Autism, SHANK, and busy highways

Two autism studies in the news. I’ve summarized them at the Thinking Person’s Guide to Autism here. Let’s just say that the headlines, stories, and news releases have hyped yet again. Indeed, the title of this post should’ve been: “Headlines hype, news releases overpromise again in autism research.”

The worst offender is easily, “Proximity to freeways increases autism risk, study finds.” Um, no. The study found that autism rates are higher among people living within 309 meters of freeways. That in no way means that living close to a freeway increases autism risk. It’s a common, basic overinterpretation of correlation and epidemiological conclusions, but it’s really starting to get old. You can read more about the fuzzy definition of “freeway” and “major road” here.

Advertisements

Did humans and their fire kill off Australia’s megafauna?

Genyornis. Courtesy of Michael Ströck & Wikimedia Commons.

Timeline, 2005: For those of us who do not live in Australia (and live instead in, say, boring old Texas), the animals that live on that continent can seem like some of the most exotic species in the world. The kangaroo, wombat, and Tasmanian devil, and most of all, the platypus, are high on the list of the unusual and bizarre in the animal kingdom.

But modern-day Australia has nothing on the Australia of 50,000 years ago when humans first arrived from Java. They encountered huge kangaroos, marsupial lions, 25-foot lizards, and tortoises the size of a subcompact car. Yet, within 5000 years, many of these animals had disappeared permanently. And since the dawn of the study of paleontology, researchers have wondered why.

Of course, it’s our fault

Of course, humans feature as the culprits in most scenarios. Just as the first people in the Americas are usually blamed at least in part for the disappearance of the American megafauna, like mammoths or giant sloths, the first people in Australia have also been suspected of hunting these animals to extinction or exposing them to diseases that decimated the populations.

As it turns out, humans may be to blame, but not through direct destruction or disease transmission. Instead, it may be the mastery of fire, the turning point in our cultural history, that ended in the extinction of many species larger than 100 pounds on the Australian continent.

Fire!

Australia’s first people probably set huge fires to signal to one another, flush animals for hunting, clear paths through what was once a mosaic of trees, shrubs, and grasses, or to encourage the growth of specific plants. The byproduct of all of this burning was catastrophic to the larger species on the continent.

The fires, according to one study, wiped out the drought-adapted plants that covered the continent’s interior, leaving behind a desert of scrub brush. The change in plant cover may have resulted in a decrease in water vapor exchange between the earth and the atmosphere with the ultimate effect of ending the yearly monsoon rains that would quench the region. Without the rains, only the hardiest, desert-ready plants survived.

You are what you eat…or ate

How could researchers possibly have elucidated these events of 45,000 years ago? By looking at fossilized bird eggs and wombat teeth. Using isotopic techniques, they assessed the types of carbon present in the bird eggs and teeth that dated back from 150,000 to 45,000 years ago. These animals genuinely were what they ate in some ways, with some isotopic markers of their diet accumulating in these tissues. Because plants metabolize different forms of carbon in different ways, the researchers could link the type of carbon isotopes they found in the egg and teeth fossils to the diet of these animals.

They found that the diet of a now-extinct species of bird, the Genyornis, consisted of the nutritious grasses of the pre-human Australian landscape. Emu eggs from before 50,000 years ago pointed to a similar diet, but eggs from 45,000 years ago indicated a shift in emu diet from nutritious grasses to the desert trees and shrubs of the current Australian interior. The vegetarian wombats also appear to have made a similar change in their diets around the same time.

Or, maybe not

And the species that are still here today, like the emu and the wombat, are the species that were general enough in their dietary needs to make the shift. The Genyornis went the way of the mammoth, possibly because its needs were too specialized for it to shift easily to a different diet. Its teeth showed no change in diet over the time period.

The researchers analyzed 1500 fossilized eggshell specimens from Genyornis and emu to solve this mystery and to pinpoint human burning practices as the culprits in the disappearance of these megafauna in a few thousand brief years. Today’s aboriginal Australians still use burning in following traditional practices, but by this time, the ecosystems have had thousands of years to adapt to burns. Thus, we don’t expect to see further dramatic disappearances of Australian fauna as a result of these practices. Indeed, some later researchers have taken issue with the idea that fire drove these changes in the first place, with some blaming hunting again, and as with many things paleontological, the precise facts of the situation remain…lost in the smoky haze of deep history.

