Butterflies are one of the most charismatic insect group on the planet, considered to be beautiful and delicate. Male butterflies use beauty to try to dazzle females into picking them as a mate, often displaying courtship behaviors that involve visual and chemical communication. Males use courtship to try to convince females that they have what it takes to produce the best offspring. It is the female’s responsibility to make the call, she has control over with whom and with how many males she will mate during her life. This is not a rule in the butterfly world, however. In some species, males have evolved strategies that can give them some level of control over female mating frequency.
Males use mating plugs to prevent female remating
An array of strategies are employed by male insects to try to delay or completely prevent females from mating more than once, including mate guarding, long lasting copulations, antiaphrodisiacs, among others. If a male is able to completely prevent female remating, he ensures that all of the female’s eggs will be fertilized with only his sperm, maximizing his reproductive success. One of these strategies are mating plugs, which are structures produced by the males and transferred to the female after the transferal of the spermatophore (a sac that holds the sperm). The mating plug physically blocks the female copulatory opening, preventing further copulations. In some butterfly groups, these plugs (also called sphragis) can be very large and externalized relative to the female body and come in many different complex shapes.
These plugs seem to be quite efficient in enforcing monogamy onto females and, therefore, serve the male’s purpose. But how does that affect females? Females of many butterfly groups remate as a way to not only acquire more sperm, but also to accumulate nutritional substances that are transferred via spermatophore by the males to the females during mating. Considering this, my colleagues and I decided to use morphology of the female genitalia to try to understand how mating plugs might be affecting females, and how females are “fighting back” for control over their mating frequency.
Females might be fighting back
We performed a study with the clearwing butterflies of the genus Pteronymia. In around 27% of the species of the genus, males use plugs to block the female genitalia and prevent female remating. After careful morphological evaluations of female specimens in insect collections, we noticed that the structure of the female genitalia in those species where males make plugs differed from those where females do not get plugged. In species with plugs, the plate where the female copulatory opening is found is very wide and bowl-like. On the other hand, the plate of species without plugs is much narrower. We believe this is a feature that evolved in females that inevitably makes male plugging less efficient because males would need to use larger amounts of plug material to completely block this area of the copulatory opening. Also, we have found that the plugs were extremely variable in regard to their size within species, and that small plugs were often more common than larger ones.
Who is winning?
It is difficult to say who is winning, but considering that variance in plug size is uncommon in several plug-making butterfly groups (e.g. Parnassius and Acraea), it is possible that Pteronymia males are reducing the amount of investment that they allocate to plugs. We also found females bearing more than one plug, which implies that the female attempted to mate with more than one male. We cannot, however, confirm if the second male was successful in transferring his spermatophore to the female.
Based on these data on female genitalia morphology, we believe that the success of the male (or perhaps conversely, of the female) likely depends on the width of the female genitalia cavity, which is harder and harder to plug as it increases in size. Also, it is probable that the individual female mating frequency is likely dependent on the size of the plug transferred by the first male to mate with her, as smaller plugs are probably less efficient in preventing remating, and more easily removed by a second male, or even lost by the female during her lifetime.
We want to continue studying how males are using plugs to enforce monogamy and, most importantly, how (and if) females are fighting back in the battle over the control of female mating frequency. Literature suggests that these types of intersexual conflicts are happening in many insect groups, through several different strategies that have evolved in males to prevent females from remating.
Citation: Ana Paula S Carvalho, Luísa L Mota, Akito Y Kawahara; Intersexual ‘Arms Race’ and the Evolution of the Sphragis in Pteronymia Butterflies, Insect Systematics and Diversity, Volume 3, Issue 1, 2019, 3, https://doi.org/10.1093/isd/ixy021