If pubic lice are not the sort of thing you want to be seen reading about, let me give you the opportunity to close your browser window right now. But if you’re at all curious about the secret that pubic lice have been keeping for over three million years, the tale of a mysterious liaison between our ancestors and the ancestors of gorillas–read on.
Darwin gave a lot of thought to the strangest creatures on this planet, wondering how they had evolved from less strange ancestors. Whales today might be fish-like warm-blooded beasts with blowholes and flukes, but long ago, Darwin argued, their ancestors were ordinary mammals that walked on land with legs. His suggestion was greeted with shock and disbelief; neverthless, scientists have found bones from ancient walking whales. Humans, Darwin argued, evolved from apes, most likely in Africa where chimpanzees and gorillas are found today. And today scientists have found about twenty different species of hominids, from chimp-like creatures that lived six million years ago to not-quite humans that lived alongside our own species. Darwin also pondered the origins of barnacles, orchids, and many other strange creatures. But for some reason–perhaps thanks to his famously weak stomach–Darwin didn’t write a single word about tapeworms. It’s a pity, because tapeworms are as strange as animals can get…
Thanks to PZ Myers for calling attention to this superb video of Corydceps, a parasitic fungus that drives its insect host up a plant before growing a spike out of its head. Leave it to David Attenborough, master of the nature documentary, to bring the beauty of this parasite to video. I’ve seen photographs of Cordyceps before, but I never knew it made such a graceful entrance.
What’s particularly cool about Cordyceps is that it is not alone. Other parasites drive their hosts to bizarre heights. Another fungus, called Entomophthora muscae, drives houseflies and other insects upwards, climbing screen doors in some cases, before springing out of its host’s body.
In the case of Entomophthora and Cordyceps, hosts go up so that parasites can come back down again–specifically, down on potential insect hosts living on the ground. But other parasites have another direction in mind. The lancet fluke drives its insect hosts up to the tops of plants so that grazing mammals may eat them. Only in the gut of a cow or some other grazer can the flukes mature and reproduce. These creatures are like the birds, bats, and pterosaurs of the parasitic world, hitting on the same brilliant solution again and again.
Well, the talk at Cornell last week went very well. Thanks to everyone who came. If you want to hear me wax rhapsodic about parasite manipulations (and explain how scientists study their evolution), you’re in luck. Cornell has put the video of the talk online. The image is pretty small on the screen, so I decided to post the slide show on my web site here. I suggest opening two screens and advancing the slides as the talk progresses.
At first the sound is a little scratchy on the video and the light balance takes a while to get properly adjusted. But don’t give up–it evens out. You may also hear a baby gurgling from time to time.
Near the end, when I talk about cuckoo birds as parasites, I refer to their host in one of the pictures as a cowbird. I should have said a reed warbler.
And if you are curious to find out more, check out my book, Parasite Rex.
Update: Apparently the video doesn’t work for some readers. I am at a loss.
Attention all Loom readers in the Cornell University area: I’m heading up to Ithaca to give a talk tomorrow on a subject near and dear to my heart–how parasites turn their hosts into puppets and slaves. I’ll be at the David Call auditorium in Kennedy Hall at 4 pm. The lecture is open to the public and will, of course, include a very creepy Powerpoint. Details here, map here.
Toxoplasma, that mind-altering, cell-manipulating, all-around awesome parasite that sits in the brains of billions of us, is back in the news.
Infection with the parasite raises the chances a woman will have a boy from 51% to 72%. The average ratio of boys to girls at birth is 51%. Women with high levels of antibodies to Toxoplasma, scientists found, have a 72% chance of having a boy. While many effects of Toxoplasma probably have something to do with adaptations that allow the parasite to thrive and spread successfully, this one seems more like a side-effect, albeit a dramatic one.
Paper abstract here.
[Thanks to BC for the fact-check.
Once again, I hear the siren song of Toxoplasma, the parasite that dwells in the brains of 50 million Americans.
