A male (top) and ovipositing andromorph female Taiga Bluet (Coenagrion resolutum)
Photo: D Gordon E Robertson (Wikimedia Commons)
It's midsummer, early evening, and I'm sitting in my canoe, surrounded by swarming damsels. From my boat, wedged among the Broad-leaved Cattail (Typha latifolia), White Water Lily (Nymphaea odorata), Bullhead Pond Lily (Nuphar lutea), European Frog-bit (Hydrocharis morsus-ranae), Watersheild (Brasenia schreberi), and a slightly bewildering assortment of rushes and sedges, I watch the comings and goings of at least 14 different species. I count four spreadwings (Lestes spp.), two dancers (Argia spp.), the colourful Eastern Forktail (Ischura verticalis) and the diminutive Sedge Sprite (Nehalennia irene). Then there are the bluets, my favourites being the beautiful yellow male Vesper Bluets (Enallagma vesperum), which as their name suggests, are vespertinal indeed. And there is the Orange Bluet (Enallagma signatum), Skimming Bluet (Enallagma geminatum), Northern Bluet (Enallagma cyanthigerum), and perhaps one or two more species that require a close examination in the hand to identify with certainty.
This lakeside marsh has a nice variety of species, but the diversity of colours is even greater because all of the damselflies here are sexually dichromatic, males and females are different colours. In sexually dichromatic damsels, it's usually the males that are brightest, more strikingly patterned, than their female counterparts. Usually, but not always. Sometimes females are just as colourful as males. In certain species, some females in a population closely resemble males (andromorph females), while others wear more subdued hues, making them easy to distinguish from males (gynomorph females). This pattern is called female-limited polymorphism, and it has evolved among many different damselfly lineages, including among most of the ones flying around my canoe.
But why has it evolved in the first place? The most favored hypotheses among odonatologists is that female-limited polymorphism has come about as a way for females to avoid sexual harassment from males. Constant sexual harassment can be problematic for females, making them more susceptible to depredation. That's because the normally cryptically-coloured girls become far more conspicuous when pursued by or attached to a brightly coloured guy. Constant harassment may also impede a female's ability to hunt, which could influence the quality or quantity of the eggs she might lay, or even reduce her lifespan.
Certain species are exposed to harassment much more frequently than others. Harassment isn't a constant problem for females of territorial species. Ebony Jewelwing (Calopteryx maculate) males, for example, establish a territory along a wooded stretch of stream. The male patrols a small parcel of waterfront real estate, chasing off other males, and wooing passing females with their iridescent bodies and ritualized displays of their jet black wings. In the case of Ebony Jewelwings and other territorial species, females can avoid unwanted harassment from males by simply steering clear of their territories. Harassment is a big problem for so-called scramble mating species, such as the Taiga Bluet (Coenagrion resolutum). These damselflies forego establishing and holding territories all together. Instead, males hang out around ponds, and chase virtually every female that comes near, attempting to catch and mate with them before other males do. In the case of Taiga Bluets and other scramble mating species, its much more difficult for females to go about unaccosted, because randy males may be hiding behind any cattail. Indeed, some male damselflies are seemingly always on the prowl for mates, and are very indiscriminate about who they try to copulate with. Male Sedge Sprites, for example, attempt mattings not only with conspecific females (whether they are receptive or not), but also occasionally other males and even females of other species.
Most female damselflies, as far as is known, only need a single mating event to have all of her eggs fertilized, but may be subjected to many advances and forced matings by numerous males, particularly so in scramble-mating species. Thus, females get way more attention than is required, or no doubt wanted. That's not to say that females are entirely at the mercy of males. Sometimes a female that has been caught by a male simply will not adopt the mating "wheel" posture needed to be fertilized. In other cases, females will simply avoid males, or flee when pursued. Females may also be able to select which male's sperm they allow to fertilize their eggs, after copulations with assorted suitors.
Female-limited polymorphism is most common in scramble-mating species and it's a result of all that sexual harassment. Gynomorphs, those females that look different from males, still experience a high degree of the sexual harassment. But andromorphs, male-lookalike females, actually manage to avoid most harassment because males simply can't distinguish females from other males, most of the time. Andromorphs promote their deception further by not only physically resembling males, but also by acting like them. Andromorph female Sedge Sprites tend to do the same kind of chasing behaviours as males do. Heteromorphs don't do that.
Unlike Ebony Jewelwings and other territorial species, scramble-mating males don't appear to use their colourful markings to display to females. So why are they so colourful then? These males use them to signal to each other. It's a form of advertisement that effectively says, I'm not a suitable mating target. It's in the best interest of males to avoid mistaken mating interactions for a number of reasons, not the least of which is the missed mating opportunity. It's exactly this form of signaling that andromorph females play on to avoid unwanted advances.
I'm still left with a couple of questions as I watch the damselflies from my canoe. Why don't all females andromorphs? That way they could all avoid sexual harassment, right? Maybe so, but perhaps there is an advantage to being a gynomorph. Maybe sexual harassment is sometimes worth it if it increases your odds of getting the very best mate. Maybe the camouflage advantage of earth tone colouration, as opposed to bright blue, orange, yellow or purple, is worth the harassment cost. The answer to that one is yet to be determined, but given that polymorphism seems to be a stable characteristic in damselfly populations, both strategies must have their advantages. What about those andromorph females, are they at a disadvantage when it comes to finding a mate? Does being a damsel in drag mean you may never get the chance to hook up with a male? Well, it turns that andromorphs do just fine at finding mates. It seems that being an andromorph doesn't make you immune to the advances of all males, it just quells the tidal wave of orgiastic enthusiasm that can ruin and otherwise fine day at the marsh.
