Explainer: Male-female flexibility in animals | Science News for Students

Explainer: Male-female flexibility in animals

Some animals behave as if they were the opposite sex; others can even change their sex — and still produce offspring
Jul 31, 2015 — 11:17 am EST
Europe’s “common frog” (shown here in Ireland) is a widespread species. But different “races” of it vary in their potentially sex-reversing lifestyles.

Europe’s “common frog” (shown here in Ireland) is a widespread species. But different “races” of it vary in their potentially sex-reversing lifestyles.

John Clare / Flickr (CC BY-NC-ND 2.0)

People tend to describe materials that can bend and be easily transformed as plastic. Most of those materials are made from polymers, often created from fossil fuels. But even behaviors can bend and morph. In that sense, these too can be considered plastic.

Paul Vasey works at the University of Lethbridge in Alberta, Canada. As a comparative psychologist, he studies behaviors in animals. And he’s noticed that how animals behave in terms of their biological sex often is not rigid or unchanging. Some behaviors can appear rather plastic.

To compare behaviors across species, it is important to keep in mind some important differences, Vasey notes. For instance: “When you have an identity, you have to have a concept of self.” In fact, identity and gender are tightly braided together in people. The two can be nearly impossible to untangle.

But outside of perhaps the great apes, he says, there’s very little evidence of a concept of “self” in animals. This means animals don’t have a sense that they are acting male or female. They merely express behaviors that are typical — and sometimes not typical — of the sex they belong to. Despite that, there are many examples of intersex conditions within the animal kingdom. Such signs of both sexes can show up both in behaviors and in physical traits.

For instance, the 1999 book Biological Exuberance points out that more than 50 species of coral-reef fish possess the ability to reverse their sex organs (egg-making ovaries and sperm-making testes). This is called  trans-sexuality. It can affect wrasses, groupers, parrotfish, angelfish and more. The fish that begin life as females, with fully functioning ovaries, can undergo a dramatic change. Voilà, they now have a fully functioning male reproductive anatomy. Even after their sex-change, both males and females can reproduce.

Several types of birds, such as warblers and ostriches, also can exhibit a mosaic of male and female characteristics. The color patterns, plumage, singing and other characteristics of one sex may show up in some members of the opposite sex. 

Researchers have documented intersex conditions in grizzly, black and polar bears. In certain populations, a small percentage of female bears possess genitalia that resemble those of male bears. Some of these sows give birth to cubs, despite looking like a boar (a male bear). Intersexuality also has shown up in baboons, deer, moose, buffalo and kangaroos. No one is sure why. But in at least some instances, water pollutants — such as pesticides — have led to clearly abnormal conditions. For instance, biologists have found eggs in the testes of some male alligators and fish that had been exposed to certain pesticides.

In some experiments, pesticide exposures even turned genetically male frogs into what appeared to be females. These Mr. Moms could bear healthy offspring — although they were always male (as each of their parents had been). In other instances, intersex conditions have arisen in completely natural settings.

But perhaps one of the best examples of sex plasticity comes from a new study in European frogs. A single species — Rana temporaria — lives in woodlands from Spain to Norway. Roughly equal numbers of males and females develop from tadpoles in the northern “race” of these frogs. But in the southern region, another race of the species produces females only. They are complete with ovaries, the organ to make eggs. Yet all of the frogs don’t stay female. About half will eventually lose their ovaries and develop testes. They are now males and able to reproduce.

The ovaries-first race relies on environmental cues to trigger the female-to-male change. Researchers reported these differences in the frogs May 7 in Proceedings of the Royal Society B.

Power Words

(for more about Power Words, click here)

anatomy     The study of the organs and tissues of animals. Scientists who work in this field are known as anatomists.

behavior  The way a person or other organism acts towards others, or conducts itself.

boar    A term for the adult male of some mammals, including pigs and bears.

fossil fuels  Any fuel — such as coal, petroleum (crude oil) or natural gas —  that has developed in the Earth over millions of years from the decayed remains of bacteria, plant or animals.

gender  The attitudes, feelings, and behaviors that a given culture associates with a person’s biological sex. Behavior that is compatible with cultural expectations is referred to as being the norm. Behaviors that are incompatible with these expectations are described as non-conforming.

genitals or genitalia    The visible sex organs.

intersex  Animals or humans that display characteristics of both male and female reproductive anatomy.

mosaic  Something made from an assembly of different types of objects. (in biology) An organism made of cells from two genetically different types or exhibiting behaviors of different types (or sexes).

ovary      The female reproductive gland that makes egg cells. 

plastic  Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

plasticity    Adaptable or reshapable. (in biology) The ability of an organ, such as the brain or skeleton to adapt in ways that stretch its normal function or abilities. This might include the brain’s ability to rewire itself to recover some lost functions and compensate for damage.

plumage   A term for the collection of feathers covering a bird. A single large feather may be called a plume.

polymer  Substances whose molecules are made of long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).

psychology  The study of the human mind, especially in relation to actions and behavior. Scientists and mental-health professionals who work in this field are known as psychologists.

sow     (in zoology) A term for an adult female in any of several large mammal species, including pigs and bears.

sperm    The reproductive cell produced by a male animal (or, in plants, produced by male organs). When one joins with an egg, the sperm cell initiates fertilization. This is the first step in creating a new organism.

testis   (plural: testes) The organ in the males of many animal species that makes sperm, the reproductive cells that fertilize eggs. This organ also is the primary site that makes testosterone, the primary male sex hormone.

trait   A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.


N. Rodrigues et al. Sex-chromosome differentiation and ‘sex races’ in the common frog (Ranatemporaria). Proceedings of the Royal Society B. Vol. 282, May 7, 2015. doi: 10.1098/rspb.2014.2726.

Bruce Bagemihl. Biological Exuberance. New York: St. Martin’s Press, 1999.

Further Reading

S. Ornes. “When frog gender flips.” Science News for Students. March 23, 2010.