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.
To celebrate World Penguin Day (which is today, April 25), here are a few fun facts about these adorable tuxedoed birds.
1. All 17 species of penguins are found exclusively in the Southern Hemisphere.
2. Emperor Penguins are the tallest species, standing nearly 4 feet tall. The smallest is the Little Blue Penguin, which is only about 16 inches.
3. The fastest species is the Gentoo Penguin, which can reach swimming speeds up to 22 mph.
4. A penguin's striking coloring is a matter of camouflage; from above, its black back blends into the murky depths of the ocean. From below, its white belly is hidden against the bright surface.
5. Fossils place the earliest penguin relative at some 60 million years ago, meaning an ancestor of the birds we see today survived the mass extinction of the dinosaurs.
6. Penguins ingest a lot of seawater while hunting for fish, but a special gland behind their eyes—the supraorbital gland—filters out the saltwater from their blood stream. Penguins excrete it through their beaks, or by sneezing.
7. Unlike most birds—which lose and replace a few feathers at a time—penguins molt all at once, spending two or three weeks land-bound as they undergo what is called the catastrophic molt.
8. All but two species of penguins breed in large colonies of up to a thousand birds.
9. It varies by species, but many penguins will mate with the same member of the opposite sex season after season.
10. Similarly, most species are also loyal to their exact nesting site, often returning to the same rookery in which they were born.
11. Some species create nests for their eggs out of pebbles and loose feathers. Emperor Penguins are an exception: They incubate a single egg each breeding season on the top of their feet. Under a loose fold of skin is a featherless area with a concentration of blood vessels that keeps the egg warm.
12. In some species, it is the male penguin which incubates the eggs while females leave to hunt for weeks at a time. Because of this, pudgy males—with enough fat storage to survive weeks without eating—are most desirable.
13. Penguin parents—both male and female—care for their young for several months until the chicks are strong enough to hunt for food on their own.
14. If a female Emperor Penguin's baby dies, she will often "kidnap" an unrelated chick.
15. Despite their lack of visible ears, penguins have excellent hearing and rely on distinct calls to identify their mates when returning to the crowded breeding grounds.
16. The first published account of penguins comes from Antonio Pigafetta, who was aboard Ferdinand Magellan's first circumnavigation of the globe in 1520. They spotted the animals near what was probably Punta Tombo in Argentina. (He called them "strange geese.")
17. An earlier, anonymous diary entry from Vasco da Gama's 1497 voyage around the Cape of Good Hope makes mention of flightless birds as large as ducks.
18. Because they aren't used to danger from animals on solid ground, wild penguins exhibit no particular fear of human tourists.
19. Unlike most sea mammals—which rely on blubber to stay warm—penguins survive because their feathers trap a layer of warm air next to the skin that serves as insulation, especially when they start generating muscular heat by swimming around.
20. In the 16th century, the word penguin actually referred to great auks (scientific name: Pinguinus impennis), a now-extinct species that inhabited the seas around eastern Canada. When explorers traveled to the Southern Hemisphere, they saw black and white birds that resembled auks, and called them penguins.
Though they're often described as "rats with wings" (a phrase popularized by the movie Stardust Memories), pigeons are actually pretty cool. From homing instincts to misleading rump feathers, here are 15 things you might not know about these avian adventurers.
1. THEY MIGHT BE THE FIRST DOMESTICATED BIRD.
The common city pigeon (Columba livia), also known as the rock pigeon, might be the first bird humankind ever domesticated. You can see them in art dating back as far as 4500 BCE in modern Iraq, and they've been a valuable source of food for thousands of years.
2. THEY WON OVER CHARLES DARWIN—AND NIKOLA TESLA.
Pigeon-breeding was a common hobby in Victorian England for everyone from well-off businessmen to average Joes, leading to some fantastically weird birds. Few hobbyists had more enthusiasm for the breeding process than Charles Darwin, who owned a diverse flock, joined London pigeon clubs, and hobnobbed with famous breeders. Darwin's passion for the birds influenced his 1868 book The Variation of Animals and Plants Under Domestication, which has not one but two chapters about pigeons (dogs and cats share a single chapter).
Nikola Tesla was another great mind who enjoyed pigeons. He used to care for injured wild pigeons in his New York City hotel room. Hands down, Tesla's favorite was a white female—about whom he once said, "I loved that pigeon, I loved her as a man loves a woman and she loved me. When she was ill, I knew and understood; she came to my room and I stayed beside her for days. I nursed her back to health. That pigeon was the joy of my life. If she needed me, nothing else mattered. As long as I had her, there was a purpose in my life." Reportedly, he was inconsolable after she died.
3. THEY UNDERSTAND SPACE AND TIME.
In a 2017 Current Biologystudy, researchers showed captive pigeons a series of digital lines on a computer screen for either two or eight seconds. Some lines were short, measuring about 2.3 inches across; others were four times longer. The pigeons were trained to evaluate either the length of the line or how long it was displayed. They found that the more time a line was displayed, the longer in length the pigeon judged it to be. The reverse was true too: If the pigeons encountered a longer line, they thought it existed in time for a greater duration. Pigeons, the scientists concluded, understand the concepts of both time and space; the researchers noted "similar results have been found with humans and other primates."
