The Lizard Project

Our second week at Palo Verde was independent projects, take two. This time, I worked with Becca, who is a junior at Bowdoin College in Maine. After researching and planning for a few days, we decided to study the use of color vision in the recognition of fruit ripeness by Ctenosaura similis. Ctenosaurs are a species of iguana that are abundant at Palo Verde. It has been shown that they use both color vision and scent in making foraging decisions, but the importance and interaction of these senses in recognizing fruit ripeness was unstudied.

Questions

Our initial questions were:
I: Does Ctenosaura similis use color vision to differentiate fruits based on ripeness?
II: When equal “I’m yummy” olfactory cues are present, does C. similis differentiate between ripe and non-ripe colors?
III: When color cues contradict olfactory cues, does color preference prevail?

Thus, the final paragraph of my final paper's introduction was as follows:

In this study, (1) we examine the ability of Ctenosaura similis to differentiate between ripe and unripe fruits using color vision. Here, we predict that C. similis will show preference for ripe-colored fruits in the absence of olfactory cues. (2) We then assess whether C. similis differentiates between ripe and unripe fruit colors in the presence of equally attractive olfactory cues. By comparing preferences shown in (1) and (2), we determine whether the role of color vision changes when chemosensory cues are available. Finally, (3) we examine the relationship between color cues and scent in the foraging decisions of C. similis, with the goal of assessing the relative importance of each cue and by what factors is it influenced.

Methods

To examine the first question, (I) we made fruit models by covering pebbles with colored plasticine (nontoxic clay). Sets of three ripe-colored and three green fruit models were placed in areas frequented by ctenosaurs, and the number of bites on each color was recorded after approximately eight hours.

(II) In the second phase, we presented ctenosaurs with a choice between a ripe-colored and green banana slice (dyed with food coloring) and recorded which banana was eaten first.

(III) In the third phase, we presented ctenosaurs with a choice between a red fruit model and green banana and scored which was (a) approached first and (b) bitten first.

Data collection for our project was fun, hilarious, and very successful (unlike my last project!). Setting out the plasticine fruits took forever, but we got a lot better at knowing where to put the fruits so as to get the most data. Collecting the banana data was highly entertaining - we would first find a ctenosaur, then sneak toward it and set a piece of cardboard with fruits on it as close as possible without the ctenosaur running away. We would then wait and watch from a far-away vantage point and record what the iguana did. Repeat, repeat, repeat. We often named our test subjects after the place where we found them - for instance, we had dock lizard, log lizard, dining hall lizard, gift shop lizard, library lizard, field lizard, and about twenty woodshed lizards. Data collection continued for five days, and on the sixth began data analysis, presentation-making, and paper-writing.

Results & Conclusions

Our results actually turned out to be really interesting and pretty cool.

(I) In the first phase, ctenosaurs strongly preferred red, yellow, and orange over green, but showed no preference for light green over dark green. None of our seventeen brown vs. green fruit model setups received bites. From this we concluded that lizards do, indeed, use color vision to forage selectively on ripe fruits.

(II) In the first phase, lizards strongly preferred red, yellow, orange, and brown dyed bananas over green, but showed no preference for light green over dark green. From this we concluded that because brown is a common color in the forest (rocks, dirt, feces, etc.), brown is not recognized as a ripe color unless coupled with positive olfactory cues. (We nicknamed this the "Don't Eat Poop" theory.) We also concluded that light green probably represents an unripe or ripening fruit, and thus an unprofitable or only marginally profitable food item.

(III) In the third phase, no difference was observed in whether the red clay fruit model or green banana was approached first, but lizards consistently ate the green banana before biting the red clay. We thus concluded that from a distance, olfactory cues and color vision interact to determine fruit ripeness, but that olfactory cues prevail at close range.

Palo Verde

Our last field station was Palo Verde, in northwestern Costa Rica. We stayed there for three weeks, an hour's drive from civilization (most of us never left the station). The OTS-owned station is in Palo Verde National Park, and is located next to a large marsh. Palo Verde is a tropical dry forest, and we arrived about four days before the dry season started. Although hot, the weather was nice and it pretty much didn't rain after the rainy season officially ended. The first week consisted of lectures, faculty-led projects, and planning for our second independent projects, which took place during the second week (more to come on that in another post). The third week was presenting and writing up our IP's, studying for final exams, taking final exams, and finishing up loose-end assignments.

