that level of biological organization is often related negatively with ease at assessing cause-effect relationships, ease of high-throughput screening of large numbers of chemicals (it is especially easier for suborganismal endpoints), and uncertainty of the ERA because low levels of biological organization tend to have a large distance between their measurement (what is quantified) and assessment endpoints (what is to be protected). In contrast, level of biological organization is often related positively with sensitivity to important negative and positive feedbacks and context dependencies within biological systems, and ease at capturing recovery from adverse contaminant effects. Some endpoints did not show obvious trends across levels of biological organization, such as the use of vertebrate animals in chemical testing and ease at screening large numbers of species, and other factors lacked sufficient data across levels of biological organization, such as repeatability, variability, cost per study and cost per species of effects assessment, the latter of which might be a more defensible way to compare costs of ERAs than cost per study. To compensate for weaknesses of ERA at any particular level of biological organization, we also review mathematical modeling approaches commonly used to extrapolate effects across levels of organization. Finally, we provide recommendations for next generation ERA, submitting that if there is an ideal level of biological organization to conduct ERA, it will only emerge if ERA is approached simultaneously from the bottom of biological organization up as well as from the top down, all while employing mathematical modeling approaches where possible to enhance ERA. Because top-down ERA is unconventional, we also offer some suggestions for how it might be implemented efficaciously. We hope this review helps researchers in the field of ERA fill key information gaps and helps risk assessors identify the best levels of biological organization to conduct ERAs with differing goals.
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The herbicide atrazine induces hyperactivity and compromises tadpole detection of predator chemical cues
Article
Full-text available
  • Jan 2016
  • ENVIRON TOXICOL CHEM
The ability to detect chemical cues is often critical for freshwater organisms to avoid predation and find food and mates. In particular, reduced activity and avoidance of chemical cues signaling predation risk are generally adaptive behaviors that reduce prey encounter rates with predators. Here, we examined the effects of the common herbicide atrazine on the ability of Cuban tree frog (Osteopilus septentrionalis) tadpoles to detect and respond to chemical cues from larval dragonfly (Libellulidae sp.) predators. Tadpoles exposed to an estimated environmental concentration of atrazine (calculated using US EPA software; measured concentration: 178 µg/L) were significantly hyperactive relative to those exposed to solvent control. Additionally, control tadpoles significantly avoided predator chemical cues, but tadpoles exposed to atrazine did not. These results are consistent with previous studies that have demonstrated that ecologically relevant concentrations of atrazine can induce hyperactivity and impair the olfactory abilities of other freshwater vertebrates. We call for additional studies examining the role of chemical contaminants in disrupting chemical communication and the quantification of subsequent impacts on the fitness and population dynamics of wildlife. This article is protected by copyright. All rights reserved.
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Effects of an herbicide and an insecticide on pond community structure and processes
Article
Full-text available
  • Aug 2005
Virtually all species live within complex food webs, and many of these organisms are exposed to contaminants. However, we know little about how community processes, such as competition and predation, influence susceptibility to contaminants or how different types of contaminants shape communities. The objective of our study was to determine how realistic concentrations of the herbicide atrazine and the insecticide endosulfan influence the structure of a pond community when the presence of common community members was manipulated. We employed a factorial design in mesocosms to evaluate the effects of pesticide treatments (25 μg/L of atrazine, 10 μg/L of endosulfan, solvent control; two pulses separated by two weeks) and the presence or absence of wood frog tadpoles (Rana sylvatica), adult snails (Planorbella trivolvis), and caged dragonfly larvae (Anax junius) on a freshwater community. Tadpoles, snails, and chironomid larvae, Polypedilum sp. (Dipterans), all competed for periphyton. As a result, tadpoles reduced the survival, mass, and reproduction of snails; snails reduced the growth, development, inactivity, and dragonfly avoidance of tadpoles; snails and tadpoles reduced the abundance of chironomid larvae; and chironomid larvae reduced snail mass. The adverse effect of snails on tadpole growth and behavior was greater in the presence of the caged tadpole predator, A. junius. Neither pesticide affected dragonfly survival, but endosulfan directly reduced zooplankton (Daphnia), and atrazine indirectly reduced chironomid abundance. Atrazine also directly decreased periphyton, and endosulfan decimated chironomid larvae, resulting in indirect increases and decreases in competition for both snails and tadpoles, respectively. Consequently, relative to endosulfan, atrazine tended to decrease snail mass and reproduction and reduce tadpole mass, development, inactivity, refuge use, and dragonfly avoidance. However, the indirect effects of pesticides depended upon the presence of heterospecifics. The indirect benefit of endosulfan on snail mass was greater in the presence of caged dragonfly larvae, and endosulfan's indirect benefit on tadpole mass was greater in the absence of snails. The effect of pesticides on tadpole activity depended on both caged dragonflies and snails. Thus, environmentally realistic concentrations of pesticides directly and indirectly shaped species responses and community composition, but the initial composition of the community influenced these pesticide effects. These results emphasize the importance of quantifying the effects of contaminants within complex natural communities.
