Endocrine disrupters with (anti)estrogenic and (anti)androgenic modes of action affecting reproductive biology of Xenopus laevis: I. Effects on sex steroid levels and biomarker expression

Adult Xenopus laevis were exposed in vivo to ethinylestradiol, tamoxifen, methyldihydrotestosterone and flutamide as (anti)estrogenic and (anti)androgenic compounds, respectively, for four weeks at a concentration of 10(-8) M and to Lambro river water, a polluted river from Italy. Effects of the treatments were analysed by mRNA expression of retinol-binding protein (RBP), transferrin (TF), transthyretin (TTR) and vitellogenin (VTG) in the liver of male and female X. laevis, to analyse the potential of these genes to detect endocrine disrupting compounds (EDC) with different modes of action. In addition, plasma VTG and sex steroid levels, estradiol-17beta (E(2)) and testosterone (T), were analysed. Sex steroids were depressed by ethinylestradiol in both sexes whereas tamoxifen increased E(2) in females. The induction of VTG protein plasma levels was more pronounced at the protein level compared to hepatic VTG mRNA expression in response to estrogenic treatment but VTG mRNA expression detected both, estrogenic and antiestrogenic EDC. The mRNA expression of TF was decreased by estrogenic and increased by antiestrogenic treatment while TTR mRNA expression was down-regulated and RBP mRNA up-regulated by estrogenic exposure. The other treatments did not affect the mRNA expression of the examined genes.     

Gonadal differentiation and hormonal sex reversal in Arctic charr (Salvelinus alpinus)

The purpose of this study was to develop a practical protocol for the production of female populations of Arctic charr (Salvelinus alpinus). Achieving this required knowledge of the timing of gonadal differentiation. Undifferentiated gonads were observed microscopically to be present by 194 degrees C-days post-hatch and definitive germ cells by 346 degrees C-days post-hatch, where " degrees C-days" denote acquired thermal units calculated as the product of temperature and days. Some of the gonads had developed a lumen by 510 degrees C-days post-hatch, and by 681 degrees C-days post-hatch anatomical divergence into two types of gonads was clear. Two protocols (immersion and feeding) were tested for hormonal sex reversal of genotypic females using the synthetic androgen 17alpha-methyldihydrotestosterone (MDHT). Six-hour MDHT immersions (0.5, 1, 3, 5 and 10 mg/L) were carried out weekly from hatch to first feeding (140 degrees C-days post-hatch), whereas daily feeding treatments (0.5 mg/kg) went from 140 to 600 degrees C-days post-hatch. The sex ratios of all immersion experimental groups were significantly different from the control, with the proportion of presumptive males increasing as MDHT concentration increased. The highest immersion treatment, 10 mg/L, yielded a population of 90% presumptive males and 10% with atypical gonads. However, the most effective treatment, yielding a population of 90% presumptive males and no fish with atypical gonads, was the feeding treatment. Given that female salmonid fishes are homogametic, sex-reversed (masculinized) genotypic females produced in this way can serve as broodstock for the creation of all-female charr populations for aquaculture.     

Life-stage-dependent sensitivity of zebrafish (Danio rerio) to estrogen exposure

The aim of this study was to identify periods in zebrafish (Danio rerio) development when estrogen exposure has long-term consequences on reproductive capabilities at the adult stage. To this end, zebrafish were exposed to 10 ng/L ethynylestradiol (EE(2)) during three stages of gonadal differentiation: (i) the juvenile hermaphroditic stage when gonads display the morphology of an immature ovary (in our zebrafish colony this lasted from 15 to 42 days post-fertilization [dpf]), (ii) the gonad transition stage when the hermaphroditic gonad differentiates into either testes or ovary (from 43 to day 71 dpf), and (iii) the premature stage of testicular and ovarian development (from 72 to 99 dpf). The consequences of stage-specific exposure to EE(2) were assessed by determining time to first spawning, fecundity (number of eggs per female per day), fertilization success (percentage of fertilized eggs) and sex ratio of the adults. Exposure during the gonad transition period induced a delay in the onset of spawning and a significant reduction of fecundity and fertilization success, whereas exposure during the hermaphroditic stage or during the premature stage had no significant impact on the reproductive parameters of adult fish. The results from this experiment pointed to the gonad transition stage as being most susceptible to persistent effects of developmental estrogen exposure. In a second experiment, the concentration dependency of the EE(2)effects was evaluated by exposing zebrafish during the gonad transition stage (43-71 dpf) to 1.67, 3 or 10 ng EE(2)/L. Significant effects of EE(2) on adult reproduction were found with 3 and 10 ng EE(2)/L, but not with 1.67 ng/L. Histological examination of the gonads revealed that at termination of EE(2) exposure (71 dpf), all individuals in the 3 and 10 ng EE(2)/L treatment possessed ovaries. However, this feminising effect appeared to be reversible since at the adult stage (190 dpf), both fish with ovaries and with testes were found. Thus, EE(2) exposure during the gonad transition stage seems to have no persistent effect on gonad histology but on reproductive capabilities.     

