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Although hormones are vital to an organism's ability to respond to environmental stressors, they can be directly altered by the environment and impact reproductive behavior. For example, in some fishes, aquatic hypoxia (low dissolved oxygen) inhibits the aromatase enzyme that converts testosterone to estradiol. Here, we examined the effects of short-term aromatase inhibition on reproductive behavior in male Pseudocrenilabrus multicolor, a widespread African cichlid, from one normoxic river population and one hypoxic swamp population. We further tested the response of females to treated and untreated males. We predicted that aromatase inhibition would decrease courtship and competitive behaviors, but the swamp population would be less affected given generational exposure to hypoxia. Specifically, we compared competition and courtship behavior of males treated with a short-term exposure to an aromatase inhibitor with control fish from the two populations. We found that both courtship and competitive behaviors were affected by the interaction between treatment and population. River fish performed fewer courtship and competitive behaviors under the aromatase inhibition treatment while the behavior of swamp males was unaffected. Additionally, we found that females from the swamp population preferred males from the aromatase inhibition treatment and river females preferred control males. While we found behavioral effects of short-term aromatase inhibition, we did not find any effects on male nuptial coloration. Overall, these results indicate that the effects of short-term aromatase inhibition on behavior could depend on local adaptation in response to hypoxia. 

Abstract Freshwater organisms face a complex array of environmental stressors that can negatively affect endocrine function and subsequent fitness outcomes. Hypoxia and turbidity are two environmental stressors that are increasing due to human activities that could lead to endocrine disruption and reduced reproductive output. Our research addresses how hypoxia and elevated turbidity affect traits related to reproductive success, specifically sex hormone concentrations, investment in reproductive tissues and body size. We used wild fish from two populations (a river and a swamp) of an African cichlid, Pseudocrenilabrus multicolor, to produce offspring that were reared in a full factorial split brood rearing experiment (hypoxic/normoxic × clear/turbid). River and swamp populations represent divergent habitat types with respect to the stressors of interest, being well-oxygenated but turbid or hypoxic and clear, respectively. Overall, we found evidence for plastic responses to both stressors. Specifically, we found that there was an interactive effect of oxygen and turbidity on testosterone in males from both populations. Additionally, males of both populations reared under hypoxic conditions were significantly smaller in both mass and standard length than those raised under normoxic conditions and invested less in reproductive tissues (quantified as gonadosomatic index). Hypoxia and turbidity are experienced naturally by this species, and these environmental stressors did not affect the number of eggs laid by females when experienced in the absence of another stressor (i.e. normoxic/turbid or hypoxic/clear). However, there was an interactive effect of hypoxia and turbidity, as females reared and maintained under this treatment combination laid fewer eggs. This research underscores the importance of considering the possibility of stressor interactions when determining how anthropogenic stressors affect fitness outcomes.