Endocrine-disrupting chemicals released into rivers affect the mating choices of fish, altering the courtship behavior of males and the mate preferences of both sexes. A new study published online this month in the journal Evolutionary Applications demonstrates how these changes lead to inter-species breeding and threaten biodiversity.
The study, by Jessica Ward and Michael Blum, focuses on the effects of Bisphenol A (BPA) on two species of common freshwater fish. BPA is an organic compound used in the manufacture of some plastics. It is controversial because it acts as a weak endocrine disrupter; in the bodies of humans and other animals, BPA can mimic estrogen. In recent years, several governments and regulatory agencies raised questions about its safety in consumer products. The chemical is currently banned from baby bottles and children's cups in 11 U.S. states.
According to a 2010 EPA report, over one million pounds of BPA are released into the environment annually, frequently ending up in rivers and streams. There are many studies on the effects of the chemical on individual animals, but Ward and Blum show BPA has the potential to impact entire populations.
Endocrine-disrupting chemicals like BPA can alter reproductive behavior and appearance by mimicking the action of natural hormones. In fishes, these hormones regulate male courtship traits, aspects of sex and species recognition, and female mating receptivity. When these get mixed up, the lines between species are blurred, and individual fishes are likely to mistake a newly introduced species as a potential mate.
Ward and Blum collected two species of fish from waterways in Georgia. Blacktail shiner (Cyprinella venusta) are native to the area, while red shiner (C. lutrensis) are an introduced species. They kept the fish separated for 14 days, exposing some to BPA. On the 15th day, the team conducted behavioral tests, observing as individual males and females from different tanks were introduced to each other.
Females of both species that were not exposed to BPA preferred males of their own kind, spending more time with them and approaching them more frequently. But the BPA-treated females did not discriminate between males of their own species and males of the other species. Results with males were similar: those exposed to BPA courted females of their own species and other females equally.
These results suggest long-term ecological and evolutionary consequences, especially in areas threatened by the introduction of invasive species. Exposure to these chemicals increases the likelihood of inter-species breeding. By weakening barriers to hybridization, BPA and other endocrine-disrupting chemicals in the environment can escalate the loss of native biodiversity and promote the establishment and spread of invasive species.