Sargantanas Pityuses Wall lizards of the Text and photography by: Nathan Dappen As summer approaches, the Mediterranean islands of Ibiza and Formentera begin to wake up Pityuses archipelago from their winter sleep. People from around the world come to soak up the summer sun and enjoy this beautiful Archipelago. But humans aren t the only sun-loving inhabitants here. Tourists will be sharing the islands with a spectacular native the Ibiza Wall Lizard which has been living here for millenia. These lizards have become the symbol, not only of nature in the Balearics, but also of the islands themselves. Lizard silhouettes are the motifs adorning towels, jewelry, t-shirts and even tattoos on people s bodies. These Sargantanas, as the locals call them, are not only popular icons, but their unique biology and natural history are giving scientists a deeper understanding of evolution. A spectacular blue speciment of Podarcis pityusensis on the island of Tagomago.
When I tell people that I m a biologist studying lizards in Formentera, they usually send me a suspicious look and say something sarcastic like, lizards...in Formentera? How convenient. After a few hundred variations of this type of exchange, I feel well prepared to deal with skepticism about my research. Normally, the skeptics and I share a few laughs about how I play with animals on a paradise island, and then, I make a concerted effort to trap them into listening to me explain just what it is that makes these lizards so special. The Ibiza wall lizard, Podarcis pityusensis, is one of about 300 terrestrial lizard species in the Eurasian lizard family Lacertidae. The species is endemic to the Pityuses Islands. That is, it lives on islands in this Archipelago and nowhere else in the world. The closest relative of this species is the Balearic lizard (P. lidifordi), found in the northern Balearic Islands (Mallorca, Menorca, the Cabrera Archipelago). In general, little is known about this species. Lizards living on different islands are often so different, that scientists aren t even sure whether different Ibiza wall lizard should be considered one species or many different species. In the last half-century various researchers have divided populations of this species into various subspecies based on morphological differences such as color, size. Hopefully, ongoing genetic research will end any controversy over how various island populations should be classified. In any case, this species is unique and merits our attention. For most people, one of the first things that come to mind when they think about reptiles is that they are cold-blooded which is, unfortunately, a misconception. Reptiles are ectothermic they get their heat from the surrounding environment. Generally speaking, this means that when its cold outside, reptiles will have cold-blood, but when it s hot, reptiles are capable of achieving body temperatures so high that they would be fatal to most warm-blooded organisms, like mammals. The way that lizards thermoregulate (regulate their body temperature) has far reaching consequences on their biology, particularly when we think about life on small, desolate islands. One benefit of getting heat from the environment is that reptiles don t need to waste energy acquired from food to stay warm. Mammals take their body temperature with them everywhere they go, allowing them to survive in many different climates. But staying warm is costly. In order to maintain their body temperature, a mammal needs to consume about ten times as many calories as reptile of similar size. This difference in thermoregulation allows lizards to survive in places with dramatically fewer resources than would be necessary for similar-sized mammals, and is the primary reason why the Ibiza wall lizard (and many other Mediterranean lizards) can persist on so many tiny, seemingly resource-deficient islets. Accordingly, lizards are usually the only terrestrial vertebrates on most small islets throughout the Balearic Islands. Scientists have been interested in the Ibiza wall lizard for decades because of it s unique evolutionary history. Many populations of this species have been isolated for many thousands of years on islands no bigger than a tennis court. Every island has a unique set of characteristics. Some islands are bigger than others, many have a different plant regime or varying degrees of predator presence. Lizards that were stranded on each island were forced to survive and adapt to unique conditions. By studying lizards adaptations to various island conditions we can learn a lot, not just about lizards, but also about the evolutionary process in general. A lizard from E spalmador sunbathing on an invasive flower patch (above). A view from the small islet of Bledes Plano, home to a population of black Ibiza Wall lizards (below).
