Split ends

Picture a stream meandering through a forest, and imagine what happens when it encounters a rock. Following the path of least resistance, the stream splits into two, each half finding its way forward on either side of the rock. Once they’ve moved past the barrier, the two streamlets flow back together, moving ahead as one. A few feet down, you won’t be able to tell that the rock was part of this stream’s history. But what if you sat beside the rock, and carefully dripped oil into one of the two streamlets? When the two arms of what used to be one stream meet on the other side of the rock, they will fail to reunite. You’ll be able to tell that they are different streams now, even though they flow alongside one another.

Now imagine that the stream, instead of being made up of droplets of water, is filled with little animals that are shaped like lizards but feel like frogs. This tale of streams splitting and then failing to rejoin becomes the tale of Ensatina salamanders in California, US. It’s the story of how new species form.

Just like frogs, salamanders are amphibians, so they need to live in a wet environment. Even the daytime air is too dry for Ensatina, so they move around at night and spend their days living under logs. When active, they potter about, looking for insects to eat on the forest floor. Not the most jet-setting of lifestyles, but over the last few million years, they’ve made their way down the entire length of California, a distance of over 1500 km. Like the stream that encounters a rock, the salamanders’ dispersal was impeded by the punishingly hot and dry Central Valley. So the population of Ensatina split and parted ways. As each half of the population encountered different environments — on the coast and on inland mountains — the salamanders gradually acquired changes. The most striking of these changes were in colour — while the coastal varieties stayed uniformly coloured, the mountainous varieties became blotchier.

When they do finally come back together, like oil and water, the coastal and mountainous varieties no longer mix. The two types of salamanders do not recognise each other as mates and therefore do not interbreed, which is the very definition of belonging to different species. But everywhere along the ring, salamanders from adjacent populations can interbreed, which means that they are one species at the top of the range, but two species by the time they get to the bottom. The discovery of this remarkable pattern was a vindication to biologists, who had long been in search of that perfect example, showing the process of how millions of species that we share our world with have formed.

Last month, I travelled in California with a colleague, Sofia Prado-Irwin, whose enthusiasm for amphibians and reptiles in general, and for Ensatina salamanders in particular, rivals any biologist’s enthusiasm for any other creature. She ascribes her enthusiasm partly to these animals’ “interesting behaviours and evolutionary stories,” but also to “some innate reason that I can never describe.” I understand this inexplicable awe in the face of a small slice of nature — I feel similarly about lizards. But my passion by no means matches Prado-Irwin’s, and I got to see her excitement about Ensatina firsthand when we went hiking in the shade of redwood trees with the explicit goal of locating some salamanders. This turned out to be an easy task. We only had to “flip over a log, and there was this bright orange gummy-bear looking thing on the ground,” as Prado-Irwin described. Minutes later, we’d found a large female and a little male, eliciting from Prado-Irwin one of the biggest smiles I’ve seen on a biologist’s face.

With just a couple of these salamanders in hand, it’s difficult to appreciate the bigger geographical and historical patterns that make these animals famous. Their bright colours are more immediately impressive. And just as Prado-Irwin had suggested, there was an interesting story to be told here too.

In some parts of California, Ensatina co-occurs with a species of newt that is incredibly toxic. In contrast, Ensatina are not toxic at all, but they take advantage of their noxious neighbours. By evolving the very same colour patterns as the newts, down to identical yellow eyes, the salamanders can fool predators into thinking they are newts. These predators have learnt, possibly through painful experience, to avoid the toxic newts at all costs.

But to an amphibian and reptile enthusiast like Prado-Irwin, clever ecological interactions like the one between the salamanders and the newts are only part of why these animals are worth our attention. Much of her fascination derives simply from the chance to see them living their daily lives. She recounts how, as a six-year-old, she saw “this giant bull snake that seemed to me like it was a million feet long. I was just so scared and impressed at the same time. It was an eye-opener [to see] that things like this live in a place where I live.” It’s an impression that has persisted for two decades, directing Prado-Irwin’s attention to frogs, lizards, and Ensatina salamanders. I suspect that if more of us were as impressed by a plant or animal in our vicinity, we’d be doing a much better job of getting along with the millions of species we share the Earth with.

( Ambika Kamath studies organismic and evolutionary biology at Harvard University)

COMMENT NOW