Why do voices crack and pimples flare up during puberty? When do boys go through puberty, and when do girls experience it? And how do other species experience puberty? Read on for pivotal facts you should know about puberty, adapted from an episode of The List Show on YouTube.

1. 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.

2. 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 “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 among males as well.

3. There are theories about why puberty is occurring sooner.

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.

4. 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.

5. There are psychosocial issues related to early puberty, too.

They include 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, "Even among early maturers, the vast majority will get through puberty fine.”

6. 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.

7. A number of anatomical changes lead to lower voices during puberty.

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.

8. Because of altered hormone production, Castrati were often tall.

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.

9. Acne is a big part of puberty.

Sexual hormones known as androgens play a big part in acne's prevalence among 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. Eighteenth- 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). The gluttons in question here were the tiny parasitic worms that people once mistook blackheads for.

10. puberty isn't just a human thing.

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 three-quarters of a pound, while males do so at about two-thirds of a pound.

11. 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.

12. Some animals that a very long time 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 adolescent mood swings is pretty frightening.

13. Dogs are less obedient during adolescence.

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.”

14. Cows experience puberty, too.

A study in the journal 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.)

15. Many types of birds develop elaborate plumage during puberty.

It's presumably, at least in part, to help them attract the opposite sex and reproduce. This can play out in different ways: 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.

16. A handful of bird species are known to have reversed sexual roles.

They include some sandpipers and button quail. 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.

17. Some birds develop breeding plumage invisible to the human eye.

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.

18. 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!

19. "guevedoces" reveal the complexity of "biological sex."

Throughout this article, we've used the terms boy and girl, male and female to refer to “biological sex”; these 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 incidence 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 12” or “testes at 12.” Many, but not all, end up living 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 popular scientific concept: 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 or expression, or how a person perceives themselves and wishes to be identified.)

20. A common drug was developed, in part, from research into guevedoces.

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.

21. Male and female brains develop differently during puberty.

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 percent increase in functional connectivity between the relevant areas of the brain during puberty, girls actually showed a 7.2 percent 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.

22. You’ll often hear that everyone goes through puberty, but that’s not entirely true.

People with Kallmann Syndrome, a rare genetic condition affecting hormone production, can have delayed or even absent puberty if they don’t receive treatment.