Most animals are one gender throughout their lives, but some species are capable of switching. Amphibians, which exhibit several interesting examples of this phenomenon, sometimes change their sex in response to environmental triggers -- which may be social, climactic or chemical in nature. In some circumstances this appears to be historically natural, while others do so as a result of herbicides or other forms of environmental pollution.
Factors Determining Gender
While chromosomes determine the initial gender of mammals and birds, environmental or epigenetic factors determine the gender of many other animals, including some reptiles, invertebrates and amphibians. Incubation temperatures are often important in determining the initial sex of an animal; but later, social or climactic variables can cause reversals for some species. Some individuals may exhibit traits associated with both genders, which can be an end stage or a step in an active transition between genders.
African reed frogs (Hyperolius viridiflavus) inhabit highly seasonal savannas and form dense breeding colonies during the wet season. In captivity, when kept in all-female populations, some individuals can become fertile and functional males, siring offspring. Some salamanders change sex as well: When scientists tested the responses of crested newt (Triturus cristatus) larvae to extreme temperatures, they found that a percentage of the animals -- which normally differentiate into males or females based on genetic factors -- changed sexes. When the temperatures were at the high end of their acceptable range, about half of the females became male; at lower temperatures, the reverse occurred, and about half of the males became females.
Making Sense of the Switch
Scientists have proposed a number of potential advantages that may cause amphibians to change sex. One such hypothesis asserts that social factors are a reason for the switch – as it appears to be for African reed frogs. Another possibility is that different selective pressures are at play for each sex. For example, in some species, large females produce far more offspring than small ones do; however, male reproductive success is largely independent of size. In such situations, an animal can maximize the number of offspring it produces -- a very important consideration for species like frogs with high adult mortality -- by being a male when small and switching to a female after attaining large size.
Chemically Caused Changes
A study by A.L. Reeder and colleagues, published in a 1998 issue of “Environmental Health Perspectives,” examined northern cricket frogs (Acris crepitans) living in polluted ponds in Illinois. Several populations exhibited sex rations that contrasted starkly with typical populations. Additionally, the team found that about 2.7 percent of the adults in the study were intersex individuals who exhibited aspects of both genders. Further studies by a variety of scientists have echoed the findings of Reeder’s team; and many have pointed to a common agricultural herbicide -- Atrazine -- as being a primary cause of such gender change. Many male frogs have turned into females and produced viable young after exposure to the chemical. While this phenomenon threatens frog populations by itself, a 2010 study published in “Proceedings of the National Academy of Sciences,” found that Atrazine also impairs immune function among the frogs.
- The Independent: Solved: Mystery of the Sex-Change Toads
- Wired: Sex-Changing Herbicide Makes Amphibians Sick, Too
- Annual Review of Ecology and Systematics: Sex Change in Plants and Animals
- Environmental Health Perspectives: Forms and Prevalence of Intersexuality and Effects of Environmental Contaminants on Sexuality in Cricket Frogs (Acris Crepitans)
- Animal Diversity Web: Hyperolius Viridiflavus
- Comstock Images/Stockbyte/Getty Images