Jaroslav Vogeltanz, Wikimedia Commons // CC BY-SA 3.0
Jaroslav Vogeltanz, Wikimedia Commons // CC BY-SA 3.0

Fleas Need Fresh Air

Jaroslav Vogeltanz, Wikimedia Commons // CC BY-SA 3.0
Jaroslav Vogeltanz, Wikimedia Commons // CC BY-SA 3.0

Fleas seem to have it made. Not only do they get to live right on their source of food, but if their host is a burrowing animal, they get to spend their days in a warm, cozy hole in the ground. Biologist Cynthia Downs knows that this good life isn’t all it’s cracked up to be, though. In a new study published in the Journal of Experimental Biology she shows that burrows aren’t safe shelters for fleas, as the carbon dioxide (CO2) that builds up in them can actually turn them into death traps for the insects. 

A few years ago, Downs was working in Israel studying the jird, a rodent closely related to the gerbil. Jirds can build sprawling, complex burrows with multiple entrances, nest and food chambers, and long tunnels. While these burrows provide shelter and a stable microclimate, the air in them can get pretty stale, and some burrows have CO2 levels that are 50 times higher than the air above ground. Downs was investigating how the layouts of the burrows affect those CO2 concentrations and what impact that had on the animals. After learning that high CO2 levels can affect the jirds’ immune systems, she started to wonder how CO2 might also affect their parasites.

To find out, Downs collected 18 Sundevall’s jirds (pictured above) from a colony kept in her lab and put each of them in an airtight plastic cage attached to an air pump. Half of the cages were supplied with regular air from the room, while the other half were fed a mix of room air and CO2 designed to mimic conditions in the jirds’ burrows. After the jirds settled in, Downs supplied each of them with some house guests—150 Xenopsylla ramesis fleas, the same number that jirds typically carry in the wild. 

These fleas normally don’t spend a whole lot of time on a jird—just a few days to fill up on blood and reproduce before moving on. To mimic the fleas’ transient ways, Downs combed the bugs from her jirds and collected them from the cages’ sandy floors every few days and then transferred in a new batch of fleas. As each group of fleas was removed, Downs placed them in an incubator so she and her colleagues could count how many had survived and how many eggs they had laid, and track how many eggs hatched.

Downs figured that because of their long shared evolutionary history with their hosts, the fleas would have evolved adaptations for living in the jirds’ burrows and could deal with the high CO2 levels. To her surprise, however, 27 percent more fleas died per day in the burrow-like cages than in the cages filled with room air, and the burrow fleas also laid 25 percent fewer eggs. In a second experiment where fleas kept in the same “burrow” or room air conditions, but without the jirds, the fleas in the simulated burrow air again had higher mortality rates and were also less mobile. 

Fleas don’t do well in stale burrow air, contrary to what Downs expected. But maybe they don’t need to, she now thinks. Fleas are fairly cosmopolitan and can infest a variety of different mammal species. With many hosts to choose from, they might not have had to adapt to underground living and the high CO2 levels in burrows.

As for why the fleas couldn’t cope with the carbon dioxide, Downs think that the gas forces them to increase their respiration to get the oxygen they need. The longer they hold their spiracles, or breathing tubes, open, the faster they dry out and die. As the fleas struggle to breathe, they also become less active, leaving them more vulnerable to being dislodged and killed when a jird scratches or grooms itself. They also spend less time feeding and drinking, which means they have fewer of the bodily resources they need to produce their eggs. It’s also possible that burrows are bad for fleas not only because of the direct effects on them, but also because of how the air conditions affect the jirds. The rodents are adapted to breathe the stale air, but the CO2 can still alter their body chemistry and immune functions, which might make their blood less nutritious for the fleas and contribute to their demise. 

Jirds and other burrowing animals have ways of limiting the CO2 levels in their homes down by keeping the entrances unsealed or adding vents. Yet, not all of them do that, and Downs says that her results may help explain why. If high CO2 levels kill fleas off and help with jirds’ parasite problems, that could be one reason they design their burrows the way they do. 

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How Does Catnip Work?
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If you have a cat, you probably keep a supply of catnip at home. Many cats are irresistibly drawn to the herb, and respond excitedly to its scent, rubbing against it, rolling around on the floor, and otherwise going nuts. There are few things that can get felines quite as riled up as a whiff of catnip—not even the most delicious treats. But why does catnip, as opposed to any other plant, have such a profound effect on our feline friends?

Catnip, or Nepeta cataria, is a member of the mint family. It contains a compound called nepetalactone, which is what causes the characteristic catnip reaction. Contrary to what you might expect, the reaction isn’t pheromone related—even though pheromones are the smelly chemicals we usually associate with a change in behavior. While pheromones bind to a set of specialized receptors in what’s known as a vomeronasal organ, located in the roof of a cat's mouth (which is why they sometimes open their mouths to detect pheromones), nepetalactone binds to olfactory receptors at the olfactory epithelium, or the tissue that lines the mucus membranes inside a cat’s nose and is linked to smell.

