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. 

Big Questions
What Makes a Cat's Tail Puff Up When It's Scared?

Cats wear their emotions on their tails, not their sleeves. They tap their fluffy rear appendages during relaxing naps, thrash them while tense, and hold them stiff and aloft when they’re feeling aggressive, among other behaviors. And in some scary situations (like, say, being surprised by a cucumber), a cat’s tail will actually expand, puffing up to nearly twice its volume as its owner hisses, arches its back, and flattens its ears. What does a super-sized tail signify, and how does it occur naturally without help from hairspray?

Cats with puffed tails are “basically trying to make themselves look as big as possible, and that’s because they detect a threat in the environment," Dr. Mikel Delgado, a certified cat behavior consultant who studied animal behavior and human-pet relationships as a PhD student at the University of California, Berkeley, tells Mental Floss. The “threat” in question can be as major as an approaching dog or as minor as an unexpected noise. Even if a cat isn't technically in any real danger, it's still biologically wired to spring to the offensive at a moment’s notice, as it's "not quite at the top of the food chain,” Delgado says. And a big tail is reflexive feline body language for “I’m big and scary, and you wouldn't want to mess with me,” she adds.

A cat’s tail puffs when muscles in its skin (where the hair base is) contract in response to hormone signals from the stress/fight or flight system, or sympathetic nervous system. Occasionally, the hairs on a cat’s back will also puff up along with the tail. That said, not all cats swell up when a startling situation strikes. “I’ve seen some cats that seem unflappable, and they never get poofed up,” Delgado says. “My cats get puffed up pretty easily.”

In addition to cats, other animals also experience piloerection, as this phenomenon is technically called. For example, “some birds puff up when they're encountering an enemy or a threat,” Delgado says. “I think it is a universal response among animals to try to get themselves out of a [potentially dangerous] situation. Really, the idea is that you don't have to fight because if you fight, you might lose an ear or you might get an injury that could be fatal. For most animals, they’re trying to figure out how to scare another animal off without actually going fisticuffs.” In other words, hiss softly, but carry a big tail.

10 Notable Gestation Periods in the Animal Kingdom

The gestation periods of the animal kingdom are varied and fascinating. Some clock in at just a few weeks, making any human green with envy, while others can last more than a year. Here are 10 notable gestation times for animals around the globe. The lesson? Be thankful that you’re not a pregnant elephant.

1. ELEPHANTS: 640-660 DAYS

Elephants are pregnant for a long time. Like really, really long. At an average of 95 weeks, the gestation period is more than double the length of a human pregnancy, so it shouldn't come as a shock that female elephants don't often have more than four offspring during their lifetimes. Who has the time?


A photo of a mother hippo and her baby in Uganda

Yes, it takes less time to make a hippopotamus than it takes to make a human.


Baby giraffes can weigh more than 150 pounds and can be around 6 feet tall. Another fascinating tidbit: giraffes give birth standing up, so it's pretty normal for a baby to fall 6 feet to the ground.


There’s a reason for the long wait: after that 17 months, Baby Shamu emerges weighing anywhere from 265 to 353 pounds and measuring about 8.5 feet long. Yikes.

5. OPOSSUM: 12-13 DAYS

A baby opossum wrapped up in a blanket

Blink and you'll miss it: This is the shortest gestation period of any mammal in North America. But since the lifespan of an opossum is only two to four years, it makes sense.


Hey, they get off pretty easy.


It's not a huge surprise that their gestational periods are pretty similar to ours, right?


A pair of black bear cubs

Also less than a human. Interestingly, cubs might only be 6 to 8 inches in length at birth and are completely hairless. 


This is the longest gestation period of any rodent. Thankfully for the mother, porcupine babies (a.k.a. porcupettes) are actually born with soft quills, and it's not until after birth that they harden up.


Baby walruses? Kind of adorable. They certainly take their sweet time coming out, though.


More from mental floss studios