Toxoplasma gondii is an extraordinary creature, whose exploits I’ve chronicled in previous posts , an article in the New York Times and my book Parasite Rex. This single-celled organism has a life cycle that takes it from cats to other mammals and birds and back to cats again. Studies have shown that the parasite can alter the behavior of rats, robbing them of their normal fear of cats–and presumably making it easier for the parasites to get into their next host.
Toxoplasma is astonishingly successful, able to live in thousands of species, including us. Billions of people are infected with Toxoplasma, which they pick up from the soil or from contaminated meat or water. In most people it remains dormant, but even in this quiet state it may also have affect human behavior. Some scientists have linked Toxoplasma to schizophrenia, while others have found personality differences between people with Toxoplasma and those who are Toxo-free. It’s possible that it uses its prey-altering strategy on our brains, too.
All well and good. But now Toxplasma is going big time. Today the Proceedings of the Royal Society of London is publishing a paper called, “Can the common brain parasite, Toxoplasma gondii, influence human culture?”
The paper’s answer? Quite possibly yes. Here’s why…
Next time I go to the doctor, I think I’ll get him to give me a test for Toxoplasma. Fifty million Americans have the parasite, so I wouldn’t be the first. And if I was carrying it around in my head, that might explain why it’s so fascinating to me.
If you keep a vegetable garden, there’s a fair chance you’ll encounter a grisly sight this summer. Some poor caterpillar will be clutching a leaf, with the pupae of parasitic wasps sprouting off its back. It has just died in a most grotesque way. A wasp has zeroed in on the caterpillar and injected eggs into its body. The eggs hatched, and the larvae devoured their hosts from within, keeping it alive until they were ready to emerge.
What makes this sight all the more grotesque is the fact that the plant the caterpillar is sitting on may have been an accomplice to the crime. When caterpillars nibble on plants, the plants sometimes respond by releasing a distinctive cocktail of chemicals. This odor then attracts parasitic wasps. The plants are not just releasing a sort of chemical scream. Wasps are very precise in the species of caterpillars they choose, and they can tell these odors apart.
But the caterpillars are not entirely helpless in this struggle. After all, they are in a sense parasites as well, and like other parasites, they have evolved ways of evading the defenses of their hosts. In the journal Public Library of Science Biology, Japanese scientists demonstrate that caterpillars are eavesdropping on the signals plants are sending and shifting their behavior to make it less likely they’ll become food for wasps.
The scientists studied a species of moth (Mythimna separata) that eats corn. Previous research had shown that the catepillars were nocturnal, munching on corn at night and then slinking into leaves during the day. But the questioned remained what sort of cues the insects were responding to. Was it the rising and setting of the sun, or was it the scent released by injured corn plants–which they only release in the day?
The Japanese researchers set up an experiment to see what was driving the insects in and out of hiding. They put caterpillars into cups covered in nylon, and gave them a folded paper tent to hide in as they wished. The scientists then observed how the caterpillars responded to different combinations of signals. They exposed them to light and dark, to the scent of uninjured plants, and to the scent of plants that had been nibbled. If the caterpillars had a supply of artificial food, they didn’t respond much to light and dark on their own. The odors released by the corn plants had a much stronger effect, regardless of the lighting.
There’s a very good reason why a corn plant should release its distress calls during the day: that’s the only time when parasitic wasps are active. Manufacturing this fragrance at night would be a waste of effort. The caterpillars have responded, it appears, by evolving to hide when they smell the odor. The ones that respond this way are the ones that tend not to turn into inspiration for science fiction movies.
This new find is just the latest addition to a list of adaptations that have evolved in caterpillars against the threat of parasites. My all-time favorite is the way some caterpillars fire their droppings out of an anal cannon to avoid building up stinking piles that will give their location away. You can read about this hygienic howitzer here, in my book Parasite Rex, and in a recent scientific review here.
Update 5/16 9 am: Link to PLOS paper added. Plus, catepillars grow up to become caterpillars