For me, identifying the species of the animal I photographed is an integral part of nature photography. But sometimes, none of the displayed images in field guides match the creature you just found. This brings me to the topic of today's educational blog entry: Color Morphs in Damselflies.
In many species of damselflies, females can be found in various color variations (morphs). However, in most field guides, only the most common color morph is displayed.
One of the most impressive color variation is the androchrome morph. Androchrome females mimic males by displaying a male coloration. Female damselflies that show the female specific coloration are called gynochrome, respectively. I will try to exemplify this topic using the Emerald Damselfly (Lestes sponsa).
Lestes sponsa males have a blue/green metallic coloration with blue eyes, whereas females usually are bronze/copper with brown eyes. The male abdomen ends with a clasper organ (cerci), used to grap the female neck during mating, whereas the female abdomen is characterized by an ovipositor, used to lay eggs. The image below shows a tandem of a male (top) and a gynomorph female (bottom). Usually, the male keeps hold of the female even after the successful mating and egg deposition. The main purpose of this behavior is to prevent the female from mating with other males.
The Emerald Damselfly (Lestes sponsa) is usually a species easy to identify. Its typical wing position while resting easily denotes them as Lestidae (spreadwings). However, one early morning I found a spreadwing I could not correctly identify at first. The shape of the abdomen clearly told me that it is a female (the ovipositor was clearly visible), but its body coloration as well as its eyes were blue (see image below). However, in my damselfly field guide books there were no spreadwing species with blue females.
Only later I discovered that I had found an androchrome female of Lestes sponsa. According to a scientific publication investigation a population in Sweden, 18,5% of the analyzed females showed an androchrome coloration. So, this color variation is not as rare as you might expect due to their almost complete absence in damselfly field guides.
For comparison, the following collage shows a male (top), a typical female (gynochrome, center) and a male-mimicking female (androchrome, bottom) Lestes sponsa.
So, why do some females mimic male coloration? One of the most prominent hypothesis considers "male harassment" as an important factor in the evolution of androchrome females. As you can imagine, being grabbed by a male damselfly by the neck and fly around as a tandem will negatively affect the females survivability; e.g. it is easy to assume, that predatory birds will be able to catch a damselfly tandem more easily than a single female.
Indeed, several studies show that damselfly males prefer to mate with the normal colored females (gynochromes), whereas androchrome females where much less observed in tandems. Thus, androchrome females successfully avoid male harassment. On a downside, androchrome females also had a reduced fecundancy (as expected). However, if the population density of the investigated damselfly species was high, male harassment on gynochrome females increased to a level where fecundancy in androchromes was actually higher.
In low population density, male harassment on gynochrome females was "acceptable", whereas many androchrome females where not mated and thus had no offspring. Under these circumstances, you would expect that the abundancy of androchrome females should drop in the next generation. But, under high population densities, gynochrome females suffer from the numerous mating attempts of males, decreasing their fecundancy below the fecundancy of the androchrome females. In these populations, you would expect to see more androchrome females in the next generation.
Thus, it is hypothesized that the occurrence of androchrome females in damselfly is mediated by a population-density dependent negative effect of male harassment on female fecundancy.
An interesting evolutionary topic, that unfortunately is not intensively researched.
1. "A preliminary study on female-limited colour polymorphism in Lestes sponsa"; Outomuro D, Söderquist L, Rodriguez-Martinez S & Johansson F; International Journal of Odonatology 2014.
2. "Density-Dependent Male Mating Harassment, Female Resistance, and Male Mimikry"; Gosden T.P. & Svensson E.I.; The American Naturalist 2008