10 Billion R Us?

How many humans can fit on Earth?

It's not a logistical question (heel-toe, or nose-to-nose?). It's a question of resources. Every other species on the planet lives with a natural limitation. Say you're a snail that lives in a scummy pond and eats algae. There's plenty of algae available, and soon there are snails everywhere. But as your population booms, and you start bumping shoulders with other snails more often, you find there's not enough algae to go around. A lot of snails die or fail to reproduce. The algae population rebounds while the snail population fades, and the cycle continues.

Humans have beaten this system--for now. We clear forests, irrigate deserts, and blow the tops off of mountains to get the resources we need. This means we've stretched our upper limit. And according to a new report from the United Nations, there's no end in sight.

Previously, the UN had predicted that the world's population would reach 9 billion during this century, then level off. The projected upper limit wasn't because we were due to run out of food or space, though. Demographer John Bongaarts says in an interview with ScienceInsider that the UN's projection, which they revisit every two years, comes from a combination of mortality rate and fertility rate: how quickly are people dying, and how many babies are they having?

As nations invest money in family planning and women gain access to birth control, the fertility rate (how many children the average woman has) tends to decline. Bongaarts says that in Africa, where most of the world's "high-fertility" countries are, there has been a lack of investment in family planning programs recently. The Bush administration, for example, cut funding for contraceptives to African nations. Additionally, Bongaarts says AIDS hasn't affected population growth in Africa quite as researchers expected it to.

The result is that the UN now expects the world's population, rather than peaking around 9 billion, to continue on to 10 billion by the end of this century. And there's no peak--the population will still be increasing in 2100.

To come up with their projections, the UN says, "Account is taken of past fertility trends in a given country plus the past experience of all other countries in the world. The model was used to generate 100,000 trajectories for future fertility for each country." Out of those 100,000 projections for each country, they used the median values to make an overall projection.

The UN's model assumes that over time, the low-fertility and medium-fertility countries will level out to their "replacement rate"--that is, each generation will have enough children to exactly replace itself. For a country where most people make it to adulthood, that means 2.1 kids per mom. Low-fertility countries include most of Europe, as well as Iran, Brazil, and (by design) China. Countries with intermediate fertility include India, Mexico, Egypt, and the United States. High-fertility countries include 39 African nations.

A small wobble away from the predicted fertility rates could have a huge impact on the global population. The UN report points out that if they've overestimated global fertility by half a child per woman, the population will peak at 8 billion and swing downward again to 6 billion by the end of the century. But if they've underestimated by a half a child, we could hit almost 16 billion by the year 2100.

Mortality is important, too. It's assumed that life expectancy will continue to increase globally. The highest-fertility countries also happen to be some of those with the lowest life expectancy, thanks to killers such as HIV and malaria. Currently, life expectancy among all high-fertility countries is just 56 years. (Low-fertility countries, where people presumably have the best access to health care, have the highest life span: 74 years, compared to 68 years for those of us in the middle group.)

All these projections, too, hinge on there not being a global cataclysm in this century that removes a large portion of our population, as the Black Death or the 1918 flu pandemic did. Just because it's the twenty-first century doesn't mean this risk is gone. Viruses are mutating all the time, bacteria are developing resistances to most of our antibiotics, climate change is altering the life cycles and habitats of animals that carry diseases--and, of course, people are getting closer together.

What's the lowest global population you remember? The world reached 3 billion people in 1959, and 4 billion in 1974. In 1987 we reached 5 billion. I remember hearing from Bill Nye (the Science Guy) that the population was almost at 6 billion; it reached that number in 1998. We're currently expected to hit 7 billion this fall, 8 billion in 2025, and 9 billion in the 2040s. Get ready to bump some shoulders.

Does. Not. Compute.