Effects of selected xenoestrogens on liver peroxisomes, vitellogenin levels and spermatogenic cell proliferation in male zebrafish

Environmental estrogenic compounds or xenoestrogens can mimic natural estrogens and cause a variety of adverse effects on aquatic wildlife. The purpose of the present work was to investigate if xenoestrogens are able to cause proliferation of liver peroxisomes using zebrafish (Danio rerio) as a model. Adult male zebrafish were exposed for 15 days to 17beta-estradiol (E2) and the xenoestrogens dibutylphthalate (DBP), methoxychlor (MXC), 4-tert-octylphenol (OP) and 17alpha-ethynylestradiol (EE2). All five tested compounds caused significant proliferation of liver peroxisomes (p < 0.05) as indicated by increased peroxisomal surface and numerical densities and elevated activities of the peroxisomal beta-oxidation enzyme acyl-CoA oxidase (AOX). In the case of DBP, MXC and E2, positive significant correlations between peroxisomal density parameters and AOX were found. The treatments did not produce gross alterations in testis histology, but spermatogenic cell proliferation was disturbed in E2 and EE2-treated groups and vitellogenin levels increased significantly in fish exposed to MXC, OP, EE2 and E2 with respect to controls. Furthermore, a significant correlation between vitellogenin levels and AOX activity was found for MXC, OP and EE2 treatments, suggesting that for the latter xenoestrogens early estrogenic effects are associated with liver peroxisome proliferation. No such association occurred with typical peroxisome proliferators such as DBP.     

Estrogens can disrupt amphibian mating behavior

The main component of classical contraceptives, 17α-ethinylestradiol (EE2), has high estrogenic activity even at environmentally relevant concentrations. Although estrogenic endocrine disrupting compounds are assumed to contribute to the worldwide decline of amphibian populations by adverse effects on sexual differentiation, evidence for EE2 affecting amphibian mating behaviour is lacking. In this study, we demonstrate that EE2 exposure at five different concentrations (0.296 ng/L, 2.96 ng/L, 29.64 ng/L, 2.96 μg/L and 296.4 μg/L) can disrupt the mating behavior of adult male Xenopus laevis. EE2 exposure at all concentrations lowered male sexual arousal, indicated by decreased proportions of advertisement calls and increased proportions of the call type rasping, which characterizes a sexually unaroused state of a male. Additionally, EE2 at all tested concentrations affected temporal and spectral parameters of the advertisement calls, respectively. The classical and highly sensitive biomarker vitellogenin, on the other hand, was only induced at concentrations equal or higher than 2.96 μg/L. If kept under control conditions after a 96 h EE2 exposure (2.96 μg/L), alterations of male advertisement calls vanish gradually within 6 weeks and result in a lower sexual attractiveness of EE2 exposed males toward females as demonstrated by female choice experiments. These findings indicate that exposure to environmentally relevant EE2 concentrations can directly disrupt male mate calling behavior of X. laevis and can indirectly affect the mating behavior of females. The results suggest the possibility that EE2 exposure could reduce the reproductive success of EE2 exposed animals and these effects might contribute to the global problem of amphibian decline.     

Reproductive condition of Akodon montensis Thomas and Bolomys lasiurus (Lund) (Rodentia, Muridae) based on histological and histometric analyses of testes and external characteristics of gonads

The reproductive activity of two species of wild rodents was studied in the Serra do Rola‐Moça State Park (PERM), in the state of Minas Gerais, Brazil. From October 1998 to October 1999 specimens of Akodon montensis (Thomas 1913) were captured in an area of seasonal semi‐deciduous forest, while specimens of Bolomys lasiurus (Lund 1840) were captured in a cerrado area (a savannah‐like grassland). The determination of the reproductive condition was based on histological and histometric analyses of testes and external characteristics of gonads. The volume of the testes, diameter of the seminiferous tubules and depth of the seminiferous epithelium were measured. Akodon montensis showed a seasonal annual reproductive cycle divided into four stages (recrudescence, activity or proliferation, regression and rest) with the reproductive activity mainly occurring during the rainy season. In contrast, B. lasiurus presented reproductive activity during almost the whole year. For both species the volume of the testes was an effective indicator of their reproductive condition. We suggest that the histological and histometric analysis of the gonads of males, together with the observations of external characteristics of gonads of males and females, may assist in identifying the reproductive condition of these animals.     

