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Why Do Fleas, Ticks, and Mosquitoes Show Individual Preferences?

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Why do fleas, ticks, and mosquitoes show individual preference?

Tirumalai Kamala:

Biting insects (bugs, fleas, flies, mites, mosquitoes, ticks) locate and bite their blood host targets from the chemical cues they release. Such cues are volatile organic compound (VOC) produced by their skin microbes after they metabolize human skin gland secretions, i.e., an individual's VOC profile is largely the product of their skin flora. Thus, biting preference is the outcome of how each biting insect's odorant receptors detect the VOCs unique to the individual it bites.

Skin glands include apocrine and eccrine sweat glands, and sebaceous glands (see below from 1).

Skin glands are differentially distributed across the body and human skin microbe abundance matches theirs (see below from 1).

The human odor profile consists of >400 compounds (2). Research on which ones are most important in attracting biting insects is very much in its infancy.

One small study (n = 48 adult male volunteers) on the African malaria mosquito Anopheles gambiae sensu stricto found that individuals the mosquitoes found highly attractive had different skin bacteria compared to individuals they found poorly attractive, specifically greater abundance but lower diversity of skin-associated bacteria (see below from 3).

In another small study (n = 48 adult male volunteers) Anopheles gambiae sensu stricto found individuals carrying the human leukocyte antigen gene Cw*07 more attractive (4). Since different individuals have different HLA haplotypes,

  • Each individual's unique HLA system generates different peptides, i.e., source material their skin-associated microbes metabolize and convert to VOCs is unique.
  • Each individual's unique HLA is involved in the immunological processes that culminate in their unique microbial profile since immune responses select which microbes to keep or reject.

Individual genetics also influence skin temperature and humidity profiles, and metabolic rate, which are other factors that influence individuals' differential attractiveness to biting insects. Metabolic rate influences local carbon dioxide levels, which along with ammonia and lactic acid and other aliphatic carboxylic acids influence landing rates of biting insects like mosquitoes (5).

Each human thus has a largely individual VOC profile, product of their unique genetics and unique skin microbial profile. In turn, biting insects each have their specific odorant receptors. Combinations of these two parameters likely make some humans more attractive to each such biting insect compared to others. Research on this topic is still nascent and there's more data for disease-carrying mosquitoes than for other biting insects.

Since human lifestyle, especially diet, can actively sculpt human microbiota profiles, it's likely future research will reveal how different diets could influence an individual's VOC profile and in turn increase or decrease a biting insects's preference for a particular individual.

Similar processes likely explain the differences between dogs who get ticks versus those who don't. However, in the case of ticks that's only the first step since immune status probably determines whether or not they successfully establish an infection, healthier dogs fending off ticks that could stably colonize less healthy ones.

Bibliography

1. Verhulst, Niels O., et al. "Chemical ecology of interactions between human skin microbiota and mosquitoes." FEMS microbiology ecology 74.1 (2010): 1-9.

2. Verhulst, Niels O., and Willem Takken. "Skin Microbiota and Attractiveness to Mosquitoes." Encyclopedia of Metagenomics. Springer US, 2015. 591-595.

3. Verhulst, Niels O., et al. "Composition of human skin microbiota affects attractiveness to malaria mosquitoes." PloS one 6.12 (2011): e28991.

4. Verhulst, Niels O., et al. "Relation between HLA genes, human skin volatiles and attractiveness of humans to malaria mosquitoes." Infection, Genetics and Evolution 18 (2013): 87-93.

5. Smallegange, Renate C., Niels O. Verhulst, and Willem Takken. "Sweaty skin: an invitation to bite?." Trends in parasitology 27.4 (2011): 143-148.

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Big Questions
Why Do Cats Freak Out After Pooping?
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Cats often exhibit some very peculiar behavior, from getting into deadly combat situations with their own tail to pouncing on unsuspecting humans. Among their most curious habits: running from their litter box like a greyhound after moving their bowels. Are they running from their own fecal matter? Has waste elimination prompted a sense of euphoria?

