In the weeks I’ve been back in Auckland I’ve been largely stuck to my desk finishing off a couple of papers and grant applications, and getting myself organised for the harvestmen project.Luckily I was able to head out yesterday to Matuku Reserve, my old PhD field site in west Auckland for a few hours of searching for giraffe weevils. Despite the weather cooling off, there were still plenty of those goofy critters around to watch and get inspiration for new projects.
It was really interesting to see how much the forest has changed since the last time I was out there. Huge karaka trees that had consistently hosted giraffe weevils each summer during the 5 years I spent at the reserve are now completely rotted or fallen down. I also spotted a few trees that were currently playing host to the weevils, either already filled with larvae, as revealed by fresh piles of sawdust piled up underneath the tree, or covered in adult pairs busily mating and laying eggs.
Although officially working on harvestmen for the new postdoc, I still have lots of ideas for taking my research on this fascinating species further. Giraffe weevils have also been on my mind over the last week as Nicola Toki chose them as her Critter of the Week on Radio New Zealand. It’s always great to spread the word about some of our smaller but no less interesting species. Also, Mike Dickison (@adzebill) and his team noticed that the Wikipedia page for the giraffe weevils was pretty awful, so they did a very speedy but thorough job in updating the page and it now looks great! It’s worth checking out if you want to learn what we already know about this species and if you’ve always wanted giraffe weevils as your desk top photo you’re in luck because I’ve donated a few photos for public use.
The reason I was out in the bush yesterday was that I’m hoping to start a new project on my favourite invertebrate, looking at trade-offs between traits that allow them to win mating opportunities through fights (i.e. their long heads used as weapons) versus traits that allow them to gain more fertilizations (i.e. more and better quality sperm).
Sperm competition in the giraffe weevil is likely to be a really important factor affecting a male’s reproductive success, because females can mate multiple times before they lay their egg. This means that competition between males continues after any physical combat that has allowed them to mate, because sperm from other males that have also mated with the female are vying to fertilize the egg. Sperm competition has led to all sorts of ways that males increase their fertilization success, including investing in more sperm (bigger ejaculates) and extensive post-copulation (after mating) guarding behaviour to stop any other males from having a chance to mate.
There is a lot of discussion in the scientific literature about these theorized trade-offs between competition among males that occurs before and after mating. The main idea is that males often have to invest in either big weapons and traits that allow them to find and secure mates, OR traits like big volumes of really high quality sperm that allow them to ensure paternity. Because energetic resources are limited, they shouldn’t be able to freely invest in both sets of traits. So, when considering that big giraffe weevil males are investing a lot into really big weapons (their long ‘noses’), we might expect them to be investing less into sperm traits than smaller males. This is because during larval development, the cells that eventually become adult body parts in beetles are formed after the larvae has stopped feeding, meaning that there is a limited pool of resources.
Sperm competition is particularly interesting when considering the giraffe weevils, as not only do female’s mate lots of times before egg-laying, there are also tiny little males running around and sneaking matings while the big males stand over females and try to guard them. As you can see from a scene I witnessed yesterday, big males are often not to so successful in preventing sneaker males from mating, as the little guys literally mate under the ‘nose’ of the guarding male.
Despite their devious behaviour, small males probably suffer from higher levels of sperm competition than big males, especially if guarding males end up mating again after they detect a sneaker’s presence as a form of retaliation.We might then expect small males to invest more into each mating than a larger male, in the form of big ejaculates in an attempt to flood the female with lots of his sperm. This has been observed in other insect systems with sneaky males, such as dung beetles, where little males ejaculate more per mating than large males and also have relatively bigger testes than their larger male counterparts.
We still have so much to learn about the mating systems of insects, and the giraffe weevils provide us with a really great opportunity to study these questions. Unlike lots of insects they are easy to observe in the wild, in big numbers, and in daylight hours (after long nights in the field watching harvestmen, the giraffe weevils seem like little angels). They also tend to stick around on one tree as adults which makes it easy to mark them with a unique colour combo on their bodies, and then track them through their lifetime.
Having an abundant population at Matuku Reserve so close to Auckland makes life as a behavioural ecologist much easier too, as we can do lots of work in nature without too much travel. It’s not a bad spot to do research either…