Weasley weevils
I love bizarre creatures, and so I count myself pretty lucky to study one of New Zealand’s most fascinating invertebrates, the giraffe weevil or tuwhaipapa (Lasiorhynchus barbicornis).
About two years ago I met with my supervisor-to-be Dr. Greg Holwell at the University of Auckland to chat about potential PhD projects.
He asked me whether I would be interested in studying giraffe weevils as he was intrigued by a combination of two things: their amazing size variation and the incredible elongation of the male’s head which gives them their giraffe-like appearance.
I too was intrigued about this and was surprised to find out that very little is known about New Zealand’s longest beetle. Both males and female giraffe weevils have an elongation of the head which is called a rostrum, although in males this is far more dramatic so that the head makes up around 50% of its total body length.
Coupled with a six fold difference in body length between the smallest and largest adult males this led us to ask questions about the mating and fighting behaviour in this species. Most importantly, how are small males (see picture of the size comparison) able to compete for females when it seems obvious that they wouldn’t stand a chance against much larger counterparts?
It turns out there are many examples throughout the animal kingdom where small males have evolved new behaviours which allow them to compete against large males. These are called alternative reproductive tactics and are often in the form of dominant versus subordinate behaviours such as guarding versus sneaking.
One of the most well-known examples is the dung beetle group, particularly the genus Onthophagus. Large males possess horns that protrude out the front of the head or thorax and are used as a weapon during combat. Small males on the other hand have tiny horns or do not grow a horn at all, so they have a pretty big disadvantage if trying to fight larger males.
However, to counteract this they have evolved sneaking behaviours where they dig side tunnels under dung pads where the females hang out and quickly copulate with them before being detected by a guarding male. Pretty amazing stuff! We wondered if something similar was happening with the giraffe weevil. I have since spent the last two summers occupying myself with this and other questions about their behaviour and ecology.
My first challenge was to find a population of giraffe weevils large enough that I could conduct observations of them in the wild. Luckily I was given a tip-off from some colleagues at Landcare Research and on my first trip in September 2009 I found 7 weevils at Matuku Reserve in the Waitakere Valley west of Auckland.
Matuku Reserve is owned and cared for by the Waitakere Branch of Forest and Bird, and has turned out to be an excellent reseach base. After that first visit I continued to return to Matuku, and over the summer of 2009/2010 I was able to find numerous trees where giraffe weevils aggregate in large numbers.
Around Auckland giraffe weevils are mostly found on their host tree karaka (Corynocarpus laevigatus), but they also bore in other New Zealand natives such as mahoe (Melicytus ramiflorus), rangiora (Brachyglottis repanda) and pukatea (Laurelia novae-zelandia). Adult weevils aggregate on these trees to copulate, during which females will drill holes into the trunk and subsequently oviposit an egg into the hole. Larvae tunnel throughout the tree feeding on wood, and hatch as adults several years later.
These aggregations of adults provided a perfect study ground for observing the interactions between these insects. I was lucky enough to find trees where the majority of weevils were aggregated from ground through to eye height so that most of the time I could get by without a ladder, although sometimes I could be found teetering half way up a tree to watch a wayward male.
Every morning, I would remove and measure each weevil, and then give them a unique colour code, so I could track their movements and behaviours. By the time summer (2010/2011) drew to a close, I’d marked several thousand weevils. After painting the weevils, I would step away from the tree and use close-range binos to observe each male for one hour periods.
What became apparent early on was that competition by males for access to females was really intense, especially on densely populated trees. On one busy tree I counted 178 weevils on a single day!
Males spend a lot of time wandering around the tree, pushing each other around to win females and sometimes engaging in combat that involved locking themselves together and attempting to dislodge their opponent from the tree.
One day I was a spectator to a clever example of subterfuge that took place between a very tiny male and a large male who was mating with a drilling female. The little male tucked himself under the pair and hid while the large male continued mating and fending off other intruding males. Eventually the large male stopped mating but stayed mounted over the female to guard her.
At this moment the sneaky male gently climbed onto the female and proceeded to mate with her, all the while being protected by the large male who didn’t seem to notice his presence! I realised I had found a very definite form of sneaking behaviour, and confirmed the presence of alternative mating tactics in this species.
Now after another field season I have another 100 hours or so of observations and a huge amount of data on mating and fighting behaviour. I’m currently trying to look more closely at small male behaviour and what conditions (such as relative size) result in the decision to sneak rather than challenge their opponent.
There are many unanswered questions and I look forward to being able to share these throughout the course of my research. Now though, as I type this on a chilly winters day, I’m mostly looking forward to the weather warming up again so that I can start another field season and delve further into the amazing world of tuwhaipapa!
Blogger: PhD Student, University of Auckland, School of Biological Sciences, Chrissie Painting
