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TWiP 73 letters

Robin writes:


Liquids are capable of forming a gas-liquid interface: maintenance of the surface requires adequate pressure in the gas. Adequate in this instance means a gas pressure greater than the vaporisation pressure in the liquid. The gas pressure is a sort of cork preventing the liquid from boiling away.

The vaporisation pressure increases with the temperature. When the vaporisation pressure equals the gas pressure, the liquid starts to boil. In the absence of any gas pressure, a liquid-gas interface cannot be maintained: that is why astronauts' blood would boil if they are exposed to the vacuum of space.

Also in the vacuum of space, when a solid is heated to its melting point it will convert directly into gas, the process being known as sublimation. In earth's atmosphere if the solid's vaporisation pressure at its melting point is greater than the atmospheric pressure, it will transit directly from solid to gas, as in the case of carbon dioxide.

Toads vs. frogs:

While toads are not a taxonomic group, use of the word "toad" implies the exclusion of or differentiation from frogs.

Toads tend to be more terrestrially adapted, with a dry skin, no cutaneous respiration, no webbing between the toes, and spending most of their adult lives on terra firma.

Reptiles are a paraphyletic group:


“a group is said to be paraphyletic if it consists of all the descendants of the last common ancestor of the group's members minus a small number of monophyletic groups of descendants, typically just one or two such groups. Such a group is said to be paraphyletic with respect to the excluded groups. For example, the group of reptiles, as traditionally defined, is paraphyletic with respect to the mammals and birds: it contains the last common ancestor of the reptiles—including the extant reptiles as well as the extinct mammal-like reptiles—along with all descendants of that ancestor except for mammals and birds. ”

Dinosaurs are tossed into the reptilian bin, but if they had been around to think and opine about it, they might have demurred:


“In addition to the living reptiles, there are many diverse groups that are now extinct, in some cases due to mass extinction events. In particular, the K–Pg extinction wiped out the pterosaurs, plesiosaurs, ornithischians, and sauropods, as well as many species of theropods (e.g. tyrannosaurs and dromaeosaurids), crocodyliforms, and squamates (e.g. mosasaurids)."

Infraclass Archosauromorpha
Order Prolacertiformes
Division Archosauria
Subdivision Crurotarsi
Superorder Crocodylomorpha
Order Crocodilia
Order Phytosauria
Order Rauisuchia
Order Rhynchosauria
Subdivision Avemetatarsalia
Infradivision Ornithodira
Order Pterosauria
Superorder Dinosauria
Order Ornithischia
Order Saurischia

Dinosaurs are lumped with reptiles:


“Although the word dinosaur means "terrible lizard", the name is somewhat misleading, as dinosaurs are not lizards. Instead, they represent a separate group of reptiles that, like many extinct forms, did not exhibit characteristics traditionally seen as reptilian, such as a sprawling limb posture or ectothermy.”

"Ship's captains flaunting this"



1. To exhibit ostentatiously or shamelessly: flaunts his knowledge. See Synonyms at show.
2. Usage Problem To show contempt for; scorn.



To show contempt for; scorn: flout a law; behavior that flouted convention. See Usage Note at flaunt.

John writes:

Dear TWiP Team,

On the latest episode you insinuated perhaps that the host was chosen for his looks and why couldn't they find someone that actually studied parasites. Well, as it happens, the host, Dan Riskin, just did an AMA (ask me anything) on reddit.com His expertise and passion is actually bats but he has become a science communicator full time.

When asked how he ended up as a science presenter his response was this:

"I was working as a scientist in a basement, when the phone rang. A production company was doing a show about evolution (called Evolve, for the History Channel), and they needed "an evolutionary biologist who is not an old man." They'd gotten my name from someone who had seen me give talks at conferences."


So yes, he was picked for his looks! Showbiz.




Robin writes:

Greetings Vincent and Dickson,

A quick note from one of your probable many silent listeners. I am enjoying the TWIV and TWIP podcasts very much! Great learning to supplement my background in ecology and public health. Quick followup on the meaning of "invasive". I think the student of ecology (Vincent) got this one right over the teacher (Dickson). I believe invasive does mean it takes over and outcompetes native species, whereas exotic is something occurring where it isn't natural. There are exotic species that are not considered invasive, although it is probably a matter of degree since any exotic species would compete with natives for resources at some level. Perhaps, that is what Dickson was implying?

Keep up the great work.



Missoula, MT

Bobbi writes:

Dear Twip group, first I'd like to say that I love your program - great work! Second, I wanted to introduce you to my Parasite Case of the Week blog at http://parasitewonders.blogspot.com

We just hit our 300th case. It's a simple blog - short cases and short answers the following week. I'd be happy to talk to you about it if you're interested. Best wishes,

Bobbi Pritt, MD, MSc, DTMH
Director of Parasitology
Mayo Clinic
Rochester, MN

John writes:


iTunes says it has been almost a month since your last episode. It is spring (15 C in Massachusetts) and time for you to emerge from hibernation.