Complex amphibian responses to past climate change

Eastern tiger salamander: Ambystoma tigrinum, courtesy of Wikimedia Commons

We were like gophers, but now we’re like voles

Timeline, 2005: There is a cave in Yellowstone packed with fossils from the late Holocene, from about 3000 years ago. We can glean from this trove of stony bone how different taxa respond to climate change at the morphological and genetic levels and define and make predictions about the current response of the world to such changes.

The cave, which is in a sensitive area of the park off limits to visitors, houses the fossilized bones of rodents, wolves, amphibians, bears, coyotes, beavers, and elk, among others. This fossil cornucopia has yielded so much in the way of stony evidence that sorting it all is in itself a mammoth task. But two climatic stories have emerged from the samples it has yielded.

A global warming story…from the Middle Ages

The first story is about salamanders and climate change. No, it’s not a 21st-century story about global warming, but a Middle Ages story about a hotter planet. From about 1150 to 650 years ago, the earth underwent a brief warming period known as the Medieval Warming Period. During this time, the sea surface temperature was about a degree warmer and overall, the planet was much drier. This climatic anomaly was followed by what many climatologists call the Little Ice Age, a period that ended around 1900.

During the warm and dry period, animals in what would become Yellowstone National Park responded in ways that left clues about how animals may respond today to our warming planet. Amphibians make particularly sensitive sentinels of environmental change, alerting us to the presence of pollutants or other alterations that affect them before larger manifestations are detectable. And they even provide us evidence in their fossils.

Hot times, smaller paedomorphic salamanders

A group from Stanford excavated the fossils of Ambystoma tigrinum (the tiger salamander) from 15 layers at the Yellowstone site and divided them into five time periods based on their estimated age. They then divided the fossils again based on whether they represented the tiger salamander in its larval, paedomorphic, early adult, or later adult stages. The tiger salamander exhibits paedomorphism, in which the animal achieves reproductive capacity or adulthood while still retaining juvenile characteristics. In the case of the tiger salamander, this translates into remaining in the water, rather than becoming a terrestrial adult, and into retaining characteristics like frilly gills. The molecular determinant of whether or not an amphibian undergoes complete metamorphosis from juvenile to adult is thyroid hormone; when levels of this internal signal are low, the animal will remain juvenile.

The researchers found that during the medieval warming period, the paedomorphic salamanders became smaller than they were during cooler times. This outcome would be expected because when water is cooler, thyroid hormone levels will be lower, and the animal will continue growing as a juvenile.

Hot times, larger adult salamanders

On the other hand, the terrestrial adult salamanders were much larger during the warm period than during cooler periods. Again, this outcome would be expected because the heat on land would encourage faster metabolism, which would result in faster growth. The researchers found no difference in actual numbers between groups at cool vs. warm periods, but express concern that drying in Yellowstone today as a result of global warming might reduce the number of aquatic paedomorphs, affecting aquatic food webs.

From amphibians to gopher teeth

The same group also studied DNA from fossilized teeth of gophers and voles discovered in the cave. They found that during the dry period, gophers, who were stuck underground and isolated, experienced genetic bottlenecking, a reduction in diversity that persists today. However, the mobile, above-ground voles sought mates far and wide during the dry, warm period and actually experienced an increase in diversity. The lead researcher in the group compares early groups of isolated humans to the gophers, saying that they would have experienced a loss of diversity. But today’s population, with our ability to travel the globe with ease, is probably undergoing an increase in diversity since we’re able to mate with people a hemisphere away.

Placoderms had the "fun kind" of sex

Dunkleosteus, a Devonian placoderm. Pencil drawing, digital coloring, Nobu Tamura, http://www.palaeocritti.com. Obtained from Wikimedia Commons.

Timeline, 2008: From about 420 to 350 million years ago, the rulers of Earth’s seas were an unattractive-looking armored fish known today as the placoderms. This group, consisting of many species, were the bulldogs of the fish world, heavy-bodied with big ugly mouths full of protruding, potentially dangerous bony plates. Some of them were quite small, but a few species grew as large as 20 feet in length. They were the dominant vertebrate worldwide for about 70 million years.

Conventional scientific wisdom would say that these ancient fish reproduced the way modern representatives of ancient lineages do: external fertilization, the sperm fertilizing the egg with a little help from water. The wisdom was so conventional, in fact, that experts placed the rise of internal fertilization—delivery of the sperm into the female via an act of copulation—a good 200 million years after the placoderms swam the seas.

