This month, scientists have built a road map to outline the mysterious origins of the world’s smallest warm-blooded vertebrate – a creature you probably know from your own backyard.
Colloquially known as hummingbirds, the Trochilidae family is a member of the order Apodiformes, as are swifts and tree swifts. The smallest species of hummingbird is called the Bee Hummingbird, and can weigh as little as 1.6 grams and perch comfortably on the top of a pencil eraser. Hummingbirds can also boast a slew of other unique adaptations including the ability to flap their wings with amazing speed – up to 80 times per second – wing strokes that can create lift on the upswing as well as the downswing, and the ability to hover in place, fly backwards, and even fly upside down.
Of course, these amazing abilities come at a cost. Their legs and feet are almost useless, allowing them to perch but not to walk or hop. Their energetic metabolisms call for high sugar food; so much of it that it is possible for a hummingbird to starve to death in a matter of hours if it cannot find nectar. For this reason, hummingbirds even sleep differently than other birds. They enter a state called torpor, more akin to hibernation than sleep, in which their metabolisms slow to as little as 1/15 their normal rate in order to keep from starving overnight.
But when and where did such unique adaptations form? That is the current debate among the paleontologists and evolutionary biologists that study Apodiformes. Until recently, little was known of the origins of the hummingbird.
Last year, Daniel Ksepka of the University of North Carolina and his team reported discovery of a detailed fossil in the Green River Formation of Wyoming: a hummingbird ancestor whose tiny proportions suggested that the small size of the Apodiformes may have evolved even before specialized flight capabilities such as speed or agility in hummingbirds.
This prompted another group of scientists led by Jimmy McGuire of the University of California, to trace the evolutionary history of these tiny birds. Their paper “Molecular Phylogenetics and the Diversification of Hummingbirds,” published May in Current Biology, details their findings as they used genetics to trace the evolutionary history of 284 species of hummingbirds in hopes of discovering exactly when Trochilidae as we know them appeared and what different paths along the evolutionary tree they have since taken.
The oldest known hummingbird fossil was found in Germany in 2004 and is about 30 million years old. This was an interesting discovery, because today Trochilidae only live in the Americas and the Caribbean. As the small and delicate bones of hummingbirds don’t fossilize well, it took McGuire’s study to trace the ancestor of every living hummingbird today to about 22 million years ago in South America. They have since diverged into nine distinct groups. Geographical changes, such as the uplift of the Andes Mountains, and the opening of distinctive ecological niches, such as rainforests or alpine meadows, have helped shape Trochilidae into the hummingbirds we know and love today.
McGuire, J. A., Witt, C. C., Remsen Jr, J. V., Corl, A., Rabosky, D. L., Altshuler, D. L., & Dudley, R. (2014). Molecular phylogenetics and the diversification of hummingbirds. Current Biology, 24(8), 910-916.
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