"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

September 22, 2020

Parapulex chephrenis

Here's the second student guest posts from the third year Evolutionary Parasitology unit (ZOOL329) class of 2020. This post was written by Patra Petrohilos and it is about the social life of Egyptian Spiny Mouse and how that relates to their fleas. (you can also read a previous post about how a muscle-dwelling worm survives under a cover of snow here).

It doesn’t require a particularly vivid imagination to appreciate that being eaten by fleas is not exactly the most stress-free experience for an animal. Neither (to the surprise of introverts nowhere) is being bullied into submission by the resident bossy boots in your social group. Surely, then, it would logically follow that being bullied by your peers AND preyed upon by parasites at the same time would be the most stressful option of all? That’s certainly what some researchers thought – and were stunned to discover that the answer was not quite what they expected.

Photo of spiny mouse from here, photo of Parapulex flea from here

Before we get any further, you may be wondering how exactly one measures the stress levels of an animal. I’m so glad you asked. Turns out, when we get stressed our bodies produce this stuff called glucocorticoids – which is such a long clunky word that I’ll just refer to it from here on in as GC. In the short term (let’s say we see a predator across the street) this is a good thing – a short burst of GC takes the energy that we’d usually spend on boring things like digesting food and diverts it to more useful activities – like running away from predators. But in the long term (let’s say we are trapped in a cage with that predator for a year) it is a very bad thing. Too much GC can do all kinds of awful things, wreaking havoc on our immune system and our fertility. Scientists can measure how much GC an animal is producing (and therefore how stressed out it is) by analysing its poo. It’s all pretty glamorous.

These particular scientists were interested in how two different negative experiences (parasitism and social interaction) interact to affect an animal’s stress levels. They decided to investigate this by studying the Egyptian spiny mouse (Acomys cahirinus) – an incredibly social little fella that is found living in groups of one male and multiple females. Within this little society, one of those females usually stakes a claim to “Queen Bee” of the group. Bizarrely, they are also especially attractive to one particular species of flea (Parapulex chephrenis), who for some reason steer clear of all other mouse species in favour of this one.

Once they had gathered their mice, the scientists split the females into two groups. The first consisted of pairs of mice, two to a cage. As tends to happen in these situations, one of the pair invariably emerged as the bossier one. This two-mouse hierarchy was well and truly established after a week, by which time the submissive one knew her place well enough to not even attempt to rock the social boat. The second group was divided into single ladies. Each mouse in this group got an entire cage to herself (and peace from any potential bickering over petty things like food).

They then divided the groups further. Half of the paired mice and half of the single ladies were infected with P. chephrenis fleas, while the other half were left flea-free. For a brief period, a male was also added to each cage (just long enough to do the kinds of things that male mice like to do with female mice) and then mouse poo was collected at various points so the scientists could gauge each mouse’s stress levels.

To their amazement, the single mice were more stressed than their paired up counterparts – even the ones being dominated by the bossy boots cagemates. Apparently company is so important to such a social species that being alone is more traumatic than being at the bottom of the pecking order. But even more astoundingly, it was the mice who were not only solitary but also flea-free that were more stressed out than anyone!

It’s possible that flea infestation made these already-anxious solitary mice more likely to indulge in a bit of grooming (a behaviour that tends to soothe rodents), but regardless – it’s fascinating that the results were the exact opposite of expected. Rather than one stressful thing exacerbating the other (like adding Carolina Reaper chili peppers to an already hot sauce would) they almost seemed to cancel each other out (like adding yogurt to a vindaloo curry).

So what’s the moral of the story? If you’re an Egyptian spiny mouse, even having awful, flea infested friends that bully you is better than having no friends at all. And for those poor waifs who don’t have friends - any distraction is preferable to the loneliness of a solitary life. Even when that distraction is being eaten by fleas.


This post was written by Patra Petrohilos


  1. Beautiful writing Patra - I really enjoyed your storytelling of this fascinating finding.

  2. Nice experiment! Maybe this behavior in these little mices is even relatable to some humans who prefer suffering in someone's hands rather than being alone.