Did you know that, on average, lefties go through puberty later than righties? In a
study of 713 females and 467 males, researchers found typical markers of puberty, such as
menarche and the onset of body hair, tended to come later for left-handers. One of the
authors of that study, Dr. Stanley Coren, subsequently suggested in a letter to the
editors of JAMA: The Journal of the American Medical Association, that this could somehow
be related to lefties’ slightly smaller stature, on average.
Hi, I’m Erin McCarthy, editor-in-chief of MentalFloss.com, and late-blooming lefties
comprise just the first of many pivotal facts about puberty that I’m going to share with
you today. From unpredictable cows to insects basically born pregnant, we’ve got a lot
of ground to cover. Let’s get started.
Lefty or righty, the average age young people go through puberty in western cultures has
been dropping for decades. Dr. Hector O. Chapa, clinical assistant professor of obstetrics
and gynecology at the Texas A&M College of Medicine, estimates that, quote, “at the
turn of the 20th century, the average age for an American girl to get her period was
16 or 17. Today, that number has decreased to 12 or 13 years.” While the markers for
puberty in boys can be a bit harder to track, there is some evidence that a similar, if
slightly less dramatic, trend is occurring amongst males, as well.
There are many theories offered to explain this change in the onset of puberty, from
exposure to synthetic chemicals to higher levels of stress, to increased rates of childhood
obesity. Some argue that the historical data is actually skewed by poor nutrition and high
rates of disease during the 19th century, which would have made delayed puberty more
common at the time.
These questions might just be academic, if it weren’t for the fact that early onset
puberty is associated with some troubling predispositions. Girls who go through what’s
known as “precocious puberty” are at greater risk of breast cancer, ovarian cancer, and
obesity later in life. This might be because both the early development and the later diseases
are related to body mass index at a young age, but more research is needed.
Just as troubling are psychosocial issues related to early puberty, such as a greater
likelihood of developing depression and substance abuse issues. Children who enter adolescence
at younger ages may have difficulty fitting in with peers, face societal expectations
that don’t match their true cognitive ages, and tend to hang out with older peers engaging
in risky behavior at higher rates. These distressing facts shouldn’t cause parents or early-onset
adolescents to panic too much, though. As University of Florida psychologist Julia Graber
says, quote, "Even among early maturers, the vast majority will get through puberty fine.”
It’s also worth pointing out that what qualifies as an “early maturer” is a bit hard to
pin down. There’s a fairly large variance in the age healthy adolescents go through
puberty. Duke Health, which integrates medical schools and organizations connected to Duke
University, says that for girls, the beginning of puberty could be anywhere from 8 or 9 to
13 years old. For boys, anything from 9 to 14 wouldn’t necessarily be considered atypical.
Bottom line: if you or your child went through puberty on the early side, it’s probably
not a huge deal, but for researchers and health professionals it is a trend that’s worth
keeping an eye on.
We know that children’s voices get deeper during puberty, especially boys, whose vocal
pitch can drop by a whole octave. And while we can make a good educated guess about the
evolutionary reason for this change—it may help attract mates or intimidate competitors—it’s
not completely clear. It’s easier to explain the physiological changes that lead to that
deeper voice. Testosterone causes the cartilage in the voice box to grow larger and thicker;
the vocal cords grow, and therefore vibrate at a lower frequency. The larynx shifts, which
can lead to a more protruding “Adam’s Apple,” and the growing bones of the face
create more room for the voice to resonate in. All this leads to a deeper voice, and,
for some unlucky adolescents, an adjustment period where the voice squeaks or cracks due
to the uneven growth of the various body parts involved.
One group that didn’t have to worry about those vocal cracks were the castrati. Typically
emerging as sweet-singing youth in church choirs, castrati were castrated before puberty
to maintain their high-pitched voices. Though it was never legal, there seems to have been
a tacit acceptance of the practice for centuries, as castrati sang for audiences far and wide,
including Pope Sixtus V in the Sistine Chapel.
And while the practice certainly seems dangerous, if not cruel, from our modern vantage point,
it does reveal some interesting insights into human development. For example: you might
expect castrati to be quite short, given the reduced levels of testosterone present during
adolescence that generally help promote growth spurts. But while some castrati were short,
they were generally known for being quite tall.
To understand why, we need to know a bit about human anatomy. Epiphyseal plates, often referred
to as growth plates, are layers of cartilage containing growing tissue found on either
side of long bones. In early adulthood they ossify and we basically stop growing. Because
typical hormone production was interrupted in castrati, their growth plates never “closed.”
