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How Your Pee Can Change The Sex Of Fish And Reptiles

How Can Human Urine Make Fish And Reptiles Change Their Sex?

Human urine and thus wastewater treatment plant output can contain hormones. Some of this is natural output, but some comes from medical hormone treatments. Scientists are discovering that populations of fish and reptiles are changing their sex because of hormones in the water. And, in some cases, fish populations are crashing because almost all of the males are changing to females and the females can't produce viable eggs.

Is It A Boy Or A Girl?

Chromosomes determine the sex of many, but not all, animals. In humans and most other mammals, plus some insects, females have two of the same sex chromosome (XX) while males have one of each (XY). Chickens and some other birds, some reptiles, and some insects are the other way around with their ZW system. Males are ZZ while females are ZW.


Other animals use schemes including XX/X0, where there is only one sex chromosome. Females have XX and males have only one X (several species of insects plus a few mammals including some rats, shrews, and voles). The platypus has 10 sex chromosomes, which seems needlessly complicated. Males have XYXYXYXYXY and females have XXXXXXXXXX.

If you find this confusing, consider the tetrahymena. That protozoan exists in seven different sexes and they can reproduce in twenty-one different combinations.

For many animals, their chromosomes control their development. Male chromosomes lead directly to development into a distinctively male form, female chromosomes to a female form.

However, with some animals you can't see any difference between males and females. Amphibians, reptiles, birds, and some fish have cloacae. A cloaca is a common opening shared by the digestive, urinary, and reproductive tracts.

Cloaca Maxima, Rome's grand sewer Cloacina, Goddess of the Sewer

Cloaca means "drain". These three diverse systems share a common drain in these animals. It's the same word the Romans used for their municipal wastewater system. The Cloaca Maxima was Rome's grand sewer, and Cloacina was its goddess.

The platypus also has a cloaca, and it lays eggs. And don't forget those 10 sex chromosomes. Plus, any group of two or more demand to be called "platypi." They're just weird. Don't pay any attention to them, it will just encourage them.

The males of a few bird species have an external phallus, but for the rest of the species with cloacae there's nothing external to this single shared opening. For some species, adult males and females have distinctive overall forms. But for young birds and the reptiles, you have to closely inspect the cloaca.

Map of central Imperial Rome showing the Cloaca Maxima running under the Forum, from https://en.wikipedia.org/wiki/File:Map_of_downtown_Rome_during_the_Roman_Empire_large-annotated.jpg

The Cloaca Maxima running under through Imperial-era Rome.

Do Some Switch?

Some fish and gastropods display sequential hermaphroditism, changing their sex during their lifespan. Clownfish, for example, are protandrous hermaphrodites, born male and changing their sex to female when they become the largest and dominant individual in their social group. The prequel to Finding Nemo should be interesting.

Protogynous hermaphroditism is more common among fish. Three out of every four of sequential hermaphrodite fish species are the other way around, they are born female and turn male at some point. Wrasses, for example, plus the groupers, porgies, gobies, parrotfish, and swamp eels.

How Did The Animal Get This Way?

Some species develop quickly, like the biology student's pal the fruit fly or Drosophila melanogaster. Fruit flies become sexually differentiated as soon as the egg is fertilized.

In other species, such as humans, there is no sexual differentiation for some time into development. Human embryology is complicated and strange. Humans first develop a cloaca, which is right next to the tail. The cloaca and surrounding structures later develop into specifically male or female reproductive organs plus an anus, while the tail disappears. This is very early, around the time when human pharyngeal grooves look like gill slits.

Human cloaca at 32 to 33 days of development, from Gray's Anatomy

Tail and cloaca of a human embryo at 32-33 days, from Gray's Anatomy.

Temperature-Dependent Sex Determination

For reptiles and fish, the temperature during a critical stage of development can determine the offspring's sex. Turtles, crocodiles, and alligators don't have sex chromosomes. Their sex is controlled by the temperature during the middle third of embryonic development.

You have to carefully examine the interior of the cloaca to determine the sex of an adult. That's a hazardous undertaking with alligators and crocodiles.


Male? Female? Sexing the alligator can be a dangerous job.