It's thought that humans process those concepts with a brain region called the parietal cortex; pigeon brains lack that cortex, so they must have a different way of judging space and time.
4. THEY CAN FIND THEIR WAY BACK TO THE NEST FROM 1300 MILES AWAY.
The birds can do this even if they've been transported in isolation—with no visual, olfactory, or magnetic clues—while scientists rotate their cages so they don't know what direction they're traveling in. How they do this is a mystery, but people have been exploiting the pigeon's navigational skills since at least 3000 BCE, when ancient peoples would set caged pigeons free and follow them to nearby land.
Their navigational skills also make pigeons great long-distance messengers. Sports fans in ancient Greece are said to have used trained pigeons to carry the results of the Ancient Olympics. Further east, Genghis Khan stayed in touch with his allies and enemies alike through a pigeon-based postal network.
5. THEY SAVED THOUSANDS OF HUMAN LIVES DURING WORLD WARS I AND II.
Pigeons' homing talents continued to shape history during the 20th century. In both World Wars, rival nations had huge flocks of pigeon messengers. (America alone had 200,000 at its disposal in WWII.) By delivering critical updates, the avians saved thousands of human lives. One racing bird named Cher Ami completed a mission that led to the rescue of 194 stranded U.S. soldiers on October 4, 1918.
6. TWO PIGEONS ALMOST DISTRACTED FROM THE DISCOVERY OF EVIDENCE OF THE BIG BANG.
In 1964, scientists in Holmdel, New Jersey, heard hissing noises from their antenna that would later prove to be signals from the Big Bang. But when they first heard the sound, they thought it might be, among other things, the poop of two pigeons that were living in the antenna. "We took the pigeons, put them in a box, and mailed them as far away as we could in the company mail to a guy who fancied pigeons," one of the scientists later recalled. "He looked at them and said these are junk pigeons and let them go and before long they were right back." But the scientists were able to clean out the antenna and determine that they had not been the cause of the noise. The trap used to catch the birds (before they had to later be, uh, permanently removed) is on view at the Smithsonian Air & Space Museum.
7. YOU CAN TRAIN THEM TO BE ART SNOBS …
Japanese psychologist Shigeru Watanabe and two colleagues earned an Ig Nobel Prize in 1995 for training pigeons, in a lab setting, to recognize the paintings of Claude Monet and Pablo Picasso and to distinguish between the painters. The pigeons were even able to use their knowledge of impressionism and cubism to identify paintings of other artists in those movements. Later, Watanabe taught other pigeons to distinguish watercolor images from pastels. And in a 2009 experiment, captive pigeons he'd borrowed were shown almost two dozen paintings made by students at a Tokyo elementary school, and were taught which ones were considered "good" and which ones were considered "bad." He then presented them with 10 new paintings and the avian critics managed to correctly guess which ones had earned bad grades from the school's teacher and a panel of adults. Watanabe's findings indicate that wild pigeons naturally categorize things on the basis of color, texture, and general appearance.
8. … AND TO DISTINGUISH WRITTEN WORDS.
In a 2016 study, scientists showed that pigeons can differentiate between strings of letters and actual words. Four of the birds built up a vocabulary of between 26 and 58 written English words, and though the birds couldn't actually read them, they could identify visual patterns and therefore tell them apart. The birds could even identify words they hadn't seen before.
9. FLUFFY PIGEON FEET MIGHT ACTUALLY BE PARTIAL WINGS.
A few pigeon breeds have fuzzy legs—which hobbyists call "muffs"—rather than scaly ones. According to a 2016 study, the DNA of these fluffy-footed pigeons leads their hind legs to take on some forelimb characteristics, making muffed pigeon legs look distinctly wing-like; they're also big-boned. Not only do they have feathers, but the hindlimbs are somewhat big-boned, too. According to biologist Mike Shapiro, who led the study, "pigeons' fancy feathered feet are partially wings."
10. SOME PIGEONS DISTRACT FALCONS WITH WHITE RUMP FEATHERS.
In a life-or-death situation, a pigeon's survival could depend upon its color pattern: Research has shown that wild falcons rarely go after pigeons that have a white patch of feathers just above the tail, and when the predators do target these birds, the attacks are rarely successful.
To figure out why this is, Ph.D. student Alberto Palleroni and a team tagged 5235 pigeons in the vicinity of Davis, California. Then, they monitored 1485 falcon-on-pigeon attacks over a seven-year span. The researchers found that although white-rumped pigeons comprised 20 to 25 percent of the area's pigeon population, they represented less than 2 percent of all the observed pigeons that were killed by falcons; the vast majority of the victims had blue rumps. Palleroni and his team rounded up 756 white- and blue-rumped pigeons and swapped their rump feathers by clipping and pasting white feathers on blue rumps, and vice versa. The falcons had a much easier time spotting and catching the newly blue-rumped pigeons, while the pigeons that received the white feathers saw predation rates plummet.