The Antiestrogens Tamoxifen and Fulvestrant Abolish Estrogenic Impacts of 17α-ethinylestradiol on Male Calling Behavior of Xenopus laevis

Various synthetic chemicals released to the environment can interfere with the endocrine system of vertebrates. Many of these endocrine disrupting compounds (EDCs) exhibit estrogenic activity and can interfere with sexual development and reproductive physiology. More recently, also chemicals with different modes of action (MOAs), such as antiestrogenic, androgenic and antiandrogenic EDCs, have been shown to be present in the environment. However, to date EDC-research primarily focuses on exposure to EDCs with just one MOA, while studies examining the effects of simultaneous exposure to EDCs with different MOAs are rare, although they would reflect more real, natural exposure situations. In the present study the combined effects of estrogenic and antiestrogenic EDCs were assessed by analyzing the calling behavior of short-term exposed male Xenopus laevis. The estrogenic 17α-ethinylestradiol (EE2), and the antiestrogenic EDCs tamoxifen (TAM) and fulvestrant (ICI) were used as model substances. As previously demonstrated, sole EE2 exposure (10−10 M) resulted in significant alterations of the male calling behavior, including altered temporal and spectral parameters of the advertisement calls. Sole TAM (10−7 M, 10−8 M, 10−10 M) or ICI (10−7 M) exposure, on the other hand, did not affect any of the measured parameters. If frogs were co-exposed to EE2 (10−10 M) and TAM (10−7 M) the effects of EE2 on some parameters were abolished, but co-exposure to EE2 and ICI (10−7 M) neutralized all estrogenic effects. Thus, although EDCs with antiestrogenic MOA might not exhibit any effects per se, they can alter the estrogenic effects of EE2. Our observations demonstrate that there is need to further investigate the combined effects of EDCs with various, not only opposing, MOAs as this would reflect realistic wildlife situations.     

Gonadal sex reversal in triploid rainbow trout (Oncorhynchus mykiss).

Sex determination in salmonids is primarily governed by sex chromosomes; however, phenotypic expression and successful development of the gonads may be influenced by additional factors. Exposure to exogenous steroids during the critical period of gonadal differentiation will reverse the expected phenotypic sex of both female and male trout. Triploidy, a viable condition in rainbow trout (RBT), alters the degree of gonadal development in a gender-specific manner. Males produce testes with similar morphology and function as diploid fish, but females produce underdeveloped ovaries devoid of growing oocytes. One possible explanation for this observed gender difference is that the timing of meiotic initiation may influence ovarian/testicular development in triploid RBT. To determine whether the early entrance of germ cells into meiosis results in the lack of ovarian development in triploid females, the objective of this study was to sex-reverse genotypic triploid female RBT (XXX) into phenotypic males and genotypic triploid male RBT (XXY) into phenotypic females. Male fish were exposed to estradiol-17beta (E(2)) and females were exposed to the non-aromatizable androgen 17alpha-methyldihydrotestosterone (MDHT). Over 90% of the male fish treated with exogenous E(2) developed gonadal structures indistinguishable from the gonads of triploid females. Triploid female RBT treated with MDHT developed testes; however, not all fish treated with this androgen were completely sex reversed. The results of this investigation are consistent with the hypothesis that the failure of ovarian development in triploid RBT is due to the early onset of meiosis and does not appear to be due to genotypic sex. J. Exp. Zool. 284:466-472, 1999.     