A catastrophe on the reef

In what is now Western Australia, something terrible happened about 380 million years ago in the shallow seas covering a coral reef: the oxygen that fed the reef suddenly plummeted, leaving the coral starved and unable to support the food web built around it. The outcome was a rapid, catastrophic loss of all of the species on the reef, including the placoderms. Thanks to stable plate tectonics and some good sediment coverage, these hapless animals remained preserved for the subsequent millions of years until a team of fossil hunters uncovered them. They now populate one of the most famous fossil finds in the world, the Gogo fossil sites, which are packed with perfect specimens of long-lost species.

The role of Sir David Attenborough, the world’s coolest naturalist

Among those perfect specimens—so perfect, in fact, that three-dimensional samples are available—is a species that now has the name Materpiscis attenboroughi. The name means “Attenborough’s mother fish” and requires a bit of explanation. Back in the late 1970s, Sir David Attenborough produced a wonderful nature and science series called Life on Earth. In the series, he highlighted the Gogo sites, and his interest led researchers to name the fish after him. But the first part of the name, the genus name Materpiscis, means “Mother fish.” Why? Because when this 10-inch fish died during that catastrophic reef loss, she died just before becoming a mother.

We know this because a couple of researchers working on her fossilized remains decided at the last minute to expose the fossil to one more round of acid treatment. They had pretty much decided to write her up as she was, which would have been plenty because of the preserved 3D perfection of her remains. But they agreed to that last treatment, which gently etches away layers of the fossil to reveal what lies beneath. They are glad they did, because what that last treatment exposed, inside of the adult fish, is a tiny, fossilized fish embryo, about a quarter of the size of its mother.

Eureka! Again, and again, and again

Anyone looking at that embryo, inside of that fish, might have had any number of “Eureka” thoughts in that moment. Eureka! It’s a fish embryo, 380 million years old! There aren’t that many of those lying around. But even more important, Eureka! It’s a fish embryo inside of the mother. That means that the egg was fertilized inside of the mother, where the embryo grew, nourished in her body, just as mammals do it. The embryo was even attached by a tiny, fossilized umbilical cord. A final Eureka! just might be that we can confirm the sex of this fish just based on the fact that she was pregnant when she died.

This just in: Sex is fun

The presence of an internally developing embryo in this placoderm sets the assumed evolutionary timing of internal fertilization back about 200 million years. No one would have guessed that these ancient, armored bulldog-like fish would represent the earliest-known internal fertilization. And the fact that fertilization was internal means that these animals must have copulated, the standard mechanism for getting sperm into the female to meet the egg. That recognition led one of the embryo’s discoverers to remark that this animal represents the earliest example a species engaging in “sex that was fun.”

Pitcher plant port-a-potty for the tree shrew

A pitcher plant (courtesy of Wikimedia Commons)

Timeline, 2009: As humans, we are a bit limited in our imaginations. For example, we’d probably never consider climbing onto the edge of a toilet seat and licking the sides while…um…employing the toilet for standard uses. Perhaps one reason—among many obvious choices—is that we’re not tree shrews living in the wilds of Borneo in Southeast Asia.

If you’re now envisioning tree-dwelling rodents enjoying the civilized development of having their own toilet, you’re not too far off. Borneo is home to a number of unusual relationships between species, but none may be stranger than the one that has developed between the tree shrew and the pitcher plant. The pitcher plant is carnivorous, and as its name implies, has a pitcher-shaped structure that it uses to trap its food.

The many uses of the pitcher plant

Normally, a pitcher plant growing on the ground is the perfect trap for hapless animals drawn to its minimal nectar output. For some species, they’re not a death trap but a place to brood offspring—one frog uses the pitcher plant to lay its eggs, where trapped, digested insects may provide some nourishment. The insects fall in because the funnel-shaped pitcher part of the plant has a slippery lip that acts as a deadly superslide for any insect that alights on it. Unable to gain a foothold, the animal slides helplessly into the plant’s interior, landing in a pool of digestive enzymes or bacteria that slowly break it down.

What does a pitcher plant do with digested insect? It does what any organism, plant or otherwise, does with its food—it extracts nutrients from it. One primary nutrient that plants (and everything else) require is nitrogen. This element is part of life’s important building blocks for DNA and RNA and the amino acids that make up proteins. Thus, to grow and reproduce, organisms must acquire nitrogen from somewhere. Some plants form a partnership with bacteria to get their nitrogen. Pitcher plants digest insects for it.

