Weather and Odonate Behavior

Anax junius in flightI’m running a little behind this week, but it’s a new year and I think it’s time to discuss my favorite scientific topic in the world: odonate behavioral responses to weather!  I have been interested in dragonflies for a long time, almost as long as I’ve been interested in insects.  I think they’re beautiful, amazing animals and I am in complete awe of them for many reasons.  When I was an undergrad, I took an ecology class in which we were required to do a personal project, so I naturally gravitated toward dragonflies.  That project turned into a senior thesis for my biology B.A. and eventually became the basis for a scientific paper.  I’ll talk about the results of my study next week.  Today I want focus on what’s known about the behavioral responses of odonates to weather, observations by other scientists who have inspired my work in this field.

As most of you reading this will know, dragonflies are powerful fliers.  However, they still have limitations.  They have massive, broad, flat wings and though they have bulky bodies, they just don’t weigh that much.  So, imagine you’re a dragonfly.  It’s a nice sunny day, but a storm is moving in.  It starts to get cooler, darker, windy.  Are you, as a light-bodied animal with gigantic wings, going to keep flying as the weather deteriorates?

If you answered no, you’re spot on!  Dragonflies and damselflies are known to be strongly affected by changes in weather.   Changes in weather throughout the day can have a significant impact on how a population of odonates behaves.  In fact, weather plays such a huge role in odonate behavior that you see some variation on this sentence in countless scientific papers:

“Data from days exhibiting inclement weather were removed from the data set before analysis.”

There’s a reason so many people do this though.  Though very few researchers have looked specifically at the role weather plays in shaping odonate behavior, these insects are well-known to behave strangely under certain conditions.  If your study is looking at, say, patrolling behaviors in a dragonfly species and a change in weather occurs that disrupts their normal behavior, that data really doesn’t help answer the questions you’re interested in.  You therefore remove that data before you analyze your results because it is not related to your study.  And ultimately, even scientists don’t like to stay outside when it’s rainy, cold, very windy, or otherwise unpleasant.  Scientists just miss a lot of what happens during those inclement weather days.  That’s unfortunate because that’s when some of the most interesting things happen!

Snowman

Snowman from one of our aquatic entomology field trips. Dragonflies would not fly in this weather! Photo by Dennis Suhre.

So what do we know about odonate responses to weather?  Several things.  Let’s consider temperature first.  Odonates are exothermic, like all insects.  If they get too cold, they can’t warm their bodies up enough to fly.  If they get too hot, all kinds of nasty chemical reactions start taking place in their bodies that can result in death if they become severely overheated.  (Long ago, I posted about obelisking behavior in dragonflies, a behavior that helps dragonflies cool down on hot days.)  Changes in temperature are known to influence odonate behavior such as the time of day that a species normally flies, flight activity levels throughout the day, and the postures of dragonflies.

Wind and a palm tree

A palm tree on a windy day.

Wind also plays a role in odonate behavior.  On very windy days, damselflies won’t fly at all, or only in places that are protected from the wind by barriers of some sort.  Dragonflies are able to fly in stronger winds, but even these larger insects have their limits and must stop flying if the winds become too powerful.  Light intensity hasn’t been looked at in as much detail as wind, but it is known to impact fight activity as well.  Activity levels tend to go up and down with light intensity.  More odonates tend to fly on sunny days than on cloudy days.

These are the obvious weather factors.  You can easily observe an increase in wind speed, a drop in the light levels, or a drop in temperature.  What about the less obvious changes in weather like barometric pressure and relative humidity?  Relative humidity isn’t considered by most researchers, so there is very little data available on its effects.  However, one Russian scientist in the 60’s published a paper proposing a link between barometric pressure and dragonfly behavior.  I happen to disagree with him, but I’ll talk about my reasons when I discuss my own study next time.

The Catalina Mountains as a storm rolls in

The Catalina Mountains as a storm rolls in.

Finally, let’s consider rain.  Glorious rain, my favorite weather!  Damselflies almost never fly in the rain.  Some of their larger dragonfly relatives have been observed flying during rain (including my favorite magnificent flier, the wandering glider), but few reports have been made of any dragonflies flying during heavy rains.  In fact, several researchers have observed that dragonflies and damselflies essentially disappear from the water’s edge right before it starts to rain.  This behavior has been named pond abandonment behavior, and it is this behavior that I am most interested in.  Why are they leaving?  Where do they go?  Are they using cues from their environment to tell them when to leave the pond and when to return?  This behavior is fascinating!

This is the sort of information I had to go on when I started my project looking at dragonfly behavioral responses to weather as an undergrad.  The few researchers that have examined the role weather plays in odonate behavior tended to analyze each weather factor separately, sometimes using data from different days.  If you’ve ever watched a storm rolling in, you know that light intensity rarely changes without a change in temperature and the wind often increases before it starts to rain.  Weather factors are clearly linked.  And most of the time you’re not lucky enough to find studies that overtly link weather and behavior to begin with.  The information about this topic tends to be hidden in papers about completely unrelated subjects, often preceded by one of those, “Data from days exhibiting inclement weather…” statements.  I happen across most information about odonates and weather when I’m reading about territoriality or dragonflies using sunny spots in forests as hunting areas.  Lots of odonate researchers know that weather plays a role in odonate behavior (a significant role in fact!), but figuring out exactly how weather does this…  Well, that’s what I am interested in figuring out!

Next up is another Friday 5, but my post early next week will focus on the things I’ve learned in my own work with odonates and weather.  This has been my pet topic ever since that first little study I did in my first ecology class in college – I love this subject!  I hope you’ll enjoy it too!

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