Rodents of unusual size

My writer friend Vivienne Tuffnell, of the lovely blog Zen and the Art of Tightrope Walking, frequently amuses her friends with photos of the antics of her pet guinea pigs.  Her love for her little pals is undoubtedly what prompted her to post a somewhat more alarming photo a couple of days ago:

She captioned it, "Guinea pigs and guinea biggers?"

What's most amazing about this is that the smaller animal isn't even a guinea pig; it's a capybara, the largest living rodent species.  On this scale, your typical guinea pig would about fit in the space underneath the capybara's belly.

So, who's the big guy?

That's Josephoartigasia, a Pliocene (five million to one million years ago) animal from southeastern South America that is thought to be the largest rodent species ever.  From skulls found in Uruguay, a full-grown Josephoartigasia weighed something like five hundred kilograms -- heavier than an adult grizzly bear.

It's hard to talk about this thing without lapsing into superlatives.  The one that blew me away was a calculation of its bite force, putting it at an estimated 950 Newtons, which is right around what an adult jaguar can exert.  Because of this, its skull was heavily reinforced; the more powerful the muscle contraction, the stronger the bones have to be (given that bones provide anchorage and leverage for the pull of the muscles).

What it needed this kind of bite force for is a matter of conjecture.  It's possible it was just for gnawing things.  Like granite outcrops, or something.  On the other hand, South America during the Pliocene was replete with huge predators including Xenosmilus, a sabre-toothed cat, and phorusrhacids -- the aptly-named "terror birds" that looked like a cross between an ostrich and a velociraptor.  So it's possible its fearsome bite was defensive.

When food is abundant and there are lots of large carnivores around, there is a significant evolutionary pressure favoring large body size, and that seems to be what happened here.  Back then, South America's fauna resembled what now lives in southern Africa -- abundant wildlife and lots of very big animals.  Josephoartigasia would have shared the habitat with giant ground sloths, glyptodonts (think "an armadillo on steroids"), toxodonts (the Pliocene answer to hippos), and the giant peccary Platygonus. 

So it was a world of megafauna, and Josephoartigasia fit right in.

But we're used to thinking of large ungulates; even giant ground sloths are familiar to anyone who's seen a kid's book on prehistoric animals.

But rodents the size of an adult longhorn steer are a little hard to imagine.

So thanks to Vivienne, who has provided cool topics for Skeptophilia before and definitely didn't fail me this time.  Me, I'm just as glad Josephoartigasia is not around any more.  I have enough of a hard time keeping squirrels out of the birdfeeder.

Having a cow-sized squirrel that could eat the birdfeeder would be another thing entirely.

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Catty behavior

Ever heard of the cui bono principle?

Cui bono? is Latin for "who benefits?"  It's been used for centuries as a central question in criminal cases; to figure out who's guilty of a crime, the first thing to determine is who benefitted from it.  But it is also critical to questions of evolutionary biology.  There are behaviors in the biological world that seem unnecessarily risky, or even suicidal, and it's hard to imagine how they'd be selected for.

So... who benefits?

Take, for example, the strange behavior of certain ants.  Ground-dwelling ants, when threatened, usually have one of two responses; rush out and try to sting or bite whatever's threatening them, or move downward (and underground) to hide.  But some ants were observed to have a third, and bizarre, response: faced with a threat, they climb upwards on plant stems, and then just docilely sit there -- and, frequently, get eaten (along with the plant) by herbivorous animals.

The reason for this weird behavior is positively grotesque.  It turns out that the seemingly-suicidal ants were infected with a brain parasite called a lancet worm (Dicrocoelium dendriticum) that, in order to complete its life cycle, has to pass through the digestive tract and liver of a ruminant (sheep, cow, or goat).  So the worm reprograms the ant's brain to make it do something that will ultimately end up with its being turned into lunch.  

Too bad for the ant.  But cui bono?  The worm, of course.  It hijacked the ant's brain to make it an unwilling participant in the worm's life cycle.

This is hardly the only example of the cui bono principle, and far from the creepiest one.  Ready to get completely skeeved out?

You may know of the pathogen Toxoplasma gondii in its connection to the recommendation by doctors that pregnant women not clean cat litter boxes.  The pathogen, which is neither a bacteria nor a virus but a protist, is carried by cats and excreted with the urine; and a pregnant woman who contracts toxoplasmosis risks birth defects in her unborn child.

Toxoplasma, however, is found in other animals besides cats, and in fact it was some recent research into hyenas that brought it to mind today.  A study out of the University of Colorado that appeared in Nature Communications a few months ago showed that wild populations of hyenas have a high rate of infection, and the weirdest result is seen in infected hyena cubs.  They, like the unfortunate ants, have a behavioral consequence of infection; they become bold, and seem to lose their perception of lions and other predators as dangerous.  They're far more likely to be killed than healthy, uninfected hyena cubs -- which, of course, benefits the pathogen because it then passes on to the lion.  The pathogen, in essence, is programming its host to engage in behavior that will make it more likely to jump to another host.

