Harry Potter, Cannibalism, and Parasites

I recently read this very interesting blog post about structuring your scientific talks like a Pixar movie. I thought it would be fun to structure some Parasite Ecology blog posts that way, so I’ve tried that for today’s post. I found it easier to model the post after a specific and well-loved story, so I’ve included the model story, too. If you want to follow along, the format is:

Once upon a time there was ___. Every day, ___. One day ___. Because of that, ___. Because of that, ___. Until finally ___.

MODEL STORY:

Once upon a time, there was a pet animal who did not seem to be particularly important to the age-old struggle between good and evil wizards: Scabbers the Rat.

Scabbers was not a glamorous animal, and was perhaps even somewhat revolting or pathetic. But anyone who knew him would have said that he was quite harmless – except, perhaps, for the people who had nearly had their fingers bitten off by Scabbers.

Every day, people went around thinking that Scabbers couldn’t possibly be a significant source of upheaval in the struggle between good and evil wizards.

After all, there were very many animals in the world, but how many had anything to do with important wizarding affairs? Even if you searched very hard, you would only find a few reports of notable pet rats. In fact, it seemed impossible that animals could ever cause much of a stir, because the actions of single animal could never spread among wizards as fast as the ideas or actions of the wizards themselves.

One day, Harry, Ron, and Hermoine found out that their simplifying assumptions about Scabbers had blinded them to a vital detail: Scabbers was actually a key player in the war between good and evil wizards who was hiding in plain sight.

By assuming that Scabbers was an animal, the Hogwarts students had not realized that Scabbers was actually a man. And that man, Peter Pettigrew, had been hiding as a rat for years after betraying The Potters to The Dark Lord.

Because of that, Harry, Ron, and Hermoine became very interested in Scabbers, and scrutinized his history carefully.

They even listened to a supposed murderer who had dragged them into a creepy old shack while he explained all about the role of animals in the struggle between good and evil wizards.

Because of that, Harry, Ron, and Hermoine found out that it is actually quite common and entirely plausible for animals to be key players in the war between good and evil wizards.

A terrifying black dog turned out to be none other than Harry’s Godfather – a very good wizard. And a very scary werewolf was actually Remus Lupin, an excellent professor and a member of the Order of the Pheonix. Even Harry’s own father had been an animagus.

Until finally, Harry, Ron, Hermoine stopped ignoring the potential for animals to be important wizards.

Hermoine even realized that the rotten Daily Prophet reporter, Rita Skeeter, was able to sneak into Hogwarts to find juicy information because she was an unregistered animagus who could turn into a beetle.

ACTUAL POST:

Once upon a time, there was an ecological interaction that did not seem to be particularly important to parasite transmission: cannibalism. 

Cannibalism is not glamorous, and is perhaps even somewhat revolting – like when big male polar bears eat little baby polar bears. But as far as parasite transmission is concerned, ecologists generally assume that cannibalism is quite harmless (except, perhaps, for the ecologists who have nearly had their fingers bitten off by cannibals?).

Every day, parasite ecologists went around thinking that cannibalism couldn’t possibly be a significant source of parasite transmission.

After all, cannibalism is very common, but how many parasite systems use cannibalism as a main transmission route? Even when parasite ecologists searched very hard, they only found a few systems where cannibalism was notably important to transmission (Rudolf and Antonovics 2007, Sadeh et al. 2007). In fact, it seemed impossible that cannibalism could be broadly important, because most acts of predation seem to involve one predator eating one prey. In order for a parasite to actually spread in a cannibalistic population, multiple cannibals would need to feed on each victim.

One day, Sadeh et al. (2016) found out that ecologists’ simplifying assumptions about cannibals had blinded them to a vital detail: cannibalism can actually be a key mechanism in parasite transmission that is hiding in plain sight.

By assuming that cannibalism occurs in homogeneous populations, ecologists had missed something important: age or stage structure may mediate the influence of cannibalism on parasite transmission.

Because of that, Sadeh et al. (2016) became very interested in cannibalism, and scrutinized the roles of cannibalism versus other modes of parasite transmission carefully.

In structured populations where older hosts are less susceptible to direct contact parasite transmission, cannibalism on younger hosts by older hosts can be an important mode of transmission, where parasites are moved from susceptible, young hosts who are experience high mortality risk to older hosts who may be more infectious when infected.

Because of that, ecologists found out that it is actually entirely plausible for cannibalism to be important to parasite transmission.

Age- or stage-structured populations are very common, and in many of the known examples of parasite transmission via cannibalism, the host population has age or stage structure. So the conditions required for cannibalism to be an important transmission route may occur in many systems.

Until finally, the person writing this blog post stopped ignoring the potential for cannibalism to be important to parasite transmission.

I can’t speak for everyone, but I think transmission-by-cannibalism is worthy of more attention by parasite ecologists. Because surely there are even more cool examples than the ones we already know about.

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