Unless no insects are available. While ground-growing pitcher plants in Borneo can subsist on available ants and other crawly critters, some pitcher plants grow on vines and trees, where ants are largely unavailable. In addition, mountainous environments are not known for harboring lots of ants, so the pitcher plant needed a new plan for getting its nutrients.

Nectar for nitrogen

The plan, it seems, was selection for making more nectar, reducing the slippery factor, and behaving like both a toilet and a food source for an abundant animal in the Borneo mountains, the mountain tree shrew. Using video cameras, researchers based at a Borneo field station captured one of the most unusual mutually beneficial relationships in nature: the tree shrew, while enjoying the abundant nectar uniquely produced by these aerial pitcher plants, also poops into the pitcher plant mid-meal. The plant, perfectly shaped for the tree shrew to park its rear just so while it eats, takes up the feces and extracts nitrogen from it. In fact, these pitcher plants may derive up to 100 percent of their nitrogen from the tree shrew poop.

Researchers think that this friendly relationship must have been in the making for a very long time. The pitcher plant opening is perfectly shaped and oriented so that the nectar collects just at the lip and the shrew must orient while eating so that the funnel-like pitcher collects any poop that emerges. The plant also has developed sturdier and thicker structures that can support the weight of a dining/excreting tree shrew, which isn’t much at less than half a pound, but quite a bit for a plant to support.

As odd as this adaptation may seem, it’s not unique. Ground-dwelling pitcher plants have formed similar mutually beneficial relationships with insect larvae that help themselves to some of the insect pickings that fall in. These larvae excrete any leftovers, and the plant harvests nutrients from these excretions. Interestingly, the tree shrew itself dines on insects, so the pitcher plant is still indirectly deriving its nitrogen from insects even when it uses tree shrew poop. It’s just getting it from the tail end of a rodent intermediary instead.

Sexual selection: Do females follow fads?

Is this male attired in the fashionable look of the season? Based on the reaction of the female in the background, perhaps not. Source: Wikimedia Commons

Timeline, 2008: Sexual selection is a mechanism of evolution that sometimes butts heads with natural selection. Under the tenets of natural selection, nature chooses based on characteristics that confer a competitive edge in a given environment. Under this construct, environment is “the decider.” But in sexual selection, either competition between the same sex or a choice made by the opposite sex determines the traits that persist. Sometimes, such traits aren’t so useful when it comes to the everyday ho-hum activities like foraging for food or avoiding predators, but they can be quite successful at catching the eye of an interested female.

Those female opinions have long been considered unchanging. In the widowbird, for example, having long, flowing black tailfeathers is a great way to attract the lady widow birds. But perhaps they don’t call them widowbirds for nothing: if those male tailfeathers get too long, the bird can’t escape easily from predators and ends up a meal instead of a mate. In these cases, natural selection pushes the tailfeather trait in one direction—shorter—while sexual selection urges it the other way—longer. The upshot is a middling area for tailfeathers length.

This kind of intersexual selection occurs throughout the animal kingdom. Probably the most well-recognized pair that engages in it is the peacock and peahen. Everyone has seen the multicolored baggage any peacock worth his plumage drags around behind him. A peacock will fan out those feathers in an impressive demonstration, strutting back and forth and waving its tail in the wind, showing off for all he’s worth. It’s a successful tactic as long as nothing is around that wants to eat him.

Frogs hoping for a mate find themselves elbow deep in the “paradox of the lek.” The lek is the breeding roundup for frogs, where they all assemble in a sort of amphibian prom. For the males, it’s a tough call, literally. They must call loudly enough to show the females how beautifully androgenized they are—androgens determine the power of their larynx—while at the same time not standing out enough to attract one of the many predators inevitably drawn to a gathering of hundreds of croaking frogs. Trapped in this paradox, the frog does his best, but natural selection and sexual selection again end up stabilizing the trait within expected grooves.

This status quo has become the expectation for many biologists who study sexual selection: natural selection may alter its choices with a shifting environment, but what’s hot to the females stays hot, environmental changes notwithstanding. But the biologists had never taken a close look at the lark bunting.

A male lark bunting has a few traits that may attract females: when it shakes off its drab winter plumage and takes on the glossy black of mating season, the male bird also sports white patches on its wings that flash through the sky and sings a song intended to draw in the ladies. But the ladies appear to be slaves to fashion, not consistently choosing large patches over small, or large bodies over lighter ones. Instead, female lark buntings change their choices with the seasons, selecting a large male one year, a dark-colored male with little in the way of patches the next, and a small-bodied male the next. Lark buntings select a new mate each year, and the choice appears to be linked to how well the male will aid in parenting duties, which both parents share. It may be that a big body is useful in a year of many predators, but a small body might work out better when food supplies are low.

