Wednesday, December 28, 2016

Go Forth

The range of European starlings in North America extends northward to Alaska. We think this range is the result of the 1890 introduction (i.e. invasion) of these birds. The native range of European starlings spreads across Europe and Asia, stretching as far East as Mongolia. Soon we will receive modern birds from Alaska, which made me remember a good question from a student a few years ago. Did starlings arrive in Alaska from their southern invasive range, or from their native range to the west? European starlings are not traditionally regarded as migratory birds. That means they do not fly over long distances seasonally to breed, though some starling populations are exceptions to this. But European starlings have been experts in rapidly and permanently expanding their range. So, some starlings go back and forth, and some just go forth.

The movement of starlings across the Alaskan landscape reminds me of a recent study about early human migrations. Though by feet not flight, humans may (or may not) have traversed a similar path, and with similar rip-roaring gusto. The traditional story is that the first humans arrived in North America across the Bering Straight from Siberia via a now long-gone land bridge. Some scientists have recently suggested that although the land bridge was present 13,000 years ago, the conditions in the region would not have allowed humans to survive (Pederson et. al, 2016). The landscape was barren with no exploitable resources for food or shelter. It was the Ice Age after all, not exactly a time of warmth and abundance. The authors go on to say that perhaps humans arrived via a different, unknown seafaring route. No matter which route they arrived, I always think of those first migrants. Human migrations did not complete themselves within the lifetime of one individual. But that does not mean that brave explorer types did not exist thousands of years ago in the human population. Undoubtedly, these individuals would make Columbus and Darwin look like wimps. The world was wilder then. Imagine going out in the morning and not knowing if continuous land lay ahead? or if you would find familiar food? And you would be a prisoner of the daily cycles of light or dark, and of the unusually long days, or continuous Alaskan nights. It makes my risk-averse body shiver.

A unique quality that humans have, that separates us from other species like birds, is that one individual can have an impact for years to come via the cultural transmission of information for generations. We have key people throughout history who, for better or worse, leave a mark on the population. Then, generations later we remember that person. But was their a starling Elvis Presley, or an Abraham Lincoln starling? The answer is no, but also kind of yes. We do see a few individuals that lead what we call the expanding front of birds. These are the birds on the edge of the group that led the species further into new localities. What was it about those individuals that made them fly farther in a new direction than any starling had before? And maybe the movement wasn’t very far, just a few new miles within their lifetime. They also didn’t know they were explorers. They weren’t leaving a flag in the ground or posing for grainy photographs, they were just eating different caterpillars and living in a new old tree. We can measure their wings, legs, bodies, and learn something about the geographic distribution of genetic diversity to understand more. The answer for why they moved is probably food or safety. This is likely the same reason early humans moved too. No one was running from war or towards religious freedom. Yet. But the new question is how did they adapt to new environments so well? Among hominins, we credit Homo sapiens with the astonishing capability to adapt to new highly varied environments. But the story of European starlings in North America is also one of amazing adaptive flexibility.

In humans, a specific genetic variant of a dopamine receptor gene called DRD4 has been linked to “novelty seeking behavior”. People with the shorter version of this gene are less likely to explore, and those with longer versions are more likely to explore. Some authors have suggested this allele played a role in the behavior of the original human explorers (Matthews and Butler, 2011). In birds, a specific genetic variant of the ADCYAP1 gene has been implicated in seasonal migratory behavior (Mueller et. al., 2011). But we know birds, just like humans, expanded their ranges too. Who were those first starlings that went forth into Alaska and didn’t go back? Why did they go? How did they adapt? They were not what we should call brave, but they were almost certainly, different.