The Tibetan ground tit (Parus humilis) is a drab, unassuming little songbird. It makes its quiet living exclusively in the highlands of the Tibetan plateau. Although the bird doesn’t look like a troublemaker, it has been an ongoing source of taxonomic controversy for over a quarter century. The ground tit was first thought to be the world’s smallest living member of the jay family (the Corvidae; Hume 1871). Due to a suite of morphological features that it shares with other Asian ground jays (genus Podoces), it was known as either the Hume’s ground-jay or the Tibetan ground-jay for over 100 years, and was even granted its very own genus, Pseudopodoces within the Corvidae. “Pseudopodoces” was indeed very similar to the jays . . . yet still distinctive enough that dissension about its true identity began to brew in the late 1970’s (Borecky 1978).
When scientists finally analyzed the bird’s genes, they found that the Tibetan ground jay was not actually the world’s smallest jay—it is not a jay at all (James et al. 2003; Gang et al. 2003). This bird is apparently the world’s largest living member of the tit family (the Paridae). These studies overturned what scientists had initially believed about the species. For those of us that don’t tend to think in terms of corvids and parids, imagine how surprised you would be if what was once thought to be the world’s tiniest bear species actually turned out to be the most gigantic dog on the planet.
It was an exciting discovery, and yet taxonomic controversy is not easily quelled. Although there was strong evidence that the bird belonged to the Paridae, how did it come to be so similar to Tibetan jays? What’s the story? Clearly, identifying the bird did not cease the speculation surrounding its origins.
A recent Nature Communications paper has helped to flesh out the story of this identity crisis and its resolution. An international group of researchers, led by Ruiqiang Li and Fuman Lei, present an updated phylogenetic assessment of P. humilis (Qu et al. 2013). They applied advanced genome sequencing techniques, which were unavailable when the species’ DNA was first analyzed ten years ago. The purposes of their study were threefold: 1) to confirm where the species fits into the bird family tree; 2) to determine how long ago it diverged from the rest of the tit lineage; and 3) to identify specific genetic mutations that have accumulated since the species split off on its own evolutionary journey.
The results of the genome sequencing reconfirmed that this much-debated species does belong within the tit lineage, not the jay lineage. The data also indicated that the Tibetan ground tit branched off from the rest of the tit species between 7.7 and 9.9 million years ago. Since that split, it has accrued a variety of adaptations related to making a living as a ground-dwelling bird at high elevation. For example, the researchers found significant increases in the activity of genes that control energy metabolism, hypoxia response, feather formation, and skeletal development. Conversely, genes related to its immune and olfactory systems have become significantly less active.
These high-altitude adaptations are the main culprits behind the decades of confusion about the bird’s identity. As the ground tit and local ground jays gradually adapted to life on the Tibetan plateau, their features became more and more similar to one another. There is a limited number of ways to make a living in extreme environments, so different bird species inhabiting the high Tibetan plateau are likely to rely on similar adaptations to survive. For example, both ground tits and ground jays have long, slender beaks that they use to probe the ground for food, and pale plumage to match their steppe habitats, which gives them a superficially similar appearance. They also both have large bodies (relative to other species in their respective taxonomic groups), allowing them to retain heat more efficiently.
The timing of the Tibetan ground tit’s divergence from other tit species is also noteworthy. The researchers point out that although the uplift of the Tibetan plateau began about 50 million years ago, the most significant increases in altitude occurred between 8-10 million years ago. This directly precedes the Tibetan ground tit’s divergence from other tit species, lending further support to the idea that adaptations to high elevation led to morphological convergence with ground jays.
This case is a prime example of a fascinating evolutionary phenomenon: convergence. Some distantly related species can easily be mistaken for close relatives, simply because selective pressures have induced them to develop similar adaptations over time. For example, the saber-tooth “tiger” form evolved independently in several different mammalian lineages, including marsupials. Those fanged carnivores may have superficially resembled on another, but didn’t share a close common ancestor at all. To put that in perspective, a saber-tooth predator from a true cat lineage, such as Smilodon, is more closely related to you than it is to Thylacosmilus, a saber-tooth marsupial. Another common example of convergence is the development of prickly spines in hedgehogs, echidnas, and porcupines, all of which are only very distantly related to one another.
The take-home message of the new study by Li, Lei, and colleagues is that similar environmental conditions, rather than shared ancestry, misled early zoologists into classifying the Tibetan ground tit as a member of the jay family. The two lineages’ similar adaptations to extreme elevation led to the morphological convergences that have confused taxonomists for decades.
This study also bolstered the genetic evidence regarding the ground tit’s evolutionary origins. In addition, the findings provide insights into gene activity controlling physiological mechanisms for survival at extremely high elevations. Further studies on ground tit behavior and ecology will likely yield clues as to why the activity of genes controlling its olfactory and immune systems has declined as it has specialized itself for life on the high plains of Tibet. The present study analyzed gene activity in the ground tit, but it will be interesting to see future work on gene activity in local ground jays as well. How many of their adaptations arise from mutations on the same genes? Since many traits are controlled by many different genes, the two birds’ genomes could have “found” different ways of tackling the challenges of high elevation living. The mystery of the Tibetan ground tit’s identity may have been solved, but it seems as though it still holds many secrets for scientists to unlock.
Anne-Marie Hodge is currently working on her doctoral degree at the University of Wyoming. She graduated from Auburn University in 2009 with a bachelor’s degree in Zoology, including a concentration in Conservation/Biodiversity and a minor in Anthropology. During her years at Auburn, Anne-Marie was a founding member of Alabama’s first chapter of the Society for Conservation Biology. She completed a Master of Science in Biology at the University of North Carolina-Wilmington in 2012, and has participated in field research expeditions in the southwestern U.S., Mexico, Belize, Ecuador, and Kenya. When she is not chasing carnivores at the equator, Anne-Marie works on her website, blogs at Endless Forms on the Nature Network and is a frequent contributor to Ecology.com.
Borecky, S. R. 1978. Evidence for the removal of Pseudopodoces humilis from the Corvidae. Bulletin of the British Ornithologists’ Club 98, 36-37.
Gang, W., Fu-Min, L., Yun-Fu, T., et al. 2003. Reclassification of the Groundpecker (Pseudopodoces humilis) as the Ground tit. Acta Zootaxonomica Sinica. 28, 554-555.
Hume, A. 1871. Stray notes on ornithology in India. Ibis 3, 403-413.
James, H. F., Ericson, P. G. P., Slikas, B., Lei, F., Gill, F. B., and Olson, S. L. 2003. Pseudopodoces humilis, a misclassified terrestrial tit (Paridae) of the Tibetan Plateau: Evolutionary consequences of shifting adaptive zones. Ibis, 145, 185-202.
Qu, Y., Zhao, H., Han, N., Zhou, G., Song, G., Gao, B., Tian, S., Zhang, J., Zhang, R., Meng, X., Zhang, Y., Zang, Y., Zhu, X., Wang, W., Lambert, D., Ericson, P. G. P., Subramanian, S., Yeung, C., Zhu, H., Jiang, Z., Li, R., & Lei, F. 2013. Ground tit genome reveals avian adaptation to living at high altitudes in the Tibetan plateau. Nature Communications 4, 2071. doi:10.1038/ncomms3071
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