Creature Feature. Lizard, lizard on the wall

Picture a scene of the Mediterranean countryside, with winding roads, olive trees, perhaps a grazing cow or two — does your imagined scene include any lizards? Mine probably wouldn’t have, but after talking to Colin Donihue, a PhD candidate at the Yale School of Forestry and Environmental Studies, I now know better. In particular, I’ve learnt that if my scene contains a wall constructed from loosely piled rocks and stones, then there’s a good chance that the wall is home to a large number of lizards belonging to the genus Podarcis. The lizards’ common name is a hint — somewhat unimaginatively, Podarcis are also known as ‘wall lizards’. Consequently, while studying Podarcis lizards on small islands in Greece, Donihue has spent a lot of time looking at walls.

Walls are a decidedly human creation — the islands themselves, and the lizards on them, are much older than the walls. So at some point in their history, in “the era of Greek mythology, of Odysseus and Homer,” Donihue suggested, Podarcis lizards encountered walls for the first time. But now, “these rock walls have been there for hundreds of years and have been built the same way for thousands of years.” So just like they would have adjusted to new plants or animals in their environment, Podarcis lizards have adjusted to the presence of walls.

To someone who spends a lot of time looking for lizards, this association between these reptiles and walls is not surprising. Think of house geckos, for example — where do you usually see them sitting? Donihue had internalised this expectation too: “I was always finding [Podarcis] on the rock walls and didn’t really think very much of it.” But careful observation led him to an unexpected conclusion: “I realised that lizards on the walls were behaving very differently from lizards off the walls.”

On walls, Podarcis lizards tend to sit in one place, basking in the sun and surveying their surroundings, waiting for an insect to pass by before quickly darting to catch and eat it. In contrast, in parts of the islands that remained wall-less, Donihue found that the Podarcis lizards “were sprinting between bushes all day long, looking for their prey,” instead of waiting for the prey to come to them. By building walls, humans have unwittingly precipitated changes in something as fundamental as how these lizards find their food.

But another sort of interaction between humans and lizards led Donihue to study Podarcis in the first place. On his first trip to the Greek islands, he noticed an interesting difference between islands of different sizes. “On small islands, lizards are not very scared of you,” he explained, and this makes catching them easy. “The way we would catch them is to dangle a little mealworm on a string. They will literally come jumping out of bushes and grab the mealworm, and you can just plop them into buckets.” In contrast, on larger islands, “they’re much more skittish, hesitant. You have to stalk them a little bit.”

While listening to Donihue, I was struck by how he described himself catching lizards in much the same way that we ecologists might describe a predator catching prey. Unsurprisingly, the difference between islands in the lizards’ response to humans could be traced to whether or not these lizards had to deal with predators like snakes and birds and cats — predators are prevalent on larger islands, and absent from smaller islands. By unintentionally mimicking an important member of its ecological community, a human could discover something important about a lizard’s biology.

Donihue is studying the ecological and evolutionary consequences of these dramatic shifts in the Podarcis lizard’s lifestyle, brought about by the absence of predators and the presence of walls. Adapting to these shifts can lead to changes in what these lizards look like, how fast they run, and how hard they bite, among other things. The intertwining of lizards’ and humans’ ecological roles, not only in bringing about these changes but also in discovering them, raises an interesting question: How do we ecologists think about our own role in the parts of the natural world we are studying?

Most of us consider ourselves distant observers of, not active participants in, the ecosystems we work in. “I like to think of myself as trying to see things as they unfold, in the way that they should be unfolding if I weren’t there,” Donihue explains. But his observations of Podarcis have begun to shift his views slightly, towards paying greater attention to human interactions with ecosystems. This doesn’t really signal a change as much as an expansion of Donihue’s view of nature: “I’m really driven by the desire to understand the connections between things.”

It’s creatures like wall lizards which can convince us that to really understand the natural world, we need to include people in these connections too.

Ambika Kamath studies organismic and evolutionary biology at Harvard University; ambikamath@gmail.com

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