Happy new year everyone!  This year New Year’s Day also happens to be Science Sunday, so I’m going to start 2012 off with a science filled post.  Woo, science!  :)

Every now and then, I’ll get an e mail from someone during the dragonfly swarming season telling me a story of how the writer walked outside and had a dozen or so dragonflies follow him/her closely as he/she walked.  Most of the people who have this experience think it’s rather magical, but it’s unexpected and they want to know what’s going on.  Happily, this behavior has actually been covered in the scientific literature!  So, I’m going to start the new year by discussing a paper that deals with this interesting behavior that was released in the odonate science journal Odonatologica in 2010, authored by O and J Holusa of the Czech Republic.

For those if you who have followed my Dragonfly Swarm Project, you know that dragonflies often swarm because there are a lot of small prey insects in the area that attract them, forming a sort of all-you-can-eat buffet for lots of hungry dragonflies.  In essence, the dragonflies are taking advantage of abundant food that is easy to capture.  Swarms just happen to form if there is enough food for several dragonflies to eat.

A similar behavior called accompanying has been documented by a few odonate researchers.  It works like this.  When a large, slow-moving animal such as a hippo, rhino, or human walks through an area, two things happen.  Insects that are attracted to the animal, such as blood sucking insects, follow them as they move.  Other insects, those found on the grass, brush, or ground, also move out of the way by flying, jumping, or running.  Small clouds of flying insects therefore form around the large animal as it moves.  If you’re a dragonfly, these little clouds of insects are a great source of food.  And, if you are a dragonfly that lives in an area where there are few bushes and flying insects are unlikely to move around much during the day, accompanying large, slow-moving animals means that you’re more likely to catch a tasty snack than if you wait for the little insects to fly on their own.

One species of dragonfly, Brachythemis leucosticta, has been well documented performing this accompanying behavior, following large mammals within its African range.  It tends to live in open, sparsely vegetated areas where movements of large animals could improve prey capture rates.  The Holusas were curious whether the European populations of B. leucosticta would exhibit the same pattern of behavior.  They also wished to document how far the dragonflies would accompany a large mammal and whether males or females were more likely to take part in the behavior.

They performed their very simple experiment with a population of B. leucosticta along the River Barbate in southern Spain.  They started in the area where the dragonflies perched in the shade near the water and walked perpendicularly away from the shore. When dragonflies followed them, they counted the number of individuals, determined the sex of the followers, and recorded the distance at which each dragonfly returned to its perch.  The researchers walked to the end of the floodplain and then made a wide arc back to the starting point, repeating the walk many more times over a two-day period.

With this experiment, the authors learned that dragonflies would usually follow them away from the water, flying close to the ground in front of them.  Fewer than 5 dragonflies would usually follow the researchers, but they recorded one group of 11.  Females were more likely to accompany than males, making up approximately 67% of the total observations.  Females also accompanied further than males (up to 111m in famales and 89m in males), though both sexes typically returned to their perches before the researchers had traveled 40m from the water.  Finally, of the 53 walks where dragonflies accompanied, dragonflies were observed catching and eating prey in only 3.

The authors made a few conclusions.  First, they noted that though the prey capture rate of accompanying dragonflies was rather low, less than 2%, this was probably a higher prey capture rate than they would experience without accompanying.  They also pointed out that few insects flew out of the grass as the researchers walked and the dragonflies captured prey every time small flying insects were observed.  Accompanying is therefore likely a valuable means of capturing prey in a habitat that is rather inhospitable for small flying insects.

The authors also attempted to explain the differences in accompanying distances they observed between males and females.  They suggest that because the females are lighter colored than males, they are not as noticable to the prey and are more successful at capturing prey via accompanying and therefore persist longer.  They also suggest that because the males are darker, they heat up more quickly upon leaving the shade.  I personally think this is highly unlikely, so I’d like to add a third possibility: that males return to their perches sooner becuase they are territorial and might lose their territory to other males if they stray too far.  Because females are not territorial, they can afford to accompany large animals further from the water to take advantage of the flying insect bounty.

Now, this behavior has not been confirmed in any US dragonfly species, but it is suspected in three: the green darner, wandering glider, and black saddlebags.  All three are commonly reported in static and migratory dragonfly swarms in the US, so these are dragonflies that are flying during the dragonfly swarming season.  This is also the time of year that those e mails start to come in from people asking why dragonflies are stalking them.  I think there’s a connection: the dragonflies in a swarming area are very likely accompanying people, forming little groups around people kicking up little flying insects as they walk.

Tomorrow I’m going to talk about another curious behavior that I haven’t been able to find any information about, but has been reported to me three times now.  It’s a bizarre behavior, so I hope you’ll check back!

Literature Cited:

O. Holusa and J. Holusa (2010). “Accompanying” behaviour of Brachythemis leucosticta (Burmeister) in Europe (Anisoptera: Libellulidae) Odonatologica, 39 (1), 63-70

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