In Part 1 of my field season wrap-up, I summarized the first two objectives of my 2011 field season: 1) breeding lizards to investigate the heritable bases of color (which failed miserably), and 2) determining whether color was under intrasexual ontogenetic conflict (my data suggest that this is happening). My third objective of the field season was to investigate what might cause divergent color evolution among lizard populations and determine whether those color difference resulted in any degree of reproductive isolation (i.e. will lizards from a blue population recognize lizards from a brown population as mates or competitors?)

The Ibiza wall lizard exhibits striking color diversity across its geographic range. On some islands lizards are blue, others they are green, or brown, or orange or even black. Different shades of blues, greens, and browns are by far, the most common colors found among lizard populations in this archipelago. Research from my 2010 field season suggests that blues and greens evolved by sexual selection. That is, some individuals are more successful at reproducing than others. If a particular trait, say color, helps them reproduce, they’ll pass on those genes for color to the next generation and eventually, that color will become common in the population, or if sexual selection is strong enough, that trait may become exaggerated over time. Brown on the other hand is likely favored because brown lizards suffer less from predation.

My research (as well as research on closely related species) suggests that color evolved in the Ibiza wall lizard as an honest signal for male fighting ability. In populations with green and blue lizards, color increases in both coverage area and saturation as lizards get older and larger. The result is that very colorful lizards are also very large. When males fight for resources like females, they can gauge whether it’s a good idea to fight with a male by looking at his color. If a male is colorful (big), run, if a male is dull (smaller), fight. Over time sexual selection has led to colorful lizards. But being colorful isn’t all its cracked up to be. Colorful lizards are also easier for predators to see. So, individuals that have the highest reproductive success are also the ones that are most likely to get attacked and eaten by predators (to see a film about this research click ).

If a geological event separates one population of lizards into several isolated populations, small differences in natural and sexual selection among islands can result in large morphological and/or physiological differences among lizard populations. If two populations that have diverged separately come back into contact, they may have changed enough while they were separated that they will no longer reproduce.
On the island of Formentera, there are three distinct lizard morphs that are restricted to specific locations. In the south-west, there is the Cap De Barbaria which is home to a population of blue lizards. In the East is La Mola with a population of turquoise lizards; and in the north-west we find the barren Trocadors peninsula – home to a population of completely brown lizards.
The geographic distribution of these populations isn’t random. The Cap de Barbaria and La Mola are the two highest points on Formentera. The rest of the island is close to sea level. In the past, sea levels were much higher, and these two areas of Formentera were most likely separated by water into two small islands. Similarly, the Trocadors Penninsula connects underwater to a shallow ridge that runs all the way to Ibiza. The highest points on this ridge now form a chain of small islands, but sea level would only have to change plus or minus 5 m to either isolate new islands or to completely connect Formentera with Ibiza. I suspect that there was a period of time in the past when the Trocadors peninsula itself was a small island (or series of tiny islands) during a period in the past when sea levels were a little higher than they are today.

Formentera is a small island – 83 km2. The two most distant locations on the island are no more than 21 km from one another. The obvious question is: how can distinct populations of the same species maintain their differences on such a small island? Gene flow between populations should quickly eliminate these differences. That is, when lizards from one population migrate to another population and reproduce, they bring their genes with them, therefore reducing the genetic (and presumably morphological) differences between populations. This doesn’t seem to be happening on Formentera. Why?

The two most distinct populations are the Blue lizards from Cap de Barbaria and the Brown lizards from the Trocadors Peninsula.  The interesting thing about these populations is that they meet and end abruptly at the base of the Trocadors Peninsula, where the vegetation stops. I set out to figure out some of the ecological forces that influence color, and see whether these changes influence the way different populations interact. If you look at the landscapes of these two locations you immediately notice that there is WAY more vegetation in the Cap de Barbaria, while there is virtually no vegetation on Trocadors, which is instead characterized by rocks, sand and sparse patches of grass.

The first thing I thought was that selection against being blue must be high on Trocadors, because there would be nowhere for conspicuous blue lizards to hide. In my previous clay-model predation experiments, we found that predators attack colorful blue lizard models more than brown lizard models and that they attack more models in the open than in the vegetation. So, predation likely selects against blue lizards trying to migrate out onto the Trocadors peninsula.
Another interesting possibility was that the lizards from these populations don’t mix because, as a result of divergent color evolution, they don’t really recognize one another as competitors or mates. We tested this in two phases. First, we captured males and females from both populations. We tethered individuals to the end of a fishing pole using thread. We then introduced these blue males and females and brown males and females to resident males on the Trocadors peninsula and recorded their response.
Brown males from Trocadors were less likely to attack blue males from Cap and were less likely to court blue females from Cap compared to brown males and females from their own population. Ok, Trocadors lizards consistently treat Cap de Barbaria lizards differently. The problem is that lizards from Cap de Barbaria are not only a different color than lizards from Trocadors, they are also twice the size and they may smell different (who knows!).

The next phase of the experiment was to determine whether Trocadors lizards treated Cap lizards differently as a result of color differences, or because of something else. So, we painted brown Trocadors lizards to look like the blue Cap lizards and introduced these painted lizards to the Trodadors lizards. We found the same thing! Trocadors males are less likely to attack Trocadors males that are painted to look like blue Cap lizards and are less likely to court Trocador females that are painted to look like blue Cap females than they are with non-painted lizards.

What does this all mean? Well, I’m not 100% sure yet. Its not clear whether brown Trocador males are hesitant to attack blue lizards because they are afraid of them or because they don’t recognize them as competitors. What is clear is that color seems to play a role in how these lizards interact and appears to result in some degree of reproductive isolation. This behavioral reproductive isolating barrier seems to be reinforced by predation on blue lizards out on the Trocadors peninsula. So, it looks like divergent sexual selection and predation are working together to maintain distinct lizard color morphs on this tiny island.

There is still a lot to do before we draw any real conclusions. Right now, I’m working in the lab to figure out how much gene flow is occurring between these two populations. The results of this lab experiment will largely influence the way we think about what’s happening. If there is little to no gene flow between Cap de Barbaria and Trocadors populations, then perhaps the lizards just don’t travel very much, or maybe behavioral reproductive isolating mechanisms are more important in maintaining population differences than predation is. On the other hand, if we find that there is a ton of gene flow between the populations, then that mean there is really strong selection against blue lizards on the Trocadors peninsula! As soon as I have that piece of the puzzle, Ill let you know!

I finished my fieldwork on June 1st and planned to spend my last week just relaxing and enjoying the island. I never ended up relaxing, but what happened that last week was pretty amazing. In about four days, I had several newspaper interviews, two live radio interviews, and gave a televised talk as the inauguaral event at the Formentera biological center. If you click on the images below you can read a few of the articles they wrote about my work and watch a news clip about my talk. Unfortunately, it’s all in Spanish or Catalan.

Formentera is small (local population about 8000 – even though about 1 million people visit each summer), and there aren’t very many lizard biologists. In fact, most of the time, I was the only one. By the time I left, everyone on the island was calling me ‘the lizard man’ or ‘Sargantana-man’ (Sargantans is what people in the Balearic islands call their endemic lizards). It was a wonderful experience being accepted into the island community. By the time I left for Miami, I felt that in some way, I had become a part of the island. Maybe it was the sense of community, or the stack of unanswered biological questions, but it was hard leaving that island and I have a strong feeling that I’ll be going back some day.