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Agrochemicals Indirectly Increase Survival of E. coli O157:H7 and Indicator Bacteria by Reducing Ecosystem Services
Article
Full-text available
  • May 2014
Storm water and agricultural runoff frequently contain agrochemicals, fecal indicator bacteria (FIB), and zoonotic pathogens. Entry of such contaminants into aquatic ecosystems may affect ecology and human health. This study tested the hypothesis that the herbicide atrazine and the fungicide chlorothalonil indirectly affect the survival of FIB (Escherichia coli and Enterococcus faecalis) and a pathogen (E. coli O157:H7) by altering densities of protozoan predators or by altering competition from autochthonous bacteria. Streptomycin-resistant E. coli, En. faecalis, and E. coli O157:H7 were added to microcosms composed of Florida river water containing natural protozoan and bacterial populations. FIB, pathogen, and protozoan densities were monitored over six days. Known metabolic inhibitors, cycloheximide and streptomycin, were used to inhibit autochthonous protozoa or bacteria, respectively. The inhibitors made it possible to isolate the effects of predation or competition on survival of allochthonous bacteria, and each treatment increased the survival of FIB and pathogens. Chlorothalonil's effect was similar to that of cycloheximide, significantly reducing protozoan densities and elevating densities of FIB and pathogens relative to the control. Atrazine treatment did not affect protozoan densities, but, through an effect on competition, resulted in significantly greater densities of En. faecalis and E. coli O157:H7. Hence, by reducing predaceous protozoa and bacterial competitors that facilitate purifying water bodies of FIBs and human pathogens, chlorothalonil and atrazine indirectly diminished an ecosystem service of fresh water.
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A Synthesis of the Effects of Pesticides on Microbial Persistence in Aquatic Ecosystems
Article
Pesticides have a pervasive presence in aquatic ecosystems throughout the world. While pesticides are intended to control fungi, insects, and other pests, their mechanisms of action are often not specific enough to prevent unintended effects, such as on non-target microbial populations. Microorganisms, including algae and cyanobacteria, protozoa, aquatic fungi, and bacteria, form the basis of many food webs and are responsible for crucial aspects of biogeochemical cycling; therefore, the potential for pesticides to alter microbial community structures must be understood to preserve ecosystem services. This review examines studies that focused on direct population-level effects and indirect community-level effects of pesticides on microorganisms. Generally, insecticides, herbicides, and fungicides were found to have adverse direct effects on algal and fungal species. Insecticides and fungicides also had deleterious direct effects in the majority of studies examining protozoa species, although herbicides were found to have inconsistent direct effects on protozoans. Our synthesis revealed mixed or no direct effects on bacterial species among all pesticide categories, with results highly dependent on the target species, chemical, and concentration used in the study. Examination of community-level, indirect effects revealed that all pesticide categories had a tendency to reduce higher trophic levels, thereby diminishing top-down pressures and favoring lower trophic levels. Often, indirect effects exerted greater influence than direct effects. However, few studies have been conducted to specifically address community-level effects of pesticides on microorganisms, and further research is necessary to better understand and predict the net effects of pesticides on ecosystem health.
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Predator diversity, intraguild predation, and indirect effects drive parasite transmission
Article
  • Feb 2015
Significance Humans are altering biodiversity globally and infectious diseases are on the rise; thus, there is considerable interest in understanding how changes to biodiversity affect disease risk. We show that the diversity of predators that consume parasites was the best negative predictor of infections in frogs, suggesting that predation on parasites can be an important mechanism of disease reduction. Follow-up experiments, field data, and mathematical models revealed that intraguild predators, predators that consume both hosts and parasites, decrease macroparasite infections per host less than predators that only consume parasites, representing a general trait of predators that predicts their ability to reduce disease. Consequently, managing assemblages of non-intraguild and intraguild predators is an underutilized tool to minimize human and wildlife diseases.
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Host life-history and host-parasite syntopy predict behavioral resistance and tolerance of parasites
Article
There is growing interest in the role that life‐history traits of hosts, such as their ‘pace‐of‐life’, play in the evolution of resistance and tolerance to parasites. Theory suggests that, relative to host species that have high syntopy (local spatial and temporal overlap) with parasites, host species with low syntopy should have lower selection pressures for more constitutive (always present) and costly defences, such as tolerance, and greater reliance on more inducible and cheaper defences, such as behaviour. Consequently, we postulated that the degree of host–parasite syntopy, which is negatively correlated with host pace‐of‐life (an axis reflecting the developmental rate of tadpoles and the inverse of their size at metamorphosis) in our tadpole–parasitic cercarial (trematode) system, would be a negative and positive predictor of behavioural resistance and tolerance, respectively. To test these hypotheses, we exposed seven tadpole species to a range of parasite (cercarial) doses crossed with anaesthesia treatments that controlled for anti‐parasite behaviour. We quantified host behaviour, successful and unsuccessful infections, and each species’ reaction norm for behavioural resistance and tolerance, defined as the slope between cercarial exposure (or attempted infections) and anti‐cercarial behaviours and mass change, respectively. Hence, tolerance is capturing any cost of parasite exposure. As hypothesized, tadpole pace‐of‐life was a significant positive predictor of behavioural resistance and negative predictor of tolerance, a result that is consistent with a trade‐off between behavioural resistance and tolerance across species that warrants further investigation. Moreover, these results were robust to considerations of phylogeny, all possible re‐orderings of the three fastest or slowest paced species, and various measurements of tolerance. These results suggest that host pace‐of‐life and host–parasite syntopy are powerful drivers of both the strength and type of host defence strategies against parasites. Future research should evaluate how often and how strongly host pace‐of‐life and host–parasite syntopy are correlated and which is the better predictor of the strength and type of host investments in anti‐parasite defences.
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