Why is that in some butterflies, the sexes are identical, whereas in other species the male and female are very different? Not an easy question to answer I think. In many of our common species, like the Red Admiral or the Peacock (below), the sexes are very similar, although the female may be slightly larger.
The technical answer as to why some male and female butterflies differ (sexual dimorphism) relates to the fact that there are different selection pressures operating on each sex. In other words, they have different roles and aspirations, and both natural and sexual selection has worked to evolve the observed differences between them. But why this only applies in the case of some species, and not others, is less easy to explain, I think. Here are a pair of courting Marsh fritillaries, where the female is slightly larger, and a bit differently coloured, than the male.
In the tropical Leopard lacewing (below), the female is a more subtle, less colourful version of the male (below).
Some males are very much brighter and flashier than the females, and it has been shown that in some such species – like the great eggfly, Hypolimnas bolina, for example, females prefer males that are bright and iridescent. The males invest physiological resources into producing shiny wing scales which generate bright UV colours to attract, and hopefully impress, females! Females use these signals as measure of the quality of the male, and make their selections of a mate, accordingly.
Another reason why the sexes might differ, is because in some species the females are mimicking another butterfly. Why only the females of a species engage in mimicry is a moot point, but again comes down to the different pressures operating on the two sexes. Females might benefit from looking like another (poisonous) species, to avoid being eaten, but males need to compete with other males – and look flashy enough to attract females of their own sex; so looking like another butterfly just does not cut the mustard! Below is a male Great Mormon (Papilio memnon agenor) and the photograph below that is the very different looking female, which is trying to mimic a totally different, poisonous species. There are similarities though; they both have the rather nice, red ‘shoulder’ flashes! On the bases of their fore-wings.
Another reason why the sexes might differ, is that larger females have the advantage of being able to lay more eggs, both in a given day and throughout their life spans; which probably explains why female butterflies are usually the largest sex. The female, is usually the one investing the most – in terms of egg protein for example – in the next generation, and evolution may have selected for a large size: more eggs, more offspring, more survivors, increased fitness.
Birdwing butterflies are excellent examples of highly sexually dimorphic species, in terms of size. In Queen Alexandra’s birdwing (Ornithoptera alexandrae) (below) – which is the largest butterfly in the world – the females are much larger than the males, and can weigh up to 12 g! Huge!
Wallace’s golden birdwing, Ornithoptera croesus, is another very obviously sexually dimorphic species (below).
Whilst larger males might benefit in competition with other males, smaller males benefit from being able to develop into adults much faster. Emerging from their pupae before females – called protandry – puts them at an advantage in terms of mating, being able to take advantage of the high receptivity of females, just after eclosion. Below are a pair of mating Gatekeepers; the female is larger so gets to do the heavy lifting!
Female butterflies can also be darker – i.e. with a greater degree of wing melanisation – than their male counterparts, because they need to generate higher body temperatures in order to mature and deposit eggs, particularly at higher elevations. Here is a female Common blue butterfly (below); the females vary a lot in terms of their brownness, but are usually very different from the much more flashy males! Although, the females also have plenty of shiny blue wing scales as well.
In tropical species, like the Archduke (Lexias pardalis dirteana) – see below – the brown females might just be sporting cryptic colouration, i.e. helping them to blend in with the background on the speckled forest floor. The flashy male is however, probably all about standing out and wanting to be seen by the females! But the male Archduke still has a little bit of speckling; he is being torn both ways. He needs to stand out but is also needs to avoid being eaten by a predator. Sex wins out I think!
There is of course, also a marked difference between the sexes in terms of their scents; in other words, the chemical composition of pheromone secretions. Males usually emit significantly stronger odours than females, although many odoriferous compounds are shared by both sexes. Not something which appears in photographs, but a male which is courting a female and rapidly vibrating his wings may well be sending chemical signals in her direction. Females also produce pheromones, but less is known about these in butterflies.
In reality, sexual dimorphism is a large and complex subject, and there may be a number of reasons why the sexes look different in some species. There is still an awful lot we do not know about these iconic species and why some of them choose to dress differently, and others do not!