Fleas sometimes make very bad decisions, and one of the worst is to hop on a burrowing rodent, suggests a new study that found concentrations of carbon dioxide in such rodents' burrows often cause their parasites to slowly dry out and die.
It is believed that fleas, as for most other organisms, feel pain, so the findings - published in the Journal of Experimental Biology - suggest how fleas might suffer too.
The research also sheds light on how rodents and other parasite hosts manage to cope with fleas, not always becoming overwhelmed by them.
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Author Cynthia Downs from Hamilton College and co-author Irina Khokhlova of Ben Burion University of the Negev, Israel, faced a squeamish task during the experiment. Simulating how the flea Xenopsylla ramesis interacts with burrowing rodents in the wild, they raised gerbil-like rodents known as "jirds," along with the fleas, which were kept in a box.
The rodents weren't the problem.
"Overall, the jirds are fairly docile and very easy to work with," Downs told Kathryn Knight of the journal.
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Downs did, however, have to overcome apprehension when plunging her hands into the box of fleas before transferring them to the rodents. Luckily, this particular flea species does not bite humans.
The researchers carefully mixed CO2 with air to reproduce the atmospheric conditions that were measured in the tunnels of some burrowing rodent species. They replaced the rodents' population of 150 fleas every 3–6 days, counting the survivors to calculate the survival rate.
When the scientists compared the fleas' death rate with that of fleas from rodents above ground, they found that it skyrocketed in the stale air. The simulated burrow flea females also produced far fewer eggs than fleas that were seemingly enjoying life on their hosts in the fresh air.
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Downs suspects that fleas in burrows might hold their breathing tubes (spiracles) open for longer than fleas above ground, causing them to dry out faster and die.
It seems that fleas have not evolved effective ways of coping with CO2-filled air underground, probably because they hop on many different types of animals.
Humans also cannot tolerate high levels of CO2 found, such as when they're exposed to volcanic and geothermal gases.
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The rodents, however, seem to do just fine in their burrows, stale air and all. The new findings suggest how they and other underground dwellers avoid parasite infestations when in their burrows and hibernating.
The study could also help to explain why several small, burrowing animals survived the mass extinction of dinosaurs and many other species (but spared birds) about 65 million years ago. High volcanic activity is thought to have occurred at that time, perhaps giving those who could burrow or fly away from the toxic gases a better chance of survival.
Photo: The flea Xenopylla cheopsis (aka "the plague flea"), which is closely related to Xenopsylla ramesis. Credit: NHM