TDK conference 2020

Competition for mating between males is present in the majority of animal species. One outcome of this is the attempted monopolisation of females by guarding. Males may either keep close to their mates during the mating season or produce a plug whilst mating, blocking the female copulatory orifice. Plugs can be found e.g. in rodents, spiders and insects, and may be surrounded by a large external shield, e.g. in Apollo butterflies. In insects, larger females often incur better reproductive success. If this is true for species producing mating plugs, the extent of male investment in the plug could be influenced by the expected female reproductive value, which might be estimated based on female size.

My question is whether female body size predicts if the male will produce a shield around the plug during mating or not, and if so, how large. I hypothesise that larger females would be more likely to get a shield rather than a plug and they would likely to receive longer shields than small females. As the mating season progresses, females would be less likely to get a shield, since male ratio, thus male-male competition in the population will decline. Simultaneously, female life expectancy will also decrease, as will their expected reproductive value.

We studied a Clouded Apollo population with mark-recapture in the Visegrád-hegység, 2015–2020. We attempted to catch each individual and mark them with a unique identifier. We measured thorax and head width, proboscis and wing length and body mass. We measured shield length for females bearing one, and documented plugs or their absence with photomacrographs where no shield was present. Using logistic regression, we investigated if size and the progress of the flight period predict whether a mated female received a plug or a shield during her first copulation. We also investigated if female size related to her shield length using linear regression.

Females with a wider head bore a shield more often than a plug in 2015, unlike other years. The rest of the body size variables did not relate to shield/plug presence. As the flight period progressed, females were less likely to bear a shield and had plugs more often. Based on these variables, body size does not influence male decision regarding shield production. Males emerge earlier than females throughout the flight period, resulting in a male-biased sex ratio and high male-male competition at the beginning. The extent of this is probably so severe, that it is not worth choosing among females, and it pays off producing a shield. Over time, sex ratio becomes female-biased, and the older females’ reproductive value decreases as does the older males’ ability to produce materials required for a shield. This may explain why the costs of shield production increases over time and why males invest less and less in the monopolisation of their mate.