Tag Archives: Mammal

Sex vs Gender

does-not-equal

America was settled by puritans. It may or may not be a coincidence that, hundreds of years later, we are still pretty weird about sex. Most people don’t even like saying the word “sex.” Enter “gender.” With the word “gender,” people can communicate their thoughts without the ickiness of having to say “sex” out loud. Or can they?

As a teacher, I often had to correct students who tried to tell me the gender of the cockroaches they were using in their experiments. Even biology professors, who really should have known better, would sometimes use the word “gender” when they meant “sex.” A friend of mine is having a baby soon. She and her husband recently made a big announcement about the gender of the baby. Official documents are notorious for items like, “Please indicate your gender: __male __female.” But what is the problem here? In short, sex and gender are not the same thing.

Sex is the biological component. Male mammals* have an X chromosome and a Y chromosome, have testicles, a penis, and produce sperm. Female mammals have two X chromosomes, have ovaries, a uterus, a vagina, and produce eggs. Sex is basically a categorical variable — you are either male or female. About 1% of people are “intersex.” For these people, there may be a disconnect between their arrangement of chromosomes and their anatomy — sometimes people will develop outwardly as male, but have two X chromosomes and no Y. The opposite can happen, too, with people developing outwardly as female, but having XY chromosomes. (People with XXY or YYX chromosomes are not considered intersex.) Sometimes people are born with ambiguous genitalia, or a mismatch between their external and internal genitalia. This throws a bit of a wrench into our concept of biological sex, but around 99% of people can be categorized comfortably as male or female. (For more information, go here.)

Gender is a psychological component. It deals with how you feel and how you present yourself. Unlike sex, which is mostly binary, gender is a smooth, continuous variable. Most human males are clustered towards the masculine end of the gender continuum, and most human females are clustered towards the feminine end of the gender continuum. But you can have males who are more or less masculine than other males, and females who are more or less feminine than other females. You can have females who are more masculine than some or most males, and males who are more or less feminine than most females. Most males identify as men, and most females identify as women, but this is not always the case. Some males identify as women, and some females identify as men — these people usually identify as “trans*” or “transgender.” (For more information, go here.)

So do cockroaches have gender? Not if the answer you are looking for is “male” or “female.” Male and female are choices for sex. Do cockroaches have a gender identity? Not as far as we know. Do cockroaches have any sense of self at all? Again, not as far as we know. There might be some scientific questions about cockroach gender, but most people don’t think about these things.

Likewise, official documents shouldn’t ask for your gender when what they really want to know is your sex otherwise the question would look like this: “Please indicate your gender on the following scale: masculine_ _ _ _ _ _ _ _  _ _ _ _ _ _feminine.”

My friend’s baby? Has a sex, but the gender will be up in the air until he or she develops a self-identity.

Both sex and gender are real things, but they are different. Make sure you’re using word you mean.

*In other types of animals, sex is determined in different ways.

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The Evolution of Flight

Out of several thousand species of birds, almost all of them can fly. They all have the ability to fly because they evolved from a common ancestor that could fly. Bats can all fly because they evolved from a common ancestor that could fly. But why can both birds and bats fly? Did they evolve from a common ancestor that could fly? While they did evolve from a common ancestor, this ancestor could not fly. How, then, are both birds and bats able to fly?

In biology, there is a concept called “convergent evolution.” Some types of organisms have similar traits because they evolved from a common ancestor that had those traits. With only a few exceptions, all mammals, amphibians and reptiles (including birds) have four limbs — two arms/wings and two legs. This is because these three lineages all evolved from a common ancestor that had four limbs. Similar traits that are due to common ancestry are called “homologous traits.”

Other types of organisms have similar traits but did not evolve from a common ancestor that had those traits. Fish and whales are a classic example of convergent evolution. They both have a tail fin that propels them through the water, forward fins that help them steer, a fusiform body that makes them hydrodynamic, and a dorsal fin that keeps them stable.

The dolphin and fish share many traits that facilitate an aquatic life. Image from wikipedia.org

But there are more differences than similarities. Here are a few:

  • The tail fin of a fish is oriented vertically, whereas the tail fin of a whale is oriented horizontally
  • Fish lay eggs, whereas whales give live birth
  • Baby fish are fed by a yolk sack in their egg, whereas baby whales are fed from mammary milk
  • Fish use gills to extract oxygen from the water, whereas whales breathe atmospheric air
  • Fish are cold-blooded, whereas whales maintain a high body temperature.
  • Fish have scales covering their skin, but whales do not.
  • Whales have typical mammalian wrist and finger bones inside their pectoral fins, but fish do not.
  • Whales have hair, but fish do not.