If you live in a city, you're familiar with the verbal ramblings of schizophrenics. You may have heard their speeches directed to no one while they're walking down the street toward you or waiting for the train. I used to occasionally share my afternoon commute with a man who liked to stand near the back of the bus and deliver a continuous and incoherent Shakespearean-style monologue, complete with accent and extravagant hand gestures.

One distinguishing feature of schizophrenic speech is disorganization. A sentence might start out normally enough but veer into nonsense. Phrases don't follow one another. Schizophrenia is also characterized by delusions--patients may believe that they're being persecuted, or that they have special powers. The causes of schizophrenia are still mysterious. But researchers at the University of Texas, Austin, have used computers to model several theories of the disease. And one of these models produced computers that talk like a schizophrenic person.

The "computational patients" studied were different iterations of a computer model called DISCERN, designed by professor Risto Miikkulainen. The model is a "neural network" that's meant to simulate how a human learns and recalls a story. Different parts of the network mimic the tasks our brain performs to understand, store, and remember words and sentences.

Miikkulainen and his graduate student Uli Grasemann fed very 28 simple stories into their computer model. Half the stories were in the first person; for example, "I was a doctor. I worked in New York. I liked my job." (I told you they were simple.) The other half were crime stories told in the third person; for example, "Tony was a gangster. Tony worked in Chicago." (Hey now...) It took thousands of repetitions to teach the computer network the stories.

They also taught three slightly less simple stories to actual human subjects, both healthy and schizophrenic. A week later, they asked the subjects to recall those three stories, and recorded the types of errors they made.

To create mentally ill computer patients, the researchers introduced a variety of errors into the DISCERN network. They modeled eight different cognitive problems that have been suggested as factors in schizophrenia. After teaching their impaired computer patients the set of simple stories, it was quiz time. The network was prompted with the first part of a story and asked to complete it. The researchers analyzed the types of errors the computer patients made, then compared them to the human patients.

Out of the eight schizophrenia models, just one had caused the computer to tell stories that sounded like the schizophrenic patients'. The computer got derailed, starting one story and drifting into another story, as did the schizophrenics. The computer had a similar tendency to mix up the characters in the stories, including confusion between first-person and third-person stories. A story about Tony the Chicago mob boss, for example, might become a story about Mary the mob boss.

The underlying error in this pseudo-schizophrenic computer simulation was what the authors called "hyperlearning." Ordinarily, scientists believe, we use a technique called prediction error to help us learn new information. We constantly make predictions, and whenever those predictions don't come true--that is, when reality doesn't match up with what we expected--we take notice and form new associations. This process is thought to involve the neurotransmitter (brain signaling chemical) dopamine. If our brains aren't prudent about this process, though, they assign too much importance to every new piece of information, and we create irrelevant associations. This is why the process is thought to be involved in schizophrenia; every detail may become meaningful to schizophrenics, and connections may appear everywhere in their speech. When the researchers adjusted the computer network's settings so that it relied too heavily on prediction error--it learned too much and ignored too little--the computer's stories sounded the most like the schizophrenic patients'.

In addition to matching the disorganization of the schizophrenic patients' stories, the computer network also provided a tantalizing hint of delusion. Swapping first-person narratives for third-person ones, it put itself at the center of scenarios it didn't belong in: "I was a mob boss."

Computers are, of course, not people. A human brain, no matter how ill, is unspeakably more complicated than a computer model. And while this computer simulation successfully modeled some speech-related symptoms of schizophrenia, it didn't address other symptoms. Still, it gave intriguing hints about prediction error and dopamine that can be used for further study in humans. That information might even lead to new treatments for the people riding your bus.

Happy Blogday!

A year ago today, I started this blog so I would have a place to vent my excitement about happenings in the science world. It's been a fun trip so far--I've learned how relativity affects your body, faced off with climate change deniers, debunked Shape-Ups, and found out why eating dirt might prevent depression. Every day brings fresh science news to my inbox, which means new stories to share and titles to pun. (I really am sorry about the puns. I work in children's publishing; it's hard to turn off.)

