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Toxoplasma gondii requires the cat digestive system for reproduction, so it hitches a ride in a rat (courtesy of flickr user cobalt123)

The life cycle of the parasite Toxoplasma gondii goes like this: Toxoplasma reproduces inside the intestine of a cat, which sheds the parasite in its feces. Rats then ingest the parasite when they consume food or water contaminated with cat feces. The parasite takes up residence in the rat’s brain and, once the rat gets eaten by a cat, it starts the cycle all over again.

Researchers have known for a few years that a rat infected with Toxoplasma loses its natural response to cat urine and no longer fears the smell. And they know that the parasite settles in the rat’s amygdala, the part of the brain that processes fear and emotions. Now a new study in the journal PLoS ONE adds another bizarre piece to the tale: When male rats infected with Toxoplasma smell cat urine, they have altered activity in the fear part of the brain as well as increased activity in the part of the brain that is responsible for sexual behavior and normally activates after exposure to a female rat.

The double messages of “you smell a cat but he’s not dangerous” and “that cat is a potential mate” lure the rat into the kitty’s deadly territory, just what the parasite needs to reproduce. Scientists still don’t know how the parasite works to alter the brain, though there apparently is a link to production of dopamine, an important neurotransmitter in the systems for decision-making and reward.

How the parasite makes the rat brain do what it needs is a particularly interesting question because rats and cats aren’t the only animals that can become infected with Toxoplasma. There is concern, for example, about the parasite’s effect on sea otters. And grazing livestock can become infected after eating contaminated vegetation. More worryingly, though, is that one-third of humans test positive for exposure to Toxoplasma (the most common ways for humans to come into contact with the parasite is through kitty litter and by consuming undercooked meat). Not only can pregnant women pass on the parasite to an unborn child (putting the child at risk of blindness or mental disability) but recent studies have also found an association between the parasitic infection and increased risk of schizophrenia and obsessive compulsive disorder.

If you’re worried about Toxoplasma, there’s no need to give up your beloved cat, but there are some precautions you can take (and definitely should take if you’re pregnant), as the CDC states:

• Avoid changing cat litter if possible. If no one else can perform the task, wear disposable gloves and wash your hands with soap and warm water afterwards.
• Ensure that the cat litter box is changed daily. The Toxoplasma parasite does not become infectious until 1 to 5 days after it is shed in a cat’s feces.
• Feed your cat commercial dry or canned food, not raw or undercooked meats.
• Keep cats indoors.
• Avoid stray cats, especially kittens. Do not get a new cat while you are pregnant.
• Keep outdoor sandboxes covered.
• Wear gloves when gardening and during contact with soil or sand because it might be contaminated with cat feces that contain Toxoplasma. Wash hands with soap and warm water after gardening or contact with soil or sand.

Does this Aston Martin V8 Vantage make your mouth water? (courtesy of flickr user Destinys Agent)

If you think about it, some of the phrases we use to express desire for inanimate, non-food items are pretty weird. We “drool” over cars. Our “mouths water” at the sight of a pile of money. Salivating makes sense when we’re talking about food—after all, salivation is part of the anticipatory phase of digestion, and saliva moistens our food to assist swallowing—but why would we drool over something we can’t eat? We do, though, as shown in a new study in the Journal of Consumer Research.

David Gal, a marketing professor at Northwestern University, conducted two experiments, each time measuring saliva production. In the first he started off with a writing assignment, asking the participants to write about either a time they felt they had power or a time when they lacked power. Those two groups were then split and shown either images of money or, as a control, office supplies. Only the people who had been assigned to write about a time when they lacked power salivated at the sight of money, Gal found. The assignment had primed those individuals to find money to be more attractive. (Office supplies, not shockingly, had no effect.)

In the second experiment, which focused on the responses of men only, the participants were primed with what Gal calls a “mating goal.” Half had to choose a picture of a woman and write about an imagined date with her; the other half had to choose a picture of a barbershop and write about an imagined haircut. The images of money and office supplies were then replaced with pictures of sports cars and fastening tools. Again, the participants who had been primed to think about what they lacked salivated over the photos of the cars. (Guys really do think that sports cars make them more attractive to girls.) “These findings show that exposure to a material reward cue stimulates salivation when the reward value is high,” Gal writes.