Intersex Occurrence in Rainbow Trout (Oncorhynchus mykiss) Male Fry Chronically Exposed to Ethynylestradiol

This study aimed to investigate the male-to-female morphological and physiological transdifferentiation process in rainbow trout (Oncorhynchus mykiss) exposed to exogenous estrogens. The first objective was to elucidate whether trout develop intersex gonads under exposure to low levels of estrogen. To this end, the gonads of an all-male population of fry exposed chronically (from 60 to 136 days post fertilization – dpf) to several doses (from environmentally relevant 0.01 µg/L to supra-environmental levels: 0.1, 1 and 10 µg/L) of the potent synthetic estrogen ethynylestradiol (EE2) were examined histologically. The morphological evaluations were underpinned by the analysis of gonad steroid (testosterone, estradiol and 11-ketotestosterone) levels and of brain and gonad gene expression, including estrogen-responsive genes and genes involved in sex differentiation in (gonads: cyp19a1a, ER isoforms, vtg, dmrt1, sox9a2; sdY; cyp11b; brain: cyp19a1b, ER isoforms). Intersex gonads were observed from the first concentration used (0.01 µg EE2/L) and sexual inversion could be detected from 0.1 µg EE2/L. This was accompanied by a linear decrease in 11-KT levels, whereas no effect on E2 and T levels was observed. Q-PCR results from the gonads showed downregulation of testicular markers (dmrt1, sox9a2; sdY; cyp11b) with increasing EE2 exposure concentrations, and upregulation of the female vtg gene. No evidence was found for a direct involvement of aromatase in the sex conversion process. The results from this study provide evidence that gonads of male trout respond to estrogen exposure by intersex formation and, with increasing concentration, by morphological and physiological conversion to phenotypic ovaries. However, supra-environmental estrogen concentrations are needed to induce these changes.     

Historical aspects of protandrous sex change in the anemonefish Amphiprion melanopus (Pomacentridae, Teleostei)

Gonadal structure and cellular composition were examined in juveniles, males and females of the protandric hermaphrodite, Amphiprion melanopus. Functional sex change was experimentally induced in the field and gonad structure was histologically examined both qualitatively and quantitatively at 10, 20, 30 and 45 days after its initiation. Juvenile gonads consist primarily of immature ovarian tissue. Functional male gonads are ovotestes with co-existing mature spermatogenic tissue and immature ovarian tissue, while females possess only ovarian tissue. The initiation of sex change is marked by a rapid maturation of spermatogenic tissue and proliferation of putative oogonia. Gonads were essentially female by 20 days into sex change, but evidence of mature female function (marked by the initiation of vitellogenesis) was not observed until 45 days. Considerable variation between individuals was seen in quantitative measures of gonadal change in the early stages of sex change, but not in later stages. Progress in sex change as indicated by histological indicators was, however, consistent within stages. Duet systems for gamete transport changed from the male to the female form after all male tissue had been replaced.     

The reproductive biology of Plagioscion squamosissimus (Heckel, 1840) in the Pará River estuary (Amazon Estuary)

This study describes the reproductive biology of the South American silver croaker Plagioscion squamosissimus (Heckel, 1840) in the Pará River estuary. Data were collected bimonthly from June 2007 to May 2008. In all, 334 specimens were examined. The gonads were processed with histological techniques, and the ovaries were subjected to morphometric analysis. The length at first maturity (L₅₀) values were 16.14 and 21.43 cm total length for females and males, respectively. The sex ratio based on the total number of individuals favoured the females (1.11 females: one male). Macroscopically, the gonads were classified as immature (A), maturing (B), mature (C) and spent (D). Microscopically, four germ cell types were distinguished based on the development of the ovaries. Chromatin nucleolus (I), perinucleolar (II), cortical alveoli (III) and full‐vitellogenic (IV) oocytes were observed in all ovarian stages, except that of the mature individuals which showed the largest number of full‐vitellogenic oocytes (IV), with a mean diameter of 565.4 μm (±147.4). Substantial percentages of individual P. squamosissimus were found to be in a reproductive condition. Reproductive activity was recorded throughout the year in the Pará River estuary, primarily in February/March and August/September.     

Allocation of nutrients during the reproductive cycle of Ophidiaster ophidianus (Echinodermata: Asteroidea)

The reproductive cycle of Ophidiaster ophidianus (strictly protected status) from São Miguel Island, in the Azorean Archipelago was studied. The reproductive strategy; the energy allocation of each sex during the reproductive cycle and the nutritional condition of the population were analyzed. Gonadal index (GI) showed a clear seasonal pattern with spawning between August and October but histological examination revealed that gamete release can occur throughout the entire year. The pyloric caeca index (PCI) showed little annual variation but with an inverse relationship with the GI. Allocation of energy to the gonads and to the pyloric caeca reflected the seasonal reproductive strategy of this species. Individuals were able to simultaneously develop gonads, pyloric caeca, and quickly regenerate lost arms. There was a major expenditure of energy by females compared to males but, sexual size dimorphism was not observed. The reproductive pattern observed in O. ophidianus combining rich food availability and seawater temperatures characteristic of a temperate zone may be the key to the success of this species in the Azorean oceanic Island.     