If you want to get me a birthday gift, you could add yourself to this blog's list of followers so my eight friends down there in the corner don't look so lonely. Or use the email button to share an Inkfish story with a friend. Comments and tips are always welcome, too.

Meanwhile, I got you a little something: Four of my favorite inkfish-related videos. Thank you for reading!

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At the bottom of the Gulf of Mexico (before the oil), a camera at a Shell oil-drilling site captured this alarming visitor. It's a Magnapinna squid, observing the drilling equipment with a posture that's hard not to imagine as menacing. 

It's said that an octopus can squeeze through any hole bigger than its eyeball. You can almost hear this one go Mmmrff! 

Cuttlefish are masters of disguise, for real. At the Marine Biological Laboratory in Woods Hole, researcher Roger Hanlon shows off some pretty impressive cuttlefish tricks. The best part is when one cuttlefish holds out its tentacles in a valiant, but unsuccessful, effort to imitate a striped background.

This video is also from Roger Hanlon. Can you spot the octopus before it sees you? That's a trick question because I know you can't.

Eternal Sunshine of the Spotless Slug

In a creature much simpler than a human, scientists have figured out how to erase a memory. Sea slugs that had received repeated electrical shocks learned to expect them again--until researchers gave the slugs an injection that returned them to blissful ignorance.

The fellow above is Aplysia californica, a hefty sea slug that's shown here releasing its mysterious magenta ink. (I suppose that makes it an honorary inkfish?) Researchers at UCLA used a tankful of these quarter-pound slugs to test the hypothesis that a certain molecule allows the slugs to store long-term memories.

At the beginning of the experiment, researchers tested the slugs' baseline sensitivity by poking them in the hind end with a broom bristle. This causes a slug to retract its siphon, a straw-like structure near the tail, for a second or two. Then they "trained" the slugs by giving them five sets of electrical shocks to the tail over the course of 80 minutes. Afterward, the slugs had learned the lesson that touches near the tail are bad. (Come to think of it, that may have been the title of a movie we watched in my fourth-grade health class). Twenty-four hours after their training session, the slugs still remembered; they retracted their siphons for 40 or 50 seconds when poked with a broom bristle. The reaction was almost as strong 48 hours after the training session.

(Two days may not seem like a very "long term" over which to remember that you were recently tormented by scientists. But short-term memory only refers to the items that we hold in our minds on the order of seconds. Anything we hang on to for longer than that is considered to be in our long-term memory.)

And then it was time for some Men in Black mind-erasing action. The molecule the researchers were interested in is called protein kinase M (PKM). A few minutes after the 24-hour test, they injected some of the sensitized slugs with a molecule that interferes with PKM and prevents it from doing its normal job--which is, in case you asked, adding phosphate groups to other proteins.

The results were straightforward and striking. At 48 hours, when the other slugs were still extremely reactive to being poked in the tail, those that had been injected with the PKM blocker were completely back to normal. Their siphon-retracting reflex was exactly what it had been before their training. The memory of the electric shocks they'd received seemed to be gone.

The scientists even tried reminding some of the slugs of their training. At 96 hours, they gave them one more set of shocks (as opposed to the five sets in the initial trial). The slugs seemed unimpressed, showing no change to their reaction.

In another experiment, the researchers left the slugs alone for a whole week after their initial shock training. On day 7, the slugs were still sensitized from their training, withdrawing their siphons for around 40 seconds when poked. Some of the slugs were injected with a PKM blocker at this point, a whole week after the training session. The next day, those slugs' reactions were right back down to zero. The un-injected slugs, though, still remembered their shocks.

The researchers also experimented on individual slug neurons--one sensory neuron and one siphon-moving neuron--that they removed from the slugs and kept in a dish. Again, they found that blocking PKM prevented the siphon neuron from retaining its "memory."

So what is PKM doing to neurons that makes it so critical to long-term memory? New memories involve the growth of connections between neurons, and the authors think the ongoing activity of PKM might be necessary to maintain these structural changes. Without housekeeping by PKM molecules, the connections are lost.

