One of our first goals on Redonda was to get a good sense for the density of lizards on the island. This is one of the key metrics that has changed following rat eradications elsewhere in the Lesser Antilles. To do that we used a few different methods, we’re hoping that each will give us a slightly different sense for densities that we can compare one year to the next.
The first was a mark-recapture study in two different locations. We’d survey a fixed area (one of the big fig trees at the north of the island and a boulder field at the south) and catch all the lizards we could find. Every time we caught a lizard we’d write a number on its belly and put a little white dot on its backside (see below). The paint spot will help us see the lizard from a distance without spending lots of time trying to catch a lizard that’d already been caught that day.
Lizard number 22 getting a number on its belly. Photo credit: Geoffrey Giller
An anole hanging out on a branch after getting a white spot. Photo credit: Geoffrey Giller
We spent two long days marking all of the lizards we could find in the tree and the rocks. After each site had had a day to rest, we returned. By figuring out which lizards we’d previously caught and how many new lizards had shown up we can get an estimate of just how many lizards are using that area. The lizards in the tree were particularly plentiful – we’re estimating a population of around 50! with unlimited time we’d continue marking, revisiting, and recapturing a few more days but our island time was short so we compromised with a single recapture day at each site. We’ll repeat the survey next year in the same places to see if there are changes in density.
Our next survey method was the tried-and-true walk around, look on both sides of your path, and count ’em up. For this Anthony did a big hike around the perimeter of the island counting all of the lizards he saw. He did this twice and found a lot of lizards! Here’s a page from my notebook showing the track. Unfortunately the map only really makes sense to us, but as you can see, he saw 61 anoles and 65 ameiva that day. The next day he ended up seeing 73 anoles and 118 ameiva! Those are pretty wide margins, but they’ll provide a baseline for next year’s repeat transects.
Now, I decided to strap a GoPro to Anthony’s head for the second survey. It’s two hours long though so I had to speed the video up. Grab your dramamine, you’re going to need it!
This video is just a short segment of the hike in the vicinity of the “Hanging Gardens.” It’s one of the prettiest parts of the island I think. You can tell too how tricky some of the traversing is. On this section Anthony saw 10 ameiva and 24 anoles. Can you find them? I slowed down the first ameiva so you can get a good look. You’ll also notice a few other surprises along the way.
I’ve posted in the past about the lizard sprint speed studies I’ve tried over the years on erhardii. Each of those experiments have come with the caveat that without a high-speed camera (shooting around 500 frames per second and costing the down-payment for a house) we can’t detect more subtle differences between lizard populations. Well, this year Menelia brought a beautiful high-speed camera from her home institution in Antwerp and we had fun finally getting a good look at these lizards on the run.
First steps, warm up the lizards. We used individual socks this year – a major innovation that made it a lot easier to be sure each lizard did their trial run before the next, and sped the process up without having to chase the lizard around the bin.
Then the white dots. These dots show up nice and clear in the video camera and help track the back of the head, the midpoint between shoulders and hips, and the middle of the back. Hopefully these will make processing the video a lot easier.
Here’s our setup. Note the two rainbow kiddie pools (hard not to notice them). This was a major improvement! The lizards would come rocketing out of the sprint speed track and land on the nice inflated pool floor, ready for us to scoop them up back into their sock.
Notice too that the sprint speed track has a couple of right-angle turns in it. One of Menelia’s questions deals with maneuverability and so we were anxious to see how well the lizards negotiated both the first, and second turns in the course.
Here’s the camera’s eye view. Beautiful! Time to run the lizards.
As I’ve mentioned before, one of my primary research interests is how these lizards change in different contexts. So far I’ve measured a whole lot of morphological traits like limb length and head width on lizards from islands all over the cyclades. I’ve also measured a whole-organism performance trait, namely bite force, which relies on many of these head-related morphological traits working together in concert to see if that varied from one island to the next. Head measurements have differed significantly, as have bite force measurements. Now I want to determine whether the differing limb lengths also result in differences in performance, specifically sprint speed.
There are many ways in the literature to measure lizard sprint speed. The most common is to use a racetrack or small treadmill, set a lizard down and record how fast it runs across the surface. This works well, to a point, but I’ve never been entirely satisfied with papers discussing the implications of a lizard that runs 4% faster than another over a thick rubber mat. I wanted to test sprint speed in a more realistic context. My first thought was to build a fenced course out in the field, but I soon realized that wasn’t going to be feasible, so I set out to replicate field conditions in the lab.
The two populations I most wanted to test were both from Naxos. One lives in a highly developed valley full of rock walls and terraces and spends almost all of its time on and near the stones. The other lives in an area without any significant human land use – no grazing and certainly no walls, and so spends its time running across loose sand from bush to bush. These two substrates formed the basis of my question – because the lizards in the two areas have different limb lengths, does that mean they have different sprinting capacity over the substrate they’re most used to traversing?
So, I built a 3 meter long plastic-lined chute in the lab. I covered the bottom with sand, set up my video camera, and started recording lizards from both populations running across it. Things got really interesting when, after I absconded with about 15 large stones from an old rock pile, I built a mini rock wall in the chute. I then re-ran all of the lizards across the stones to see if some managed better than others. I now have a few hours of video to go through, frame by frame to calculate the sprint speeds of each individual, but fingers crossed something interesting comes of it! Here are a few pictures of the set up.
Just a quick update to let you know I’m alive and well. Sorry for the long delay since last posting. I’ve quite literally been flat out every single day. Last week Tuesday thru Friday I was up and in the field at or before 6:00 am. I’m a herpetologist! We’re supposed to get up with the lizards at 10:30! There’s a reason I didn’t want to study birds.
Still, the early mornings have generated lots of adventures, which I fully intend to fill you in on asap. Until then though, here’s the product of my first early morning: the second set of sticky traps and pitfall traps I collected at the crack of dawn on Tuesday. All in all I had 144 sticky traps and 216 pitfall traps, which should hopefully give me a good snapshot of the insect communities around the rock walls.
After the craziness in Africa trying to smear tanglefoot on 3×5 index cards (read about it here) I decided to splurge and buy the real thing (for an additional $7) DEFINITELY a good investment. I also had the idea to preserve them in the plastic sheathes that fit into a 3 ring binder. Much better than my previous debacle with seran wrap in the field. So here they are… now all I have to do is sort through all of them, identify their order and count ’em up! Oh, and then sort through the soggy insects in the pitfall traps… I really need to do that soon…