How do we Hear?

By Scott Mattison (@FoolsPizza)

Hearing is one of the five traditional senses, but by the time individuals reach 60 years old, one in three will develop significant hearing loss in one or both ears. To treat this hearing loss, we must first have a basic understanding of how sound and our hearing work.

Sounds travel by mechanical vibrations of particles that contact one another. Air, water, and solids all allow for these vibrations to occur; however, in space, no one can hear you scream because outer space is a vacuum with no small particles to transfer sound (sorry Star Wars fans).

The ear is divided into three regions: the outer ear, the middle ear, and the inner ear. The outer ear is what is most familiar to people, it consists of what we traditionally call ears and a canal that leads to the eardrum. The role of the outer ear is to collect sound and direct it to the middle ear through the ear drum. The unique shape of the ear enables directed sensing of sounds which gives people the ability to determine where sounds come from.

850px-Auditory_ossicles-en.svg
A nice illustration of the middle ear bones (ossicles). Courtesy of Wikipedia.

The middle ear is an open, air filled space in the skull that separated from the outer ear by the ear drum. The active portion of the middle ear consists of three bones known as the malleus, incus, and stapes. The malleus is attached to the eardrum, and when the eardrum vibrates in response to sounds, those vibrations are transferred to the malleus. Movements of the malleus move the incus, which in turn move the stapes. The stapes then vibrates against what is called the oval window of the inner ear. The bones of the middle ear form a lever arm that provides an amplification of sound by around 130%. Additional amplification occurs due to the size difference between the eardrum and the oval window of the inner ear. Overall, sound vibrations entering the inner ear are around 18 times larger than sound vibrations against the eardrum. The middle ear also has special muscles that when activated will reduce the intensity of loud sounds. This is called the acoustic reflex and may reduce sound intensity by around a factor of 10. The acoustic reflex protects the delicate structures of the inner ear.

The inner ear is where all the magic of hearing happens. The inner ear takes the mechanical vibrations of sounds and converts them into electrical impulses within the brain. How this actually works is both a lot cooler and a lot more complicated than it might seem.

At their best, humans can hear a range of sound frequencies (think notes on the piano, each note is a different frequency) between 20 Hz and 20,000 Hz. 20 Hz is a really deep bass sound and 20,000 Hz (If you can still hear it) is an extremely high pitched whine. As we age, sensory cells in our inner ear begin to die and we lose the ability to hear higher pitched frequencies. By age 20, most males have lost the ability to hear 20,000 Hz, females typically begin to lose higher frequency hearing later into their twenties. Fortunately most of human communication is limited to the 100s of Hz frequency range. The fact that we can differentiate between sounds of different frequencies means that our inner ear has to respond differently to different sound waves.

Nervous cells have a recovery period of 1 – 5 ms after each time they fire, meaning the fastest a nerve could respond is 1000 Hz. If this is true, how can our ears generate nervous signals in response to sounds vibrating up to 20 times faster? The answer to this question is a fun word: Biomechanics. The inner ear, known as the cochlea, consists of 3 fluid-filled tubes that spiral up to a blind end. Typically this is drawn as its own snail-shaped solid, but in reality the cochlea is a fluid filled cavity with the temporal bone of the skull. Sounds enter the cochlea through the oval window into the bottom of the 3 tubes. Sound waves travel to the top of the spiral of the cochlea then back down through the top tube and vibrate against the oval window. This top tube and bottom tube mainly play the role of sound conductor; the nervous tissue is all sitting within the middle tube.

Inner Ear
A rough sketch of the cochlea with the spiral stretched out into a strait line. Sound waves enter through the oval window.  Higher frequency sounds are propagated through the basilar membrane toward the start of the cochlea whereas lower frequency sounds pass through the basilar membrane toward the end.