Experts—if anyone is said to qualify as an expert in post-poop moods—aren’t exactly sure, but they’ve presented a number of entertaining theories. From a biological standpoint, some animal behaviorists suspect that a cat bolting after a deposit might stem from fears that a predator could track them based on the smell of their waste. But researchers are quick to note that they haven’t observed cats run from their BMs in the wild.

Biology also has a little bit to do with another theory, which postulates that cats used to getting their rear ends licked by their mother after defecating as kittens are showing off their independence by sprinting away, their butts having taken on self-cleaning properties in adulthood.

Not convinced? You might find another idea more plausible: Both humans and cats have a vagus nerve running from their brain stem. In both species, the nerve can be stimulated by defecation, leading to a pleasurable sensation and what some have labeled “poo-phoria,” or post-poop elation. In running, the cat may simply be working off excess energy brought on by stimulation of the nerve.

Less interesting is the notion that notoriously hygienic cats may simply want to shake off excess litter or fecal matter by running a 100-meter dash, or that a digestive problem has led to some discomfort they’re attempting to flee from. The fact is, so little research has been done in the field of pooping cat mania that there’s no universally accepted answer. Like so much of what makes cats tick, a definitive motivation will have to remain a mystery.

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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Big Questions
Why Do Baseball Managers Wear Uniforms?
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Basketball and hockey coaches wear business suits on the sidelines. Football coaches wear team-branded shirts and jackets and often ill-fitting pleated khakis. Why are baseball managers the only guys who wear the same outfit as their players?

According to John Thorn, the official historian of Major League Baseball since 2011, it goes back to the earliest days of the game. Back then, the person known as the manager was the business manager: the guy who kept the books in order and the road trips on schedule. Meanwhile, the guy we call the manager today, the one who arranges the roster and decides when to pull a pitcher, was known as the captain. In addition to managing the team on the field, he was usually also on the team as a player. For many years, the “manager” wore a player’s uniform simply because he was a player. There were also a few captains who didn’t play for the team and stuck to making decisions in the dugout, and they usually wore suits.

With the passing of time, it became less common for the captain to play, and on most teams they took on strictly managerial roles. Instead of suits proliferating throughout America’s dugouts, though, non-playing captains largely hung on to the tradition of wearing a player's uniform. By the early to mid 20th century, wearing the uniform was the norm for managers, with a few notable exceptions. The Philadelphia Athletics’s Connie Mack and the Brooklyn Dodgers’s Burt Shotton continued to wear suits and ties to games long after it fell out of favor (though Shotton sometimes liked to layer a team jacket on top of his street clothes). Once those two retired, it’s been uniforms as far as the eye can see.

The adherence to the uniform among managers in the second half of the 20th century leads some people to think that MLB mandates it, but a look through the official major league rules [PDF] doesn’t turn up much on a manager’s dress. Rule 1.11(a) (1) says that “All players on a team shall wear uniforms identical in color, trim and style, and all players’ uniforms shall include minimal six-inch numbers on their backs" and rule 2.00 states that a coach is a "team member in uniform appointed by the manager to perform such duties as the manager may designate, such as but not limited to acting as base coach."

While Rule 2.00 gives a rundown of the manager’s role and some rules that apply to them, it doesn’t specify that they’re uniformed. Further down, Rule 3.15 says that "No person shall be allowed on the playing field during a game except players and coaches in uniform, managers, news photographers authorized by the home team, umpires, officers of the law in uniform and watchmen or other employees of the home club." Again, nothing about the managers being uniformed.

All that said, Rule 2.00 defines the bench or dugout as “the seating facilities reserved for players, substitutes and other team members in uniform when they are not actively engaged on the playing field," and makes no exceptions for managers or anyone else. While the managers’ duds are never addressed anywhere else, this definition does seem to necessitate, in a roundabout way, that managers wear a uniform—at least if they want to have access to the dugout. And, really, where else would they sit?

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