One of the highlights of our stay at Palo Verde ended up being Thanksgiving, which we all expected to be completely depressing. We had been told that nothing special would happen for Thanksgiving, since it isn't celebrated in Costa Rica. It fell during IP week, so we spent the day collecting data as planned. We had been banned from the dining hall for the day as it was being fumigated (or so we thought). Turns out the whole thing was a cover-up for an absolutely delicious turkey dinner, complete with mashed potatoes, cranberry sauce, pumpkin pie, and the like. On a day when a lot of us were sad not to be with our families, it was exactly what we needed.

To escape all the academic craziness, a few friends and I made it a habit to play soccer before dinner every day with a few guys at the station. Although I'm absolutely terrible at soccer, it became something I looked forward to every day. Our Tico friends were also pretty amused at our attempts to yell soccer-related things in Spanish. The only downside was that the absolute worst time for mosquitoes at Palo Verde is at about 5:30, and despite my constant self-dousings with DEET, my legs were completely mutilated by the end of our stay. Oh well - it was worth it.

A few of my favorite memories from Palo Verde involve the various creatures that inhabit the area around our bunk rooms. There are men's and women's bathrooms at the station, which are next to each other. The wall between them doesn't go all the way up to the ceiling - as in, there's a space between the top of the wall and the ceiling. One night, a few girls and I were in the bathroom brushing our teeth and getting ready for bed. Suddenly, we hear one of the guys in the other bathroom telling us to back up away from the wall. "Why?!" we asked. The reply was something like, "Just do it!" (so we did) and suddenly a scorpion flew over the wall and into our bathroom. After the obligatory screaming and laughing, Becca carefully picked up the scorpion with a paper towel and attempted to throw it back over the wall. Paper towels, however, are not at all aerodynamic and the scorpion instead slammed into the wall. Attempt two, and repeat. That was the end of the scorpion, but the event has become one of our favorite jungle stories.

Another hilarious occurence occurred during the last week, right in the middle of IP and finals stress. Two of my roommates (Claire and Becca) and I were sleeping peacefully when suddenly we were awakened by our third roommate (Courtney) whispering in a very panicked voice. It was about 1:00 AM, so it took me a minute to become conscious, but when I did I realized that Courtney was informing us all that there was a rat in our room. Sure enough, we turned on our headlamps and saw a rat (probably about five inches in length without the tail) digging around in some ziploc bags in the middle of the room. Upon being spotted, it quickly ran and hid under Courtney's bed. I was exhausted, and thus decided to just go back to sleep, but it quickly became obvious that Courtney was not about to let us all sleep when there was a rat living under her bed. Thus, Plan B: I put on my shoes and ran out to the classroom to get one of the insect-catching nets. Back in the room, I crouched down and the floor and somehow (miraculously) managed to swipe the rat into the net on my first try. Laughing hysterically (although trying not to wake everyone), we put the rat outside, thinking that was the end of that.

Wrong. The next night, I was again awaked by Courtney's frantically whispering voice. This time, though, she was saying, "Hilary! Hilary!" When I woke up, she informed me not only had ANOTHER rat found its way into our room, but this time it had climbed into my bed. This, of course, set me flipping my sheets everywhere thinking the rat was going to come flying into my face at any second. Neither of us knows where this second rat went, but it didn't seem to actually be in my bed although Courtney had watched it crawl up towards me. The general consensus is that the rat (thankfully) never actually made it inside my mosquito net. I always knew that thing protected me from things besides mosquitoes; I just never knew from exactly what until then.

Luckily, the next night was our last, and we spent most of it in the classroom revising our papers - thus, no more rat incidents. Good times.

^My, Courtney's, Claire's, and Becca's room at Palo Verde. My bunk, into which the rat climbed on our second-to-last night, is on the bottom right.

^Cows and birds in the Palo Verde marsh. Cows are grazed in the marsh to control invasive cattails.

^View of the sunset over the marsh from the soccer field.

^Tarantula we found during a faculty-led project.

^Measuring scorpions during a faculty-led project.

^White-faced monkey with a baby on its back.

^View from an overlook to which we hiked during the first week.

^Sunset over the marsh - Tim is in the birdwatching stand.

^A few of us decided to dress up for dinner one night, just to be ridiculous (and to make ourselves feel a little bit less disgusting)

After finishing our final exams at Palo Verde, we spent two days essentially vacationing at a touristy place called Buena Vista Lodge. This gave our professors time to grade our exams and us time to unwind from a pretty intense week. During one of our days there, a group of us took a bus to a nearby national park and hiked almost 7 miles round-trip to a waterfall. We spent a while swimming in the water beneath it, which I found absolutely freezing but of course was the only one who thought so.

^Swimming beneath the waterfall.

^We saw lots of cactus-like bromeliads on our hike to the waterfall.

^Hanging out with three of my favorites at Buena Vista Lodge. From the left: Becca, Jess, Courtney, and me.