Why Do Some Male Fish Grow Eggs? – Endocrine Disruptors & The Environment


Industry in Detroit (photo by Nina Munteanu)

Intersex—the presence of both male and female characteristics in the same fish—is currently observed in fish in more streams in North America than in the past. Intersex is one manifestation of endocrine disruption in fish.

endocrine2The endocrine system is a set of glands and the hormones they produce that help guide and regulate the development, growth, reproduction and behavior of most living things (yes plants too!). The endocrine system is made up of glands, which secrete hormones, and receptor cells which detect and react to the hormones. Glands release hormones that travel throughout the body, acting as chemical messengers. Hormones interact with cells that contain matching receptors. The hormone binds with the receptor, like a key into a lock.

Chemicals, both natural and human-made, that interfere with the hormonal system are called endocrine disruptors. Studies have linked endocrine disruptors to adverse biological effects in animals, giving rise to concerns that low-level exposure might cause similar effects in human beings.

Disruption of the endocrine system via Endocrine Disrupting Chemicals (EDCs) or Hormone Disrupting Chemicals (HDCs) happens in different ways. Some chemicals mimic a natural hormone, fooling the body into over-responding to the stimulus, or responding at inappropriate times. Other endocrine disruptors curtail the effects of a hormone from certain receptors by blocking the receptor site on a cell. Still others directly stimulate or inhibit the endocrine system and cause overproduction or underproduction of hormones.


Pesticide Atrazine turns male frogs into females

Aquatic scientists have documented the presence of endocrine disrupting contaminants in rivers and streams in North America with severe impacts on fish populations. Endocrine disruption doesn’t end with fish. The effects of endocrine disruptors in humans include:

  • Reduction of male fertility;
  • Abnormalities in male reproductive organs;
  • Female reproductive diseases;
  • Earlier puberty; and,
  • Declines in the numbers of males born.
  • Some EDCs can also affect the development of the nervous and immune systems.

endocrine disruptors chemicalsSome well-known examples of EDCs are 17-alpha ethinylestradiol (the contraceptive pill), dioxins, PCBs, PAHs, furans, phenols and several organic pesticides (most prominently DDT and its derivatives). Research results suggest that chemicals are inadvertently disrupting the endocrine system of humans and wildlife. Several chemicals were shown to disrupt the endocrine systems of animals in laboratory studies, and strong evidence exists that chemical exposure has been associated with adverse developmental and reproductive effects on fish and wildlife. The relationship of human diseases of the endocrine system and exposure to environmental contaminants, however, is still poorly understood and remains a controversial issue among scientists (Kavlock et al., 1996, EPA, 1997).

A USGS scientist dissecting a fishOne example of the devastating consequences of the exposure of developing animals, including humans, to endocrine disruptors is the case of the potent drug diethylstilbestrol (DES), a synthetic estrogen. Prior to its ban in the early 1970s, doctors mistakenly prescribed DES to as many as five million pregnant women to block spontaneous abortion and promote fetal growth. It was discovered after the children went through puberty that DES affected the development of the reproductive system and caused vaginal cancer. In addition to disruption of reproductive hormones, modulation of adrenal, thyroid and growth hormone function have also been described for various compounds in both humans and some animals, although the significance of these effects have not yet been fully determined.

Chemicals with affinities for estrogen receptors may cause lowered sperm count and other related reproductive abnormalities. Male fetuses exposed to high doses of estrogens may develop with many female characteristics. Lower doses may alter the differentiation and multiplication of the germ cells that eventually give rise to sperm. Dr. John A. McLachlan, director of intramural research at the National Institute of Environmental Health Sciences noted that “some of the environmental chemicals that have estrogenic activity also seem to have a long half-life and can bioaccumulate” in the body’s fat tissue. Most endocrine disrupting chemicals are fat-soluble. This means that they are not rapidly flushed from the body, but in fact stored in fat. These chemicals bioaccumulate up the food chain; individuals will accumulate more of these chemicals throughout their lifetime.

In 1992 researchers at Lake Apopka in Florida associated a declining alligator population with a depressed reproduction rate. Many of the male alligators had tiny penises that prevented successful reproduction. These developmental problems were connected to a large organochlorine pesticide spill several years earlier; although the water tested clean, the alligators and their eggs contained detectable levels of endocrine disrupting pesticides. Fish in the Great Lakes, which are heavily contaminated with polychlorinated biphenyls (PCBs) and other organochlorines, exhibit numerous reproductive function problems and swelling of the thyroid gland. Fish-eating birds, such as eagles, terns, and gulls show similar health effects; as does mink, a mammal that also eats fish from the Great Lakes. These findings are consistent with lab studies that indicate that PCBs interfere with thyroid function and with sex hormones.

Recommended Reading:

Colborn, Theo; Dianne Dumanoski; and John Peterson Myers. Our stolen future : are we threatening our fertility, intelligence, and survival? : a scientific detective story. New York : Dutton, 1996. 306 p. ISBN 0452274141

Executive Summary (PDF). Global assessment of the state-of-the-science of endocrine disruptors. International Programme on Chemical Safety, World Health Organization (2002).

Endocrine Disruptors (PDF). National Institute of Environmental Health Sciences (2006).

Natural Resources Defence Council (NRDC). Endocrine Disruption—an Overview. Website: http://www.nrdc.org/health/effects/bendrep.asp


nina-2014aaaNina Munteanu is an ecologist, limnologist and internationally published author of award-nominated speculative novels, short stories and non-fiction. She is co-editor of Europa SF and currently teaches writing courses at George Brown College and the University of Toronto. Visit www.ninamunteanu.ca for the latest on her books.



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