OK, so under the proper circumstances, we might drool over a non-food item. But why would this be? As Gal notes, “Salivation to material reward is not of any obvious function.” He has two theories, though: One, that we are conditioned from early in life to associate material rewards with food. More likely, though, might be number two, that salivation is a side-effect of the natural reward system. If there’s just one system in our brains that rewards us for everything—from drugs to money to chocolate chip cookies—then it makes sense that we could salivate over any of those things. As Jonah Lehrer writes on the Wired blog Frontal Cortex:

Although our dopamine neurons evolved to process and predict biological necessities, they’ve since learned to embrace a more catholic set of desires, so that pieces of green paper filled with pictures of dead presidents get them very excited. While relying on a single pathway to process all of our rewards normally works quite well—the dopamine reward pathway is some well-tested cognitive software, since the same basic code is present in nearly every mammal—it does lead to a few unintended side-effects. Just ask a drug addict, or that man who starts to drool whenever a Ferrari drives by.

Hummingbirds can bend their beaks in the middle using muscles in their head, but no one has checked to see whether other birds can do the same thing. (photo courtesy of flickr user Amyn Kassam)

As a biologist at North Carolina State University, Rob Dunn studies the complex and diverse world of ants. In addition, he’s part of a fascinating—and, to some, slightly disgusting—project looking at the diversity of microbes that live in the human belly button. Here at Smithsonian, we know Dunn because he’s also a great science writer. Dunn is the author of two books (Every Living Thing and The Wild Life of Our Bodies) and numerous magazine and web articles, including several of my recent Smithsonian favorites—”The Mystery of the Singing Mice,” “The Top Ten Daily Consequences of Having Evolved” and “The Untold Story of the Hamster, a.k.a. Mr. Saddlebags.” Even better, Dunn was a great sport when I asked him why he liked science:

No one can tell you for sure what the appendix does. No one knows how deep into the Earth life goes. No one knows how high into the sky life goes. No one is sure what the mites that live on human foreheads do, though they are there while you are reading.

Most species on Earth remain unnamed, not to mention totally unstudied. New species are easy to find in Manhattan, walking around alongside celebrities. No one can tell me what the species of bacteria living on my body, hundreds of species, are doing. No one can say for sure if there is another, yet to be discovered, domain of life. Parasites in my body might be affecting my behavior, and even the sorts of things I write late at night.

There are ant species that farm fungus in the Amazon. There are beetle species that farm fungus in my backyard. Both do so with greater sophistication than I or any other human can farm fungus. No one is sure why weaver ants have green abdomens. No one knows why we have specialized glands in our armpits that feed bacteria that produce the smells we think of as body odor. No one is sure why we have such large sinuses. There exists active discussion about why our bodies are warm and not cold.

There is a bacteria species that lives in hot water heaters, but nowhere else yet studied on Earth. Hummingbirds can bend their beaks in the middle using muscles in their head, but no one has checked to see whether other birds can do the same thing. Most mice on Earth might be singing, but only a few have been listened to.

I like to do and write about biology for these reasons, because in biology most of what is knowable is still unknown, because in biology we are still ignorant, because in biology the very body I use to type these words, with its crooked fingers and twisty mind, is only partially, modestly, understood, because biology will never fully be understood, because biology is a tapestry being unraveled, because the lives of the people unraveling the stories are, even when superficially humble and human, always fascinating, because biology is like biography with better characters, because I find deep and wondrous joy in biology, because even when an editor writes me late at night to ask why I write about and do biology my first response is to smile at how much I love biology, smile and wonder, the way we all wonder before the grandeur of the stars but sometimes forget to wonder before the grandeur of life.

If you’d like to participate in our Why I Like Science series, send a 200- to 500-word essay to WhyILikeScience@gmail.com; I’ll publish the best entries in future posts on Surprising Science.

The moose likely got drunk eating apples fermenting on the ground. AP Photo/Per Johansson

You may have seen the story earlier this week of the drunken Swedish moose (or elk, as they call the antlered behemoth in Sweden) that got stuck in a tree. “I thought at first that someone was having a laugh. Then I went over to take a look and spotted an elk stuck in an apple tree with only one leg left on the ground,” Per Johansson, who spotted the inebriated mammal in the garden next door to his house in Särö, told The Local. The moose likely got drunk eating apples fermenting on the ground and got stuck in the tree trying to get fresh fruit. “Drunken elk are common in Sweden during the autumn season when there are plenty of apples lying around on the ground and hanging from branches in Swedish gardens,” The Local states.

Moose aren’t the only non-human animals with a taste for alcohol, though.

