Welcome to the opening installment of a new series on what we can learn about human sex and gender by looking at the natural world.

Sex—one small word with huge implications. To most humans, being male or female implies a certain set of inseparable biological and sociological characteristics, but the natural world around us is rarely so black and white. For every characteristic that we associate with a particular sex, the animal kingdom harbors at least one surprising exception; concepts that we believe are inextricably linked are uncoupled, and even reversed, in other species.

Our sex exerts an incredible influence on our lives, influencing anatomy, appearance, behavior, and countless other traits. Socially, our sex also matters in terms of how we are understood and treated by others, because we see each other through a gendered lens. Female college graduates are hired more often than male graduates, for instance, but earn 17 percent less than their male counterparts. Police pull male drivers over at a much higher rate than they do female drivers. In high school math classes, teachers direct questions toward, call on, and interact with male students much more frequently than females.

We tend to recognize differences between males and females and try to explain them as a function of sex; we classify some things as masculine and others as feminine, often couching these divisions in evolutionary terms. But these justifications are often inaccurate. As we'll see below, many of the sociological differences between males and females have little, if any, universal relationship to sex.

“Sex” vs. “gender”

Most humans make no real distinction between the words “sex” and “gender”—one term often replaces the other. When asking about a friend’s pregnancy, we might ask about either the sex or the gender of the baby with the same intent, and the US Census Bureau even uses the words “sex” and “gender” interchangeably. The general confusion between these terms made headlines recently when a transgender contestant wanted to compete in the Miss Universe Pageant; a kerfuffle ensued when nobody could quite figure out if that should be allowed.

Sex is a scientific concept, referring to the biological and physiological differences between males and females. In humans, males have penises, an X and a Y chromosome, and generally lower voices and higher amounts of body hair. Vaginas, two X chromosomes, and the ability to lactate and to menstruate are sex characteristics usually belonging to females. These traits aren’t universal in males and females of other species, but they are representative of the biological concept of sex.

Confusion over sex and gender norms runs rampant. Scientists tend to assign traditional human gender roles to animals—and even to plants.

Gender, meanwhile, is a sociological construct. Most often, the term “gender” describes how men and women fulfill certain cultural norms defined by their sex. The World Health Organization says that gender “refers to the socially constructed roles, behaviors, activities, and attributes that a given society considers appropriate for men and women.” Gender encompasses everything from preferences to hobbies to roles in the home and the workplace, and the term applies only in a human context; since it is a social concept, gender does not apply to other species.

That's not to say that cultural gender roles have no roots in biology. The tendency for women to take on many of the parental duties, for example, likely stems from mothers’ ability to lactate, providing necessary nourishment for their children. Biologically, men are drawn to a woman’s hourglass figure because it signals health and fertility; many women, in turn, try to alter or enhance their body shape to fulfill this standard of beauty.

However, ties to biology are not necessary for gender roles to develop, and many of these roles have nothing at all to do with sex. For instance, we often associate girls with pink and boys with blue. But it was standard in the US during the early 1900s to dress American boys in pink and girls in blue. With its hint of "powerful" red, pink was considered a “stronger” color, while blue was thought to be “daintier” and more suitable for girls. These preferences changed in the mid-20th century; today, many parents wouldn’t dare dress their sons in pink.

Because gender roles are so pervasive in human culture, we tend to use them to help us understand the rest of the world. Unfortunately, viewing nature through a human lens doesn’t always give us an accurate picture of what is going on. Nature constantly engineers new and creative solutions to all sorts of problems—turning our stereotypes about sex upside-down along the way.

Appearances can be deceiving

The spotted hyena may have one of the most arresting sexual incongruities in the animal kingdom. Female hyenas possess what scientists call "pseudopenises," since they so closely mimic male penises. These female organs are fully erectile, can be up to seven inches long, and are accompanied by a “pseudoscrotum.” Stranger still, a female hyena must urinate, copulate, and give birth through her pseudopenis. No wonder that, for centuries, people thought hyenas were hermaphrodites—watching a mom with a large penis nurse her babies can be a little confusing.

In a different kind of genital reversal, many males in the animal kingdom lack a penis entirely. In more than 97 percent of bird species, males have no external sex organ at all; instead, both males and females have vents called "cloacas." In these species, mating involves the romantic-sounding “cloacal kiss,” where the two birds line up their genital vents for the transfer of sperm.

We generally expect the males of a species to be big and burly, but biologically speaking, it doesn’t always pay for males to be larger than females. In some spider species, for instance, the females are much larger than males. Researchers believe that this is due to the males’ need to move around efficiently, as lightweight males can better scamper across “bridges” to get from place to place and spread their genes.

In other species, being large provides a definite advantage for females. Sometimes referred to as the BOFFFF ("Big Old Fat Fecund Female Fish”) hypothesis, the idea builds on the fact that large females can carry and spawn an exponentially greater number of eggs than smaller females. Since sending more offspring out into the world is a huge biological advantage for these females, larger bodies are favored, and these species have little or no similar selection pressure for large males.

Males are often the showier sex, of course; male deer have elaborate antlers, male lions have huge manes, and most male birds and fish are more colorful than their female counterparts. But little about appearance is a constant. In a species called the empedid dance fly, for instance, females are actually the sex with flashier signals. While male dance flies are unadorned, the females have “ornaments” on their abdomens and legs that may trick males into thinking that the female's eggs are more mature that they really are. But these adornments, like many of the male sex ornaments, are costly—they make it much harder for females to escape from spider webs.