[Image licensed under the Creative Commons New Jersey Birds, Spotted hyena cubs in Limpopo, CC BY-SA 2.0]

So, a weird and gruesome outcome of being infected with a tropical disease, right?  Nothing for us humans to worry about, right?  Well, what you may not know is that there is a significant likelihood that you have toxoplasmosis right now.  In fact, if you have ever owned a cat, the probability stands close to 100%.

A study done a while back by Kevin Lafferty, of the University of California, suggests that as many as three billion people may have a dormant Toxoplasma infection.  Yes, dear readers, you read that right; that's three billion with a "b," as in a little less than half of the human population.  Turns out that Lafferty's research indicated that when you get toxoplasmosis, you get flu-like symptoms for a couple of days, and then the symptoms abate -- but for most of us, the protist goes dormant, and we carry around the parasite for life.

This is creepy enough, but wait'll you hear what it does to you.

Lafferty's research showed that in mammalian hosts, the Toxoplasma organism invades, and becomes dormant in, the host's brain cells.  Not only hyenas become bolder around predators; mice and rats do, as well, aiding in the passage of the germ between rodents and domestic cats.  Lafferty's study, though, goes a step further, and looks at what latent Toxoplasma infection does to humans -- and he found  it seems to cause significant personality changes.

Now, it doesn't make us have a high affinity for cats, which would make sense, and would explain Crazy Cat Lady Syndrome, in which some people think it's normal to own thirty cats, and somehow seem immune to the truly cataclysmic odor that their houses attain.  No, what actually happens is more subtle.  Apparently, if you have Toxoplasma, you're more likely to be neurotic.  People who tested positive for antibodies for Toxoplasma scored far higher on personality assessments in the areas of guilt-proneness, anxiety, and risk of depression.  These effects were so pronounced that Lafferty speculates that it could account for certain differences between cultures.

"In some cultures, infection is very rare," Lafferty said, "while in others, virtually everyone is infected.  The distribution of Toxoplasma gondii could explain differences in cultural aspects that relate to ego, money, material possessions, work, and rules."

I find this speculation fascinating.  The idea that my neuroses might not be due to my genes or upbringing, but because I'm carrying around a parasite in my brain, doesn't create the level of Icky-Poo Factor that you might expect.  Of course, I'm a biologist, and so I'm at least on some level accustomed to thinking about creepy-crawlies.  But the idea that some sort of a microorganism could affect my behavior strikes me as weirdly interesting, particularly since I've had at least one cat in my household for a significant chunk of the past forty years.

So, maybe our personalities aren't as static as we'd like to think -- they can be influenced by a great many circumstances outside of our control.  Add parasite infestations to that list.  And if that whole idea upsets you too much, take comfort in the fact that Lafferty's research has spurred medical researchers to try to find a drug that can destroy the germ.  Nothing's been certified for human use so far, so don't cancel your appointment with your therapist just yet, but there are a couple that are looking promising.  What's uncertain is whether, if the pathogen were eradicated, it would reverse the changes in the brain -- if, for example, nervous, neurotic people would find themselves less anxiety-prone -- or if the alterations in the brain are more or less permanent.  But I, for one, would volunteer to give it a try, once (or if) the medication becomes available.

Until then, you should probably shouldn't worry.  What's a few brain parasites among friends, after all?  In fact, just forget I brought it up.  Relax, go and sit in your recliner, and pet your cat, Mr. Fluffkins, for a while.

You'll feel better.  Trust me.

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I've mentioned before how fascinated I am with the parts of history that still are largely mysterious -- the top of the list being the European Dark Ages, between the fall of Rome and the re-consolidation of central government under people like Charlemagne and Alfred the Great.  Not all that much was being written down in the interim, and much of the history we have comes from much later (such as History of the Kings of Britain, by Geoffrey of Monmouth, chronicling the events of the fourth through the eighth centuries C.E. -- but written in the twelfth century).

"Dark Ages," though, may be an unfair appellation, according to the new book Matthew Gabriele and David Perry called The Bright Ages: A New History of Medieval Europe.  Gabriele and Perry look at what is known of those years, and their contention is that it wasn't the savage, ignorant hotbed of backwards superstition many of us picture, but a rich and complex world, including the majesty of Byzantium, the beauty and scientific advancements of Moorish Spain, and the artistic genius of the master illuminators found in just about every Christian abbey in Europe.

It's an interesting perspective.  It certainly doesn't settle all the questions; we're still relying on a paucity of actual records, and the ones we have (Geoffrey's work being a case in point) sometimes being as full of legends, myths, and folk tales as they are of actual history.  But The Bright Ages goes a long way toward dispelling the sense that medieval Europe was seven hundred years of nothing but human misery.  It's a fascinating look at humanity's distant, and shadowed, past.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

Mice, rats, and sunk costs

One of the most difficult-to-fight biases in human nature is the sunk-cost fallacy.