Researcher David Glanzman, who led the study, believes that understanding these processes could lead, in the future, to targeting and erasing specific memories in humans. "Almost all of the processes that are involved in memory in the snail [or sea slug] also have been shown to be involved in memory in the brains of mammals," he said in a press release.

It's a spooky idea, but erasing memories might be of help in treating post-traumatic stress disorder or drug addiction. The process might even be reversed to treat Alzheimer's disease, which is currently incurable. Let's hope that when that day arrives, someone remembers to thank the humble sea slugs.


Image: Genny Anderson/Wikimedia Commons

Do Boys Drool?

Researchers in Austria have discovered that females are much better than males at noticing when a tennis ball rolling behind a wall appears to change size. In fact, males don't seem to notice at all. Don't worry, guys--I'm talking about dogs.

Corsin Müller and his colleagues studied 25 male dogs and 25 female dogs. Each dog was led into a room by its owner and allowed to play with a large and a small blue tennis ball; this let the dogs get familiar with the objects in the experiment. Then the owners walked the dogs out of the room and back in again. Owners sat in a chair--blindfolded, so as not to give any inadvertent clues to their pets--and sat their dogs between their knees. Meanwhile, a hidden experimenter watched the dog with a camera. When the dog was calmly looking in the direction of a low wall, the experiment began. The experimenter tugged on hidden strings so that a blue tennis ball rolled behind the wall. A moment later, a second ball rolled out from the opposite side of the wall. For half the dogs, the second ball was the same size as the first. For the other half, it was different (either a big ball changing to a small ball or vice versa). The male and female dogs were evenly divided between the same-size and different-size groups.

As the second ball rolled out, cameras around the room recorded the dog's reaction. The experiment used a principle that's common in psychology experiments done with baby humans. If a baby sees something it doesn't expect to see, the assumption goes, the baby will stare at that object for longer that it would otherwise. Infants learn "size constancy," the rule that things should stay the same size from one moment to the next, during the first year of their lives. If they see an object appear to change size, they stare at it.

Since dogs are kind of like infants, I guess, the principle can be transferred. So the researchers studied videos of the dogs' reactions to see how long they stared at the second blue ball. The result was striking: Male dogs looked at the ball for the same amount of time, no matter what. Size constancy, schmize constancy. But female dogs stared for significantly longer when the ball appeared to change size.

(I won't tell anyone if you want to take a couple seconds and secretly cheer for the female dogs. But be aware that a girl dog is going to look dumb later on in this story.)

Why might this be? It's always tempting to invoke an evolutionary explanation. A scientist interviewed by ScienceNOW (and not involved in the study) takes the bait, suggesting that female dogs have evolved to pay more attention to visual cues so they can keep track of their puppies. The problem with such an explanation is that you can never really know whether it's true. And in the case of human sex differences--say, a difference in innate math ability that hasn't been convincingly shown to exist--some people use them as excuses to cling to outmoded generalizations such as "men like things, women like people." (Yes, I'm talking about John Tierney.)

Müller doesn't speculate about an evolutionary basis for the difference he found between male and female dogs. Instead, he writes that the most likely explanation is a "by-product of other sex differences." In other words, male and female brains are sculpted differently by the hormones they receive during development, and affected differently by hormones throughout life--but not all of these differences have to be evolutionary adaptations.

The uninvolved dog expert in the ScienceNOW story does add, interestingly, that male dogs tend to be more scent oriented than female dogs, and are preferred for tracking. Could it be that male dogs, for whatever reason, depend more on their noses, while females depend more on their eyes?