The researchers who uncovered this secret of lark bunting female fickleness watched the birds for five years and based their findings on statistical correlations only. For this reason, they don’t know exactly what drives the females’ annually varying choices, but they speculate that environmental factors play a role. Thus, sexual selection steps away from the realm of the static and becomes more like—possibly almost indistinguishable from—natural selection.

Note: This blog post has been submitted for the ScienceOnline 2011 Travel Award Contest sponsored by NESCent, the National Evolutionary Synthesis Center. Here’s hoping that the judges find sexual selection to be this year’s travel award fad.

The narwhal: a serious case of nerves

"Narwhal or unicorn"

Timeline, 2006: The narwhal has a history as striking as the animal itself. Vikings kept the narwhal a secret for centuries even as they peddled its “horn” as that of a unicorn. Narwhal tusks were so prized that monarchs paid the equivalent of the cost of a castle just to have one. They were thought to have magic powers, render poison ineffective, cure all manner of diseases, and foil assassins.

A tooth and nothing but a tooth

As it turns out, the horn is really just a tooth, an extremely long, odd, tooth. The narwhal tusk, which usually grows only on males from their left upper jaw, can reach lengths of six feet or more. Sometimes, males will grow two tusks, one on each side. The tooth turns like a corkscrew as it grows, stick straight, from the narwhal’s head. They are such an odd sight that scientists have been trying to figure out for centuries exactly what that tusk might be doing there.

Some have posited that the narwhal uses the tusks in epic battles with other male narwhals. Others have fancifully suggested that the animal might use the long tooth to break through the ice, ram the sides of ships (nevermind the disconnect between when the tusk arose and when ships entered the scene), or to skewer prey—although no one seems to have addressed how the narwhal would then get the prey to its mouth.

Gentle tusk rubbing

The facts are that the narwhal rarely, if ever, appears to duel with other narwhals. Its primary use of the tusk appears to be for tusking other males, in which the animals gently rub tusks with one another. They also may be used in mating or other activities, although that has not yet been demonstrated. But what has been discovered is that the narwhal ought to be suffering from a severe case of permanent toothache.

Arctic cold strikes a narwhal nerve

Anyone who has ever had exposed nerves around their teeth knows that when cold hits those nerves, the pain usually sends us running for the dentist. Now imagine that your tooth is six feet long, has millions of completely exposed nerve endings, and is constantly plunged in the icy waters of the Arctic. You’ve just imagined being a narwhal.

Dentist on ice

A clinical instructor at the Harvard School of Dental Medicine who thinks of nothing but teeth made this discovery about the narwhal. The instructor, Martin Nweeia, can wax rhapsodic about teeth and how central they are to our health and the stories they can tell even about how we lived and died. He has carried his tooth obsession beyond his own species, however; his passion led him to spend days on Arctic ice floes, watching for the elusive narwhal, or at least one of the tusks, to emerge from the deadly cold water. He also befriended the local Inuit, who rely on the narwhal as a source of food and fuel oil.

His fascination and rapport with the Inuit people ended with his viewing several specimens of narwhal tusks. What he and his colleagues discovered astonished them. The tusks appeared to consist of open tubules that led straight to what appear to be millions of exposed nerve endings. In humans, nerve tubules are never open in healthy teeth. But in the narwhal tusk, which is an incredible example of sexual dimorphism and the only spiral tooth known in nature today, these open tubules were the norm.

Sensory tooth

The researchers speculated that the animals may use this enormous number of naked nerves as a finely sensitive sensory organ. In addition, it is possible that the teeth transmit voltage through a process called the piezo effect, in which crystals generate voltage when a mechanical force rattles them. In the case of the narwhal, who swim quickly through the water, water pressure might provide the force. Because narwhals are among the most vocal of whales, the tusks could also be sound sensors.

Why would dentists be so interested in the tusks of a whale? Examinations of the narwhal tusks have revealed that they are incredibly flexible, unlike our teeth, which are strong but also rigid and comparatively brittle. It is possible that understanding the narwhal tusk might have clinical applications for developing flexible dental materials for restoring pearly whites in people.

%d bloggers like this: