My field season – perhaps my last as a graduate student – ended almost two months ago. Most people who follow Day’s Edge know that I study color evolution in an endemic lizard species found on the charming Spanish Mediterranean island of Formentera. Honestly, it all went by in a flash. Seems like yesterday that I was online and disapproving of Google’s annual April Fools hoaxes. Over the last three years, I spent about ten months in the islands conducting my PhD research and enjoying island life. I posted info about some of our results from 2010 here. Also, I posted a little description of Formentera last year. You can check that out here.
This year, I spent April, May and the first week of June on Formentera. I had three main goals to accomplish: I wanted to 1) set up a breeding experiment to investigate the heritable basis of color; 2) test whether intrasexual ontogenetic conflict over color expression was occurring (i.e. are the costs and benefits of expressing color different between juveniles and adults of the same sex); and 3) investigate what might cause divergent color evolution among 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?). Needless to say, these were ambitions goals for only two months of work. Luckily, I didn’t have to do all the work alone. I brought four University of Miami undergraduates with me as field assistants. It’s amazing what you can accomplish with a team!
The breeding experiment failed miserably. Here’s a Field Vision video about my set up. Watching it now, I’m not surprised it didn’t work. Lizards kept escaping and breeding pairs seemed to hate each other. In total, I only collected four eggs—not nearly enough to estimate heritability. Additionally, I didn’t have the time or money to stay until those eggs hatched, much less rear them until adulthood. So, ixnay on the breeding experiment. It was a full-fledged failure. That’s OK. Research doesn’t always go as planned, which is why its important or researchers to be flexible.
On to project number two: figuring out whether color was under intrasexual ontogenetic conflict. In 2010 I noticed something interesting: in areas were there was patchy vegetation interspersed with open habitat, I kept seeing juveniles out in the open—tons of them. Adults, on the other hand, seemed to stay closer to vegetation. At first, I ignored this observation. But after visiting a few new islands, I also noticed that the juveniles of some islands were more colorful than juveniles of other islands. And it wasn’t simply that juvenile color was correlated with adult color from the same population. On a few islands, juveniles looked very much like their adult counterparts, while on others they were much less colorful than the adults.
The next thing I know, I see six cannibalism events in the span of a few weeks (to see a video of these lizards cannibalizing juveniles watch our 3-minute video Cold-Blooded Cannibals). And suddenly, I had a new hypothesis. Maybe vegetative patches are these lizards’ favorite habitats. This makes sense. There’s more food in these areas and there’s less risk of getting attacked by a bird when a lizard is in the bushes. But these patches are limited – there are only so many of them and in each patch there’s only enough room for so many lizards. Adult lizards fight amongst themselves for the best patches. The biggest, baddest males end up in their favorite bushes therefore excluding losers to marginal habitat. Juveniles can’t possibly compete with adults for these resources – especially since these little guys need to be careful not to get eaten! So, they abandon the vegetation for open areas until they are big enough to compete. But now these juveniles are out in the open, where they are vulnerable to other predators like birds. The price of being colorful in the open is much higher than it is for residents in the vegetation. Therefore, there should be strong selection on juveniles to be less colorful. The problem is that adult males use conspicuous color to advertise their fighting ability. In sum, being colorful is bad for juveniles because it makes them more conspicuous to other lizards (who eat them) and more vulnerable to detection by avian predators (more vulnerable than adults because juveniles are forced in to marginal open habitats). But being colorful is good for adult males, because it helps them win fights, which allows them access to more resources.
I think that color expression is under conflict among age groups. If colorful adult males sire colorful juveniles, the juveniles will be at a disadvantage. If less colorful males give birth to less colorful lizards, they may fare well as juveniles, but as adults, they aren’t going to win very many fights. Because color expression is probably controlled by many of the same genes in both juveniles and adults, its tough for lizards be successful in both these stages of development (hence ontogenetic conflict).
Natural selection is a pretty amazing process. Generations of selection on hormonal responses (which play a prime role in the expression of color genes) have likely resulted in juveniles that are much less colorful than adults. But the fact that in some populations juveniles are still colorful suggests that there are some populations where this conflict has not been fully resolved. Wow. That was a long explanation.
I tested this hypothesis with four simple experiments. First, I did a survey to find out if there really were more juveniles in patches than vegetation by putting pitfall traps in vegetation of open habitats and seeing what we’d catch. Second, we performed a clay model predation experiment, making hundreds of clay lizard models, putting them in vegetation and open areas and looking at where they were attacked most and by what types of predators (you can see these experiments in action by watching Field Visions II and III). Third, we collected juveniles and inspected them for scars that came from being attacked by other lizards (see Field Vision V). And finally, we tethered juveniles to a fishing pole and introduced them to adult lizards to see whether the adults would try and eat them (again, if you want to watch some lizard cannibalism, check out Cold-Blooded Cannibals).
Unlike the breeding experiment, everything work out just as I expected. We found more juveniles in open areas and more adults in vegetation. In the predation experiment, lizard models were attacked more by birds in open areas and by lizards in vegetated areas. In both areas, colorful models were attacked more than cryptic models. Out of 15 juveniles that we caught, all but one of them was covered in scars from lizard attacks and almost every adult lizard, male and female, immediately attacked tethered juveniles as if they were food items. It doesn’t get cleaner than that, folks. The only data missing (data that I will probably never collect) to show that color is under intrasexual ontogenetic conflict, is data showing that colorful adults actually sire offspring that are more colorful than less colorful adults. For now, this is just something that I assume happens.
This post is already WAY longer than I planned, so I’m going to stop here and tell you about my final experiment in a few days.
One last note: this week was the annual Animal Behavior conference. They have a film festival at this conference. This year, Neil and I had three videos in the festival –Who’s your Neighbor, Battle of the Sexes, and Alpha Male. Who’s your Neighbor and Alpha Male can be seen on our Days Edge site and Battle of the Sexes should be up there by tomorrow. We don’t know yet how our films fared, but if you get a chance, they are worth checking out.