One dog doesn't make for much of a sample size, but can still be (I think) an entertaining example: In this MythBusters video, the two hosts of the show wear highly detailed masks of each other's faces. They also swap clothes. Host Jamie Hyneman's dog, who is trained to run to her owner when asked, "Where's Jamie?" demonstrates her trick--and runs straight to Adam, who's wearing the Jamie mask. Pretty embarrassing for an animal with such a sensitive nose. To her credit, she then turns around, shakes herself and barks. "She's very confused," Adam observes. Tennis balls changing size is one thing, but humans changing faces is a challenge she's not equipped for.

Cephalopods in Space

The space shuttle Endeavour* is scheduled to blast off this afternoon at 3:47 EDT. It will be the second-to-last launch before NASA retires their shuttle fleet for good. The shuttle's commander, Mark Kelly, has spent much of the last few months at the bedside of his wife, senator Gabrielle Giffords, while she recovers from being shot in the head. It's been a long countdown, to say the least.

Now the shuttle is almost in the air, and along with its human passengers, it will be carrying some tentacled cargo: Squids in Space. No really, that's the official name of the project. (It's also referred to as Squid in Space, since the plural of "squid" is whatever you feel like.)

University of Florida scientist Jamie Foster is leading the project, which will study the development of squid embryos in the near-zero gravity of spaceflight. Her Squids in Space team also includes college students and high schoolers who, one hopes, appreciate that they are doing the coolest class project ever.

Foster's concern isn't really about the squid or squids, though; her research interest is bacteria. "Animals, including humans, are walking (or swimming) microbial ecosystems that interact daily with billions of microbes," she said in a press report. Humans' most important microbial interaction is with the bacteria that line our guts. But the Hawaiian bobtail squid, Euprymna scolopes, utilizes bacteria differently: as soon as it hatches, it absorbs a glow-in-the-dark species of bacteria from the water around it. The bacteria, Vibrio fischeri, lend the squid their bioluminescence. The squid houses the bacteria in a light organ on its body, and their glow obscures the squid's silhouette to predators lurking below.

What happens when squid embryos hatch into a low-gravity environment? "The effects of microgravity on these mutualistic associations are yet unknown," Foster said. In space, "Do good bacteria go bad?" When applied to cephalopods on a shuttle, the question has a bit of a goofy, Snakes on a Plane feel. But it's a question with serious health implications for human astronauts--whatever they're doing after the shuttles are retired.

What will they be doing, anyway? I've especially wondered about this since so many MUSE readers are space aficionados. Actually, "aficionado" might be putting it gently. One reader wrote online that she's observing today's launch by plastering her school with posters, wearing a special shuttle-launch outfit, and handing out cards to her fellow high schoolers. Some of our readers want desperately to be involved in the space program when they're older, even though they have no idea what that program will look like in 20 or 30 years.

So I'm hoping to pose the question to Mark Kelly himself and put his answer in the magazine. PBS is doing a live interview with the shuttle commander on Monday, and they're inviting questions from the public through their YouTube channel. Some of the questions with the most votes will be read during the interview (leaving out, I assume, the many iterations of "have u ever seen a ufo on ur flights, thx, anonymous"). I posted my question online, and if you'd like to vote for it, you can find it by clicking here, scrolling down to the search bar on the right-hand side of the screen, and entering "muse." Voting ends tomorrow night.

Godspeed, astronauts and cephalopods!

UPDATE: Un-shockingly, NASA has had to delay the launch until at least Monday due to technical problems. (But you should still vote for my question. Please?)

UPDATE, MAY 2: Thanks so much for your votes! My question ended up as the top rated in the "Students and Classrooms" category, which included some cute video questions from kids (who will probably get chosen anyway) as well as some hard-hitting journalistic questions ("just plz hear me out was the landing on the moon fake im relly wANT TO KNOW"). Meanwhile, NASA has again delayed the shuttle launch, this time until at least May 8. The astronauts have been sent back to Houston for more training. No word yet on the squid.

*Why the British spelling that makes your computer send up squiggly red underlines everywhere? The shuttle, christened in a nationwide student contest, is named after a ship sailed by James Cook. Even NASA has found the British affectation tricky at times; they misspelled the name of the shuttle on a giant launchpad sign in 2007.

