Let’s Talk About Sex

Blues mating

It’s spring at last, butterflies are emerging and love is in the air. Butterfly lifecycles are neatly divided when it comes to the important things in life – caterpillars are all about eating and growth and adult butterflies are all about reproduction and, to a lesser extent, dispersal.

So let’s talk about sex. First the basics; like humans, butterflies are either male or female. They mate, joining the tips of their abdomens, and the male passes sperm to the female in order to fertilize her eggs. The female then lays her eggs on plants or on the ground.

All very straightforward. However, there is a wealth of interesting, sometimes amazing, behaviour and biology when it comes to butterflies and sex.

Courtship, for example, is important for many butterflies. Generally, female butterflies choose which males they want to mate with and males therefore compete for the attention of females. Some males search for females, patrolling almost incessantly during good weather in the hope of encountering a receptive female, while others perch on a good vantage point to keep watch.

Male Peacock butterflies set up territories on the ground, for example, often in a sunny spot at the corner of a field, hedge or woodland ride, while Speckled Woods often perch on plants in small pools of sunlight on the woodland floor, waiting to intercept passing females. Such territories are vigorously defended.

Spectacular aerial fights take place between rival males, with the butterflies spiralling upwards, each trying to get above the other. Most often the resident male wins this ritual battle and returns to his perch. Other insects, human observers and even twigs thrown past a perching male all tend to elicit the same ‘up and at ‘em’ response.

The Small Heath butterfly, a small and rather drab species, takes mate location to another level. Small Heaths form leks, behaviour more famously associated with large birds such as Black Grouse and Capercaillie. A lek is a place where males and females gather together to select mates. Small Heaths congregate at landmarks such as trees and bushes in their open grassland habitats where climate seems to be the attraction; the males competing for the most sheltered spots. Virgin females deliberately avoid the attentions of searching males as they make their way towards the lek but, once there, they encourage advances with a conspicuous circling flight.

Once male and female have found each other, more intimate courtship takes place. This ranges from the aerobatic feats of the Silver-washed Fritillary, where the male flies loops around the female in midair, to the delicate, face-to-face, antennae-waggling display of Wood Whites.

Chemical signals can be important too, and not only in attraction. Male Large White butterflies, for example, transfer an anti-aphrodisiac chemical to females during mating. This deters other males from courting the recently mated female. Incredibly, certain tiny wasps can detect this same chemical and use it to track down Large White eggs, which they then parasitise with their own offspring.

In many butterfly species, females mate only when they first emerge. This is why males tend to emerge a few days ahead of females. After mating, females then try to avoid the unwarranted attentions of amorous males while they get on with the important task of egg-laying.

However, the female Green-veined White will typically mate with several males over the course of her lifetime and, by doing so, will increase the total number of eggs that she can lay. This is because, during mating, male Green-veined Whites not only transfer sperm but also a so-called ‘nuptial gift’ of nutrients that the female can assimilate and use to increase egg production. Exceptionally, male Green-veined Whites may transfer 25% of their own body mass to females during mating, though typically this is more like 15%.

Butterflies don’t go in much for parental care – in fact the parent butterflies have usually died by the time their offspring hatch out as caterpillars. However, female butterflies often put a lot of effort into deciding where to lay their eggs, so as to give their offspring a good start in life. They choose the right types of plants for their future caterpillars to feed on and can be very selective about the size, shape and location of particular plants. Duke of Burgundy females prefer to lay eggs on bigger, prominent cowslip plants with longer leaves, and Small Tortoiseshells select the younger, more nutritious nettle leaves for their eggs. Often, plants growing in particular micro-climates are selected (e.g. Silver-spotted Skippers preferentially lay eggs on foodplants growing in particularly warm situations). Orange-tips can even distinguish between plants on which another Orange-tip egg has already been laid and unoccupied plants, thanks to a pheromone ‘marker’ left by egg-laying females – handy, as Orange-tip caterpillars are cannibalistic so there’s only room for one on each foodplant! 

Richard Fox

Surveys Manager

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