[Odonata-l] Sexual dimorphism question?

Wed Feb 7 10:04:14 PST 2007

There are so many aspects of this discussion that are absolutely fascinating, and it is great sport for a cold winter's day.

  Evolution, natural selection, and in particular, the sexual selection aspect of natural selection I have always found intriguing. Female polymorphism, and in particular male-like females seem, at first glance, to make little sense, but make an absolutely fascinating subject. 

  First, I have some thoughts on sexual dimorphism. I understand that more may be at work here than sexual selection, and there may be other explanations for bright and flashy colors and patterns, but anyone would have an extremely difficult time convincing me that all of this leads, at least in the Order Odonata, in one way or another, to aspects of sexual selection and reproduction. To me, I don't think any one fact drives this home more than looking at this order without sex. In other words, look under water. With Odonata larvae, at least as far as I am aware, there is nothing but subtle and cryptic coloration and patterns. Bring sex into the picture, and that picture changes dramatically!

  A couple other points I would like to make and which we should all keep in mind, and which I hope are not too great of simplification. From a standpoint of natural selection and reproduction, what is best for a female, and what is best for a male are usually two different things. Males and females, have different investments in the offspring, even when care after hatching is not an issue. Because it takes more energy to produce eggs than sperm, males can distribute and contribute their genes to far more offspring than females. For a male, it is usually to their advantage to mate as often as possible. A female can produce far less offspring who will pass along her genes. Because of this, it is often to her advantage to be much more selective.  

  I can think of a couple good illustrations of the selectivity of females. One is with Sylurus scudderi, the Zebra Clubtail. I don't know how many of this striking, and I would say beautiful, species I have seen. Say it is 100 individuals that I have seen, and maybe I have had a good and close look at 50 of the 100. How many females of this species have I had a good close look at? Zero point zero. Maybe the males are not truly territorial, but they at least seem to have rolling and temporary territories. The male finds a nice prominent branch, or downed log, or maybe even more often, and prominent and sunny rock, and he waits. Maybe you might see a half dozen other males nearby, on their choosen and prominent perch. The males are having arguments over whose perch is whose. They are fighting each other off, and performing game of musical perches. The females are nowhere to be seen, at least by intrepid photographers, but they are there. They are up in the treetops keeping
 an eye on things. A particular male catches a females eye for a reason known only to her, and she suddenly appears, swooping down from the treetops. In an instant, she and the male are paired, and fly back up to the treetops in copula. The point is, it is the female doing the selecting. The male will be back, and he'll mate with any female of the species to come along and make herself available.

  Another good illustration is Somatochlora hineana. I have spent some very enjoyable hours observing the mating behavior of these emeralds. It's maybe 8 or 9AM on a sunny day in some remote and beautiful bog. Male Hine's Emerald dragonflies are cruising about, looking for a nice little spot that would impress a female, and where she might want to oviposit. He stakes this area out for himself, which may be only a little depression in the sphagnum, filled with water. He hovers above it, and darts back and forth, hovering, darting, turning, hovering again. His emerald eyes are absolutely gleaming in the sun. He may have to fight off other males, and may get displaced by a more aggressive male. Other Hine's males are trying to maintain their own favorable territories nearby. The jockying and sparring continues. Again, where are the females? They are hanging around (literally, of course) at the edge of the woods, blending in with the trees they are hanging from, still in the
 shade, so gleaming eyes don't give them away. But they do rather suddenly show up, choose a male, mate, and oviposit while the male is hovering nearby, guarding. But again, the male is staking out and fighting to keep the favorable breeding territory, and the female seems to be the one doing the selecting.

  Females of the order odonata are so often less visible, both due to more cryptic coloring, and to their behavior. They are so often skulkers, hiding in the vegetation or treetops. This is an advantage in keeping from becoming prey, but also, wouldn't being less visible allow the females to be more selective in mating? Of course, as the point was made, insects look different to other insects than they do to us. I suspect there is some correlation, however, and maybe a large correlation, between what we see as flashy or hidden,and what they see. I suspect that, more often than not, an insect that is flashy to us, or well hidden to us, is likewise flashy or well hidden to other insects, even though it may be to a greater or lesser degree.

  I like Dennis Paulson's point "When you think of how quick males should be to respond to incoming females, when the sex ratio may be 50:1 at a pond swarming with male Enallagma, you can see that males colored like females might be harassed constantly." True enough, and even more so for females! It seems to me that cryptic coloration and behavior allows a female to be more selective. Another interesting thought here, might not, a female looking like more like a male, allow that female to be more selective in choosing a mate? Someone in this discussion, and I'm sorry, I don't remember who it was, made the point that males were observed mating with male-like females far less often than with the other females. But that may not mean that these male-like females are not mating successfully. It may mean that they are mating more at other times or in other locations, and I could see that they might be able to be more selective about the males they are mating with. They show
 their, hmmmm....femaleness, when it is to their advantage, that is to say when they choose to mate. When I have pondered the very intriguing phenomena of male-like females, this was one of the few plausible explanations that I could come up with. Whether it is right or not, I do not know, but I think it is plausible.

  Sorry to be so verbose here, but it is a good way to pass time in the dead of winter!

  Glenn Corbiere

Glenn Corbiere 
100 Prospect St. 
Chester, MA. 01011-9657 

www.dragonhunter.net 

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