Testosterone plays a key role in the hardening of that growing tissue, and it wasn’t present
in sufficient quantities to “close” the plates. This means that many castrati kept
growing for longer than they would have had the surgical intervention never taken place.
One surprising side note to put this in historical perspective: there’s actually a recording
of the man who might be the last castrato. Alessandro Moreschi didn’t retire until
1913, and there’s audio of him from 1902 available on YouTube today.
Somehow both less beautiful and less horrific (though maybe not for the young people experiencing
it), we have acne. Sexual hormones known as androgens play a big part in its prevalence
amongst young people going through puberty. These hormones can stimulate the sebaceous
glands, leading to more oil production, which can clog pores and provide a source of food
for acne-causing bacteria.
Fun fact: the medical term for a blackhead is comedo, which comes from the Latin word
for glutton. 18th century doctors weren’t blaming blackheads on a diet of greasy pizza
or French fries (any connection between diet and acne, by the way, has limited scientific
proof thus far). No, the gluttons in question here were the TINY PARASITIC WORMS that people
once mistook blackheads for.
Humans aren’t the only ones that go through puberty. And for some animals, age isn’t
the primary determinant for when it occurs. According to the BBC’s Science Focus, a
rodent native to Cuba reaches sexual maturity based on its weight. Female Cabrera’s hutia
apparently reach sexual maturity right around 3 quarters of a pound, while males do so at
about 2/3rds of a pound.
And not every animal needs to go through sexual maturation to produce offspring. Aphids are
“essentially born pregnant,” according to Ed Spevak, curator of Invertebrates at
the St. Louis Zoo. While the insects can turn to sexual reproduction when environmental
factors necessitate it, they can also reproduce asexually, resulting in new females hatching
with eggs already growing inside them. Kind of terrifying. Cool, but terrifying.
On the opposite end of the spectrum are animals that take way longer than us to reach sexual
maturity. Researchers studied a number of Greenland sharks and estimated that the slow-developing
fish they looked at had lived upwards of three to four hundred years. They also estimated
that females might not reach sexual maturity until around 150 years old. Given that Greenland
sharks are apex predators who have been found to have consumed everything from polar bear
jaws to entire reindeer carcasses, the prospect of their 130-year-old adolescent mood swings
is pretty frightening. Kinda makes me wonder why they weren’t featured in Shark Night
3D. ...Was I the only one who saw that movie?
Just like humans can experience greater mood swings and rebel against authority figures
during puberty, research from a consortium of universities in the UK reported evidence
that dogs become less obedient during adolescence. Direct observation by researchers revealed
that 8-month-old dogs took longer to respond to the “sit” command than 5-month-olds.
Dog owners who answered a questionnaire also indicated that dogs in puberty were harder
to train. Interestingly, the disobedient doggie behavior was particularly associated with
interactions with the dog owner. When a stranger was giving the commands, the dogs were more
obedient. One of the researchers conducting the study, Dr. Naomi Harvey, likened this
to “taking it out on your mum.”
It’s not just dogs. A study in the Royal Society Open Science determined that cows’
behavior is less predictable during puberty. And if you were hoping that science writers
could resist suggesting that adolescent cows get “mooooody,” you’re going to be udderly
disappointed. I never said I was above a bad pun!
Many types of birds develop elaborate plumage during puberty, presumably, at least in part,
to help them attract the opposite sex and reproduce. This can play out in different
ways, though. While only male birds of paradise are known for their brilliantly colored feathers,
a diet rich in carotenoids means that flamingoes of both sexes can turn a bright shade of pink
by the time they hit sexual maturity. The bright colors are probably still serving a
reproductive function for flamingoes, though; for either sex, vibrant pink feathers can
advertise a healthy bird.
Two more fun facts before we get out of this rabbit-hole...no, flaming-hole, of colorful
plumage. A handful of bird species, like sandpipers and button quail, are known to have reversed
sexual roles. Males incubate eggs while females defend their territories and fight for access
to males. As we might expect, then, it is the females of these species which tend to
develop more ornamentation in sexual maturity. They’re the ones with more competition and
more incentive to stick out.
Even cooler: some birds which seem, to human beings, to be dull or monochromatic, actually
undergo a similar process in which they develop breeding plumage. It’s just that we can’t
see it. Unlike humans, most birds have four, not three, types of cone cells in their eyes,
and most can see ultraviolet light, which we generally can’t. That means there’s
reason to believe that birds perceive color and light differently than we do, and the
evidence seems to back that up. In one study, female European starlings were shown to prefer
males with greater amounts of ultraviolet reflectance (as measured by spectrophotometers).