Whales do, of course, share a common ancestor with fish, but this common ancestor is not the reason that whales have their aquatic adaptations. The ancestors of whales first evolved into a terrestrial life, then evolved back into the water, much later in life.

When two or more different types of organisms evolve a similar trait independently, these traits are called “analogous traits” and the process of evolving these analogous traits is called “convergent evolution.”

Off the top of my head, I can think of nine independent evolutionary origins of flight — that is, nine separate events of convergent evolution. There are probably more that I don’t know about. Let’s start with the three best fliers that are currently alive: Birds, bats and insects.

Birds and bats are both tetrapods, so they are stuck with four limbs. They both use primarily their front limbs for flight, but they do it differently. Bird hand and wrist bones are fused together to make a short, stumpy end bone. Feathers produce the area required to produce lift.

The bones of a bird wing. Image from wikipedia.org

When birds are in flight, they keep their legs and feet tucked out of the way so they do not interfere with flying.

Canada goose in flight. Note that the legs are not used in flight. Image from wikipedia.org

Bats have a membrane of skin that stretches between their arms and legs that help produce lift. The legs and feet of bats are very important for flight.

Bat in flight. The legs are important in forming the wings. Image from wikipedia.org

Bats have elongated fingers that make up most of the wings. They use skin that is stretched between their fingers to create the area required to produce lift.

The arm bones in the bats and birds are homologous to one another, but their wings are the result of convergent evolution.

Insects have six legs and two pairs of wings. Insect wings are inflexible, except for where the connect to the body; a little bit like the oars on a boat. There are no bones or muscles inside the wings. Birds and bats have aerodynamic bodies that allow them to pass through the air efficiently. Some insects, like the dragonflies, have aerodynamic bodies, but bees and beetles do not.

Dragonfly. Image from wikipedia.org

The pterosaurs were not technically dinosaurs, but they were close relatives. Modern birds, which are dinosaurs, are not direct descendants of the pterosaurs, but birds are more closely related to the pterosaurs than they are to bats. Despite the closer genetic relatedness, the pterosaurs flight ability resembles bats more than birds in a variety of ways. First, they did not appear to have had feathers. Instead, they probably used a membrane of skin to form their wings much the way bats do.

Bats use fingers 2-4 (index through pinkie) for flight, and finger 1 (the thumb) for limited gripping. Pterosaurs only had four fingers, and only finger 4 was used for flight, whereas fingers 1-3 were used for gripping.

Pterosaur wing. Image from http://www.geol.umd.edu

 

Other, lesser fliers:

These are animals that fly sort of like a paper airplane. They cannot propel themselves once they are in the air — they have to jump to get their initial momentum. But once they are in the air, they can control their direction and create an air foil to slow their falling. Humans can do this with the aid of a wingsuit:

Flying squirrels: A little bit like bats, flying squirrels have a membrane of skin that stretches between their front and rear legs. This allows them to glide over longer distances than they would otherwise be able to jump.

Flying squirrel in flight. Image from wikipedia.org

 

Flying lizards: Although the word “dinosaur” literally means “terrible lizard,” lizards and dinosaurs are completely different types of reptile. Flying lizards in the genus “Draco” are not very closely related to the flying dinosaurs. The flying lizards are very unusual because they do not use any of their four limbs for flying. Instead, they are able to spread out their ribs to form fairly immobile wings which allow them to glide for short distances.

 

Flying dragon. Image from wikipedia.org

Flying fish: Flying fish are much better at flying than you would expect. They use their tail to get out of the water and get speed. Once they are in the air they can glide for fairly long distances. If they want to increase their speed, they can put their tail back into the water and give themselves another push. This makes them the only glider that I know of that can add energy to their glide without landing.

Flying Fish. Image from wikipedia.org

 

Flying frogs: Like bats, flying frogs create “wings” by stretching skin between long fingers. Unlike bats, the “wings” of the flying frogs are limited to their feet, and do not include any skin on the arms or legs.

Flying frog in flight. Image from http://endangeredliving.files.wordpress.com/

Flying snakes: To people who are afraid of snakes, nothing sounds more horrifying than snakes that can fly. But don’t worry — the flying snakes are the worst flyers of the group. They are able to flatten out their bodies to create a very minimal air foil. Their “flight” looks a lot like jumping or falling, but research has shown that they are able to steer themselves in the air. It may not seem like I should have included these in a list of things that evolved to fly, but remember that everything that evolved to fly had to go through many stages of flying ability. In the first stages, the animals would have just been jumping. In later stages, they would have a rudimentary ability to glide and navigate. For this reason, I firmly consider these snakes to be an example of incipient flight.

 

 

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