Effects of 17α-ethynylestradiol on sexual differentiation and development of the African clawed frog (Xenopus laevis)

Several studies have shown that exposure of amphibians, including the African clawed frog (Xenopus laevis), to potent estrogens at critical times during development results in feminization and/or demasculinization. However, genotyping of X. laevis has only recently become possible, so studies performed in the past were rarely able to make explicit linkages between genetic and phenotypic sex. Therefore, to further characterize this relationship, X. laevis tadpoles were exposed during development to 0.09, 0.84, or 8.81μg/L 17α-ethynylestradiol (EE2), which is the estrogen analog commonly used in oral contraceptives. Exposure to all concentrations of EE2 tested resulted in significant delays in time to metamorphosis. Genotyping showed that genetic sex ratios were similar among treatments. However, morphological evaluation revealed that a significant number of individuals with a male genotype displayed mixed sex and abnormal phenotypes. Additionally, both genetic males and females exposed to EE2 exhibited greater presence of vitellogenin protein relative to the respective controls. Since estrogens function downstream of the initial molecular signals of sexual differentiation, it is likely that genetic male animals received mixed endogenous male and exogenous female signals that caused disordered sexual development. The production of vitellogenin was probably temporally separated and independent from primary effects on sexual differentiation, and might have contributed to delays in metamorphosis observed in individuals exposed to EE2.     

Reproductive strategy of the Patagonian catfish Hatcheria macraei

This study describes the reproductive strategy of the stream‐dwelling catfish Hatcheria macraei in the Pichileufu River, Argentina. Gonad maturity phases, classified on the basis of histological analysis, stages of gamete development and the frequency distribution of oocyte size, were correlated with macroscopic features of the gonads. Hatcheria macraei has a cystovarian ovary, asynchronous oocyte development and lobular testes. Five oocyte and four spermatogenic stages were identified and related to macroscopic gonad characteristics, making it possible to divide gonad development into five phases for females and males. Mature oocyte diameter ranged from 922 to 1935 µm. Absolute fecundity in mature females varied from 115 to 480 oocytes. Hatcheria macraei has multiple spawning during a protracted reproductive season that extends from December to April. This, together with its small size, is characteristic of an opportunistic reproductive strategy, commonly found in species that inhabit adverse and unpredictable environments, such as the low‐order rivers of Patagonia.     

A Biological Model of Accelerated Senescence. 3. Histological Characteristics of Age-Related Changes of Spermatogenic Epithelium in SAM (Senescence-Accelerated Mouse) Mice

The data characterizing the age-related morphological changes in the spermatogenic epithelium of SAMP1 (senescence-accelerated prone) and SAMR1 (senescence-accelerated resistant) mice are presented. In many tubules, early spermatogenesis was accompanied by the formation of many morphologically abnormal germ cells on histological sections of the gonads of sexually immature (three–four weeks) mice of both strains. At this stage, destructive processes in the spermatogenic epithelium were more pronounced in SAMR1 mice. In sexually mature (two–three months) SAMP1 and SAMR1 mice, spermatogenesis as a whole proceeded normally. The first signs of regressive changes in the inner structure of most tubules (disintegration, detachment of spermatogenic epithelium from basal membrane) and morphology of germ cells (pycnosis, nuclear and cytoplasmic vacuolization) were found in SAMP1 mice at the age of six–seven months. In the older age groups (9–10 and 12–15 months), all types of spermatogenic cells were represented in both SAMP1 and SAMR1 mice, but most of these cells were atypical. Mitotic figures were recorded in a population of highly differentiated Sertoli cells.     

Effects of 17alpha-ethynylestradiol on sexual development of male western mosquitofish (Gambusia affinis)

Juvenile male western mosquitofish (Gambusia affinis) were exposed to different concentrations of 17alpha-ethynyl estradiol (EE2) in the diet during the period of sexual maturation. A clear inhibiting influence of EE2 on sexual development was apparent. The proportion of males in each treatment group that failed to complete gonopodial development during the 150-day observation period increased significantly with EE2 concentration. There were significant nonlinear trends toward shorter gonopodia in groups exposed to higher EE2 concentrations. Vitellogenin (VTG) was detectable in the blood of all fish exposed to 1.0 or more micro/g food and the concentration increased dramatically with increasing EE2 exposure. A significant negative association was seen between EE2 concentration and spermatophore counts. This study has demonstrated deleterious effects of EE2 exposure on sexual maturation and several indirect measures of reproductive fitness. It supports the biological relevance of vitellogenin in the blood and reduced gonopodium length as biomarkers for estrogen exposure and endocrine disruption in mosquitofish.     

