Welcome to the third and final segment of the Dragonfly Trilogy – and another installment of From the Literature! If you don’t know anything about dragonfly territoriality, I recommend reading part two of my trilogy for more information on how dragonflies and damselflies set up and defend territories. You’ll get more out of this post if you know something about territoriality before diving in.
Last time, I discussed how odonates benefit from being territorial, why they set up territories and defend them from other males. Like many things in biology, it boils down to sex: males that defend high quality territories generally mate with more females than males with low quality territories. Likewise, males that defend territories generally mate with more females than males that do not defend territories. It is usually better to be a male with a territory than a male with no territory, but there are often many more males than there are available territories and some males are inevitably left out. Presumably the stronger, better males (the most fit males) end up successfully claiming and holding territories while the weaker, wimpier males are left without territories and become wanderers.
This is the situation that a group of researchers in Finland recently investigated. They chose to study the damselfly Calopteryx virgo, a European damselfly also known as the beautiful demoiselle. This gorgeous creature is pictured at left and is one of the damselflies known to be territorial.
The researchers asked a simple question: are C. virgo males that defend territories larger than males that do not defend territories? They wanted to know if the damselflies that were able to protect a territory from other males were somehow better suited to being territorial than the damselflies doomed to be wanderers and unable to gain their own territory. They also wanted to know if this changed over time, whether the non-territorial males eventually became territorial.
To answer these questions, the researchers captured mature C. virgo males arriving at their study stream in Finland, then marked them (so they could tell them apart), measured their right hindwings, weighed them, and released them back into the study area. They observed the damselflies on two different days ten days apart and determined whether the males were territorial (they stayed within a small, 2 meter area for at least three hours) or non-territorial (they did not remain within a 2 meter area). Males were considered wanderers if they moved more than 100 meters during the observational periods.
The team discovered that there was a significant difference in size between territorial and non-territorial males. Territorial males had longer wings and were heavier than wandering males, so the bigger males were the ones claiming and holding territories. The researchers also discovered that time didn’t have much to do with whether a damselfly male was territorial or not. The wing length and weight of the wandering males was about the same and wandering males were consistently smaller than the territorial males on both days. Wandering males made up about the same percentage of the population both days too. So, the smaller males weren’t ever getting territories and were consistently being excluded.
The data that Koskimaki and colleagues presented in their paper suggest that the bigger, more physically impressive males get more mates. To better understand the significance of these results, let’s consider a similar situation in humans that many people will recognize. Think about what you know about stereotypical high schoolers. Who gets more dates: the jocks (usually the bigger, more physically impressive males) or the nerds (usually the smaller, less physically impressive males)? When I was in high school (I myself was firmly rooted in the nerd category!), the jocks got most of the girls while the nerds usually only admired the girls from afar. The jocks outcompeted the nerds physically, and because they were generally more attractive, they excluded the nerds from finding dates by hogging all of the available girls. If you ignore the possible confounding affects of wealth, intelligence, and overall personality that come into play in human mating behaviors, almost the same thing is happening in the high school students that we see in the damselflies that Koskimaki and colleagues studied! In effect, the jocks among the damselflies were getting all the girls because they were better suited to protecting territories, and thereby attracted more mates, than the nerds who were unable to gain a territory.
I’ll end with two important questions: if it is so much better for males to be bigger so that they can more successfully hold valuable territories, 1) why are any males small and 2) why aren’t the damselflies getting bigger and bigger over time? Koskimaki and colleagues suggest that that territorial and non-territorial males might form two distinct subgroups within the damselflies, each with their own strategies and goals for mating. Even males without territories are able to mate with some females. They could end up with the same number of offspring, thus ensuring the continued existence of smaller males in the population, if they have means for compensating for their relative lack of mating opportunities. The team cites several other studies that suggest that this is happening in several territorial damselfly species, that non-territorial males are equally successful in producing offspring compared to territorial males. It is likely that there are some benefits to being smaller or some costs to being larger that have not yet been accounted for. Further studies in this area would be a great avenue for future research!
I hope you’ve enjoyed the dragonfly trilogy! It’s been a lot of fun delving into the dragonfly literature for a few weeks and sharing information about my favorite group of insects. I’m sure to post more odonate research in the future, but next time I’ll be telling a story of a centipede and a woman who is very, very scared of them – me!
Koskimaki, J., Rantala, M.J., & Suhonen, J. (2009). Wandering males are smaller than territorial males in the damselfly Calopteryx virgo (L.) (Zygoptera: Calopterygidae). Odonatoligica, 38 (2), 159-165.
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