Mating system, female fecundity and sexual selection in wild Cassida viridis beetles

Popular Abstract

What makes a beetle attractive?

My project looks at how green tortoise beetles (Cassida viridis) choose partners, and which physical traits have a higher success in obtaining a mate. I did this by tracking individuals in three wild populations of Skåne, Sweden, and taking photographs of mating beetles. To document differences between the sexes and populations, see how many partners a beetle has, and understand how body size and body shape affect mating success, in a species where little is known.

Selection is one of the drivers of evolution and therefore an important aspect to understand how diversity in nature is built. Selection favors individuals with traits that have increased survival or reproductive success in a population. This study focused on sexual selection, scrutinizing individual mating success through the number of mating partners. In addition, it is a complex topic with several different potential factors affecting it and possible variation between populations and years.
Across two reproductive seasons (May-August 2022 and 2023), I marked adult beetles with a unique ID and recorded mating events during weekly visits to the wild populations. I also took photographs of all captured beetles. Later, I measured body size and shape of each individual beetle using the photographs taken in the field and a computer software, Phenopype. In the end, I had information on the number of matings of each beetle, their sex (male or female), population, body size and body shape, to test my hypothesis. Additionally, in 2022, I collected data on the number of egg collections a female laid, to complement the main analysis.
Results and Implications
For my study populations of the C. viridis beetle, I demonstrated that females are bigger than males, likely linked to fecundity advantages, since bigger females laid more egg collections than smaller females. Both males and females mated with multiple partners, yet the choice of partner varied between years. In 2022, beetles mated with similarly shaped partners (e.g. more circular beetles mated with more circular beetles) and females copulating later in the season or remating chose a bigger male. Nevertheless, in 2023, beetles chose a partner dissimilar to themselves (e.g. big females mated with small males and vice-versa) and females mating later or remating chose a smaller male. Moreover, contrary to popular belief, bigger is not always better. And so is the case in this beetle, where smaller beetles had more partners overall, in both years. Additionally, results varied between sexes in terms of the selection acting on body size and its strength. In males, both smaller and bigger than average individuals had mating advantages, with populations favoring different sized males and female preference changing as the season progressed. Nonetheless, in females it was more beneficial to have an intermediate size, likely a result of opposing selection pressures acting on the same trait (e.g. bigger females have higher fecundity but lower mating success and survival). Lastly, beetle shape was not a direct target of selection and only selected in combination with beetle size (e.g. smaller and narrower beetles had more sexual partners).
As you might have noticed, beetle attractiveness in terms of body size is not as straightforward as one might think. In C. viridis, the preferences and mating system seemed flexible. Even though smaller beetles had higher mating success overall, sexual selection on body size varied between sexes, years and populations. Demonstrating a complex system with different potential factors in play, such as: population density, beetle mobility in the search for mates, differential harassment and longevity. As well as exhibiting that selection can act on multiple traits combined (body size and shape). To conclude, future studies should expand on these results, as this was the first study of sexual selection done in this species and so much more is yet to be uncovered.

Master’s Degree Project in Biology, 45 credits, Department of Biology, Lund university.

Advisor: Maja Tarka, Kristina Karlsson Green, Erica Winslott.
Department of Biology, Lund university.

• author: Botelho, Ana
• supervisor: Maja Tarka; Kristina Karlsson Green; Erica Winslott
• organization: Degree Projects in Biology
• course: BION02 20232
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9178967
• date added to LUP: 2024-12-19 16:12:09
• date last changed: 2024-12-19 16:12:09