Logs are heroes

Imagine living under a log for your entire life just to not die. Who would want that? Of course, it is those nasty looking bugs that you see in large groups and want to step on at first sight. More specifically, I am talking about isopods, or more commonly known as sowbugs, pillbugs, and roly-polies. To these cutely named creatures, a log is their sanctuary.

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A group of isopods hanging out on a log

So what is it about logs that make them sanctuaries? Well, that is exactly what this year’s biology 326 class wanted to find out. On a rainy day in January, we went to Pacific Spirit Regional Park and collected isopods, which were found only underneath logs, where it was dark, wet and cold. That sounds awful, right? Actually, they desire a place like this. We observed this behavioural preference experimentally, where we gave many isopods the option of choosing a wet or dry, dark or light, and cold or warm environment. Indeed, the majority preferred to settle in the wet and cold area. However, they were neutral about living in the light or dark area.

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Experimental set up for isopods choosing between a dark and light environment

There are numerous possibilities why isopods prefer the moisture, coldness and perhaps darkness of logs. One reason is that these animals are vulnerable to desiccation, which is severe body water loss (through evaporation) that may lead to death. If isopods were in the open where sunlight is prevalent and moisture is low, desiccation would likely happen. Contrastingly, by living under a log, the coldness and dampness make body water loss less likely. Isopods are also food, at least to birds. Therefore, if they stay covered under the logs, the birds that look down on the forest floor will not be able to detect and then eat them. However, logs can only do so much because isopods have other predators such as spiders, which can travel under there. In fact when we were turning over logs to collect isopods, we also observed a few spiders. That being said, some do have a defence mechanism of rolling up into a ball, hence the name roly-poly.

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An isopod rolled up into a ball

Lastly, and probably the most interesting possibility of logs being sanctuaries is the fact that isopods have modified gills on their legs. These gills allow them to breath and therefore live, but in order to work properly, they need to be in a moist environment. Fortunately, the logs are here to save the day.

As you can see, logs are not boring static objects lying on the forest floor. Isopods rely on these to survive for numerous reasons: prevent desiccation, avoid predators and allow for proper breathing. Logs are in fact living heroes. To learn more about isopods such as their life cycle and predators, I would recommend looking at the article Sow Bugs and Pill Bugs, by Susan Jones.

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