Scientists know the basics of the chemical structure of nepetalactone, but how it causes excitement in cats is less clear. “We don’t know the full mechanisms of how the binding of these compounds to the receptors in the nose ultimately changes their behavior,” as Bruce Kornreich, associate director of the Cornell Feline Health Center, tells Mental Floss. Sadly, sticking a bunch of cats in an MRI machine with catnip and analyzing their brain activity isn’t really feasible, either from a practical or a financial standpoint, so it’s hard to determine which parts of a cat’s brain are reacting to the chemical as they frolic and play.

Though it may look like they’re getting high, catnip doesn’t appear to be harmful or addictive to cats. The euphoric period only lasts for a short time before cats become temporarily immune to its charms, meaning that it’s hard for them to overdo it.

“Cats do seem to limit themselves," Michael Topper, president of the American Veterinary Medical Association, tells Mental Floss. "Their stimulation lasts for about 10 minutes, then it sort of goes away.” While you may not want to turn your house into a greenhouse for catnip and let your feline friend run loose, it’s a useful way to keep indoor cats—whose environment isn’t always the most thrilling—stimulated and happy. (If you need proof of just how much cats love this herb, we suggest checking out Cats on Catnip, a new book of photography from professional cat photographer Andrew Martilla featuring dozens of images of cats playing around with catnip.)

That said, not all cats respond to catnip. According to Topper, an estimated 70 percent of cats react to catnip, and it appears to have a genetic basis. Topper compares it to the genetic variation that causes some individuals to smell asparagus pee while others don’t. Even if a cat will eventually love the smell of catnip, it doesn’t come out of the womb yearning for a sniff. Young kittens don’t show any behavioral response to it, and may not develop one until several months after birth [PDF].

But some researchers contend that more cats may respond to catnip than we actually realize. In one 2017 study, a group of researchers in Mexico examined how cats might subtly respond to catnip in ways that aren’t always as obvious as rolling around on the floor with their tongue hanging out. It found that 80 percent of cats responded to catnip in a passive way, showing decreased motor activity and sitting the “sphinx” position, an indicator of a relaxed state.

There are also other plants that have similar effects on cats, some of which may appeal to a wider variety of felines than regular old catnip. In a 2017 study in the journal BMC Veterinary Research, researchers tested feline responses to not just catnip, but several other plants containing compounds similar in structure to nepetalactone, like valerian root, Tatarian honeysuckle, and silver vine. They found that 94 percent of cats responded to at least one of the plants, if not more than one. The majority of the cats that didn’t respond to catnip itself did respond to silver vine, suggesting that plant might be a potential alternative for cats that seem immune to catnip’s charms.

Despite the name, domestic cats aren’t the only species that love catnip. Many other feline species enjoy it, too, including lions and jaguars, though tigers are largely indifferent to it. The scent of the plant also attracts butterflies. (However, no matter what you’ve heard, humans can’t get high off it. When made into a tea, though, it reportedly has mild sedative effects.)

The reason Nepeta cataria releases nepetalactone doesn’t necessarily have to do with giving your cat a buzz. The fact that it gives cats that little charge of euphoria may be purely coincidental. The chemical is an insect repellant that the plant emits as a defense mechanism against pests like aphids. According to the American Chemical Society, nepetalactone attracts wasps and other insect predators that eat aphids, calling in protective reinforcements when the plant is in aphid-related distress. That it brings all the cats to the yard is just a side effect.

Because of this, catnip may have even more uses in the future beyond sending cats into a delighted frenzy. Rutgers University has spent more than a decade breeding a more potent version of catnip, called CR9, which produces more nepetalactone. It’s not just a matter of selling better cat toys; since catnip releases the compound to ward off insects, it’s also a great mosquito repellant, one that scientists hope can one day be adapted for human use. In that case, you might be as excited about catnip as your cat is.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Good News, Dog Parents: You Can Teach Puppies as Well as Their Canine Moms Can
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If you’ve ever adopted a puppy, you probably know how frustrating it can be to teach your new family member the basic tenets of common decency, like not to pee on the carpet or tear up a whole roll of toilet paper.

In other areas, though, pups are rather impressive learners, capable of mimicking some human behaviors. In fact, for some tasks, they learn just as effectively from watching people as they do from watching other dogs, including their own mothers, a new study in Nature revealed.

Researchers from Hungary and the UK took 48 young puppies of various breeds and studied the conditions under which they can be taught to open a puzzle box containing food. The experiment revealed that the puppies were able to learn how to open the box regardless of whether the task was first demonstrated by a person, their mother, or an unfamiliar dog. In other words, not only are puppies capable of social learning, but they're able to learn tasks from humans they don't know—in this case, the experimenter.

However, researchers were surprised to learn that the puppies were more likely to learn how to open the box by watching an unfamiliar dog than by watching their own mothers. That may be because puppies spend more time looking at—and thus, learning from—an unfamiliar dog that intrigues them. This differs from other species such as kittens, which “learn to press a lever for food more rapidly from their mother than from an unfamiliar adult,” the study notes.

In addition, the puppies were able to perform the task again after a one-hour break, indicating that they had retained some memory of the learning experience.

The ability of dogs to learn from humans has been recorded in previous research. A 2015 study revealed that dogs learn better by demonstration (or the “do as I do” method) than training techniques that involve a system of punishments and rewards. The "do as I do" approach probably isn't the most practical method of teaching your pup to do its business outside, but if you already have an adult dog at home, your new puppy can follow the older dog's lead and learn by example.

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