The pen-tailed treeshrew of Malaysia gets credit for having the world’s highest alcohol tolerance. Seven species of animals, including the treeshrew and the slow loris, feed on fermented nectar from the flower buds of the bertam palm plant. But though the treeshrew quaffs this brew all day long, it doesn’t get drunk, scientists found in a 2008 PNAS study. “They seem to have developed some type of mechanism to deal with that high level of alcohol and not get drunk,” University of Western Ontario microbiologist, and study co-author, Marc-André Lachance told LiveScience. “The amount of alcohol we’re talking about is huge—it’s several times the legal limit in most countries.”

Fruit bats also appear to tolerate the effects of fermentation on fruit better than the Swedish moose did. In a 2010 PLoS ONE study, scientists fed wild-caught fruit bats sugar water laced with alcohol and sent them through a maze. Though many of the bats would have gotten a FUI (flying under the influence) citation, they had no more trouble navigating than did bats given sugar water alone. The researchers think that being able to tolerate alcohol lets the bats have access to a food source—fruit—for a longer period than only when it’s ripe.

Rhesus macaques, however, are more like humans than treeshrews, according to a 2006 Methods study in which the monkeys were given access to an alcoholic drink in a series of experiments. “It was not unusual to see some of the monkeys stumble and fall, sway, and vomit,” study co-author Scott Chen, of the National Institutes of Health Animal Center, told Discovery News. “In a few of our heavy drinkers, they would drink until they fell asleep.” The macaques frequently drank until their blood reached the .08 level that would disqualify them from driving a car in most states. And when the researchers looked at patterns of drinking, macaques that lived alone tended to drink the most. In addition, they drank more at the end of the day, like humans after a long day of work.

But stories of drunk elephants on the African savannah are likely just stories, according to a 2006 study in Physiological and Biochemical Zoology. Local lore says that elephants get intoxicated from the fermented fruit of the marula tree. Elephants do have a taste for alcohol, but when scientists sat down to look at the claim, they found several problems. First, the elephants don’t eat the rotten fruit off the ground. They eat the fresh fruit right off the tree. Second, the fresh fruit doesn’t spend enough time in the elephant to ferment and produce alcohol there. And, third, even if the elephant did eat the rotten fruit, the animal would have to eat 1,400 pieces of exceptionally fermented fruit to get drunk.

The study probably won’t change the widespread belief in inebriated pachyderms, though. As the study’s lead author, Steve Morris of the University of Bristol, told National Geographic News, “People just want to believe in drunken elephants.”

Sweet sorghum may be grown for biofuel (courtesy of flickr user Pethan)

If one of the goals of growing plants for biofuel is to be kinder to the environment than you are by extracting oil from the earth, you wouldn’t want to plant anything that could be harmful to the environment. But how could a plant harm the environment? Well, it could become invasive, outcompeting native species, altering the habitat and driving other species into extinction. The damage from and control of invasive plants already costs the United States more than $34 billion each year, according to one estimate. Bioenergy shouldn’t add to that number.

Recognizing this potential for danger, a group of biologists at the University of Florida recently set out to predict whether a dozen species being considered for biofuel cultivation could become invasive. Their study appears in Biomass and Bioenergy.

The researchers note that the characteristics that make a plant attractive as a biofuel source—high productivity, low input requirements, wide breadth of habitat—overlap with those of non-native invasive species. And when the biologists analyzed a dozen non-native species using an assessment system already used by Australia and New Zealand for more than a decade, only four species (miscantus, plume grass, sugarcane and sweet sorghum) had acceptable scores. Seven other species were rated as likely to become invasive, and the last needed further evaluation.

These results may be surprising to the people who proposed these species as biofuels because nearly all of the plants have been grown in Florida for decades for ornamental or agricultural purposes. And they may think, therefore, that this study can be ignored. But growing a tree in a garden is not the same thing as growing acres of them for regular harvesting. “Cultivation of large acreages of a species previously cultivated and introduced in low numbers over relatively low acreages, might so significantly alter propagule press that shifts in dispersal and colonization frequency occur,” the scientists write. In other words, growing something in large numbers can create the opportunities necessary for the species to take off and grow in even larger numbers in places you never intended.

And that has happened in the past. In Australia, for example, people grew a type of ornamental tree called Mimosa pigra for at least 60 years with no problems. But when the tree was moved to a new riparian habitat—the land near rivers or streams—the tree quickly became invasive; it’s now one of Australia’s worst invasive plants.

Not that long ago biofuels were touted as the easy solution to our energy future. We now know that’s not the case. And this study shows that it’s even more complex that we thought.