The enemy of my enemy is my friend


An acanthocephalan parasite

Acanthocephalan parasites are infamous for their ability to dramatically alter the behavior of their hosts.  These parasites often have multi-stage life cycles, meaning that they die unless the host that they’re currently residing in gets eaten by the next host in the cycle. Acanthocephalans are remarkably efficient at manipulating their intermediate hosts into ending up on the dinner plate of the next host in the cycle. 

One well-known example of this manipulation involves the acanthocephalan parasite Polymorphus minutus and its crustacean intermediate host Gammarus roeseli.  The definitive host of P. minutus is a waterbird which scoops G. roeseli out of the water.  The usually defense by G. roeseli to avoid being consumed by the waterbird is to hunker down at the bottom of a streambed and hide in the rubble. The parasite is capable of counteracting this defensive mechanism, presumably by altering the concentration of serotonin in the crustacean’s nervous system. This manipulation causes the the host to swim to the water surface and clamp down on the surrounding vegetation.  In a closely related system (a different species of Gammarus and a different Polymorphus parasite), this behavior has been found to significantly increase consumption of infected gammarids by waterbirds.  In essence, the parasite causes its tiny host to swim up to where the predators are feasting and hang out until it becomes dinner. 


Gammarus roeseli

Gammarus roeseli

The interests of G. roeseli and the parasite P. minutus are clearly not aligned when it comes to waterbird predation. There is, however, one thing that they can agree on.  Neither of them want to end up in the stomach of other predators (fishes, crustaceans, etc.). Non-waterbird predators represent a dead end for both members of the party.  The old addage “the enemy of my enemy is my friend” goes a long way here, and recent studies suggest that the parasite enhances the host’s ability to stay out of harm’s way when their interests converge. 

Three-spined sticklebacks
Three-spined sticklebacks

Medoc et al. 2009 have shown that gammarids infected with P. minutus have a leg up on their uninfected counterparts when it comes to avoiding predation by three-spined sticklebacks.   Infected gammarids spent more time hiding in vegetation near the water surface and suffered much lower predation rates.  Additionally, another recent study found that infected gammarids are up to 35% faster when escaping a predatory crustacean.  

This phenomenon has received little attention in the literature.  The few studies that have looked at whether or not parasites “help” their hosts escape from mutually unfavorable predators have reported mixed results.  Some studies have found that infection increased the host’s susceptibility to all predators, whether or not they’re included in the parasite’s life cycle.  

The two studies decribed above showed that the gammarids could escape from predators if escape involved moving fast or hiding near the water surface. But, although the parasites have evolved an excellent host manipulation, it’s probably not perfect.

For example, in the above system where parasites make gammarids cling to water surface vegetation in order to be eaten by birds, it is likely that by doing so the gammarids become vulnerable to other predators. At that water level, there should be a number of other predators that can take advantage of the defenseless gammarids. How fine-tuned the parasite’s control of the gammarid can be is an interesting question deserving future study.

On another note, I’m heading to Santa Barbara tomorrow to meet Armand Kuris and Kevin Lafferty, two immensely awesome parasitologists who I hope to collaborate with for my dissertation work.  When I return on Thursday, I’ll surely have fun stories about the stuff I learned.  Wish me luck!


13 thoughts on “The enemy of my enemy is my friend

  1. Good luck!

    It makes sense that teasing out subtle behavioral changes that have multiple conditions affecting them would be hard.

  2. Yes! I love hearing about behaviour-modifying parasites! Thank you for bringing this particular one to my attention, as I had not heard of it before. But then again, it is more of a hobby for me than something I’m studying outright.

    I hope the meeting went well!

  3. Hey Kelly, Hope you had a blast in SB. Isn’t it interesting that relatively advanced vertebrates (i.e. predatory birds) haven’t learned to avoid fish that are acting strange. Which birds are these and to they tend to be “stupid” birds? I’ll probably corner you in lab next week and pester you about these studies. Great writing!

    • Hi, Ann! Actually, the parasites don’t cause much damage to the birds. The parasites reproduce in their digestive tract and their offspring leave the birds when they defecate. It’s in the parasite’s best interest for the bird to continue surviving and defecating and it actually seems like parasites therefore have a minimal impact on the bird’s health. That being said, it probably actually is in the bird’s best interest to go ahead and eat the easy to catch fish since the repercussions for doing so are fairly minimal.

      Thanks for reading, Ann! Hope you’re having a good week!

  4. I’ve never commented before but I greatly enjoy your posts, you have the science life I’ve wanted since childhood but have never even tried to reach for. At 25 I’ve still never looked into college and just pretend to be an expert on these things through my site articles.

    Parasites are my VERY favorite subject, and I can’t help seeing the entire natural world in terms of “parasites” and “living vehicles for parasites.”

    That some parasites would help protect hosts from undesirable predators makes entirely too much sense, I don’t know why this is the first time I’ve heard it. Even Parasite Rex (the parasitologist’s bible) didn’t touch much on that.

  5. But.. but.. if a parasite could augment the host’s ability to evade some predators, in a way that’s unrelated to its main function of getting it eaten by the proper predators, then why wouldn’t the host evolve that ability itself?

    • Hm, I don’t think it’s unrelated to its desire to be eaten by the proper predators. If the parasite’s host is eaten by the wrong predator, then the parasites dies as well.

      We’re not sure exactly how the parasites manage to make their hosts faster, but it doesn’t necessarily follow that the hosts should be able to evolve this ability on its own. Hosts are constantly making energy trade-offs. If all of their time and energy went into antipredator behaviors, then they would have no energy left for reproduction, body maintenance, etc. This means that hosts don’t always behave in the best way possible when a predator is encountered. If they had infinite energy, then they would always have the energy required to move really fast while also being able to still breed and grow. Unfortunately, they dont have infinite energy and therefore may be left a bit on the low energy side when they happen to encounter a predator.

      • And in this case, the fish evades the stickleback in a way that makes it more accessible to the birds, so while it would normally spread its defenses thin to cover all the major threats, the parasite gives it a specific advantage and specific weakness.

        Like a pokemons.

        I’ll shut up since I just reiterated what you said in a stupider manner 🙂

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