I wanted to call your attention to two related papers on nonhuman parasites. I mean, parasites infecting nonhuman hosts.

I was disappointed to learn from an early TWiP episode that tapeworms do not cause ravenous hunger. So my parents were wrong about the reason for my appetite as a child.

Some rodent biologists (that is, human biologists studying rodents) got to wondering what parasites did to the appetites of their favorite mammals.

Researchers treated African ground squirrels with ivermectin and fipronil. Treated squirrels stopped itching and spent more time eating. Surprisingly, they did not gain weight. Parasites had depressed the squirrels' resting metabolic rate. Parasite-free squirrels had to eat more to maintain normal weight.

Despite not gaining weight, treated squirrels did have a lot more babies. So they were healthier.

Now when is Merck going to donate a billion doses of ivermectin to the squirrels of the world?


Scantlebury M. et al. 2007. Energetic costs of parasitism in the Cape
ground squirrel Xerus inauris. Proc R Soc Lond B Biol Sci


Hullegass, M. et al. 2010. Parasite removal increases reproductive
success in a social African ground squirrel. Behavioral Ecology


Cape Ground Squirrel on Wikpedia


Johnathan writes:

Dear Drs. Racaniello and Despommier,

Thanks for the wonderful job you do with TWIP and TWIV. I appreciate all the time and effort you put into the podcasts and look forward to each new episode.

I was wondering why you haven't yet given much if any attention to bed bugs on TWIP? They're arguably the most important human parasite in the United States and growing at an alarming rate. Twenty years ago bed bugs were extremely rare in the United States, but now cost the US economy an estimated $3 billion annually. The emergency department where I work is forced to close and decontaminate a main-treatment area room approximately every three days for an average of 17 hours at a time due to a bed bug being found on a patient. While bed bugs are not vectors of human disease, they can cause itchy rashes and less commonly asthma and angioedema. Bed bugs are also associated with depression, social isolation, suicidal ideation, anxiety, nightmares and other mental health concerns. Of all the parasites you reasonable could acquire in the United States, bed bugs would have to be the least desired given their itchy bites, social stigma, difficulty with diagnosis and treatment, and the time and costs associated with eradicating them. The CDC neglected to place bed bugs on their list of the top neglected parasitic infections yet I can't think of a human parasite which more adversely affects the health and well being of Americans than bed bugs. Any thoughts?

Thanks and keep up the wonderful podcasting!

-John Sheele

Johnathan Sheele, MD, MPH, MHS
Assistant Professor, Department of Emergency Medicine
Associate Fellowship Director, Global Emergency Medicine
University Hospitals Case Medical Center
Case Western Reserve University

Jim writes:

Nothing new here, but it's in your region, so just in case it's of use: Ticks in NY

Jim, Smithfield, VA

Heather writes:

Hi Dr.s R&D!

Just a quick clarification: all invasive species are exotic species, but not all exotic species are invasive. Invasive species generally cause ecological, health, or economic problems. Dr. D.'s beloved trout are exotic across much of their current global range. In many areas where non-native trout, such as rainbows, have been introduced they are not considered invasive but they are exotic. Large and smallmouth bass are in a similar situation. However, brown trout in public waterways in the US are considered invasive. Pike in Alaska salmon rivers are one of the most recent and serious human-caused aquatic invasions.

Scott writes:

Dear Dick and Vincent,
Sarcoptic mange is having a significant impact on the current wolf population in the Yellowstone area. In addition mange is also affecting the fox and coyote population in the front range of Colorado, and I suspect the problem extends even further. I think people would enjoy listing to a program on mange. Where did it originate, is it the same scabies mite humans can get? I also have photographs of fox and coyote with mange you could post on your website.

Greg writes:

Greetings professors,

I am a soon-to-be first year veterinary student and long-time lover of parasitology. I just discovered TWiP and am working my way through the early episodes, where you discuss, among other things, the discovery of Trichinella spiralis. You mention Paget and Virchow, but do not mention Joseph Leidy. Leidy, a noted anatomist, paleontologist, parasitologist, and overall polymath (and one of my favorite scientists) is often credited with the discovery of encysted worms in pig flesh. In 1846 (several years before Virchow's work), Leidy made the connection between undercooked pork and human trichinosis, and advocated for the thorough cooking of meat before consumption. Apologies if you touch on this subject in a later episode. I'm working through them as fast as I can! Big fan of the show.





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