Finding a mate is one of the biggest challenges facing any animal which relies on sexual reproduction. For butterflies, the process of finding, recognising and attracting a mate usually rests on a combination of sight and scent (1). The task of finding, or locating a mate is usually carried out by males, although females can facilitate the process by placing themselves in the vicinity, for example flying past perching males who then give chase. The male butterfly cannot force himself on a female, so the outcome is not a foregone conclusion and depends on the predilection – receptivity and choosiness – of the female. 

If the female has recently mated, she does not want to be hassled by a lot of eager males. It would be an annoyance and a waste of her time: she needs to get on with laying her already fertilised eggs. To avoid this conflict of interests, many species of butterfly have evolved specific behaviours by which already mated females can avoid wasting time in unwanted courtship. Butterflies in the family Pieridae (Whites, Sulphurs, Yellows) have evolved a very specific piece of behaviour, or body language, called the mate refusal posture: the wings of the female are held wide open –  at or below the horizontal – and the abdomen is raised up, almost to a vertical position, often with the genitalia extruded (2, 3). If you Google ‘mate refusal posture’ you will find a few images of this type of behaviour on the internet, including some nice photographs of an incredibly worn female cabbage white butterfly in this posture, on another blog site (Cabinet of Curiosities).

I happened to come across this behaviour in the green-veined white butterfly, Pieris napi. The light at the time was not great, as the butterflies were in the shade and I was not using flash, but I think I managed to catch some of the components of this behaviour. See sequence of photographs that follows below.

The reason why the female butterfly is unreceptive and adopting this characteristic mate refusal posture is because she has already mated and is in receipt of a spermataphore. A spermatophore is transferred from the male to the female during copulation and is composed of a tough, indigestible outer envelope, an inner matrix, and a bolus of sperm.  There is a picture of a spermatophore from a male Pieris rapae butterfly in this excellent article by the University of Cincinnati.

The females maintain this posture as long as there are males in the vicinity. After a few days the female has broken down the spermatophore and is ready to mate again. Virgin females are also unreceptive to advances from males for a short period after they have emerged from their pupae. Their refusal posture is however, somewhat different, with wings closed.  After about 30 minutes, her wings have hardened and she becomes receptive and demonstrates this by remaining motionless with closed wings.

In butterfly species like this one (Pieris napi) , which mate more than once, the females gain by receiving nutrients from the males during sex, hereby building up stores of protein to maximise their fecundity. Multiple matings have been shown to increase the production of offspring. Yet despite this obvious advantage, the average number of female matings is often less than two in the field, because mating is a risky business! Copulating pairs are relatively inactive – whilst they are in copula – and therefore vulnerable to attack by predators.

Males usually mate more frequently than females. His sperm can fertilise all of her available eggs for a few days, until she is ready to mate again. So although an adult female can live for a week or so in the field, she probably only mates a couple of times on average and needs therefore, to put off ardent males while she gets on with the business of laying the eggs for the next generation.

In conclusion, it is worth remembering that although all of these butterflies look white to us (albeit with black markings) the butterflies themselves see each other in colour! This is because their wings contain pigments which absorbs ultraviolet light (4). Since we humans cannot see UV light the wings appear mostly white to us (there are some yellow pigments as well), but butterflies have UV photoreceptors, so the wings appear brightly coloured to them. The actual colours of the wings, as seen by the butterflies – the small white, Pieris rapae, in this case – are thought to be lavender (for females) or bold violet (for males).

For reference, these photographs of the mate refusal posture were taken on 25 June 2017, in Galicia, Spain. The subspecies P. n. napi is the same as that found in the UK.

References

1. Vane-Wright, R. I., & Boppre, M. (1993). Visual and chemical signalling in butterflies: functional and phylogenetic perspectives. Philosophical Transactions of the Royal Society of London B: Biological Sciences, 340(1292), 197-205.
2. Obara, Y. (1964). Mating behavior of the cabbage white, Pieris rapae crucivora. II. “The mate-refusal posture” of the female. Zool Mag
   (Dobutsugaku Zasshi) 73: 175 –178 (in Japanese with English abstract).
3. Itoh, J., & Obara, Y. (1994). Visual stimuli eliciting mate refusal posture in the mated female of the cabbage white butterfly, Pieris rapae crucivora (Lepidoptera: Pieridae). Applied Entomology and Zoology, 29(3), 377-388.
4. Wijnen, B., H. L. Leertouwer, and D. G. Stavenga. “Colors and pterin pigmentation of pierid butterfly wings.” Journal of insect physiology 53.12 (2007): 1206-1217.