So even when we can’t see it with the naked eye, birds may be undergoing visual changes
to help themselves stand out in the reproductive market.
19. Male red colobus and olive colobus monkeys change their appearance during puberty, but
it’s not to help them reproduce—or at least not immediately. As they’re just reaching
puberty, males develop “pseudo-swellings.” The skin around the anus swells up, as if
to mimic the appearance of a sexually mature female. This physical change eventually disappears.
One theory for this development is that it helps protect the young males from being kicked
out of their pods by the mature, dominant males. Pretty clever!
Let’s switch gears back to puberty in humans. Throughout this video, we used the terms boy
and girl, male and female to refer to “biological sex.” Those terms are broadly used when
it comes to puberty and sexual maturation. And when looking at human beings on a population-wide
level, we can often track differences in development between people with XY chromosomes and those
with XX. But even the concept of “biological sex” isn’t always so simple. Case in point:
There’s a small, geographically-isolated community in the Dominican Republic with particularly
high incidences of a rare condition affecting sexual development.
Basically, babies are born with apparently female sex organs and facial features, but
around puberty they develop testes, a penis, and more typically male physiognomy. These
adolescents are known as “guevedoces,” which roughly translates to “penis at twelve”
or “testes at twelve.” Many, but not all, end up living out their adult lives as men.
Research conducted by Dr. Julianne Imperato-McGinley revealed that guevedoces have XY chromosomes,
but are deficient in an enzyme which helps to turn testosterone into dihydrotestosterone
and leads to the development of biologically male sex organs. It’s an example of an increasingly
understood scientific principle, that knowledge of our chromosomes isn’t sufficient to understanding
sex. To say nothing of XXY or single-X individuals, it’s important to understand the role genetic
signals play in development, as well. (It’s also worth pointing out that we’re talking
about “biological sex,” even if it isn’t always a simple concept, rather than gender
identity of expression, which deals with how a person perceives themself and wishes to
be identified).
In the case of guevedoces, when a second surge of testosterone occurs during puberty, the
body responds, but some meaningful differences persist between them and males who develop
their sex organs in utero. One such difference is a tendency for guevedoces to have small
prostates, which led to a pretty fascinating bit of medical history. After hearing about
research into guevedoces in the 1970s, Roy Vagelos was intrigued. Vagelos was the head
of research at the pharmaceutical company Merck at the time, and he knew that enlarged
prostates are a relatively common affliction in older men. Merck would go on to use the
insights gleaned in Imperato-McGinley’s research to develop finasteride, a drug that
continues to be used today to treat enlarged prostates. It’s also prescribed to treat
male pattern baldness, sometimes under the brand name Propecia. The story of finasteride
is still being written: there have been lawsuits about the drug’s side-effects in recent
years.
OK, so we can recognize that sex is more complicated than we might have grown up understanding,
but on a population level, we can still glean average differences between males and females.
Here’s one interesting, if not fully understood, difference that arises between the sexes during
puberty. Researchers analyzed brain scans of about 150 boys and 150 girls at various
stages of puberty. They looked specifically at regions of the brain potentially associated
with a risk of mood problems in adolescents and noticed an interesting divergence. While
the boys in the study showed a 6.5% increase in functional connectivity between the relevant
areas of the brain during puberty, girls actually showed a 7.2% decrease of connectivity in
the same areas. More research is needed to determine whether these contrasting developments
might help explain differing rates of mood disorders, like depression, in adolescent
boys and girls, but it’s an interesting window into how much more we have to learn
about human development and the brain.
You’ll often hear that everyone goes through puberty, but that’s not entirely true. People
with Kallmann Syndrome, a rare genetic disease affecting hormone production, can have delayed
or even absent puberty if they don’t receive treatment.
But for those that do go through puberty, especially any adolescents out there watching
this video, my final fact is this: puberty, when it does come, eventually passes. So much
of what they say about puberty is true: it can be awkward, unsettling, and if the kids
at my school were any indication, pretty smelly at times. You couldn’t pay me to go back
to age 13, and that’s taking into account the fact that I had reliable access to Dunkaroos.
So while adolescence expresses itself in infinite ways, and puberty can take different young
people different amounts of time to go through, know this: it will end, eventually. And by
the way, Dunkaroos are back, and when you’re an adult you can eat them at any time of day.
Our next episode is about our home. No, not my apartment, although I have compiled way
too much information about this place over the last 5 months. No, it’s all about Earth.
If you have a cool fact about our planet, drop it in the comments for a chance to be
featured in that episode. It’ll be up on September 16th. We’ll see you then!