Endocrine Disruptors

But then there are the endocrine disruptors, molecules that interfere with the body's hormone systems. These can cause cancerous tumors and developmental disorders. They can also change the male/female balance of individuals, especially when they involve sex steroids such as estrogens and androgens.

Endocrine disruptors may be used intentionally. Groupers are commonly raised in fish farms in Asia. Adults take several years to change from female to male, so male-stage groupers are especially valuable members of the operation's broodstock.

Aquaculture operations may use artificial hormones to adjust the sex ratio. Tilapia are raised on food containing high levels of 17α-methyltestosterone.

Methyltestosterone molecule


Methyltestosterone molecule

17α-methyltestosterone is given to humans for a variety of medical purposes, and abused to enhance athletic performance and bodybuilding.

Added to tilapia feed in large quantities, it changes the females into faster-growing males.

The Sex-Changing Crocodiles and Fish

Science reported that American crocodiles in Costa Rica's Palo Verde National Park had a strange sex ratio, with males outnumbering females four to one. The warming climate should be pushing the ratio the other way, with more females after warmer temperatures during development.

The park is surrounded by fish farms that raise tilapia using feed filled with the synthetic hormone 17α-methyltestosterone. The fish farms want to skew the sex ratio of their tilapia heavily male. It appears that their overflow and runoff are getting into the water in the park and changing the crocodile sex. The crocodiles' tissues contained 17α-methyltestosterone.

That same article reported that scientists in the 1990s found that male alligators in Florida's Lake Apopka were femininized, with shrunken genitalia and reduced testosterone levels. That lake is polluted by chemicals that mimic estrogen.

Trenbolone molecule

Trenbolone or 19-nor-δ9,11-testosterone, very similar to 17α-methyltestosterone.

Cattle have trenbolone acetate pellets implanted in their ears while they are in U.S. feedlots being prepared for market. Trenbolone, or 19-nor-δ9,11-testosterone, is an anabolic-androgenic steroid that increases muscle growth and appetite. At the Sochi Winter Olympics in 2014, Russian officials gave their athletes a mixture of anabolic-androgenic steroids including trenbolone, metenolone, and oxandrolone.

Regulations are supposed to limit runoff from feedlots into streams, but compliance and enforcement are lax. Endocrinologists have found intersex minnows downstream from feedlots. That is, females with male characteristics and vice-versa. Meanwhile the E.U. banned growth hormone use in both domestic and imported meat in 1990. See the Overview and the journal article for more.

Human Hormonal Runoff

Enough people are taking hormones, especially testosterone and its derivatives, that sewage treatment plant output can be a significant source of contamination.

An article in Scientific American reported that only about half of the prescription pharmaceuticals and other newly recognized contaminants are removed by sewage treatment plants. This comes from a study by the International Joint Commission, a consortium from the United States and Canada studying the Great Lakes.

The European Union may restrict hormone use in cattle, but humans still use it and excrete it into the wastewater flow. A study in Galicia in northwestern Spain found three hormones (estrone, 17β-estradiol, and 17α-ethinylestradiol, used in birth-control pills) plus other pharmaceuticals in the output of a municipal wastewater treatment plant.

A different study in Spain examined the sludge at two sewage treatment plants. They found nine hormones and their derivatives, plus 20 pharmaceuticals including anti-inflammatories, lipid regulators, and antibiotics,

A study in northwestern Ontario found that natural estrogens (estrone or 17β-estradiol) and the synthetic estrogen used in birth-control pills (17α-ethinylestradiol) led to feminization of male fishes and disruptions in the egg production of female fishes. This led to the near extinction of fathead minnows from a lake over a 7-year period. For this study they added the hormones to an isolated lake, using estrogens that are not completely broken down in typical municipal wastewater treatment plants.

Treatment plant upgrades can help. A study in Canada measured the incidence of intersex fish downstream from a large wastewater treatment plant that had gone through an upgrade of its sludge treatment process. When the larger of two treatment plants at Kitchener, Ontario used carbonacious activated sludge treatment, 70% to 100% of nominally male rainbow darter found downstream in the Grand River were intersex. After an upgrade to nitrifying activated sludge treatment, the intersex rate dropped to less than 10%.

Work is underway to measure and control pharmaceuticals in sewage treatment output in Italy, Finland, and many other countries.

Scholarly papers on:
pharmaceuticals in sewage hormones in sewage

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