Parasites and Predators: Partners in Crime

This week, I wanted to post about a paper that was just too sexy to resist blogging about.  However, as I was writing my post, I stumbled across Jeremy Yoder’s even better post about the paper.  So, you should click through to the Denim and Tweed blog, read his post, and then admire my only best attempt at drawing a flea.

Raveh et al. (2011) performed an experiment where gerbils were either infested or uninfested with fleas.  They put the gerbils in field enclosures, and then exposed the gerbils to a muzzled fox predator for half of the nights that the gerbils were in the enclosures.  Gerbils were provided with sand boxes containing buried seeds, and the gerbils had to balance foraging in the sand boxes for seeds and avoiding getting pounced on by the fox.  Among other things, Raveh et al. (2011) measured the “giving up density” in the sand boxes.  That is, how much food remained in the sand box when the gerbils left the food patch?  Higher giving up densities meant that the gerbils spent less time foraging, which would be bad news for gerbil seed consumption and storage.

Gerbils infested with high densities of fleas left food patches at higher giving up densities than gerbils without fleas.  And when gerbils had fleas and a fox was present, the gerbils left food patches at the highest giving up densities.  So, fleas distracted/irritated gerbils so much that the gerbils spent less time foraging for food, and that change in foraging behavior was amplified when both parasites and predators were present.  In a world full of things trying to eat gerbils, how’s a gerbil gonna eat?

Here are some things that I liked about this paper:

  1. Beautiful, mathy hypothesis testing.  Seriously, go read this paper.
  2. In the wild, 97-100% of gerbils have fleas!  Wow!
  3. Parasites may facilitate predation on the host, even when the parasites aren’t trophically transmitted.
  4. This relates to my post about vicious circles of body condition (or susceptibility) and parasite infection.  If gerbils with many fleas forage less than gerbils with few/no fleas, their body condition might decline.  And if their body condition declines, they might be more susceptible to fleas.  Etc.  This might lead to vicious circles of declining health/fitness.  Or, perhaps the vicious circles don’t have much time to act, because foxes come along and eat the distracted gerbils.

Serious business.

Reference:

Raveh, A., B. P. Kotler, Z. Abramsky, and B. R. Krasnov. 2011. Driven to distraction: detecting the hidden costs of flea parasitism through foraging behaviour in gerbils. Ecology letters 14:47–51.

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