Close observation revealed that the white patches distract birds of prey. In the wild, falcons dive-bomb other winged animals from above at high speeds. Some pigeons respond by rolling away in midair, and on a spiraling bird, white rump feathers can be eye-catching, which means that a patch of them may divert a hungry raptor's focus long enough to make the carnivore miscalculate and zip right past its intended victim.
11. DODOS WERE RELATED TO TODAY'S PIGEONS.
Though most of this list focuses on the rock pigeon, there are 308 living species of pigeons and doves. Together, they make up an order of birds known as the columbiformes. The extinct dodo belonged to this group as well.
Flightless and (somewhat) docile, dodos once inhabited Mauritius, an island near Madagascar. The species had no natural predators, but when human sailors arrived with rats, dogs, cats, and pigs, it began to die out, and before the 17th century came to a close, the dodo had vanished altogether. DNA testing has confirmed that pigeons are closely related to the dodo, and the vibrant Nicobar pigeon (above) is its nearest genetic relative. A multi-colored bird with iridescent feathers, this near-threatened creature is found on small islands in the South Pacific and off Asia. Unlike the dodo, it can fly.
12. AT ONE POINT, MORE THAN ONE-QUARTER OF ALL THE BIRDS LIVING IN THE U.S. MAY HAVE BEEN PASSENGER PIGEONS.
Wild/feral rock pigeons reside in all 50 states, which makes it easy to forget that they're invasive birds. Originally native to Eurasia and northern Africa, the species was (most likely) introduced to North America by French settlers in 1606. At the time, a different kind of columbiform—this one indigenous—was already thriving there: the passenger pigeon (Ectopistes migratorius). As many as 5 billion of them were living in America when England, Spain, and France first started colonizing, and they may have once represented anywhere from 25 to 40 percent of the total U.S. bird population. But by the early 20th century, they had become a rare sight, thanks to overhunting, habitat loss, and a possible genetic diversity issue. The last known passenger pigeon—a captive female named Martha—died on September 1, 1914.
13. THEY'RE REALLY GOOD AT MULTITASKING.
According to one study, they're more efficient multitaskers than people are. Scientists at Ruhr-Universitat Bochum put together a test group of 15 humans and 12 pigeons and trained all of them to complete two simple jobs (like pressing a keyboard once a light bulb came on). They were also put in situations wherein they'd need to stop working on one job and switch over to another. In some trials, the participants had to make the change immediately. During these test runs, humans and pigeons switched between jobs at the same speed.
But in other trials, the test subjects were allowed to complete one assignment and then had to wait 300 milliseconds before moving on to the next job. Interestingly, in these runs, the pigeons were quicker to get started on that second task after the period ended. In the avian brain, nerve cells are more densely packed, which might enable our feathered friends to process information faster than we can under the right circumstances.
14. PIGEONS PRODUCE FAKE "MILK."
Only mammals produce genuine milk, but pigeons and doves (along with some other species of birds) feed their young with something similar—a whitish liquid filled with nutrients, fats, antioxidants, and healthy proteins called "crop milk." Both male and female pigeons create the milk in the crop, a section of the esophagus designed to store food temporarily. As is the case with mammal milk, the creation of crop milk is regulated by the hormone prolactin. Newly-hatched pigeons drink crop milk until they're weaned off it after four weeks or so. (And if you've ever asked yourself, "Where are all the baby pigeons?" we have the answer for you right here.)
15. ONE STUDY SUGGESTS THAT, GIVEN THE RIGHT CONDITIONS, THEY'RE AS GOOD AT IDENTIFYING CANCER AS DOCTORS.
We've already established that pigeons are excellent at differentiating between artists and words, but a 2015 study revealed they can also distinguish between malignant and benign growths in the right conditions. Researchers at University of California Davis Medical Center put 16 pigeons in a room with magnified biopsies of potential breast cancers. If the pigeons correctly identified them as either benign or malignant, they got a treat, According to Scientific American.
"Once trained, the pigeons' average diagnostic accuracy reached an impressive 85 percent. But when a "flock sourcing" approach was taken, in which the most common answer among all subjects was used, group accuracy climbed to a staggering 99 percent, or what would be expected from a pathologist. The pigeons were also able to apply their knowledge to novel images, showing the findings weren't simply a result of rote memorization."
Mammograms proved to be more of a challenge, however; the birds could memorize signs of cancer in the images they were trained on but could not identify the signs in new images.
No matter how impressive their results, "I don't anticipate that pigeons, no matter how good they become at pathology or radiology, will be playing a role in actual patient care—certainly for the foreseeable future," study co-author Richard M. Levenson told Scientific American. "There are just too many regulatory barriers—at least in the West."