Some male butterflies plug up their mates鈥� reproductive tracts to prevent them from having sex with rivals. Now, there鈥檚 evidence that they might not be doing so indiscriminately. The type and size of the mating plugs seems to correlate with how much males are willing to invest in their partners.
Males of several groups of animals, such as beetles and water bugs, are known for mate-guarding: sticking around after copulation to keep others away from their partner and increase the chance that they alone will fertilise all of a female鈥檚 eggs.
But it鈥檚 an energy-consuming strategy, so some male butterflies have developed an alternative. They secrete a mate-guarding seal 鈥� the sphragis 鈥� to literally plug their partners鈥� genitals and make further mating difficult or impossible.
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It鈥檚 an unusual approach, only occurring in 1-2 per cent of butterfly species. In an effort to find out how some butterflies use the sphragis, at the University of Veterinary Medicine Budapest, Hungary, and his colleagues studied clouded Apollo butterflies (Parnassius mnemosyne) 鈥� one of the rare species that produce them. Between 2015 and 2020, they examined 492 female butterflies that had recently mated.
Their research confirmed earlier observations that female butterflies are subjected to one of three different types of plugs: thin internal filaments that are not particularly effective for preventing further mating; a slightly larger plugging device the researchers dubbed a 鈥渟topple鈥�; or an even larger plug that includes a stopple and an external shield, which is most effective for preventing rivals from mating with the female.
Kis and his colleagues realised when competition was higher, males dispensed more shields. And crucially, female butterflies with wider thoraces 鈥� animals that have a better chance of surviving 鈥� were the most likely to receive one of the large shields. These females also tended to be younger, according to Kis, meaning that they had more eggs to lay and so were the most desirable mating partners.
鈥淚f [confirmed], the likely rule is at a high level of competition, produce a shield, at a medium level of competition, invest more in the larger females,鈥� says Kis. 鈥淎t a low level of competition, produce a cheap stopple or filament, and use the remaining resources at further [mating] opportunities.鈥�
The male butterflies seem to be assessing the situation and deciding on the best strategy to boost their reproductive output, according to Kis. How they gather and evaluate this information is still a mystery, though. 鈥淯nfortunately, we only have ideas on the mechanisms with which males are able to assess female quality,鈥� says Kis.
鈥淭hese findings are just one tiny piece of a rather large and baffling jigsaw puzzle,鈥� says at Griffith University in Australia, who was not involved in the study. He鈥檚 been saying since the 1980s that the sphragis is probably 鈥渢he best example of conflict between mates producing evolutionary change鈥� known to science.
鈥淭he sphragis is an evolutionary oddity; it is a system of co-adaptations,鈥� he says. 鈥淚t is not just 鈥榓 big plug鈥�.鈥�
Ecology and Evolution