Between the bottom tube and the middle tube is what is called the basilar membrane. This membrane has changing mechanical properties all the way down its length so that at the entrance of the cochlea it vibrates strongly in response to higher frequency sounds and at the top of the spiraled cochlea the basilar membrane vibrates best in response to lower frequency sounds. Nervous cells in the inner ear respond to the movement of the fluid within that middle tube so when the basilar membrane vibrates more strongly due to higher frequency sounds at the base of the cochlea, the nervous cells in that region know to tell the brain that you just heard a high pitched sound. And when nervous cells respond to movement in the upper part of the cochlea, the brain knows you just heard a lower pitched sound.

1251px-Organ_of_corti.svg
An illustration of what is known as the organ of Corti. This rests on top of the basilar membrane and plays a large role in our hearing. Image source: Wikipedia.

The basilar membrane is a really cool example of biomechanics in of the body solving complex challenges, and Georg von Békésy even won the Nobel Prize in Physiology or Medicine in 1961 for his work in in demonstrating its function. However, movement of the basilar membrane alone does not completely account for how well the hearing of humans and other mammals can tell the difference between two frequencies. This is the role of the organ of Corti, which I will introduce in a later blog post. Obviously, there is a bit more to the inner ear than I have explained here, and this is an exciting question for which scientists are still exploring answers!

 

Are Koopas Sex-Role Reversed?

By: Andrew Anderson (@AndersonEvolve)

In my previous post, I presented what koopas are and what forms (phenotypes) are considered the same species.  Of course, after the post I found this page, which lists what character types are within the koopa species complex.  The bad news is I missed a couple—I had no idea there were so many Mario games. The good news is the ones I designated as koopas are on the list and honestly it’s all made up anyways.  In addition, the groups I failed to include aren’t necessary to determine the parental and mating system of koopas.

The first question—what kind of parental care do koopas have?— is straightforward in its answer:  male brood care. Running down the list of familial relations and we see Koops has a father, Koopley, Kolorado has a father, Koopa Krag is a grandfather, and Bowser is just a prolific father.  Notice something missing? No mention of mothers. Even Baby Bowser’s caretaker is Kamek. Now we can debate as to how dedicated these fathers are since most of the time the father is absent, but in most cases the father has some interest in the well-being of his children.  We do know female koopas exist, so why aren’t they seen with offspring? Quite simply, females don’t participate in broodcare, leaving males with those duties. It is true we can’t sex most koopas, but I think that’s because they haven’t reached maturity yet and address that below.  Such a conclusion means that the Koopa Troop is sending young koopas into battle that Mario mercilessly pounds on.

The next question—which sex is under stronger sexual selection (more intense competition for mating and producing offspring)?—is a bit more challenging to answer.  There has been some discussion among biologists about how best to measure and determine sexual selection, but this paper has a good description and mathematical explanation for those who want a little more. I’ve gone over the various things known about koopas and honestly can’t come to a certain conclusion, so I’ll just lay out the evidence for both sides.

Typical sex-roles–Stronger Male Sexual Selection

There are three pieces of evidence that supports the typical sex-roles for koopas.  The first is the variance in reproductive success. Bowser has a bunch of kids, while the other known fathers seem to just have one.  Unequal reproductive output creates opportunities for sexual selection. Bateman proposed that males mating with more females lead to more offspring, but more matings does not produce more offspring in females.  A quick glance across all of Bowser’s kids from Mario 3 and there is a wide variety of forms, suggesting that they may have different mothers. There seems to be some male koopas who mate A LOT, some who mate a little, and some who don’t mate; a clear indication that males could be under sexual selection.

hairkoopa
Male koopas with maturation (secondary sex?) trait of hair. Notice the prolific Bowser’s luxurious locks. Left:  Koopley–Koops father, Top:  Koopa Krag, Bottom:  Bowser

The other evidence is koopas seem to have secondary sexual characteristics.  Look at the known fathers in koopa-dom and the non-fathers. Notice that the fathers have hair, I propose that hair is a trait that shows the males are ready to mate, much like antlers on deer.  Now, there are koopas with hair that don’t have any (known) offspring, but it could be that they are in mating condition but have not found a mate or produced any offspring yet. The more important aspect is there are no successful males without hair.  While there are other traits, like a spiked shell and horns that Bowser has, no other fathers have them and there are other koopas that posses spikes and horns without children, so these traits may not play a role in reproductive success (Bowser is attractive because of his long flowing hair).

immature
Koopas without hair and no known offspring. None of these koopas are mature.