17alpha-ethinylestradiol reduces the competitive reproductive fitness of the male guppy (Poecilia reticulata)

Whether endocrine disruption in an individual male is actually translated into reduced reproductive success in a natural competitive environment is extremely difficult to predict. Here, we have used paternity analysis to provide new information on the ability of an endocrine disruptor to deleteriously affect male guppy reproductive fitness by including the effect of intermale competition. Groups of male guppies were exposed to 10.5, 44.4, or 112 ng/L of the synthetic estrogen 17alpha-ethinylestradiol (EE2) from birth to adulthood. Subsequently, an exposed male competed against an unexposed male for the opportunity to fertilize a receptive female. The successful males siring the majority of the offspring in each brood were then identified using microsatellites in genetic paternity analysis. Only the highest dose of EE2 produced harmful effects with a significantly female-biased sex ratio, significant reductions in male sperm count, testis weight, body coloration and courtship behavior, and a significant increase in body size. These feminizing effects were translated into a highly significant reduction in fertility, where only 1 of the 17 exposed males sired offspring in competition with unexposed males. The evidence suggests that EE2-treated males have reduced reproductive fitness compared with untreated males, possibly the result of EE2 effects on multiple fitness traits. To our knowledge, this is the first study providing evidence of endocrine disruption at the population level that has included the ecologically highly relevant effect of sexual competition on male reproductive fitness.     

Photoperiod and temperature affect reproductive and nonreproductive functions in male prairie voles (Microtus ochrogaster)

Adult male prairie voles (Microtus ochrogaster) were housed for 10 wk and exposed to long (16L:8D) or short (8L:16D) photoperiods at 21 degrees or 5 degrees C. Maintenance in short day lengths reduced testicular, epididymal, and seminal vesicle mass and also significantly depressed spermatogenic activity. Cold ambient temperature further suppressed gonadal size in voles exposed to short days. Several pelage characteristics were affected by photoperiod, but not by temperature. Increased fur density, fur depth, and length of guard hair and underhair were observed in voles exposed to short days. Intrascapular brown fat and gonadal fat pad mass as well as body mass were significantly less in voles housed in cold temperatures than in voles exposed to warm ambient temperatures; photoperiod did not affect these parameters. Approximately 30% of the male voles exposed to short days maintained their reproductive systems, yet they clearly processed photoperiodic information; all short-day males, regardless of reproductive condition, had comparable winter pelage development. Our results suggest that in prairie voles, photoperiod may be a predictive cue for reproductive function in nature; however, it appears that pelage development is a more obligatory response to photoperiod than is reproduction.     

Endocrine Disrupting Compounds Alter Risk‐Taking Behavior in Guppies (Poecilia reticulata)

Endocrine disrupting compounds (EDCs) enter aquatic habitats from a variety of anthropogenic sources and can mimic, block, or modulate the synthesis of natural hormones. EDCs affect both reproductive and non‐reproductive behaviors because hormones mediate responses associated with aggression and fear. We examined the effects of two EDCs on risk‐taking behaviors in guppies (Poecilia reticulata). We quantified risk‐taking in terms of propensity to forage in a risky location and tendency to join groups in the presence of a predator. We found that male and female guppies responded oppositely to environmentally relevant concentrations of an estrogenic EDC, 17α‐ethinylestradiol (EE2), or an androgenic EDC, 17β‐trenbolone (TB). Males decreased risk‐taking with increasing EE2 concentration (as predicted), but females increased risk‐taking (contrary to prediction). In contrast, females increased risk‐taking with increasing TB concentrations (as predicted), but males decreased risk‐taking (contrary to prediction). These results did not match our expectation that EE2 would reduce risk‐taking and TB would increase risk‐taking in both sexes. We suspect EE2 and TB produced these counterintuitive effects by downregulating their corresponding hormone receptors and thus reducing levels of circulating endogenous hormones in females and males, respectively. These results show that EDCs can alter fish behavior and potentially reduce fitness in unexpected ways.