How the hummingbird ended up in the Andes

Hummingbirds, fast and busy birds, live full lives – and some species do so even in the thin air of the Andes.

Just how these birds do so has been unclear: a bird that needs as much oxygen as does the hummingbird seems ill suited for the oxygen-poor Andean highlands. Animals, of course, tend to live where their resource needs are met.

But a new paper published in PNAS this week reports that different species of high-altitude South American hummingbirds have, independent of each other, alighted on the same genetic mutations to solve their high-altitude problems: these birds have optimized oxygen transport systems that keep their bantam bodies flush with oxygen.

“Natural selection has hit upon the same solutions time and time again,” says Jay Storz, co-lead author on the paper and a geneticist at The University of Nebraska-Lincoln.

The hummingbird, all dressed up and ready to dance in ruby reds and sapphire blues, looks like a living jewel. In Victorian England, fashionable girls wore the bright, sparkling birds as taxidermic baubles swinging from their lobes. The birds are still the darlings of jewelers, but as inspiration, not material.

But these decadent-looking birds live their lives at speeds unbefitting the leisure class. The hummingbird’s life is fast – so fast that is has the highest metabolic rate among all vertebrates. A hovering hummingbird burns energy at about 10 times the rate of a standout human athlete operating at peak performance. When the diminutive bird is afraid, its heart can beat up to 1260 beats per minute.

So, the hummingbird needs oxygen, and lots of it. But, in a puzzling point, about a quarter of these little oxygen guzzlers are found in the rarified air of elevations above about 10,000 feet. That kind of elevation is anathema to a human athlete. How do these birds do it?

To find out, the research team collected blood and tissue samples from 10 species of hummingbirds distributed at different elevations in the Andes, up to about 15,020 feet (hummingbirds are found at up to around 16,400 feet in the Andes). The team found a strong relationship between these species’ elevations and the likelihood that their hemoglobin had the high oxygen-binding affinities needed to take up enough oxygen in oxygen-poor environments.

The authors then reconstructed the ancestral states of each species’ hemoglobin. This analysis showed that multiple species of hummingbirds had, independent of each other, colonized the mountain range’s high-altitudes and evolved the same adaptive trait required to do so.

“Hummingbird species that live at high altitude have had to evolve highly efficient oxygen-transport systems to fuel their high rates of aerobic metabolism,” says Dr. Storz.

The team next sequenced 63 hummingbird species’ DNA to investigate how optimized oxygen binding appeared at the molecular level. Previous work has shown that different species often happen on the same adaptive solution to a problem without mirroring each other at the genetic level. In other words, the mutation that codes for superior oxygen binding might be found in different places in different species’ DNA. Or, different mutations in the same gene might produce the same changes in the animals’ oxygen transport systems.

But, in high-altitude hummingbirds, the change in how the birds’ hemoglobin binds oxygen occurred in mutations at the same two amino acid sites in their DNA. One of these mutations occurred in at least 13 separate hummingbird species. The other one occurred in four different species.

“This is the single most remarkable case of parallel evolution ever discovered,” says Christopher Witt, a co-lead author on the paper and an ornithologist at the University of New Mexico. “The hummingbird example is spectacular because it's the same changes to the same gene that occur repeatedly, each time in the exact same environmental context.”

That several species of high-altitude hummingbirds have alighted on the same molecular change to their oxygen transport systems “makes sense,” says Dr. Witt, “because there are probably very few possible ways that they could tweak such a finely honed system.”

 “We imagine that the vast majority of possible mutations would be damaging,” he says, “so natural selection insures that we never find them in nature.”