Photo: NASA FSGC

The New Atkins

Do mothers who diet during pregnancy predispose their children to be heavier? That's the intriguing suggestion of a new study done in the United Kingdom, and the latest addition to a field known as epigenetics.

Epigenetics is a funny story (though not raucously so) in biology. In my first biology textbooks in middle or high school, an 18th-century French biologist named Jean-Baptiste Lamarck was the chump of the chapters on evolution. Before Charles Darwin formulated his ideas on adaptation and natural selection, Lamarck speculated that organisms could pass down traits that they'd acquired during their lifetime. For example, if a giraffe gradually stretched out its neck by reaching for high leaves, it could pass on that stretched-out neck to its young. Poor Lamarck's theory was tragically easy to disprove. If you cut off a mouse's tail, after all, it still has babies with tails. Genes are the units of heredity. We get our DNA from our parents, and nothing we do during our lifetime changes our DNA.

Except for when it does. These days, scientists are gaining appreciation for epigenetics: factors above the level of the gene that we can pass down to our children. Sorry for all the joshing, Jean-Baptiste. Genes are still the most important unit of inheritance, but we now know that the way our DNA is coiled and packaged inside our cells affects how its instructions are carried out. Factors in our lifestyles can cause changes in that packaging, and we can pass on those changes to the next generation.

Epigenetic change can also happen in utero. A human's DNA is locked in as soon as sperm and egg combine, but factors in the womb--our first environment--can have epigenetic effects on the DNA in our rapidly multiplying cells.

That brings us back to pregnant mothers. Previous studies have shown a connection between famine in utero and obesity in adulthood. If a mother can't get enough food, does she somehow mark the genes of her fetus so that it clings to calories in later life? A slow metabolism is helpful in a time of famine, but not so much in a time of fast food. Studies in animals have suggested that epigenetic changes are responsible for the link between a pregnant mother's diet and her offspring's adult weight.

Looking at DNA from the umbilical cords of newborns, the authors of the new study set out to find epigenetic changes that were linked to the children's weight many years later--and to their mother's diets.

The authors studied two groups of women and their children living in the UK. They interviewed the women when they were 15 weeks pregnant about their diets. When the babies were born, their umbilical cords were frozen. Six or nine years later, the researchers tracked down these children (between the two groups, there were 317) and measured their body fat. They also extracted DNA from the frozen umbilical cords.

The researchers looked at a certain kind of epigenetic change called methylation--a chemical tag attached to the DNA that makes it less legible to the cellular machinery. In both groups, they found that increased methylation around one particular gene in the umbilical cord DNA was strongly correlated with higher body fat when the children were older. This marker in the newborns' DNA, that is, partly predicted what their bodies would be like as six- or nine-year-olds.

Additionally, methylation at that same gene seemed to be linked to the mothers' carbohydrate intake in early pregnancy. Mothers who reported eating low levels of carbs had babies with higher levels of methylation. The increase in methylation was not linked to higher fat or protein intake, just to low levels of (reported) carbohydrates.

It's hard to rigorously measure the nutrients in a person's diet from their answers on a questionnaire. And even if the correlation is real, it doesn't prove the mother's diet is causing the methylation or the higher fat level in her child. Furthermore, we don't know whether these children will grow up to be overweight adults. But the study's suggestion is tantalizing. Could women who follow Atkins or South Beach, or another diet sold to them in the name of health, be predisposing their unborn children to a lifetime of weight struggles?

As researchers continue to solidify the links between our lifestyles and epigenetics, new diagnostic tests and treatments may become possible. A DNA test at birth could reveal a person's risk for various diseases based on epigenetic factors--instead of, or in addition to, their genetic risk. And scientists might even develop treatments that target DNA packaging and methylation.

Or maybe someone will just start selling another fad diet. Epigenetics: the new Atkins!