In this blog I consider why there are different female colour forms (or morphs) in some damselflies, such as the Common Bluet, and briefly discuss some of the theories which attempt to explain their existence.
In Enallagma species (Coenagrionidae) – called Bluets – males usually start searching for mates before females have appeared. Some males hover over the water and attempt to intercept females, or try to break up pairs that have already formed and are in the characteristic tandem formation, where the male clasps the female by the neck (below).
The pair eventually form a mating wheel, when the female bends her body around to his reproductive organs (below). I detailed the mating process in a previous blog: Mating damselflies; there’s a lot going on, unseen!
Other males perch on floating or emergent vegetation (below) and investigate any damselflies or potential prey that fly nearby. However, the majority stay close to the shoreline, hovering and perching on vegetation in veritable swarms of blue darting damsels.
Mature females can come in up three colour forms (or morphs): ‘blue’, ‘brown’ (or yellowish-orange) and typical (greenish), although not necessarily at the same site. The juvenile males are a drab pink/purple, but turn blue a few days after emerging. However, they can be distinguished from females by having a solid blue colour on the bottom 2 abdominal segments.
Bluet damselflies are non-territorial and engage in what is called scramble competition to find mates. In non-territorial damselflies like Enallagma cyathigerum, which actively search for mates rather than defend specific territories, female-specific, colour polymorphisms often occur. The female morphs which are a similar colour to the males (blue in this case: see below) are called andromorphs, whereas females which are that are markedly different from males in colouration, are called heteromorphs.
One theory, is that andromorphs are male mimics, that have evolved to closely resemble males – in both colouration and behaviour – in order to avoid harassment by males at high population densities. The downside of this adaptation is that andromorphs would be at a distinct disadvantage at low population densities, suffering mating failure by virtue of being overlooked by searching males. An alternative hypothesis (learned mate recognition) suggests that males should respond sexually to andromorphs at greater rates in populations as their proportions increase. In other words, they learn to recognize female variants as potential mates. In Enallagma cyathigerum, the andromorphs have female-specific, dark abdominal dorsal markings (below) that provide a cue to their sex for mate-seeking males.
However, there is a large literature exploring these sexual conflict hypotheses, with quite a few studies concluding there is no evidence that female colour morphs suffer differential harassment. So, if the female forms which look most like the males get equally harassed, these female polymorphisms must by maintained by selective forces other than sexual conflict.
So, if the male-like forms (andromorphs) did not evolve to avoid being harassed by the randy males, then what exactly is the function of the different coloured females? A novel theory was put forward by Schultz and Fincke (2013), who studied the two female morphs – blue andromorphs and green heteromorphs – in polymorphic Enallagma species from North America. They suggest that the two forms represent alternative forms of deception: mimicry and background crypsis. At breeding sites, when perched among a large number of males, the blue, male-mimicking Enallagma andromorphs are not very apparent – i.e. they are hard to see – and are not recognised by the males. So, they do suffer from less attention or harassment!
However, away from the breeding sites, in vegetation, where the females usually occur when not reproducing, the blue females are highly detectable to males, as well as to other predators. The colouration of the heteromorphs – variable shades of green in these American species (and our Common Blue) – on the other hand, renders them more cryptic in this habitat, i.e. amongst the background vegetation (see below). So, it is tempting to speculate that these two different morphs may be sustained by opposing selective forces: natural selection for crypsis, and sexual selection for avoiding harassment by males?
I get the impression however, from my foray into the literature on this subject, is that the jury is still out on why different colour morphs of damselflies (and some dragonflies, of course) appear year after year. There must be good reasons for their maintainence, but it still remains rather a mystery.
Cook, P., Rasmussen, R., Brown, J. M., & Cooper, I. A. (2018). Sexual conflict does not maintain female colour polymorphism in a territorial damselfly. Animal Behaviour, 140, 171-176.
Miller, M. N., & Fincke, O. M. (1999). Cues for mate recognition and the effect of prior experience on mate recognition in Enallagma damselflies. Journal of Insect Behavior, 12(6), 801-814.
Fincke, O. M. (2004). Polymorphic signals of harassed female odonates and the males that learn them support a novel frequency-dependent model. Animal Behaviour, 67(5), 833-845.
Fincke, O. M., Fargevieille, A., & Schultz, T. D. (2007). Lack of innate preference for morph and species identity in mate-searching Enallagma damselflies. Behavioral Ecology and Sociobiology, 61(7), 1121-1131.
Fincke, O. M., Jödicke, R., Paulson, D. R., & Schultz, T. D. (2005). The evolution and frequency of female color morphs in Holarctic Odonata: why are male-like females typically the minority?. International Journal of Odonatology, 8(2), 183-212.
Schultz, T. D., & Fincke, O. M. (2013). Lost in the crowd or hidden in the grass: signal apparency of female polymorphic damselflies in alternative habitats. Animal behaviour, 86(5), 923-931.
Ting, J. J., Bots, J., Jvostov, F. P., van Gossum, H., & Sherratt, T. N. (2009). Effects of extreme variation in female morph frequencies on the mating behaviour of male damselflies. Behavioral ecology and sociobiology, 64(2), 225-236.
And the colour morphs of the female Blue-tailed Damselfly (Ischnura elegans) where there is no blue version is even more confusing.
I think the typica form is the andromorph in I. elegans.😊