The idea is the more time, effort, and/or money we've put into a decision, the less likely we are to abandon it -- even after it has been proven a bad choice.  It's what makes people stick with cars that are lemons, investments that are financial disasters, marriages that are horrible, and politicians who have proven themselves to be unethical and self-serving, long after cut-and-run would, all things considered, be the most logical course of action.

The tendency is so ubiquitous that it's often taken for granted.  You even see it in far less logical scenarios than the ones I mentioned above, where there could be at least some rational reason for sticking with the original choice.  A good example is games of pure chance, where gamblers will keep on wasting money because they are certain that a losing streak is bound to end.  "I'm already a thousand dollars in the hole," they'll say.  "I can risk five hundred more."  Here, sunk-cost makes no sense whatsoever; the lost thousand is not an investment that could pay off in any sense of the word, and losing streaks in games of pure chance are not bound to do anything.

That's why they're called "games of pure chance."

So the ubiquity of the sunk-cost fallacy is undeniable, but what's less obvious is why we do it.  Sticking with a bad choice is rarely ever advantageous.  But despite its dubious benefits to survival, what seems certain is that it's a very old behavior, evolutionarily speaking.  Because researchers at the University of Minnesota have just shown that sunk-cost decision making not only occurs in humans, but in...

... mice and rats.

[Image licensed under the Creative Commons Rasbak, Apodemus sylvaticus bosmuis, CC BY-SA 3.0]

In a paper entitled "Sensitivity to 'Sunk Costs' in Mice, Rats, and Humans" that appeared last week in the journal Science, neuropsychologists Brian M. Sweis, Samantha V. Abram, Brandy J. Schmidt, Kelsey D. Seeland, Angus W. MacDonald III, Mark J. Thomas, and A. David Redish showed that even our very distant relatives engage in sunk-cost errors.  The authors write:

Sunk costs are irrecoverable investments that should not influence decisions, because decisions should be made on the basis of expected future consequences.  Both human and nonhuman animals can show sensitivity to sunk costs, but reports from across species are inconsistent.  In a temporal context, a sensitivity to sunk costs arises when an individual resists ending an activity, even if it seems unproductive, because of the time already invested.  In two parallel foraging tasks that we designed, we found that mice, rats, and humans show similar sensitivities to sunk costs in their decision-making.  Unexpectedly, sensitivity to time invested accrued only after an initial decision had been made.  These findings suggest that sensitivity to temporal sunk costs lies in a vulnerability distinct from deliberation processes and that this distinction is present across species.

In both the experiments with humans and rodents, the setup was the same -- the subject navigates a maze looking for rewards (a food pellet for the mice, and hilariously, a video of kittens playing for the humans, showing that cat videos really are an incentive for us) which are scattered randomly through the maze.  Each time a reward is encountered, the subject is told how long it will take for the reward to be delivered (a tone the mice and rats are trained to associate with wait time, and a countdown timer for the humans).  Because the rewards are plentiful and some of the waits are long, what would make logical sense is to abandon a reward if the wait time is too long, so more time could be spent searching for rewards with short wait times.

But that's not what happened.  Both the rodents and the humans would often stick with rewards with very long wait times -- and the ones who said, "Screw it, this is too long to sit here twiddling my thumbs" all gave up early on.  The longer the test subject stuck with the wait, the more likely they were to hang on to the very end, even at the cost of a considerable amount of time that could have been spent foraging more productively.

"Obviously, the best thing is as quick as possible to get into the wait zone," said David Redish, who co-authored the study.  "But nobody does that.  Somehow, all three species know that if you get into the wait zone, you’re going to pay this sunk cost, and they actually spend extra time deliberating in the offer zone so that they don’t end up getting stuck."

What this research doesn't indicate though, its why we all do this.  Behaviors that are common throughout groups of related species -- what are called evolutionarily-conserved behaviors -- are thought to have some kind of significant survival advantage.  (Just as evolutionarily-conserved genes are thought to be essential, even if we don't know for certain what they do.)  "Evolution by natural selection would not promote any behavior unless it had some — perhaps obscure — net overall benefit," said Alex Kacelnik, a professor of behavioral ecology at Oxford, who was not part of the study, but praised its design and rigor.  "If everybody does it, the reasoning goes, there must be a reason."

But what that reason is remains unclear.  We have to leave it at "we're not as logical as we like to think, and our motivation for decision-making not as based in solid fact as you might expect," however unsatisfying that might be.

But it is something to consider next time we're weighing the benefits of sticking with a decision we already made -- whether it's the wait time for downloading a kitten video, or continuing our support for a politician.

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This week's Skeptophilia book recommendation is a must-read for anyone concerned about the current state of the world's environment.  The Sixth Extinction, by Elizabeth Kolbert, is a retrospective of the five great extinction events the Earth has experienced -- the largest of which, the Permian-Triassic extinction of 252 million years ago, wiped out 95% of the species on Earth.  Kolbert makes a persuasive, if devastating, argument; that we are currently in the middle of a sixth mass extinction -- this one caused exclusively by the activities of humans.  It's a fascinating, alarming, and absolutely essential read.