Brian Barton
Frogs are more sensitive to hormone-disrupting environmental pollutants than previously thought. A study by UC Berkeley biologists found that atrazine, a widely used herbicide, wreaks havoc with the sex lives of adult male frogs, emasculating three-quarters of them and turning one in 10 into females. Another study by Uppsala University found that male tadpoles that swim in water with relevant levels of hormone-disrupting pollutants become females. While it was initially believed that pollutants were the primary cause of sex change in frogs, recent studies have shown that frogs can change sex even in natural, pollution-free settings, possibly due to local changes in temperature or other environmental factors.
| Characteristics | Values |
|---|---|
| Pollutants | Atrazine, a potent endocrine disruptor, synthetic estrogens, and herbicides |
| Effect on Frogs | Turning male frogs into females, decreasing testosterone levels, decreasing fertility, causing hermaphroditism |
| Study Location | Suburban ponds, natural settings, laboratory |
| Species Affected | African clawed frog (Xenopus laevis), leopard frogs (Rana pipiens), green frogs (Rana clamitans), agile frogs (Rana dalmatina) |
| Factors Influencing Sex Change | Environmental factors, temperature, genetic constitution |
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What You'll Learn
The herbicide atrazine can turn male frogs into females
The herbicide atrazine is one of the world's most widely used pesticides. Some 80 million pounds of atrazine are applied annually in the United States on corn and sorghum to control weeds and increase crop yield. Atrazine is also one of the most common pesticide contaminants of ground, surface, and drinking water.
Tyrone Hayes, a professor of integrative biology at the University of California, Berkeley, and his colleagues conducted a study in 2002 on African clawed frogs (Xenopus laevis). They found that tadpoles raised in atrazine-contaminated water become hermaphrodites – they develop both female (ovaries) and male (testes) gonads. This occurred at atrazine levels as low as 0.1 parts per billion (ppb), 30 times lower than the levels allowed in drinking water by the EPA (3 ppb).
In a subsequent study, native leopard frogs (Rana pipiens) collected from atrazine-contaminated streams in the Midwest were found to often have eggs in their testes. Many males had lower testosterone levels than normal females and smaller than normal voice boxes, presumably limiting their ability to call mates. The study also showed that atrazine-exposed males suffered from depressed testosterone, decreased breeding gland size, demasculinized/feminized laryngeal development, suppressed mating behavior, reduced spermatogenesis, and decreased fertility.
Hayes's research also established that many frogs in Midwestern streams contaminated by atrazine and other pesticides have compromised immune systems, leading to increased mortality from bacterial disease. Atrazine has also been found to increase northern leopard frogs' susceptibility to potentially deadly parasites.
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Frogs can change sex in natural settings
Frogs are sensitive to hormone-disrupting environmental pollutants, such as the herbicide atrazine, which is one of the world's most widely used pesticides. Studies have shown that exposing frog eggs to even 0.1 parts per billion of atrazine leads to a larger proportion of females than a control group of frog eggs with no artificial chemicals added. Atrazine has been shown to emasculate three-quarters of male frogs and turn one in ten into females.
However, frogs can also change sex in natural settings, relatively free from human-caused pollution. A study found that green frogs (Rana clamitans) can reverse their sex in forested ponds, with no high levels of pollution. The authors of the study speculate that the frogs are reacting to local changes in temperature or other environmental factors. Sex reversal in frogs may have the function of boosting genetic diversity. For example, a female with a Y chromosome can breed, allowing the Y chromosome to create new, healthier versions and possibly rid itself of some bad mutations.
Indeed, a study from Uppsala University found that low, environmentally relevant concentrations of the contraceptive pill estrogen ethynylestradiol cause sex reversal in frogs. This study showed that male tadpoles that swim in water with relevant levels of such substances become females. Furthermore, exposure to a heatwave induced female-to-male sex reversal in tadpoles, while exposure to ethinylestradiol did not alter the masculinizing effect of heat.
While it was previously thought that pollutants were the main cause of sex changes in frogs, it now seems that frogs can adjust their sexual destiny to local circumstances, including variations in temperature or other environmental variables.
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Synthetic estrogens and herbicides induce sex reversal
Frogs have been shown to reverse their sex in both polluted and non-polluted environments. While some studies have found that certain pollutants can induce sex reversal in frogs, others have shown that frogs can also change sex in natural, pollution-free settings.
In laboratory settings, synthetic estrogens and herbicides have been shown to induce sex reversal in genetically male frogs, causing them to develop outwardly as females. For example, exposure to the herbicide atrazine, a potent endocrine disruptor, has been found to disrupt the sexual development of amphibians. Studies have shown that tadpoles raised in atrazine-contaminated water can become hermaphrodites, developing both female and male gonads. Atrazine has also been linked to decreased testosterone levels in male frogs and smaller voice boxes, which may limit their ability to call mates.
Field studies have indicated that atrazine may have similar effects on frogs in the wild, contributing to the global decline in amphibian populations. In 2014, a paper in the Proceedings of the National Academy of Sciences found that pollution-laden runoff into suburban ponds in the US could be turning larval male amphibians into females. However, subsequent research by the same group found that sex reversal also occurs in more pristine forest ponds, suggesting that it may be a natural phenomenon for certain frog species.
In addition to atrazine, other synthetic estrogens have been implicated in frog sex reversal. For example, a study from Uppsala University found that low concentrations of the contraceptive pill estrogen ethynylestradiol, which is often released into waterways through wastewater, can cause sex reversal in male tadpoles. This study provides further evidence that frogs are more sensitive to hormone-disrupting environmental pollutants than previously thought.
While the exact mechanisms are not yet fully understood, it is clear that synthetic estrogens and herbicides can induce sex reversal in frogs, particularly during their early development as tadpoles. These findings highlight the potential consequences of anthropogenic environmental change on species where genetic sex determination can be influenced by the chemical environment.
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Frogs can adjust their sex in response to local circumstances
Frogs are more sensitive to hormone-disrupting environmental pollutants than previously thought. Research has shown that exposing frog eggs to certain pollutants, such as synthetic estrogens and herbicides, can induce genetically male frogs to develop outwardly as females. One such pollutant is atrazine, a widely used herbicide that has been shown to have detrimental hormonal effects on frogs, causing them to develop into the wrong sex.
Studies have found that male tadpoles that swim in water contaminated with even low levels of atrazine become hermaphrodites, developing both female and male gonads. This can lead to a decrease or even a wipe-out of a frog population due to the skewed sex ratios. In addition, male frogs exposed to atrazine often have lower testosterone levels and smaller voice boxes, impacting their ability to call mates.
However, it is important to note that sex reversal in frogs is not solely due to pollution. Recent studies have shown that frogs can also change their sex in natural, unadulterated settings relatively free from human-caused pollution. For example, a study on green frogs (Rana clamitans) found that they changed sex even in forested ponds with low levels of pollution. This suggests that frogs can adjust their sex in response to local circumstances, such as variations in temperature or other environmental factors.
Indeed, a study on agile frogs (Rana dalmatina) found that high temperatures induced female-to-male sex reversal, while exposure to a xenoestrogen, EE2, did not have the same effect. This highlights the complexity of sex determination in amphibians, which is influenced by both genetics and the environment. While mammals are primarily bound to genetics when it comes to sex determination, amphibians like frogs have more flexibility, allowing them to adjust their sexual destiny based on local conditions.
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Endocrine-disrupting chemicals can induce sex reversal
Frogs have been shown to reverse their sex in both polluted and pristine environments. While this phenomenon may be influenced by local circumstances, such as temperature or other environmental factors, pollutants are also a significant contributing factor. Endocrine-disrupting chemicals, including synthetic estrogens and herbicides, have been found to induce sex reversal in frogs, particularly during their tadpole phase.
One well-studied example of an endocrine-disrupting chemical is atrazine, a widely used herbicide and pesticide. Studies have shown that exposure to atrazine can disrupt the sexual development of amphibians. In one experiment, tadpoles raised in atrazine-contaminated water developed both female and male gonads, becoming hermaphrodites. This occurred at extremely low levels of atrazine, far below the concentrations allowed in drinking water by regulatory bodies.
Further research has provided additional evidence of the impact of atrazine on frog populations. Native leopard frogs collected from atrazine-contaminated streams often displayed unusual characteristics, such as eggs in their testes and lower testosterone levels than normal female frogs. These frogs also exhibited smaller voice boxes, potentially impacting their ability to call mates. The findings suggest that atrazine can lead to hormonal imbalances, causing frogs to develop into the wrong sex, contrary to their genetic constitution.
In addition to atrazine, other endocrine-disrupting chemicals have been implicated in frog sex reversal. A study by Uppsala University found that low concentrations of ethynylestradiol, a synthetic estrogen found in contraceptive pills, caused sex reversal in frogs. This discovery highlights the detrimental effects of hormone-disturbing environmental pollutants on frog populations.
While the ability of frogs to reverse their sex may offer potential genetic benefits, such as boosting genetic diversity, the presence of endocrine-disrupting chemicals in the environment remains a significant concern. These chemicals can have wide-ranging consequences for frog populations, affecting their survival, development, and overall fitness. As such, it is crucial to recognize the impact of human activities on the environment and to address the release of endocrine-disrupting chemicals to mitigate their potential harm to fragile ecosystems.
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Frequently asked questions
Research has shown that exposure to certain pollutants like synthetic estrogens and herbicides can cause genetically male frogs to develop as females. One such pollutant is the herbicide atrazine, which is one of the world's most widely used pesticides.
Atrazine has been found to wreak havoc with the sex lives of adult male frogs, with three-quarters of those exposed to it becoming chemically castrated and one in ten turning into females.
A study found that high temperatures induced female-to-male sex reversal in tadpoles, while exposure to ethinylestradiol (EE2) did not alter the masculinizing effect of heat.
Yes, studies have found that pollutants can affect the sex of frog species in both polluted and pristine natural settings.
