29 April 2018

Sexual Dimorphism : Part 1

Sexual Dimorphism in Butterflies : Part 1
Featuring Male-Female Differences

A mating pair of Orange Emigrant.  Left : Male, Right : Female

Sexual dimorphism in butterflies is the condition where the two sexes of the same species exhibit different characteristics beyond the differences in their sexual organs. These differences may include size, colour, distinguishing markings and secondary characteristics. In some cases the flight characteristics between the males and females are also distinctly different. The differences in sexual dimorphism can range from negligible to exaggerated, and may be subjected to sexual selection.

The degree of differences in sexual dimorphism in butterflies can be classified under a range that falls between negligible to exaggerated. I have further broadly categorised the differences into the following 4 categories :
1) Negligible : Males and Females appear so similar that it takes a bit more effort to scrutinise secondary characteristics closely to ascertain and differentiate between the males and females.
2) Subtle : Males and Females appear similar but there are minor giveaways that can instantly distinguish between the males and females
3) Distinct : There are clear differences between males and females that a glance will be able to separate between the two sexes in these species.
4) Exaggerated : The males and females of these species of butterflies appear so different that one could even assume that they are two different species.

A mating pair of Dark Brand Bush Brown.  Left : Female, Right : Male

These categorisations are by no means scientifically based and not intended as formal scientific classifications. As in any general layman categorisations, they are based on personal observations and there are certainly outliers and exceptions that may or may not fall clearly into any of these categories.

A mating pair of Pale Mottle

Then there are examples of dorsal and/or ventral sexual dimorphism. This is where the differences between males and females of a species are either on the dorsal (upperside) or ventral (underside) of the wings. In some cases, the differences are both on the dorsal and ventral sides of the wings.

A mating pair of two different morphs of the Lemon Emigrant

In some exceptional species, the sexual dimorphism is further expanded to polymorphism (i.e. more than just one form in either sex). This phenomenon may occur in just the females that display many different forms, or just the males that display many different forms. And then in some extreme examples, both the males and females display polymorphism. But by and large, only a handful of species fall into these categories. In Singapore, some examples are the Great Mormon, Malay Baron and Lemon Emigrant.

1 . Sexual Dimorphism (Negligible differences)

A pair of Lime Butterfly.  With a close look at the abdominal tips of the two individuals, it can be ascertained that the female is the top butterfly and the male is at the bottom

We now take a look at some of the examples of butterflies that fall into the 4 broad categories of sexual dimorphism. The first category is the most straightforward - where the males and females of a butterfly species are physically similar and it may take careful scrutiny to establish between a male and female of the butterfly. Examples shown here are certainly not exhaustive but just to illustrate some species that fall into this category.

Top : Male Lime Butterfly Bottom : Female Lime Butterfly. Note the size of the tornal spot and proportion of the blue-edged black spot to the red spot

Our first examples from the Papilionidae family would be the Lime Butterfly (Papilio demoleus malayanus). Ventrally (or seen from the underside), this species' sexual dimorphism can be considered 'negligible'. On the dorsal (upperside) of the wings however, the dimorphism may be considered 'subtle' as the main difference between the male and female of this species is the tornal spot on the hindwing. In space 1b on the hindwing, there is a red spot in both sexes. In the male, this spot is capped with a narrow blue lunule with a very narrow intervening black gap. In contrast, the red spot and the blue lunule in the female have a rather large black spot between them.

A mating pair of Psyche.  Difficult to separate between male and female from the underside

An example from the family Pieridae would be the Psyche (Leptosia nina malayana), in which the male and female are virtually indistiguishable when viewed from both the upperside and underside of the wings. The mottled underside and the predominantly white upperside with a black apical patch and a black spot on the forewing is almost identical in both the male and female of this species. This makes it near impossible to separate between the male and female of this species unless the primary sexual organs are examined.

From the Nymphalidae family, a good example of negligible difference in sexual dimorphism would be the Peacock Pansy (Junonia almana javana). This pair of mating Peacock Pansy demonstrates that there is almost little or no perceptible dimorphism between the male and female of this species. When viewed ventrally, it is difficult to separate the two. The two sexes are also near identical on the upperside with the usual ocelli present in both the male and female.

This pair of Common Caerulean (Jamides celeno aelianus) represents an example of negligible difference category as far as sexual dimorphism is concerned. Ventrally, the male and female is difficult to set apart from just the physical markings on the underside of the species. However, on the dorsal side, the sexes can be distinguished by the broader black border on the forewings of the female, making this species a dorsally distinct category.

Mating pairs of Skippers showing very indistinct differences between the males and females

Amongst the skippers, there is less sexual dimorphism in quite a number of the species in the family Hesperiidae. Quite a large number of the species can be classified under the negligible difference category. Examples shown here are the Large Snow Flat (from the sub-family Pyrginae), the Chestnut Bob and Grass Demon (from the sub-family Hesperiinae).

2. Sexual Dimorphism (Subtle differences)

Mating pair of Tailed Jay.  Female is the one on top, with its comparatively longer tail

The next category would be butterflies that demonstrate subtle differences in males and females of a species. The degree to which we can consider the "subtleness" in the difference between a male and female of butterflies is rather subjective, and here, we just give examples where there are minor features that are able to give away the difference between the sexes.

Left : Male Tailed Jay and Right : Female Tailed Jay.  Note the comparative length of the tails

A Papilionidae example of subtle difference between the male and female of the species is probably the Tailed Jay (Graphium agamemnon agamemnon). In this species, the key difference between the sexes is the relative length of the tail of the butterfly. Whilst the colour, spots and size are usually similar (with the male typically slightly smaller), the individual with the longer tail (usually twice as long) is the female. This very subtle difference separates the male from the female of this species.

Mating pair of Orange Emigrant.  The female, on the right, has darker and more pronounced spots on the wings

A typical example of a Pieridae that displays subtle differences between the male and female would be the Orange Emigrant (Catopsilia scylla cornelia). In the photograph shown of a mating pair, the female has more spots on the hindwings compared to the male, and on the dorsal side, the female has a black marginal border on the hindwing (absent in the male).

Mating pair of Plain Tiger.  The male, on the top, has an extra spot on the hindwing

From the subfamily Danainae, the secondary sexual characteristics of a sex brand on the hindwings is a subtle diagnostic feature that separates the male from the female. In the case of this mating pair of Plain Tigers (Danaus chrysippus chrysippus), the male (top) has an additional "spot" in the form of a sex brand on the hindwing. The female does not have this brand. This subtle spot separates the sexes in this species.

Mating pair of Apefly.  The female, on the left, has a more rounded forewing compared to the sharper and more angular shape of the male's forewing.

In this pair of Apefly (Spalgis epius epius), the structure of the wings is a very subtle giveaway in separating the male and female butterflies. The forewing of the male is more pointed and pronounced compared to the more rounded forewing of the (usually) larger female. The male, on the right of this photo, displays the more angular forewing and is obvious enough to distinguish between the sexes.

A mating pair of Common Mormon with clearly distinct features between the male and female.  These two categories will be discussed in Part 2 of this article next week.

In Part 2 of this article, we take a look at species with more distinct physical appearances between the males and females. We also look at the evolution of some of these examples of sexual dimorphism and ask some questions with regard to some of the reasons behind some of these evolutionary adaptations, probably by natural selection.

Text by Khew SK : Photos by Chng CK, Antonio Giudici, Khew SK, Koh CH, Loh MY, Tan BJ and Anthony Wong