Lastly, the dearth of females suggests the sex ratio is tilted towards males.  In cases where one sex is rarer than the other,the rare sex is able to be choosier about their mate, putting selection on the majority sex.  In most species the split of sexes is 50/50 so the better descriptor is Operational Sex Ratio (OSR) and it counts only individuals who are reproductively available. If one sex takes longer to mature or is engaged in broodcare/pregnancy, it doesn’t count.  Even though male koopas take part in broodcare, Bowser demonstrates that they can brood from multiple females so they aren’t removed from the OSR calculation (something that is common in fishes).

Reversed Sex-Roles: Stronger Female Sexual Selection

So while I have assumed Bowser has lots of kids from multiple females, it is currently Nintendo’s stance that he only has one child–Bowser Jr.  Personally, I disagree since the Koopa Kids and the Koopalings have been called his children in previous games. In any case, if Bowser only has one child then the variance in offspring across males drops precipitously.  Now the number of children any male koopa has is either 1 or 0, which isn’t much variance. In this case, the males don’t seem to get more offspring from multiple females (i.e., little opportunity for sexual selection). This doesn’t mean females are higher, but it certainly undercuts the evidence for males.

300px-Koopalings_-_New_Super_Mario_Bros_U
Koopalings.  Notice the wide variety of forms.  It seems unlikely that all were made from the same pairing.

Alternatively, Bowser could be cuckolded with the possibility he was given all his children by one female although not all of them he sired.  This is known to happen in some polyandrous (multiple males one female) mating systems such as sandpipers and jacanas. The result is Bowser mated with one female, who mated with multiple males, that gave him seven eggs (the Koopalings) for him to care for.  Certainly if you were to look at all the Koopalings many of them bear little resemblance to the King of Koopas. Without genetic tests, we have no way to know the true parentage and thus the mating system for sure–although it would make for a great Maury episode.

female
Female Koopas.  From Left to Right and Top to Bottom:  Pom Pom, Koopie Koo, Wendy O. Koopa, Lakilulu, Kammy Koopa, Holly Koopa, Kylie Koopa.  Notice the females with hair either have a mate or are considered mature. 

Just as reproductively active male koopas have hair, it appears females have the same trait to indicate maturation.  Take a look at the 7 named female koopas: Koopie Koo, Lakilulu, and Pom Pom all have boyfriends and have hair. Holly, Kylie, and Wendy O Koopa are not shown with love interests and do not have hair.  Kammy Koopa has hair and has no shown boyfriend, but given her apparent age she is likely mature. Since both males and females have hair it can’t be a secondary sex trait (it has to be different between the sexes to be a sex trait), but it can be a sign of maturation.  

Both Lakilulu and Koopie Koo are extremely protective of their boyfriends, perhaps this is similar to mate guarding if females are under stronger selection.  While Koopie Koo’s boyfriend, Koops, is not mature (no hair), she encourages him to adventure and come back a stronger koopa. It may be she has secured a mate and wants to be sure he is ready to provide care, a long term strategy if ever there was one.  So why are the female so protective of their mates? We can go back to the operational sex ratio; if males are engaged in brood care and not many are mature yet (very few have hair) then there are less males than females available for mating thereby forcing females to protect the mates they have found.

I would like to point out that it is entirely possible, and even not uncommon, for both sexes to experience sexual selection.  When we talk about the roles though, we are asking which sex undergoes greater selection than the other as that can drive how evolution works.  While I’m not certain, I would like to think the females are under stronger selection; but then again I work with reversed species so I’m biased.  What do you think? Sound off on our twitter @SciSidequests!!