One of the more remarkable adaptations in this species is the ability of these lizards to be omnivorous. Unlike their mainland counterparts, who eat insects almost exclusively, various lizard species living on Mediterranean islands, including the Ibiza wall lizard, have evolved the ability eat flowers, nectar, stems, fruit, washed-up jellyfish, and even each other. These lizards are true omnivores and this adaptation has dramatically influenced the morphology, physiology, and behavior of the lizards, as well as the small island ecosystems that they live in. The evolution of omnivory in these lizards is, in large part, the result of the unusually high rate of plant-lizard interactions observed on small island ecosystems in the Mediterranean. The lack of resources on small islands led hungry lizards to look for food in unusual places one of those places was in plants. But, it s not just lizards that benefit from this relationship. While plants supply various resources to lizards, lizards unknowingly provide plants with several useful services. The Ibiza wall lizard is one of only a handful of reptiles worldwide that can be considered a true pollinator. Lizards visit flowers to eat their petals, drink their nectar, and capture insect pollinators. As they do so, pollen grains attach themselves to the lizards bodies. Lizards then visit different plants, and the pollen attached to their body comes off and pollinates new flowers. On many islands, lizards act as one of the most important pollinators. The coevolution of plants and lizards doesn t stop at pollination. One of the main challenges for plants is to disperse their seeds. If a seed falls near its parent plant, it will be competing for resources with its close relative. Further, if that seed survives to reproductive maturity, it has a good change of pollinating, or being pollinated by, the parent plant as it is so close by. Because competing with relatives and inbreeding (reproducing with close relatives) is usually not good in nature, it s best if plants can disperse their seeds as A male lizard from the island of Formentera eats another lizard s egg (top left). Its common to fnd lizards atop flowers drinking nectar, eating petals, or waiting for insect pollinators to fly by (top right). far away from themselves as possible. That s where lizards come in. Lizards are often the only animals on small islands that will eat fruit. After eating the fruit and moving on, lizards defecate seeds away from the parent plant and in doing so are often a principal seed disperser on small islands. Another interesting behavioral adaptation observed in insular lizard populations is an increased rate of cannibalism. On many islands resources are so scarce and competition is so high, that it s not uncommon to see lizards eating other lizards eggs or one another. This may seem distasteful and maybe a little savage, but many animals resort to cannibalism when forced to live in situations with intense competition for limited resources. Just look at what happened to humans on Easter Island, the Marquesas Islands, or pioneers in the American West (e.g., the Donnor Party). Most small islets have no terrestrial predators and one of the biggest threats to juveniles is the possibility of being eaten by an adult. After thousands of years of cannibalism by larger and older lizards, juveniles must have developed a plethora of survival strategies that influence their morphology or behavior. For instance, juvenile lizards are often much less colorful than adults. One possible explanation for this age-based color change is that juveniles lizards may
A male lizard from the island of Torrerta has killed and is eating a juvenile lizard. Cannibalism is common on small islands like Torreta. benefit from being harder to see. As they get older, and less vulnerable to predation by larger individuals, the cost of expressing vibrant adult coloration decreases. Unfortunately, the effects of this cannibalism on the evolution of this species (for instance on age-specific color expression) are poorly studied. To me, the most spectacular characteristic of this species is the striking color diversity across its geographic range. The Ibiza wall lizard exhibits some of the greatest color variation of all reptiles. These lizards live only on Ibiza, Formentera, and about 42 small coastal islands. Yet among these islands, this species can be found in every shade of blue and green. On a few islands, the lizards are orange, or brown, or even black with metallic purple bellies. On some islands lizards are a single color, while on others there are many. Additionally, males and females express color differently (a phenomenon called sexual dichromatism) with males generally exhibiting more saturated and vibrant colors. With all this diversity, scientists can ask lots of interesting questions about color evolution. In my research, I investigate factors that influence or maintain color variation to better understand evolutionary processes. The question that immediately came to my mind when I first read about the Ibiza wall lizard was, why is there so much color variation? The short answer is that we don t really know yet; but, the preliminary results from my research suggest that conspicuous colors like blue and green may have evolved by a form of natural selection called sexual selection. Sexual selection arises from to variation in reproductive success due to competition for mates. These lizards are semi-territorial. Males will aggressively defend overlapping home ranges, and the females in them, from other males. We think that color is used as an honest signal for fighting ability in males because conspicuous color expression increases with as lizards increase in age and size. Larger individuals can win more fights. So, by evaluating a lizard s color, other lizards can assess, from a safe distance, whether it s a good idea to fight with them or not. By communicating fighting ability with a signal like color, big males save time and energy by not fighting males that aren t really competitors, while small males avoid fighting with males that they don t have chance of beating. It makes sense for males to express this color, but why would females also be so extravagantly colored? As I said earlier, females are generally not as conspicuously colored as males, but female color varies dramatically, and on some islands, female color expression is identical to that of males. So far, we ve been unable to figure out any functional role that color might play in females. One possibility is that females are colorful because they share most of their genes with males. As a result, when females mate with big colorful males, they won t just sire colorful sons, but they ll also have colorful daughters, regardless of whether color actually helps females. This possibility, that sexual selection on male color is driving the evolution of male-typical color in females, is looking more and more likely as our research continues. In some areas lizards are brown and blend in perfectly with the sand or rocks. One spectacular population of brown Ibiza wall lizards is found on Formentera s Trocodors Penninsula. If you visit this Peninsula,
you ll notice that there is very little vegetation and that sand, grass and rocks dominate the landscape. Anything that was bright green or blue would stand out in this environment and it s likely that lizards here lack conspicuous color as a result of higher predation on conspicuously colored lizards. Variation in predation, habitat, diet, the strength of sexual selection and many other factors could influence color evolution. While we have a good guess for why lizards might be blue, green or brown, we still don t any idea about why there are blue lizards with yellow backs, black lizards with purple bellies, or orange lizards (just to name a few). Our project is only just beginning and there is a great deal to be discovered! From top left down: male lizards from the islands of (1) Bledes Plano, (2) Es Vedra, (3) Trocadors (Formentera), (4) Ca Mari (Formentera). On the next page, from the top right down: male lizards from (1) Tagomago, (2) Ibiza, (3) Illas Negras, and (4) the Cap de Barbaria (Formentera). Nathan Dappen is a biologist at the University of Miami. In addition to his research career in evolutionary biology, Nathan is an active photographer and filmmaker. See more of his photography and film work at: www.nathandappen.com & www.daysedgeproductions.com. If you have questions about his research or media work, please contact him at: nathan.dappen@gmail.com